Evapotranspiration - University of Arizonaswhydro.arizona.edu/archive/V7_N1/SWHVol7Issue1.pdf · John M. Baker Micrometeorological methods of measuring ET, such as eddy covariance

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Evapotranspiration

Volume 7/Number 1 January/February 2008

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Evapotranspiration—the sum of water evaporated from bare ground and open water plus water transpired by vegetation—is a significant component of the water balance in the Southwest, and one of the hardest to measure. It varies enormously across an area, depending on such factors as how much sun reaches the surface, how much is reflected, the temperature and moisture content of the air, the wind speed, how much moisture and heat are in the ground, the type and density of plants growing, and many more hard-to-measure and highly variable factors.

It’s often simpler to measure precipitation, then subtract runoff and groundwater infiltration and assume ET is what’s left. For a rough estimate, that approach works. But more precise ET rates are important to some. Irrigators need to know how much water to order. Groundwater modelers want to calibrate their models accurately. Water managers need to know how much water they can properly transfer from an irrigation right to a municipal one. In this issue, our feature authors describe various ways ET can be measured, and where the errors in measurement come in.

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Richard G. Allen Martha Anderson John M. Baker

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T h e R e s o u r c e f o r S e m i - A r i d H y d r o l o g y

Atomic force microscope image of the lower surface of a plant leaf showing stomata, which regulate transpiration, the transfer of water vapor and other gases to the atmosphere. Photo: nanoAnalytics GmbH. See www.nano2life.org.

� • January/February 2008 • Southwest Hydrology

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Inside This Issue

18 Why Do We Care About ET?Richard G. AllenWhat is ET and why do we need to measure it? Find out why hydrologists, engineers, groundwater modelers, agronomists, ecologists, and water managers care about ET and how its measurement figures into their decision-making.

20 Approaches to ET MeasurementBetsy WoodhouseRelatively precise means to measure ET over crops have been developed, but what if you need to know ET for an entire watershed and don’t have an NSF grant handy?

22 Evapotranspiration Measurement MethodsW. James ShuttleworthMethods for direct measurement of ET generally involve water budget and vapor flow measurements. This handy table compares the assumptions, strengths, weaknesses, measurement scale, and error rates of the more commonly used approaches ... and a few of the up-and-coming ones.

24 Challenges and Cautions in Measuring EvapotranspirationJohn M. BakerMicrometeorological methods of measuring ET, such as eddy covariance and the Bowen ratio/energy balance, have improved the currency and accuracy of ET data, or have they? Potential users need to be aware of pitfalls, limitations, and costs of these methods.

26 Quantifying Riparian EvapotranspirationRussell L. Scott, David G. Williams, Travis E. Huxman, Kevin R. Hultine, and David C. GoodrichEfforts to improve measurement of riparian ET using micrometeorological and plant physiological techniques are resulting in greater appreciation of the importance of riparian vegetation in the water balance of Southwest basins and of the impacts of vegetation change from grassland to shrubland.

28 ET – The Key to Balancing the Water Budget in the SouthwestMichael T. Moreo, Nancy A. Damar and Randell J. LaczniakThe accuracy of basin water budgets is an essential factor for models to effectively predict the potential effects of groundwater pumping. But obtaining basin-wide ET to include in the water budget is no small task. Read about one such approach in eastern Nevada.

30 From High Overhead: ET Measurement via Remote SensingRichard G. Allen, Jan M.H. Hendrickx, David Toll, Martha Anderson, Jan Kleissl, and William KustasCombining spatial ET data from satellites with point measurements of ET and soil moisture and modeling data is proving to be a cost-effective way to improve accuracy of ET mapping. The approach is being used in a wide range of water management and monitoring applications.

Departments8 On the Ground

Visualizing GW trends, by Fred D. Tillman and Stanley A. LeakeThe Lower Basin’s water future, by Mark Lellouch

12 GovernmentDelta fish trump farmersCA perchlorate standard implementedYucca Mt. drilling woesUSGS lab employees keep jobsEl Paso desal plant opensNavajo Nation wants CO River shareCCSP progress assessedNew NM desal research lab opensBoR awards drought relief fundsNew EPA publicationsFeds outline water sci/tech strategy

36 R&DPolonium-210 in NV wellsNRDC: Manage for droughtAuto-schedulers for irrigation soughtAlgae production for biofuelsTrees on levees OK in CADrug use monitoring via wastewater

38 Business Directory

39 Society PagesWateReuse success storiesGuidance for NM water systemsCA groundwater meeting wrap-upMarvelous ISMAR

41 PeopleReclamation names new directorsNew chief hydrologist at ADWRSnow honors climate scientists

42 Calendar

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EvapotranspirationWith all the sunshine and warm temperatures we have in the Southwest, we have the potential for very high rates of evapotranspiration—if water is available. The question is how much ET actually occurs? ET is a difficult parameter to measure, but various methods have been developed to try to get at that upward flux of moisture. These range from direct measurement at a single point to direct measurement from space, or indirect measurement over an equally large range of scales. Who is using these methods? For what uses? What sorts of values are being measured? Our contributors address these questions and more.


T h e R e s o u r c e f o r S e m i - A r i d H y d r o l o g y We thank the following sponsors for their support:

P.O. Box 210158B, Tucson, AZ 85721-0158 · visit our web site: www.swhydro.arizona.edu · 520.626.1805

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January/February 2008 • Southwest Hydrology • �

ON THE GROUNDA Visual Representation of Trends in Groundwater ConditionsFred D. Tillman and Stanley A. Leake – U.S. Geological Survey

Groundwater databases maintained by the Arizona Department of Water Resources (ADWR) and the U.S. Geological Survey (USGS) are an important component of many studies of water resources in Arizona. In the past few years, the USGS Ground-Water Resources Program has been studying the possibility of using these databases to present basic interpretations of groundwater conditions over large areas, as has been done for surface water in the form of networks of stream gauging stations (for example, see WaterWatch at water.usgs.gov/waterwatch/). A web-based system that presents basic interpretations of groundwater conditions would give water managers, politicians, and the general public a way to understand this valuable resource. A prototype system has been developed for a number of alluvial basins in Arizona.

Turning Data into Trend MapsOne indicator of groundwater conditions that can benefit from an expanded-area approach is the trend in recent water levels measured in wells. For surface water, “recent” might encompass periods of days or weeks, but for groundwater in the alluvial basins of Arizona, it could encompass years.

Water-level data from USGS and ADWR databases have been combined and a computer algorithm developed to evaluate trends in the water-level data. The algorithm requires, at a minimum,

a unique well identifier with water-level observations and dates, along with location information for the well. Sufficient spatial coverage of water levels is necessary to show trends in groundwater conditions. Sparse data, especially evident in rural areas, present challenges to interpreting trends in poorly monitored regions. Regular and frequent data collection is also required to capture temporal changes in water levels.

The algorithm computes linear trends in water-level observations based on a user-defined date range, minimum required number of observations, and goodness of linear fit. The algorithm seeks the best linear fit of data in the time period, based on the user’s choice of any or all of the following: 1) all observations; 2) maximum and minimum amplitude observations of cyclical periodic data; and 3) user-specified months of observation. Water levels recorded during pumping are not used in computing these trends, and all hydrographs are visually inspected to ensure that the

computed linear trend qualitatively represents the trend in the data.

Linear trends from individual wells are projected to a regional scale by a second algorithm that constructs modified Thiessen polygons around each well, utilizing a user-defined maximum distance to constrain the representative area for each well. These regional trends are presented in a prototype online interactive

map service with linked well hydrographs that highlight data used in computing the trend (montezuma.wr.usgs.gov/website/azgwconditions/).

Lower Colorado River Basin ExampleAn investigation of recent trends in groundwater conditions that employs this approach is currently underway on the most developed of the 72 alluvial basins in the Lower Colorado River Basin in Arizona (see map), with the results to be made available as part of a broader “Arizona Groundwater Conditions” website. For

these analyses, trends are computed for the most recent 10-year period (1997 through 2006). Areas are

labeled “falling” if the rate of water-level decline exceeds one foot per year. “Rising” areas indicate rates of water-level rise exceeding one foot per year, and “nearly stable” areas indicate rates of change between these two thresholds.

Recent groundwater-level trends in some of the most-developed basins in Arizona reveal areas under stress and those that may be responding positively to water-resources management actions such as reduction in withdrawals and managed recharge. In less-developed areas, trends may be more indicative of climatic variability. Trends from defined

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Areal representation of trends in groundwater levels for selected basins in Arizona, 1997–2006. Only wells with at least three observations during this time period and a goodness of linear fit of 0.75 are shown. Trends are represented to a maximum distance of five kilometers (if no other well is closer).


time periods, such as subsequent to the enactment of a groundwater policy, can be evaluated with this method as well. Other trend categories also can be shown, such as regions experiencing declines in water levels greater than some critical threshold.

Groundwater levels are one good measure of the health of an aquifer system. Publicly available tools that are easy to understand are needed to make use of these valuable datasets. This new tool is one such approach. Trend analyses of groundwater observations take into account anthropogenic and climatic impacts on our aquifers and allow insight into systems where lengthy historical records are unavailable. Although estimates of recharge and groundwater extractions and forecasts based on groundwater models are useful, ultimately only “facts under the ground” are proof that groundwater resources are being managed sustainably.

Contact Fred Tillman at [email protected]. www.golder.com/water

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January/February 2008 • Southwest Hydrology • �

ON THE GROUND (continued)

Where Will the Lower Basin’s Water Come From?Mark Lellouch – Sonoran Institute

Two powerful forces are affecting the Lower Colorado River Basin’s water supplies: rapid urban growth and climate change. As California, Arizona, and Nevada are already using their full allocation of Colorado River water (14.5 million acre-feet per year [afy]), increases in use will have to be offset by transfers from the agricultural sector or from the Upper Basin, or development of new supplies both inside and outside the basin. This article focuses on the last of these sources, augmentation of supplies, including augmentation projects, efficiency projects, and conservation programs. The table below summarizes the supply gains and costs of the major augmentation and efficiency projects.

Augmenting the ResourceRecent research by the U.S. Bureau of Reclamation estimates that cloud seeding could generate an additional 1 million afy in basinwide snowpack during an average precipitation year and approximately half that amount in a drought year, assuming the process produces a 10 percent increase in precipitation. However, studies over the past sixty years have not proved that cloud seeding does enhance water supplies on a basin scale. More studies are needed to understand the atmospheric processes that are involved.

Desalination is another option being closely examined. As of spring 2006, 21 desalination plants were proposed along the California coast that, as a whole, could produce 600,000 afy. If implemented, some of these could supplement existing Colorado River supplies. While reverse osmosis is a very energy-intensive and expensive process, costs continue to decrease.

One of the more ambitious alternatives being considered is importing water from other basins. For example, Mississippi River water from southern Missouri could

be imported into the Navajo River in southwestern Colorado through a 1,000-mile-long pipeline to provide an estimated 675,000 afy. However, the political and institutional hurdles associated with such a project are tremendous.

Nevada, with a Colorado River allocation of only 300,000 afy, is aggressively pursuing groundwater supplies in Clark, Lincoln, and White Pine counties to provide approximately 160,000 afy to the Las Vegas Valley. Surface water diversions from the Virgin and Muddy rivers, currently on hold, could provide the Southern Nevada Water Authority (SNWA) with another 125,000 afy.

Increasing EfficiencyA number of water efficiency projects have been proposed to reduce leaks in the Lower Basin’s water delivery system. The benefits from two of these projects—the lining of the All-American Canal (AAC) and the construction of the Drop 2 reservoir along the AAC—will accrue at the expense of agricultural stakeholders in the Mexicali Valley and riparian and wetland areas in the Colorado River Delta, straining relations between the United States and Mexico. Reclamation is working closely with environmental groups to ensure that the operation of the Yuma Desalting Plant does not threaten the existence of the Ciénega de Santa Clara wetlands, a key stopover for migrating birds on the Pacific Flyway.

A new treatment plant south of Mexicali is designed to capture all of the effluent from the city that was previously draining into the New River and the Salton Sea. There are also significant opportunities to improve water delivery in Mexico. The lining of canals and other efficiency measures in the Mexicali Valley could save 150,000 afy. The water conserved could provide replacement supplies in the face of shortages, reduce dependence of local farmers on groundwater, and restore key riparian areas in the Delta.

Leaf beetles (Diorhabda elongata) have been used effectively to destroy water-consuming invasive species such as tamarisk in pilot projects in Nevada. Their large-scale use along the Lower Colorado might yield significant amounts of water at a very low cost per acre-foot. More studies are needed to quantify the savings after replacing tamarisk by native vegetation.

Conservation OptionsMunicipal water savings are being pursued across the Southwest. In Las Vegas, SNWA imposes drought restrictions on outdoor use and offers incentives to residents who replace their lawns with water-efficient landscaping. In Tucson, a four-tiered pricing system, where residential consumers who use a lot of water pay rates more than three times those in Las Vegas, has reduced per capita water use to only 60 percent that of Las Vegas.

Projected supply gains and cost of various water augmentation and efficiency projects.

Supply gains (afy)

Cost ($ million)

Cost per acre-foot3

Augmentation projectsNevada ground and surface water 285,000 2,600-3,100 9,123-10,877

Desalination–ocean water 20,000-100,000 22-160 1,100-1,600

Desalination–brackish groundwater1 152,000 61-289 400-1,900

Mississippi River importation2 675,000 925 1,370

Efficiency projectsLining of All-American and Coachella canals

200,000 354 1,770

Canal lining in Mexico 150,000 56 373

Drop 2 storage reservoir 40,000 147 3,675

Mexicali II treatment plant 22,500 26 1,1561 Includes 108,000 afy from the Yuma Desalting Plant 2 Costs are speculative 3 One-time cost for an acre-foot annually in perpetuity

10 • January/February 2008 • Southwest Hydrology

Decisions in the agricultural sector, while likely rational at the farm level, are skewed by incentives (water prices and crop subsidies) that ignore the real scarcity of water. The sector, which consumes roughly 85 percent of the water used in the Lower Basin, can reduce its water consumption in three main ways: by investing in irrigation efficiency, by switching to lower water-use crops, and by retiring agricultural land. Studies estimate that the first two methods could generate water savings on the order of 800,000 afy in Arizona alone.

Creativity and resourcefulness have always been key traits of Westerners, especially when it comes to finding water. Barring particularly fierce impacts from climate change, the real threat to unlimited growth in the Southwest may be related to quality-of-life issues rather than water scarcity.

See Chapter 4 of the report “Ecosystem Changes and Water Policy Choices: Four Scenarios for the Lower Colorado River Basin to 2050”at www.sonoran.org. Contact Mark Lellouch at [email protected].

January/February 2008 • Southwest Hydrology • 11

GOVERNMENTDelta Ruling Favors Fish, Not FarmersAn August 31 ruling by the U.S. District Court determined that pumping from California’s Sacramento-San Joaquin Delta must be reduced to protect endangered fish species. Under the ruling, pumping will be restricted from the end of December until June to protect the fish; it could result in as much as 37 percent less water pumped in an average year, reported the Christian Science Monitor, although the judge did not set a precise figure. The decision sent shockwaves throughout California as farmers, municipalities, and industry considered its implications. Although defendants—the U.S. Department of the Interior, California Department of Water Resources, state water contractors, and other water authorities—argued strongly that factors besides pumping were responsible for the decline of the delta smelt population, an indicator of overall delta health, the judge was unmoved.

The lawsuit was filed in 2005 by Earthjustice, representing the Natural Resources Defense Council, Friends of the River, California Trout, the Bay Institute, and Baykeeper. It charged that the biological opinion on the impacts of pumping on the fish that was developed by the U.S. Fish and Wildlife Service and used as the basis of water project operations, was flawed in numerous ways and that pumping from the delta was a major factor in the decline of the species.

The Christian Science Monitor reported on potential impacts to Californians: farmers will have to idle fields and cut back on water-intensive crops such as lettuce, cotton, and rice; Silicon Valley computer chip makers may need to find more efficient means to manufacture their products or dramatically increase the prices of them; cities from Sacramento south will have to determine whether voluntary water conservation efforts will be sufficient or if water rationing and fines for excess use will be necessary.

Visit www.earthjustice.org and www.csmonitor.com.

CA’s Perchlorate Standard Takes Effect

On October 18, 2007, perchlorate became a regulated drinking water contaminant in California, with a maximum contaminant level (MCL) of 6 parts per billion (ppb). No federal MCL for perchlorate in drinking water has been established, and California is only the second state to implement its own standard. Massachusetts established an MCL of 2 ppb in 2006.

Several states have health-based guidance levels—not legally enforceable and not necessarily representing a level of health hazard—for perchlorate in drinking water, ranging from 1 ppb in New Mexico (the interim groundwater screening level) to 18-ppb public notification levels in New York and Nevada.

Initial testing in 1997 by the California Department of Health Services (now the California Department of Public Health) and subsequent monitoring showed perchlorate to be a widespread drinking water contaminant in several hundred wells, primarily in Southern California. Perchlorate is also found in the Colorado River, an important source of water for drinking and irrigation, where its presence results from contamination from ammonium perchlorate manufacturing facilities in Nevada.

Visit www.cdph.ca.gov/certlic/drinkingwater/Pages/perchlorate.aspx

Judge Unsympathetic to Yucca Mountain Water Needs

In September, a federal judge ruled in favor of the State of Nevada in rejecting a demand by the U.S. Department of Energy (DOE) to receive 8 million gallons of water allegedly needed to drill test holes at Yucca Mountain, the nation’s proposed high-level nuclear waste dump.

DOE’s request goes back to 1992, when Nevada granted DOE temporary water permits for use in site characterization at

Yucca Mountain. Those permits expired in 2002, so in 2001, DOE requested an extension. The state denied the extension when, three months later, DOE expressed its intent to recommend the Yucca Mountain site to the President, indicating site characterization was complete. President Bush signed the Yucca Mountain Development Act in 2002, giving DOE 90 days to file a license application for the facility to the Nuclear Regulatory Commission. That application has yet to be filed; DOE has announced it will be submitted by June 30, 2008.

Meanwhile, in 1997, DOE applied for permanent rights to 430 acre-feet of groundwater in anticipation of site characterization and eventual construction and operation needs. The application was denied in 2000, was subsequently appealed, and has passed between state and federal courts. DOE continued, mostly unsuccessfully, to fight for water rights.

In 2003, correspondence between DOE and the state engineer first began to mention the need for water to drill boreholes at the site, and for other needs. According to court documents, the number, depth, and purpose of the boreholes, and the amount of water needed, changed frequently and inconsistently over the next several years. DOE argued the boreholes were mandated by federal law and that the state’s unwillingness to provide the water rights was causing federal drilling deadlines to be violated. Over time, the amount of water required for the boreholes increased from the original request of 300,000 gallons to 8 million gallons in 2007.

DOE has maintained that the boreholes are necessary for site characterization. But court documents point out that that need appears inconsistent with DOE’s presentation to Congress in 2002 that Yucca Mountain had met its criteria as a suitable site: if the criteria have already been met, why is more site characterization needed?

Nevada eventually allowed DOE to use some water for specific phases of the


Yucca Mountain project. But in May 2007, the state determined that DOE was using water for purposes other than those permitted and on June 1 issued a cease-and-desist order to DOE to stop pumping.

In the September ruling, Judge Roger Hunt determined that DOE’s arguments for the need to conduct borehole drilling were contradictory and without merit. He wrote, “The Court entertains the suspicion that either DOE wants to look busy, or it wants to keep its contractor occupied during its lengthy delays in filing for a license.”

The judge also determined that DOE’s claim that various drilling deadlines have been mandated by Congress or federal law and therefore drilling must be allowed are also without merit, along with claims of the actual amount of water needed for drilling. In addition, Hunt’s ruling states that he learned from counsel for the DOE that “in argument it was admitted that only 10 acre-feet was needed to complete the drilling project. That is approximately 3,258,510 gallons, not 4 million or 8 million gallons demanded by DOE. This admission, among the many others, demonstrates that the DOE has intentionally multiplied its demands to create a crisis of its own making.”

According to the Las Vegas Sun, DOE continued drilling—and using groundwater—after the cease-and-desist order, claiming some drilling was exempt from it. In September, the newspaper reported that the state would go back to a judge if necessary to compel DOE to stop.

The ruling is available at www.state.nv.us/nucwaste/news2007/pdf/usdc070831doe.pdf. Also visit www.lasvegassun.com.

USGS Wins Bid for Own AnalysesLast fall, the U.S. Geological Survey concluded that its National Water Quality Laboratory (NWQL) in Lakewood, Colorado, should continue to be staffed and operated by government employees.

continued on next page

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GOVERNMENT (continued)A one-year “A-76 Competitive Sourcing Study” required USGS NWQL employees to compete with private companies by submitting proposals for operating the facility more efficiently. A-76 refers to a U.S. Office of Management and Budget circular that specifies the procedures for identifying inherently governmental and commercial activities, and requires the federal government to demonstrate through competition that it offers superior performance and lower cost than the private sector. In 1999, Congress passed the Federal Activities Inventory Reform Act, which requires such a study.

The USGS NWQL analyzes more than 35,000 samples per year, resulting in more than 1.3 million water quality measurements. The laboratory staff consists of approximately 125 federal employees who provide data for USGS national, regional, and local programs and projects.

More information about the lab is available at nwql.usgs.gov/Public/pubs/NWQL_Summ.pdf.

World’s Largest Inland Desal Plant OpensLast August, El Paso Water Utilities dedicated the Kay Bailey Hutchison Desalination Plant, an $87 million facility that produces 27.5 million gallons of water per day, the largest inland desalination plant in the world.

The desalination plant is a joint project of El Paso Water Utilities and the U.S. Army/Fort Bliss. It produces potable water by treating brackish groundwater from the Hueco Bolson aquifer using a reverse osmosis treatment system.

Seawater desalination plants are more common than inland plants such as El Paso’s. They have an ample supply of raw water, and the residuals are returned to the ocean. El Paso’s plant has attracted industry-wide attention due to its inland location and innovative method of concentrate disposal.

The concentrate is injected underground through wells. A geologic formation confines it and prevents its migration to fresh water. The storage volume is sufficient for 50 years of operation.

The desalination plant increases El Paso’s fresh water production by 25 percent, augmenting existing supplies and ensuring that El Paso and Fort Bliss have water for growth and development for the future.

Visit www.epwu.org/water/desal_info.html.

Navajo Nation Wants Share of CO River WaterWhile the seven states that share Colorado River water continue to work out a shortage-sharing agreement, tribes in the basin are beginning to lose patience over when or if they’ll receive their water claims. An Arizona Republic article last August highlighted the contrast between thriving cities with fountains and golf courses and the Navajo Nation, where nearly 40 percent of the people have to haul their water.

Water rights settlements between tribes and states, especially Arizona and New Mexico, have been in the courts for decades. As the states develop Colorado River shortage sharing plans for times of drought, they cannot ignore the fact that some of that water may belong to the tribes. In 2004, after 30 years of work, federal, state, and local officials reached an agreement with the Gila River Indian Community (GRIC) which, according to the Republic, “gave the community control of nearly half the Colorado River water that flows down the Central Arizona Project canal and set aside more than $400 million to pay for pipelines and canals.” GRIC’s allocation is 328,000 acre-feet per year.

The Navajo Nation estimates it needs at least 76,000 acre-feet a year from the Colorado River and as much as 63,000 acre-feet a year from the Little Colorado River, said the Republic, and several other tribes in Arizona could potentially

make their own claims in the future. Tired of delays, the Navajo Nation sued the federal government in 2003 for ignoring its water needs, but a settlement still has not been reached. Negotiations in Arizona were complicated by a deal the tribe reached with New Mexico in 2005, in which its claims to the San Juan River were settled and a proposal for badly needed infrastructure was developed, said the newspaper. However, Congress has not authorized the deal, in part due to its $1 billion price tag. Arizona and some of its large water providers have sided with Congress; they advocate a comprehensive settlement between the tribe and all parties involved—including both states.

Meanwhile, the Republic said, the Navajo Nation watches while the seven basin states and the Department of Interior work out their own agreements without including the tribe. According to the newspaper, Navajo Nation President Joe Shirley warned Washington lawmakers that although willing to negotiate, “the tribe will take its claims to court if settlement attempts don’t progress.”

Visit www.azcentral.com/arizonarepublic/news/articles/0827water-navajodeal0827.html.

Weaknesses in Federal Climate Science IdentifiedIn 2002, President Bush created the U.S. Climate Change Science Program (CCSP) to consolidate and improve government-wide management of climate and related environmental science. The Committee on Strategic Advice on the CCSP was subsequently created to, among other tasks, evaluate progress made during the first four years of CCSP. Their findings were released in a public review draft in September, with the following conclusions:

• The director of CCSP and agency principals lack authority to allocate or prioritize funding across agencies, which limits overall progress in the program.

• Good progress has been made in documenting and understanding climate

1� • January/February 2008 • Southwest Hydrology

change, but use of that knowledge to support decision-making and manage the risks and opportunities of climate change is lagging. More work is needed to synthesize research results and engage decision makers.

• Progress in understanding and predicting climate change has improved more at global, continental, and ocean basin scales than at regional and local scales. This may explain in part why water managers have been slow to incorporate climate science in their decision-making process.

• Our understanding of the impact of climate changes on human well-being and vulnerabilities is much less developed than our understanding of the natural climate system. This is attributed to lack of consistent funding and the relatively small number of social scientists qualified to undertake this research.

• Science observation systems have fueled advances in climate change science and applications, but many existing and planned observing systems have been cancelled, delayed, or degraded, threatening future progress. The loss of existing and planned satellite sensors is perhaps the single greatest threat to the future success of CCSP. Funding cuts to ground-based monitoring networks such as stream gauge networks and snow observation systems will also affect progress.

• Progress in communicating CCSP results and engaging stakeholders is inadequate. Scientists generally don’t know what water managers need, and managers don’t know what types of products scientists could provide.

The President’s 2007 budget request for the CCSP was $1.715 million.

The public review draft is available at www.climatescience.gov/Library/sap/sap5-1/public-review-draft/. The executive summary is available through National Academies Press at books.nap.edu/catalog.php?record_id=11934.

Desal Research Facility Opens in New MexicoThe Brackish Groundwater National Desalination Research Facility in the

Tularosa Basin in Alamogordo, New Mexico, officially opened last August. The new facility, created through a partnership between the U.S. Bureau of Reclamation,


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GOVERNMENT (continued)Sandia National Laboratories, and New Mexico State University, is dedicated to testing desalination of brackish and impaired groundwater and speeding the transfer of technologies to end users.

The new facility hopes to attract researchers to develop cost-effective, efficient desalination technologies that, when applied to brackish and impaired groundwater, result in new supplies of usable water for municipal, agricultural, industrial, and environmental purposes. Reclamation believes that improving research and processes will help to increase output and lower the costs of desalinating groundwater.

Visit www.usbr.gov/pmts/water/research/tularosa.html.

Reclamation Funds Drought Relief Projects In September, the U.S. Bureau of Reclamation awarded nearly $2.3 million

to 14 drought-relief projects in the West. Funding went to Arizona ($1.2 million), California ($100,000), Idaho ($204,000), North Dakota ($100,000), and South Dakota ($535,000). In general, the funded projects involved replacement, rehabilitation, or installation of wells; installation of new stream gauges; and fire suppression for small communities.

The Hualapai Tribe of Arizona received $1.2 million to provide water for livestock, wildlife, and fire suppression activities. Four projects to be undertaken include drilling a new well, installing a temporary pipeline and solar pump, purchasing new storage tanks, and hauling water for livestock.

Visit www.usbr.gov.

New EPA Documents ReleasedLast summer, U.S. EPA released several new documents designed to help with watershed-based permitting, the Ground Water Rule, and water quality trading.

Watershed Permitting: “Watershed-Based NPDES Permitting Technical Guidance” was designed to help integrate National Pollutant Discharge Elimination System (NPDES) permits into watershed management plans. It is a follow-up to the 2003 implementation guidance and leads interested parties through the analysis of watershed data and how to develop a framework for implementing an NPDES program.

The guidance is available at www.epa.gov/npdes/watersheds.

Ground Water Rule: Three new documents were developed to help states and public water systems understand requirements of the Ground Water Rule (GWR) finalized in November 2006. The GWR was designed to increase protection against microbial pathogens in public water systems that use groundwater as a source of drinking water.

“Complying with the Ground Water Rule: Small Entity Compliance Guide”

outlines GWR requirements and how they apply to small public water systems. “Consecutive System Guide for the Ground Water Rule” describes the specific responsibilities of both wholesale providers and consecutive systems that purchase water from wholesalers and provides recommendations to help them meet those responsibilities. The “Ground Water Rule Source Water Monitoring Methods Guidance Manual” includes information about the basis for groundwater monitoring, how to determine the appropriate fecal indicator for monitoring, and how the different analytical methods work.

The documents are available at www.epa.gov/safewater/disinfection/gwr/compliancehelp.html.

Water Quality Trading: “The Water Quality Trading Toolkit for Permit Writers” aims to help the regulated community design and implement voluntary water quality trading programs consistent with EPA’s 2003 National Water Quality Trading Policy.

The new guidance provides permitting authorities with tools for incorporating trading provisions into required permits. It focuses on trading nitrogen and phosphorus, but other pollutants may be considered.

The publication is available at www.epa.gov/owow/watershed/trading/WQTToolkit.html.

Feds Outline Water Science Support StrategyFollowing a recent Government Accountability Office study that determined that 39 states anticipate water shortages within the next decade, the Subcommittee on Water Availability and Quality (SWAQ) of the National Science and Technology Council’s Committee on Environment and Natural Resources was established. In September, the group outlined a general strategy to address national water challenges.

The strategy calls for the development of: a national water census; technologies

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for enhancing reliable water supply; innovative water-use technologies and tools to enhance their public acceptance; and collaborative tools and processes for U.S. water solutions. It also seeks improved understanding of water-related ecosystem services and ecosystem water needs and better hydrologic prediction models and their applications.

SWAQ also identified the following critical actions needed by “appropriate” (yet unspecified) agencies:

• Strengthen, integrate, and transform existing hydrologic models into tools for making decisions on watershed and subwatershed scales.

• Link hydrologic models to climate models that simulate the water cycle over broad geographic areas and long time periods, and couple them with institutional models to provide a full suite of physical, economic, and technological decision tools for water managers.

• Make advanced hydrologic models operational through establishment of a “community hydrologic prediction system.”

Regarding just who will do this work and how it will be paid for, the SWAQ report stated: “For a given priority implementation, SWAQ will convene a multiagency team to review the needs within these areas and benchmark skills, capabilities, and tools currently available within appropriate federal agencies to meet these needs. The teams will recommend pathways to improve current capabilities to meet our emerging and future needs. Those pathways will be pursued through coordination of agency planning and budget processes, and through coordination with the Office of Management and Budget. This will ensure the best use of current capabilities and past investments, and ensure that future investments are efficient, coordinated, and avoid duplication.”

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January/February 2008 • Southwest Hydrology • 1�

In desert areas of the Southwest, ET rates are so high that nearly as much water goes up into the atmosphere as falls from it.

They are all around us: those crazy, vibrant water molecules yearning to break away from their liquid

siblings and become free wanderers in the sea of atmosphere. All they need is the addition of a little energy and space in the air above, and they are off!

Thus, evaporation. It occurs from open water, from wet or moist soil, and as transpiration from stomata of plants. It is an age-old process that has pumped enormous quantities of water vapor from the oceans each year for millenia onto land masses in the form of rain or snow. A significant portion of this rain or snow in turn evaporates or transpires from the land surface and flows with the air masses to where the air cools below the saturation point so that the molecules condense and precipitate once again.

It Takes EnergyJust as evaporation of the water contained in a teapot requires the addition of energy, evapotranspiration (combined evaporation and transpiration, or ET) requires energy from the surrounding environment. Engineers and scientists make use of this energy requirement to estimate the rate of ET by measuring the energy consumed during the ET process. This energy is termed latent heat. Measured or estimated energy consumption is translated into an equivalent depth of ET over the land surface by dividing the latent heat by a constant that varies only slightly with temperature.

The significant energy requirement of ET helps place a cap on the amount of water that can evaporate from soil

and vegetation surfaces, since there is a limited amount of energy at the land surface at any particular time. The amount of energy is governed by the amount of solar radiation, the amount of heat flowing from beneath the soil surface, and by the amount of heat that can be drawn out of the air as it passes over the surface. The heat exchange from passing air is a function of the temperature of the air relative to the surface, the velocity of the air, the “roughness” of the vegetation, and the dryness of the air. These impacts are often incorporated into “combination” equations, such as the widely used Penman-Monteith equation that uses weather measurements of solar radiation, air temperature, air humidity, and air movement to estimate ET rates.

ET is the second largest component of the hydrologic water balance, behind only precipitation. In desert areas of the Southwest, ET rates are so high that nearly as much water goes up into the atmosphere as falls from it. Globally, more than 50 percent of solar radiation is returned to the atmosphere through evaporation and transpiration. This return replenishes atmospheric moisture and leads to precipitation recycling. Yet ET is often the most difficult of the water balance variables to measure due to its wide spatial variation and invisibility.

Who Is Using It?The irrigation engineering community was perhaps the first to push for more precise estimation of ET to help irrigated agriculture apply water more efficiently and to ensure that water supplies are

Why Do We Care About ET?Richard G. Allen – Research and Extension Center, University of Idaho


adequate. Results of this push have been scientific-based (i.e., ET-based) irrigation scheduling programs for farm water management and standardized procedures for calculating ET requirements that have been published by the American Society of Civil Engineers and the United Nations Food and Agriculture Organization.

Today, irrigation engineers use daily ET rates to design and size irrigation conveyance, pumping, and application systems (such as sprinkler and drip), and seasonal ET volumes to size storage systems or ensure water supply for food production. They need ET information to monitor irrigation system performance (for example, the ratio of ET to diversions) and to assist farmers in irrigation scheduling.

Hydrologists covet estimates of ET over large land domains so that they can assess river basin and watershed water balances and estimate soil water storage and recharge (deep percolation) to groundwater systems. ET and infiltration are often the two large unknowns in a river basin or study area, so if ET can be determined, an improved estimate of the groundwater recharge term can be derived as a “residual.” These watershed water balances are used to make critically important management decisions regarding current and future allocation of water supplies. Thus, uncertainties and measurement errors in major water components—particularly when measuring ET over natural vegetation and agricultural systems—have significant implications for those decisions. Historically, too much trust has been placed on somewhat loose numbers; our communities of technical professionals and water managers must tighten up their estimates in light of increasing competition for water in many river basins.

Groundwater modelers, who focus on the hydraulics of liquid water flowing through geologic strata, often wish for more accurate knowledge of rates, timing, and spatial distribution of ET, since this impacts both the extraction of groundwater used for irrigation and the surplus of soil water that can percolate through the strata and replenish the groundwater.

Civil engineers need to know ET rates and volumes and their variation in time in order to size water storage and conveyance structures (including for irrigation of landscapes and crops). Knowing the ET component of the water balance allows them to estimate sustainable yields of water from river basins under present and future land-use scenarios, and to optimize water management.

Agronomists and plant physiologists use precision measurements of the transpiration component of ET to estimate crop yields and to examine physiological behavior when soil water is in shortage. Shortage of soil moisture creates water stress in plants, which in turn causes the partial or complete closure of stomata. Stomatal closure, although critical to conserving life-sustaining water in plant tissue, also reduces the influx of carbon dioxide required for photosynthesis and the creation of biomass and reproductive seed. Happy, water-unstressed plants make for near-maximum levels of ET. Transpiration creates a flow of water from soil to roots and leaves that functions as a conduit for nutrient transport. Additionally, transpiration can help keep leaves somewhat cooled during hot periods and perhaps at temperatures more conducive to biological and enzymatic processes.

Ecologists study the relationships among precipitation, ET, and health of plant communities, including competition among species. Plant species can impact ET rates and create a sort of feedback between soil water and plant health. Deeper-rooted species have an advantage over shallow-rooted species by accessing water stored deep in the soil, thus increasing their water supply. Species that green up earlier in the growing season or nearer to the beginning of a wet season can convert available soil water reserves into ET sooner than their competition, helping them to dominate a landscape. Non-native species are often able to invade a biome because of this competitive edge.

River basin managers also care about ET from native plant species versus ET

from invasive species, and should take riparian ET consumption into account when releasing reservoir flows. Native plants, while consuming valuable water, are generally desirable and require sustaining ET supply streams. Invasive species, on the other hand, sometimes consume precious water to the detriment of native plant systems.

Why We Must CareDespite the interest in ET across many disciplines, ET quantification remains an imprecise science. Improvements in accuracy of data and new measurement techniques will have important and positive impacts in a number of scientific disciplines and in practical applications in engineering, agriculture, and water management. A number of court cases at local, state, and even U.S. Supreme Court levels, along with hydrologic issues related to endangered species management, are helping to move the science of quantifying ET forward.

Contact Rick Allen at [email protected].


Evapotranspiration (ET) is the most difficult parameter of the water budget to measure, as it

involves transfers of both energy and mass that often change rapidly in space and time, and can require measurement of a number of related parameters. Methods for measuring ET range from relatively direct but resource-intensive methods, to more easily obtained empirical estimates.

The most precise and direct method for measuring ET is generally considered to be a weighing lysimeter in which vegetation is grown on a scale (sometimes quite large). Weight changes due to precipitation and drainage are monitored and ET is measured in terms of the weight of water lost. Other methods measure changes in soil moisture, water vapor, or latent heat energy, or water budget parameters from which ET can be readily determined. Shuttleworth (page 22) presents significant aspects of lysimeters and other methods commonly used for relatively direct measurements of ET. Baker (page 24) describes a few of these in more detail, highlighting the care that is warranted in instrumentation set-up and data interpretation. Direct-measurement methods are generally expensive, time-consuming, and data-intensive, and are most often employed by the research community. However, these methods provide high-quality data important for evaluating or calibrating estimates obtained by the empirical, but perhaps more routine, approach, which is the focus of this article.

ET Without a Research GrantWhat do those who need ET values but lack large research grants do? Commonly, ET is estimated using empirical or analytical equations. A number of these equations calculate ET using relatively inexpensive and readily available weather data such as temperature, relative humidity, solar radiation, and wind speed.

The equations were originally developed to estimate ET over crops, which are flat and homogeneous. Do these calculations

translate to natural settings, at scales of watersheds? Not likely without some error, but in many situations this is the best that can be done. Precipitation and other water budget data (such as runoff) can provide boundary values for such ET estimates.

Estimating ET: the Natural WorldWhile the simplest approximation of ET is the difference between precipitation and runoff, most estimators recognize the significant control that climate exerts on ET and incorporate local weather parameters into the calculation.

Weather data have usually been included in an ET estimate using a two-step approach. First, potential ET is calculated using equations such as Penman, Priestley-Taylor, Thornthwaite, or Blaney-Criddle. While various and sometimes contrasting definitions of potential ET have been used by different researchers, in general, potential ET is the amount of ET that would occur in a given setting under conditions of unlimited water availability to meet the evaporative demand of the local climate. In the semi-arid Southwest, potential ET is usually far greater than actual ET from the water-limited soils and plants, such that the sum of potential ET and runoff far exceeds precipitation. Therefore, the second step is to adjust ET estimates to account for the water budget discrepancy by assuming actual ET is a fraction of potential ET. Measured runoff or other water budget parameters are used to tweak the potential ET down to a value that is reasonable.

Increasingly, an estimation of “reference crop evaporation” (ETo) is being used instead of potential ET. ETo is an estimate of what ET would be over a highly studied

Approaches to ET MeasurementBetsy Woodhouse – Southwest Hydrology, University of Arizona

Measured runoff or other water budget parameters are used to tweak the potential ET to a value that is reasonable.

ET rates for various natural vegetation types across the Southwest. Data are from federal and state agencies and universities; ET rates were obtained by various direct and empirical methods.

Vegetation type Location ET rate (inches/year)Saltcedar Gila River, AZ 56Saltcedar Middle Rio Grande, NM 42-57Saltcedar Middle Rio Grande, NM 34-49Saltcedar Colorado River near Blythe, AZ 28-30Cottonwood Middle Rio Grande, NM 65-85Cottonwood Middle Rio Grande, NM 44-53Cottonwood San Pedro River, AZ 19-28Mesquite San Pedro River, AZ 27Mesquite San Pedro River, AZ 25-26Honey mesquite Colorado River near Blythe, AZ 19Russian olive Middle Rio Grande, NM 42-50Ponderosa pine Northern AZ, high elevation 20Ponderosa pine Nevada and northern NM 11-19Pinyon-juniper Northern AZ, mid-elevation 16Pinyon-juniper Nevada 12Grass Middle Rio Grande, NM 2.8-23Shrub Middle Rio Grande, NM 0-14Mixed; low elevation Middle Rio Grande, NM 0-16Xerophytes Nevada 9-12Sagebrush Nevada 12Sage and bitterbrush Nevada 10-18


“reference” vegetation, that is, well-watered and actively transpiring grass of a certain height. ETo is calculated using the Penman-Monteith equation, and expresses the energy available to evaporate water and the wind available to transport water vapor from the ground into the air, for the reference vegetation type. The second step again is to relate this value to the actual vegetation in the area of interest by multiplying ETo by a crop-specific factor to obtain “crop evapotranspiration.” Such factors are now being developed for natural vegetation types and landscapes.

The table below left illustrates the ranges of ET estimates that have been measured and calculated for various types of natural vegetation in the Southwest.

Crop Factors: the Farmer’s AdvantageFarmers, who need to predict their irrigation needs as precisely as possible, have an advantage over those estimating ET from natural areas, as crop factors for

many types of well-watered crops already have been determined. To do this, ET is measured directly for a specific crop, usually using a lysimeter in a research environment, and the specific crop factor is assigned to account for the difference between the measured and ETo values. Farmers can calculate the ETo for their location from local weather measurements and multiply it by the established crop factor to estimate actual ET demand.

This approach assumes that the value of the crop factor derived at one site under one set of climate conditions applies to the same crop elsewhere, independent of the climate conditions, which may not be the case. Nonetheless, it is currently the standard approach recommended by the United Nations Food and Agriculture Organization (see Allen and others, 1998). Recently, Shuttleworth (2006) proposed the one-step Matt-Shuttleworth equation, which uses crop factors but also calculates the effect of the aerodynamic characteristics of the crop based on its height.

A Further Step: Predicting ChangesCan estimates of ET rates be used to model the effects of vegetation change? While the earliest meteorological models ignored vegetation, meteorologists now commonly use the Penman-Monteith equation. The vegetation parameters required in the equation are either defined as constants or obtained through submodels for each ecosystem. Thus meteorological models now make predictions of the impact of land use changes, such as Amazonian deforestation, or of climate change itself, given changes in land cover. Recently, the more advanced hydrological models are using this approach as well.

ReferencesAllen, R.G., L.S. Pireira, D. Raes, and M. Smith,

1998. Crop evapotranspiration—Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, www.fao.org/docrep/X0490E/x0490e00.htm.

Shuttleworth, W.J., 2006. Towards one-step estimation of crop water requirements, Trans.of Amer. Soc. of Agr. and Biol. Engineers, 49(4): 925-935.

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Evapotranspiration Measurement MethodsW. James Shuttleworth – SAHRA, University of Arizona

In the United States and several other countries, the term “evapotranspiration” (ET) is used

when considering evaporation from vegetation-covered ground. It describes the total evaporation from the soil and wet plants plus transpiration from dry plants. The two most common types of direct-measurement methods, water budget and

water vapor transfer measurements, are described first in the table below. Water budget measurements deduce ET as a loss of liquid water by measuring or estimating all the other components in a water budget. Such methods are long-established and have been refined over the years.

Water vapor transfer methods measure the

Brief Description Assumptions Strengths and weaknesses Scale of Mea-surement* Error

Wat

er b

udge

t mea

sure

men

ts

Evaporation pan Directly measures change in water level over time for a sample of open water in a “pan” with well-specified dimensions and siting.

Assumes relationship between measured evaporation from pans and actual evaporation from adjacent area can be calibrated, and calibration is transfer-able between locations and climates.

A long-established and well-recognized method, simple to understand and implement, and reasonably inexpensive; but because it relies on the validity of an extrapolated calibration factor previously defined elsewhere, is primarily used for crop ET estimates rather than heterogeneous natural vegetation covers.

PlotVaries with reliability and relevance of calibration factor, but 10 to 20% errors are possible for crops, with greater errors likely for natural vegetation because calibration may be unknown.

Water balance of basin

The unmeasured difference between other measured components of the basin water balance, including incoming precipitation, surface and groundwater outflow, and soil water storage.

Assumes all other components of the basin water balance can be measured as spatial averages with sufficient accuracy for evaporation to be reliably calcu-lated as the difference between them.

Gives an area-average measurement for vegetation covers for a hydrologically significant region, however, area-average measurement of the other water balance terms can be expensive and difficult, especially groundwater flow and soil moisture. Consequently only longer time-average estimates are possible.

Basin Varies with quality of implementation and size and nature of basin, but errors as low as 10 to 20% may be achievable in research basins with persistent care.

LysimetryMeasures change in weight of an isolated, preferably undisturbed, soil sample with overlying vegetation (if present) while measuring precipitation to and drainage from the sample.

Assumes the sample of soil and overlying vegetation on which measurements are made are representative in terms of soil water content and vegetation growth and vigor of the plot or field in question.

If the soil and vegetation sample is truly representative, the lysimeter is widely accepted as being an unparalleled standard against which to compare and validate other evaporation measurements and models of crop evaporation. Modern high-precision lysimeters are expensive (~$50,000) and require expert supervision.

SampleState-of-the-art lysimeters can provide daily measurements with high accuracy (few %), but errors can become substantial (few x 10%) if the sample is unrepresentative.

Soil moisture depletion

Measures change in water content of a representative sample of undis-turbed soil and vegetation while measuring precipitation and run-on/runoff and estimating deep drainage for the sample plot.

Assumes that soil water measuring devices (resistance blocks, tensiometers, neutron probes, time-domain reflectometers, capacitance sensors) adequately determine change in soil water, the effects of deep roots and sensor placement are small, and deep drainage can be estimated adequately.

Most often used in crop-covered plots. Measurement is reasonably inexpensive and, in principle, repre-sentative of the plot in which it is implemented, but disturbance during installation of soil water sensors and deep roots extending below the measurement depth can negatively influence the measurement. Deep drainage is hard to estimate.

Plot

Varies with quality of implementation but neutron probe errors of less than 10% are achievable; TDR and soil capacitance sensors are highly variable but can attain as low as 10 to 20% error.

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Bowen Ratio - En-ergy Budget

Calculates evaporation as latent heat from the surface energy budget using the ratio of sensible to latent heat (Bowen ratio) derived from the ratio between atmospheric temperature and humidity gradients measured a few meters above vegetation.

Assumes the turbulent diffusion coefficient for sensible heat and latent heat are the same in the lower atmosphere in all conditions of atmospheric stability, and that plot-scale measurements of energy budget components (net radiation, soil heat) are representative of upwind conditions.

Well-established method. Relatively inexpensive proprietary systems can be purchased that work for both short crops and natural vegetation. Problematic over tall vegetation when atmospheric gradients are low. Often cannot be used near dawn and dusk when the Bowen ratio is minus one.

Field Errors associated with assumptions and representativeness plus errors in required supplementary sensors result in overall errors of around 5 to 15%.

Eddy correlation(also called eddy

covariance)

Calculates evaporation as 20- to 60-minute time averages from the correlation coefficient between fluctuations in vertical windspeed and atmospheric humidity measured at high frequency (~10 Hz) at the same location, a few meters above vegetation.

Assumes only turbulent transfer of water vapor at sample point, and that cor-rections for water vapor transfer in turbulence at time scales less than ~0.1 seconds or greater than the selected averaging time are acceptable.

Currently preferred method for field-scale measurements in research applications. Implemented using relatively expensive proprietary logger and colocated sensors, but prone to systematic underestimation of fluxes. Perhaps best used to measure Bowen ratio, with evaporation deduced from surface energy budget.

Field

Systematic underestimation up to 25% can occur in the basic evaporation measurement. If sensible heat is also measured to determine Bowen ratio and energy balance is used to calcu-late evaporation, error can be reduced to 5 to 15%.

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Transpiration measurement by

porometry or moni-toring sap flow

Porometry: measured from humidity increase in a chamber temporarily enclosing transpiring leaves/shoots. Sap Flow: measured from rate of sap flow in trunk, branches, or roots using heat as a tracer, with an estimate of the area of wood through which flow occurs.

Porometry assumes the enclosure of leaves and shoots in the chamber does not significantly alter transpiration rate. Sap Flow assumes installation of sensors does not alter sap flow rate, and cross-sectional area over which flow occurs can be determined accurately.

Porometry: a manual measurement that allows determination of environmental influences on stomatal control at leaf level. Sap Flow: allows routine unsupervised measurement of transpiration from whole plants or plant components over extended periods.

Leaf-to-plant; plot scale

with multiple sampling

Porometry: small for leaves (~few %). Sap Flow: errors as-sumed to be 5 to 15% for individual plants. Both: at plot scale, errors are strongly determined by the number of samples taken and the variability in these samples.

Rainfall intercep-tion loss from tall

vegetation

Measured as difference between cumulative rainfall above/below tall (usually forest) canopy. Requires careful below-canopy sampling with gauges/troughs that sample at spatial scale of canopy features, preferably randomly relocated after each measurement interval.

Assumes below-canopy sampling is adequate, a requirement rarely met for a typical 1-2 week measurement interval. It becomes feasible over several measurement intervals if gauges are regularly and randomly relocated.

Allows separate identification of wet canopy contribution to ET for tall vegetation. Rarely if ever attempted for short vegetation and crops, but possible in principle.

Plot

Strongly depends on below-canopy sampling. One-gauge arrangement provides only order of magnitude estimate, but time average with many gauge relocations can reduce error to around 5 to10%.

Soil evaporationA small-scale, shallow implementation of lysimetry or soil moisture depletion methods for a near-surface soil sample below vegetation using several “microlysimeters” or sequential gravimetric multisampling.

Assumes the average of all small soil samples, regardless of their below-canopy location, are representative of the entire soil surface.

A comparatively simple and inexpensive manual measurement. Gravimetric approach is time-intensive and the sample is destroyed, preventing repeated measurement at same place.

Plot, with mul-tiple sampling

Strongly depends on below-canopy sampling, but errors as low as 10 to 20% are possible with many samples and care.

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Scintillometer measurements

Uses theoretical relationship between sensible and latent heat fluxes and atmospheric scintillation introduced into a beam of electromagnetic radiation between source and detector by temperature and humidity fluctuations.

Applies strictly in an ideal turbulent field close, but not too close, to a surface with uniform aerodynamic roughness. However, field experiments suggest a worthwhile measurement is possible over a mixture of vegetation covers.

The only micrometerological method that can be used to provide an (albeit indirect) measurement of the line-average sensible and latent heat over several kilometers.

Field to landscape

Field comparisons between the line-average flux over several types of vegetation and eddy-correlation measurements for each vegetation type agree at the 10 to 20% level or better.

Remote sensing estimates

Evaporation is deduced indirectly from the surface energy balance, with sensible heat calculated from the difference between air temperature and the temperature of the evaporating surface, along with an estimate of the aerodynamic exchange resistance between these two.

Assumes the “aerodynamic” surface temperature (that which controls sensible heat transfer from the surface), is the same as (or can be estimated from) the “radiometric” surface temperature (that which can be measured using an airborne or satellite radiometer).

Provides opportunity for instantaneous snapshots of evaporation over large areas in clear sky condi-tions, but uncertainties in the effective surface emissivity and effective aerodynamic exchange resistance can give systematic errors—both being worst for sparse canopies. Therefore, ground-truth evaporation measurements are usually required.

Field to regional

With ground-truth measurements, snapshot maps of evapora-tion in clear sky conditions may be accurate to 10 to 20%, but time-average estimation from these snapshots introduces additional uncertainty.

LIDAR (LIght Detec-tion And Ranging)

method

The local time-average vertical gradient of water vapor is sampled remotely using LIDAR. Local evaporation flux is calculated from this using similarity theory and supplementary measurements of friction velocity and atmospheric stability.

Assumes Monin-Obukov similarity theory applies and the supplementary mea-surements of friction velocity and atmospheric stability are locally applicable within the measurement field of the LIDAR.

Gives detailed and frequent 3-D mapping of the water-vapor gradient, valuable in assessing variations over areas with heterogeneous evaporation. However, equipment costs are extremely high and indepen-dent ET measurement is required to assess accuracy.

Field to landscape

Provides a useful measure of spatial ET variations but requires independent validation/calibration.

*Scales: Leaf-to-plant: the size of the basic canopy, typically square centimeters to a few square meters; Sample: area of the soil and vegetation sample, typically a few square meters; Plot: typically a few to tens of square meters; Field: typically a few hundreds of square meters; Landscape: typically a few thousands of square meters; Regional: typically a few square miles; Basin: varies from landscape to regional scale and beyond.


flow of water vapor into the atmosphere using meteorological sensors mounted above the surface. Sometimes these sensors measure evaporation not in mass terms, but in the context of the surface-energy balance as latent heat flux. This is the flow of energy that is transferred with the water vapor and that leaves the surface in the form of latent heat.

It can be useful to measure the separate contributions to ET: transpiration from

plants, rain or snowwater evaporated from the plant canopy, and evaporation from the soil surface. Some of these methods are described next in the table.

Other recent ET measurement efforts attempt to measure area-average ET. Examples of these also are included.

An alternative to the direct measurement methods described below is to model ET rates using local climate data in empirical

and analytical equations. This approach is not covered here.

ET measurement methods tend to have their champions, individuals who are convinced their method is best. When appraising the strengths, weaknesses and likely errors of the different methods, I have sought to be impartial and conservative, but the appraisal is to some extent subjective and it is personal!

Contact Jim Shuttleworth at [email protected].

Brief Description Assumptions Strengths and weaknesses Scale of Mea-surement* Error

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Evaporation pan Directly measures change in water level over time for a sample of open water in a “pan” with well-specified dimensions and siting.

Assumes relationship between measured evaporation from pans and actual evaporation from adjacent area can be calibrated, and calibration is transfer-able between locations and climates.

A long-established and well-recognized method, simple to understand and implement, and reasonably inexpensive; but because it relies on the validity of an extrapolated calibration factor previously defined elsewhere, is primarily used for crop ET estimates rather than heterogeneous natural vegetation covers.

PlotVaries with reliability and relevance of calibration factor, but 10 to 20% errors are possible for crops, with greater errors likely for natural vegetation because calibration may be unknown.

Water balance of basin

The unmeasured difference between other measured components of the basin water balance, including incoming precipitation, surface and groundwater outflow, and soil water storage.

Assumes all other components of the basin water balance can be measured as spatial averages with sufficient accuracy for evaporation to be reliably calcu-lated as the difference between them.

Gives an area-average measurement for vegetation covers for a hydrologically significant region, however, area-average measurement of the other water balance terms can be expensive and difficult, especially groundwater flow and soil moisture. Consequently only longer time-average estimates are possible.

Basin Varies with quality of implementation and size and nature of basin, but errors as low as 10 to 20% may be achievable in research basins with persistent care.

LysimetryMeasures change in weight of an isolated, preferably undisturbed, soil sample with overlying vegetation (if present) while measuring precipitation to and drainage from the sample.

Assumes the sample of soil and overlying vegetation on which measurements are made are representative in terms of soil water content and vegetation growth and vigor of the plot or field in question.

If the soil and vegetation sample is truly representative, the lysimeter is widely accepted as being an unparalleled standard against which to compare and validate other evaporation measurements and models of crop evaporation. Modern high-precision lysimeters are expensive (~$50,000) and require expert supervision.

SampleState-of-the-art lysimeters can provide daily measurements with high accuracy (few %), but errors can become substantial (few x 10%) if the sample is unrepresentative.

Soil moisture depletion

Measures change in water content of a representative sample of undis-turbed soil and vegetation while measuring precipitation and run-on/runoff and estimating deep drainage for the sample plot.

Assumes that soil water measuring devices (resistance blocks, tensiometers, neutron probes, time-domain reflectometers, capacitance sensors) adequately determine change in soil water, the effects of deep roots and sensor placement are small, and deep drainage can be estimated adequately.

Most often used in crop-covered plots. Measurement is reasonably inexpensive and, in principle, repre-sentative of the plot in which it is implemented, but disturbance during installation of soil water sensors and deep roots extending below the measurement depth can negatively influence the measurement. Deep drainage is hard to estimate.

Plot

Varies with quality of implementation but neutron probe errors of less than 10% are achievable; TDR and soil capacitance sensors are highly variable but can attain as low as 10 to 20% error.

Wat

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Bowen Ratio - En-ergy Budget

Calculates evaporation as latent heat from the surface energy budget using the ratio of sensible to latent heat (Bowen ratio) derived from the ratio between atmospheric temperature and humidity gradients measured a few meters above vegetation.

Assumes the turbulent diffusion coefficient for sensible heat and latent heat are the same in the lower atmosphere in all conditions of atmospheric stability, and that plot-scale measurements of energy budget components (net radiation, soil heat) are representative of upwind conditions.

Well-established method. Relatively inexpensive proprietary systems can be purchased that work for both short crops and natural vegetation. Problematic over tall vegetation when atmospheric gradients are low. Often cannot be used near dawn and dusk when the Bowen ratio is minus one.

Field Errors associated with assumptions and representativeness plus errors in required supplementary sensors result in overall errors of around 5 to 15%.

Eddy correlation(also called eddy

covariance)

Calculates evaporation as 20- to 60-minute time averages from the correlation coefficient between fluctuations in vertical windspeed and atmospheric humidity measured at high frequency (~10 Hz) at the same location, a few meters above vegetation.

Assumes only turbulent transfer of water vapor at sample point, and that cor-rections for water vapor transfer in turbulence at time scales less than ~0.1 seconds or greater than the selected averaging time are acceptable.

Currently preferred method for field-scale measurements in research applications. Implemented using relatively expensive proprietary logger and colocated sensors, but prone to systematic underestimation of fluxes. Perhaps best used to measure Bowen ratio, with evaporation deduced from surface energy budget.

Field

Systematic underestimation up to 25% can occur in the basic evaporation measurement. If sensible heat is also measured to determine Bowen ratio and energy balance is used to calcu-late evaporation, error can be reduced to 5 to 15%.

Com

pone

nts

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Transpiration measurement by

porometry or moni-toring sap flow

Porometry: measured from humidity increase in a chamber temporarily enclosing transpiring leaves/shoots. Sap Flow: measured from rate of sap flow in trunk, branches, or roots using heat as a tracer, with an estimate of the area of wood through which flow occurs.

Porometry assumes the enclosure of leaves and shoots in the chamber does not significantly alter transpiration rate. Sap Flow assumes installation of sensors does not alter sap flow rate, and cross-sectional area over which flow occurs can be determined accurately.

Porometry: a manual measurement that allows determination of environmental influences on stomatal control at leaf level. Sap Flow: allows routine unsupervised measurement of transpiration from whole plants or plant components over extended periods.

Leaf-to-plant; plot scale

with multiple sampling

Porometry: small for leaves (~few %). Sap Flow: errors as-sumed to be 5 to 15% for individual plants. Both: at plot scale, errors are strongly determined by the number of samples taken and the variability in these samples.

Rainfall intercep-tion loss from tall

vegetation

Measured as difference between cumulative rainfall above/below tall (usually forest) canopy. Requires careful below-canopy sampling with gauges/troughs that sample at spatial scale of canopy features, preferably randomly relocated after each measurement interval.

Assumes below-canopy sampling is adequate, a requirement rarely met for a typical 1-2 week measurement interval. It becomes feasible over several measurement intervals if gauges are regularly and randomly relocated.

Allows separate identification of wet canopy contribution to ET for tall vegetation. Rarely if ever attempted for short vegetation and crops, but possible in principle.

Plot

Strongly depends on below-canopy sampling. One-gauge arrangement provides only order of magnitude estimate, but time average with many gauge relocations can reduce error to around 5 to10%.

Soil evaporationA small-scale, shallow implementation of lysimetry or soil moisture depletion methods for a near-surface soil sample below vegetation using several “microlysimeters” or sequential gravimetric multisampling.

Assumes the average of all small soil samples, regardless of their below-canopy location, are representative of the entire soil surface.

A comparatively simple and inexpensive manual measurement. Gravimetric approach is time-intensive and the sample is destroyed, preventing repeated measurement at same place.

Plot, with mul-tiple sampling

Strongly depends on below-canopy sampling, but errors as low as 10 to 20% are possible with many samples and care.

Larg

e-sc

ale

eva

pora

tion

Scintillometer measurements

Uses theoretical relationship between sensible and latent heat fluxes and atmospheric scintillation introduced into a beam of electromagnetic radiation between source and detector by temperature and humidity fluctuations.

Applies strictly in an ideal turbulent field close, but not too close, to a surface with uniform aerodynamic roughness. However, field experiments suggest a worthwhile measurement is possible over a mixture of vegetation covers.

The only micrometerological method that can be used to provide an (albeit indirect) measurement of the line-average sensible and latent heat over several kilometers.

Field to landscape

Field comparisons between the line-average flux over several types of vegetation and eddy-correlation measurements for each vegetation type agree at the 10 to 20% level or better.

Remote sensing estimates

Evaporation is deduced indirectly from the surface energy balance, with sensible heat calculated from the difference between air temperature and the temperature of the evaporating surface, along with an estimate of the aerodynamic exchange resistance between these two.

Assumes the “aerodynamic” surface temperature (that which controls sensible heat transfer from the surface), is the same as (or can be estimated from) the “radiometric” surface temperature (that which can be measured using an airborne or satellite radiometer).

Provides opportunity for instantaneous snapshots of evaporation over large areas in clear sky condi-tions, but uncertainties in the effective surface emissivity and effective aerodynamic exchange resistance can give systematic errors—both being worst for sparse canopies. Therefore, ground-truth evaporation measurements are usually required.

Field to regional

With ground-truth measurements, snapshot maps of evapora-tion in clear sky conditions may be accurate to 10 to 20%, but time-average estimation from these snapshots introduces additional uncertainty.

LIDAR (LIght Detec-tion And Ranging)

method

The local time-average vertical gradient of water vapor is sampled remotely using LIDAR. Local evaporation flux is calculated from this using similarity theory and supplementary measurements of friction velocity and atmospheric stability.

Assumes Monin-Obukov similarity theory applies and the supplementary mea-surements of friction velocity and atmospheric stability are locally applicable within the measurement field of the LIDAR.

Gives detailed and frequent 3-D mapping of the water-vapor gradient, valuable in assessing variations over areas with heterogeneous evaporation. However, equipment costs are extremely high and indepen-dent ET measurement is required to assess accuracy.

Field to landscape

Provides a useful measure of spatial ET variations but requires independent validation/calibration.

*Scales: Leaf-to-plant: the size of the basic canopy, typically square centimeters to a few square meters; Sample: area of the soil and vegetation sample, typically a few square meters; Plot: typically a few to tens of square meters; Field: typically a few hundreds of square meters; Landscape: typically a few thousands of square meters; Regional: typically a few square miles; Basin: varies from landscape to regional scale and beyond.


Challenges and Cautions in Measuring EvapotranspirationJohn M. Baker – USDA-ARS and University of Minnesota

In recent decades, our ability to measure evapotranspiration (ET) has improved dramatically with

the availability of new instrumentation and field-portable computing power. The resulting methodologies include micrometeorological techniques,

remote sensing approaches, and lysimetry. Potential users should be aware of the pitfalls, limitations, and costs of these methods when evaluating their applicability to a site.

ET Measuring TechniquesRemote sensing is appealing because it can be done without on-site instrumentation and provides estimates for broad geographical areas. However, it is still primarily a research application, with no commercially available equipment or software for routine applications.

Weighing lysimetry involves placement of a load cell beneath soil that is then planted with the same vegetation as its surroundings. When properly installed, these probably yield the most accurate ET data, but installation is challenging and expensive.

Non-weighing lysimetry, also known as a water-balance approach, consists of installing soil moisture sensors at various depths beneath the surface to measure changes in soil moisture due to ET. This is probably the least expensive and most commonly used method for estimating ET, but it has numerous limitations. Chief among them is temporal resolution: ET usually cannot be measured at less than weekly resolution.

Micrometeorological methods include Bowen ratio/energy balance (BREB) and eddy covariance. Both have commercially available systems and can provide hourly data. They are now considered mature technologies, creating the impression that continuous, accurate measurement of ET is routine. However, the requirements and assumptions associated with these methods impose limits on the practical application and absolute accuracy of the data in ways that potential users should understand.

Bowen Ratio/Energy BalanceThe BREB method is an indirect method based on conservation of energy. Net radiation (Rn) is measured above the surface, and soil heat flux (G) is measured below it. The difference between these two must be consumed by either evaporating water (latent heat) or heating the air (sensible heat). The ratio of sensible to latent heat, known as the Bowen ratio (ß), can be estimated from temperature and humidity measurements, each made concurrently at two heights above the surface. Measurements of Rn, G, and ß can thus be used to solve directly for the ET rate. However, all three measurements are subject to error.

The requirements and assumptions associated with these methods impose limits on the practical application and absolute accuracy of the data in ways that potential users should understand.


Challenges and Cautions in Measuring Evapotranspiration

New and properly calibrated net radiometers are probably accurate to within 5 percent, but age and dust deposition make them subject to drift, and they are not easily recalibrated. Long-term accuracy is typically to within 10 percent. Soil heat flux is generally measured with calibrated plates installed at a defined depth below the surface, supplemented by thermocouples in the surface layer. The accuracy of this measurement is probably not better than 10 to15 percent. The gradient measurement of air temperature is usually made with thermocouples through which air is pulled by a fan. The temperature difference between the two heights is often quite small, but data loggers can measure it with relatively high accuracy. The humidity gradient is more difficult. In most systems it is measured with either an infrared gas analyzer (IRGA) or a dew point hygrometer. Either instrument must be periodically recalibrated, and the tubing through which the air is drawn must be kept clean. Collectively, these multiple measurements with their associated errors means the overall accuracy of BREB-based ET measurements will probably not be better than 20 percent unless extreme care is taken.

Eddy CovarianceThe primary attraction of eddy covariance is its conceptual simplicity. Consider a point in the atmosphere not too far above a vegetated surface. Parcels of air (eddies) will move past that point constantly. While the principal direction of movement will be horizontal, each eddy may have a vertical component as well; that is the nature of turbulence. When water is evaporating from the surface below, the upward-moving eddies will have, on average, a slightly higher mixing ratio than the downward-moving eddies. If the vertical velocity, w, and mixing ratio, h, of each eddy passing this point can be measured, then the vertical transport of water vapor through the plane containing the point can be calculated from the mean covariance of w and h.

Sensor Requirements: Eddy covariance requires instruments capable of measuring vertical wind speed and water vapor mixing

ratio at the same point, at a sampling rate sufficient to capture all eddies contributing to the transport, typically 10 hertz. Suitable sonic anemometers are available, but the humidity measurement again presents a challenge. IRGAs are sufficiently fast and accurate if properly calibrated, but a choice must be made between open-path and closed-path systems. In closed-path systems, a pump draws air from the sampling point through a tube and routes it to the IRGA, in a shelter. In open-path

systems, the source and detector of the IRGA are suspended in the air with open space between them. With no pump, an open-path system is easier to operate and requires less power, but is more susceptible to interference from rainfall, condensation, dust, and bird droppings. And since open-path systems measure vapor density rather than mixing ratio, the ET measurements must be corrected for density fluctuations due to sensible

see Challenges, page 33

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Riparian Evapotranspiration

Russell L. Scott and David C. Goodrich – USDA-ARS Southwest Watershed Research Center, David G. Williams – University of Wyoming, Travis E. Huxman – University of Arizona, and Kevin R. Hultine – University of Utah

Along the Upper San Pedro, mesquite groundwater use was the dominant component of total riparian water use.

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Quantifying

Riparian corridors are hot spots of biological activity and provide valuable habitat, supporting

significant biodiversity in semiarid regions such as the southwestern United States. Yet rural and urban developments increasingly are impacting the vitality of riparian areas by changing land use, diverting water, and lowering the water table. The San Pedro River in southeastern Arizona is a good example of such a situation. Population growth and the resulting increase in groundwater pumping in the Upper San Pedro Basin have created concern that the water table may fall below the rooting zone of the riparian vegetation, leading to abrupt changes in many ecosystems.

A multidisciplinary group of government scientists and university researchers has been working to better understand the hydrological functioning of riparian systems in the Southwest, particularly the quantification of riparian evapotranspiration (ET). Groundwater modeling studies have long shown that water use by riparian vegetation is an important component of the basin water balance. Yet because the quantification of ET was based on indirect techniques like streamflow data or by untested empirical approaches, its magnitude was highly uncertain.

Refining Riparian ET Measurements

Our focus over the last 10 years has been on making

direct measurements of ET using micrometeorological and

plant physiological techniques. Micrometeorological techniques

like Bowen ratio or eddy covariance quantify ET over an area of around 0.2 to

0.4 square miles, so measurements using these methods were made in carefully chosen sites with uniform stands of vegetation, like the floodplain grasslands

and mesquite shrublands and woodlands along the San Pedro. Shrublands and grasslands along the old alluvial terraces were found to have similar annual ET rates of around 24 to 28 inches per year, while the more mature and dense mesquite woodlands typically have annual rates greater than 28 inches (Scott and others, 2006a; Williams and Scott, in press). This represents significant groundwater use: ET in excess of precipitation, as annual rainfall totals have ranged from only about 10 to 12 inches. On a leaf-area basis alone, mesquite transpiration is considerably higher than that of grass. This finding has management implications because mesquite are readily expanding into grassland areas, which likely has resulted or will result in increasing groundwater use for the whole riparian system (Scott

and others, 2006b). These investigations also revealed that grasses can only access groundwater at depths of 11 feet or less, but the deeper-rooted trees access groundwater at depths greater than 36 feet.

In riparian plant communities like the long, narrow cottonwood galleries or stands of seepwillow (a dominant understory plant along the river) that were not amenable to micrometeorological techniques, sap-flow sensors were deployed in our studies to quantify water flow in roots, branches, and stems of the dominant plant types (see sidebar at right). This technique was used in combination with plant surveys of total sap wood area and canopy cover to determine transpiration. Cottonwood and willow forests along a perennial flow reach (depth to groundwater ranging from 3 to 7 feet) had the highest water use with rates exceeding 37 inches for a growing season. Cottonwoods along an intermittent reach of the river where the depth to groundwater ranged from 10 to 13 feet were more water stressed and used only about 20 inches over the same time period (Gazal and others, 2006). Transpiration rates for seepwillow were about 31 inches, similar to the cottonwood overstory despite the reduced atmospheric demand of the understory environment (Scott and others, 2006a).

Scaling Up from Site to ReachTwo approaches were used to scale up the site-based measurements to obtain total riparian vegetation water use along entire reaches of the San Pedro. The first used a detailed vegetation map and the second used vegetation indices and surface temperature from satellites to provide spatially explicit data.

Vegetation mapping: Detailed vegetation maps were used to quantify the total

Sapflow measurement on mesquite tree roots in the San Pedro Basin.


area of the different riparian vegetation communities and then multiply these areas by their respective estimates of ET obtained from the micrometeorological or sap flow measurements (Goodrich and others, 2000; Scott and others, 2006a). Riparian groundwater use in 2003 for a 30-mile reach along the San Pedro River from the international boundary to the USGS stream-gauging station near Tombstone was calculated to be about 7,300 to 9,000 acre-feet (around 9 to 11 million cubic meters) per year. For the entire Sierra Vista subwatershed, estimates were 25 to 57 percent greater than the amount determined by the Arizona Department of Water Resources based solely on stream gauge information. Mesquite groundwater use was the dominant component of total riparian water use (58 percent), owing to its high abundance, followed by cottonwood-willow, open water, sacaton, and tamarisk.

Satellite data: More recently, a second approach to scaling up site-based measurements has been to connect satellite measurements of surface temperature and

vegetation greenness with multiple years of site-specific ET data in a statistical modeling framework (Scott, 2007). Because the satellite data have spatial resolution of about 800 feet and temporal resolution of every 16 days since the year 2000,

multiyear ET estimates, which implicitly account for the spatial heterogeneity of the vegetation functioning, were possible over the entire basin. Annual riparian

see Riparian ET, page 34

(Top) Total nighttime sap flow of the taproot and a lateral root of a mesquite tree, calculated from half-hourly measurements between 8 p.m. and 5:30 a.m. Negative values represent reverse flow (away from the crown). A significant negative correlation is observed between nocturnal sap flow in the taproot and in the lateral root (R2 = 0.85, P < 0.0001). (Bottom) Daily precipitation totals at the field site during the study. Adapted from Hultine and others (2004).

The Water-Banking Mesquites

One of the most fascinating results of our research was the the discovery of “hydraulic redistribution” by mesquite, or the transfer of soil water via plant roots in response to water potential gradients. Growing evidence suggests this process is prevalent in any ecosystem that contains plants with roots that span moisture potential gradients. Hultine and others (2004) discovered that riparian mesquites have the ability to redistribute near-surface soil moisture to the deeper vadose zone throughout the entire year (see chart at right). Measured nighttime sap flow in a mesquite taproot was upward before the monsoon onset, but became downward when the surface soil was moist, and sap in lateral roots moved toward the stem. Moisture redistribution followed the moisture potential gradient with upward “lifting” of deep vadose zone moisture or groundwater during the dry season and downward descent of precipitation during times of abundant surface moisture. In this way, they found that mesquite can “store” rainfall deeper in the vadose zone, away from scavenging understory plant roots and bare soil evaporation processes, and then later use this moisture to support transpiration. We are currently examining the ecohydrological significance of this process in nonriparian mesquites; preliminary results suggest it plays a pivotal role in their successful expansion into grassland ecosystems.

ReferenceHultine, K.R., R.L. Scott, W.L. Cable, and others, 2004. Hydraulic redistribution by a dominant, warm desert phreatophyte: seasonal patterns and response to precipitation pulses. Functional Ecology, 18: 530-538.

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2015 N. Forbes Ave. Suite 105Tucson, Arizona 85745

Phone: 520.628.9330 • Fax: 520.628.1122

www.geosystemsanalysis.com

Groundwater Recharge Studies

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Innovative Solutions in Hydrology


Throughout the Southwest, state and federal water-resource managers are becoming increasingly

concerned about the impacts of future groundwater development on the region’s limited water resources, environmentally sensitive ecosystems, and rural lifestyle. To address their concerns, scientists and engineers are deploying physically based mathematical models to assess and predict the potential effects of increased groundwater pumping. The accuracy of these predictions is directly related to how well water budgets are quantified and balanced at basin and regional scales.

Groundwater Discharge Via ETOf the three main components of a predevelopment groundwater budget—natural discharge, natural recharge, and subsurface flow—estimates of natural discharge are the most straightforward to obtain, and can be used to constrain the other, more difficult to quantify water budget components. In the Southwest, groundwater discharges naturally in low areas of intermontaine basins by 1) spring and seep flow; 2) transpiration by local phreatophytes, and 3) evaporation from soil and open water.

Evapotranspiration (ET) is the combined process that transfers evaporated and transpired water from the land surface to the atmosphere. ET measurements from discharge areas typical of the Southwest include spring and seep flow because most of the discharged water evaporates from

pools or open drainages, or infiltrates downward to the shallow water table where ultimately it is transpired by local vegetation. Thus, hydrologists often use ET to estimate regional groundwater discharge in the Southwest.

One method commonly used to estimate regional groundwater discharge is to compute the difference between ET and local precipitation. First, ET is calculated as the product of ET rates and the acreages of vegetation, open water, and moist soil through which ET occurs. Next, the calculated volume of ET is partitioned into local precipitation and regional groundwater sources. This method, illustrated at right, was applied recently in Spring Valley, Nevada (part A on figure), where groundwater discharge was estimated as part of a congressionally mandated evaluation of the water resources of White Pine County, Nevada (Welch and others, 2007). Although Spring Valley is used to illustrate the method; it is only one of 12 basins for which ET and discharge were estimated using this regional approach.

Mapping ET UnitsThe ET rate in groundwater discharge areas varies with vegetation type and density, and soil characteristics. In general, the more dense and healthy the vegetation and the wetter the soil, the greater is the ET. Remote-sensing techniques using satellite imagery in combination with field mapping have been used in the Southwest

to group areas of similar vegetation and soil conditions within groundwater discharge areas (Laczniak and others, 2001). These “ET units” represent areas

of similar ET rates. Reliable estimates of groundwater discharge require accurate mapping and grouping of local ET units and a sound knowledge of local ET rates.

ET units typical of the Southwest range from areas of no vegetation, such as open water, dry playa, and moist bare soil, to areas with vegetation often dominated by phreatophytic shrubs, grasses, rushes, and reeds. The use of remote sensing to delineate the units is an improvement over earlier studies, which relied only on time-consuming and costly field mapping that often resulted in fewer and less precise ET-unit groupings.

The Spring Valley discharge area, defined by the extent of the phreatophytic shrub greasewood (Sarcobatus vermiculatus), was mapped using satellite data, aerial photography, and elevation models

Michael T. Moreo, Nancy A. Damar, and Randell J. Laczniak – U.S. Geological Survey, Nevada Water Science Center

ET–The Key to Balancing the Water Budget in the Southwest

One method used to estimate regional groundwater discharge is to compute the difference between ET and local precipitation.


along with established control points to a 1:24,000 scale. Ten ET units were identified within the Spring Valley discharge area and other such areas in White Pine County and delineated using Landsat Thematic Mapper (TM) imagery-based methods.

Shrubland, grassland, meadowland, and moist bare soil ET units were delineated using the Modified Soil Adjusted Vegetation Index and a Tasseled Cap transformation of a single TM image (Smith and others, 2007). Dry playa, marshland, and open water ET units were delineated using a published land cover map based on multiple-date TM imagery (Southwest Regional Gap Analysis Program). Recently irrigated acreage was delineated from multiple-date TM imagery. ET units delineated by these techniques were combined into a single ET-unit map of the groundwater discharge area (parts B and D on figure). The accuracy of the ET-unit map was assessed and ground-truthed through field observations.

Measuring ET RatesIdeally, ET rates should be measured in each of the dominant ET units until a long-term average is established. Although it now is possible to measure ET continuously over long time periods with micrometeorological instrumentation, time and funding constraints often limit the acquisition of field data. Researchers therefore often must rely on ET rates measured over less-than-optimal time periods or on reported rates measured outside their study areas, adding uncertainty to estimates because of differences in measurement techniques, climate, soil, and vegetation.

ET rates reported in recent literature for vegetation and soil moisture conditions similar to those of White Pine County were normalized to local long-term precipitation averages to develop a reasonable range of the annual ET rate for each of the ET units in the county. The rates were then scaled based on vegetation density to develop an estimate of the ET rate for each of the ET units in Spring Valley.

ET site

0 25 miles

SpringValley

Recharge area

MarshlandMeadowlandGrassland

Dense shrublandModerate shrublandSparse shrubland

Moist bare soil

Open water

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Agriculture

Precipitation ET

PrecipitationGW discharge

06 070

2.0

1.0

0.5

1.5

2.5

ET, i

n fe

et p

er w

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r

ET units

180,000 acres total

ET u

ntis,

in th

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ands

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0

20

40

60

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Open water < 10 acresET

rate

, in

feet

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er y

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0

1

2

3

4

5

6

ET

Precipitation

ET units

GW discharge

ET units Spring Valley ET total

ET, i

n th

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ands

of a

cre-

feet

ET, i

n th

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ands

of a

cre-

feet

0

20

40

60

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100

0

40

80

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Groundwatersource

ET

Precipitationsource

Satellite image Areas of ground-water recharge and discharge

Mapped ET units

Measured ET ratesMicrometeorological equipment

Area of ET unitsRates of ET per unit

ET and groundwater discharge

Conceptualization of the ET units and the groundwater discharge estimation process for Spring Valley.see Budget, page 33

Phot

o: H

owar

d Gr

ahn


The wide variability in evapotranspiration (ET) in both space and time poses a huge

challenge to quantify total volumes of evaporation over large areas and time periods. To address this, satellite-based ET maps are being integrated with land-surface modeling or point measurements of ET to provide accurate estimates for large areas. The 1999 launch of Landsat 7 lowered the cost of medium-resolution satellite imagery by a factor of ten to about $400 per image. Numerous surface energy balance algorithms were subsequently developed or refined. Most utilize thermal (infrared) imagery collected by a satellite system to estimate actual ET, which may be less than potential ET due to shortage of soil water.

Deriving regional ET solely by ground-based “flux” measurements using eddy covariance (EC) and Bowen ratio (BR) or other systems can result in large uncertainty at substantial expense. Even with the best of equipment and exercise of care, ground-based flux measurements can sample only a single representation of spatial features that are rich in variations in soil type, soil moisture, vegetation type, vegetation cover, water table depth, slope, and aspect. Extrapolating these ground-based flux measurements over large areas becomes difficult and speculative, even with the help of numerical models.

Potential ET is approximated using maps of vegetation cover, which can be generated with reasonable accuracy

using short-wave imagery available from many remote-sensing satellites. Actual ET, however, also depends on ecosystem health, which is affected by water shortages, as well as on weather

conditions such as wind, radiation as affected by cloud cover, air temperature, and humidity. The thermal images from satellites function as a sort of

“thermometer” of ecosystem health by integrating all the factors that influence actual ET. Given the same vegetation cover, the higher the temperature is, the greater the water shortage exhibited, and thus the lower the actual ET rate is relative to the potential rate.

Combining spatial ET data from satellites with point measurements or modeling data of ET and soil moisture provides a cost-effective means to improve accuracy in ET mapping. For example, data assimilation methods have been used by NASA’s Land Information System to integrate satellite and surface observations into model runs to construct local to global hydrologic forecasts with resolutions beginning at about one-half mile and ranging to 15 miles, with time steps of less than one hour (Cosgrove and others, 2003).

Richard G. Allen – University of Idaho, Jan M.H. Hendrickx – New Mexico Tech, David Toll – NASA Goddard Space Flight Center, Martha Anderson and William Kustas – USDA-ARS-Beltsville, and Jan Kleissl – University of California-San Diego

From High Overhead: ET Measurement via Remote Sensing

NOAH Mean Annual ET (mm/yr)0 200 400 600 800 1000 1200+00

-135

60

-90 -45 0 45 90 135

30

00

-30

Annual ET, mm/year, for the terrestrial regions of the globe, modeled using the Global Land Data Assimilation System NOAH model (Rodell and others, 2004b).

Combining spatial ET data from satellites with point measurements or modeling data provides a cost-effective means to improve accuracy in ET mapping.


Errors and UncertaintyLoescher and others (2005) and Hendrickx and others (2007) have examined, respectively, land-based EC and scintillometer flux measurement uncertainties. Linear regressions for sensible heat fluxes of eight EC instruments in neutral and unstable conditions revealed differences of up to 30 percent, with typical uncertainties of 7 to 11 percent. Two studies with five large-aperture scintillometers in New Mexico showed typical differences among paired instruments of 5 to 10 percent.

Uncertainties associated with ET derived from remotely based energy balance systems under ideal conditions may be no greater than those common to ground-based systems. Uncertainties in satellite-based estimates result from random error and systematic biases in: estimation of surface reflectance and albedo, net radiation, surface temperature, air temperature gradients used in sensible heat flux calculations, aerodynamic resistance estimation, atmospheric corrections of reflectance, soil heat flux estimation, and estimation of wind speed fields. Most polar-orbiting satellites travel 440 miles above the earth’s surface, yet the transport of vapor and sensible heat from land surfaces is strongly impacted by aerodynamic processes including wind speed, turbulence, and buoyancy, all of which are invisible to satellites.

Most of the common satellite-based energy balance systems have automatic techniques to calibrate around many of these errors (Allen and others, 2007a). Quality-controlled ground-based measurements or routinely produced indices such as reference ET are frequently used to calibrate remotely sensed ET estimates to improve accuracy. Flux towers can provide time-continuous validation at discrete points within the remote-sensing and modeling domain, while high-spatial resolution remote-sensing data can inform models calibrated or forced using flux measurements.

Scaling Up and DownRemote-sensing algorithms can infer long-term carbon, water, and energy budgets

across areas ranging from watershed to global scales. One approach to “upscaling” fluxes is to use land-atmosphere transfer schemes and computer models linked to remotely sensed boundary conditions (Anderson and others, 2007a). The gridded model output can be validated at the positions of available ground measurements; if the agreement is reasonable and the observation sites are representative, the remote sensing-derived flux maps can be used to represent conditions beyond the ground measurements. This upscaling bridges the gap between flux tower footprint scales (around 200 to 1,000 feet) and the target scales of interest (hundreds to thousands of miles for the continental United States).

Daily fluxes have been mapped using the ALEXI process of Anderson and others (2007b) across regional and continental scales at 3- to 6-mile resolution using the Geostationary Operational Environmental Satellite (GOES). These coarse-scale flux estimates can also be downscaled to a finer resolution at sites of particular interest (such as around a flux tower, aircraft flightpath, or experimental site) using higher-resolution imagery from the thermally equipped Landsat, ASTER, or MODIS satellites. Maps of actual-to-potential ET ratio from ALEXI correspond well with patterns of antecedent

precipitation and are being evaluated for use in operational monitoring of drought conditions at local to continental scales.

Applications at All ScalesUse of actual ET derived by surface energy balance models using high- to medium-resolution thermal-based satellite imagery has been adopted by a number of western states for their routine water operations and planning programs. The Idaho Department of Water Resources and University of Idaho have partnered to use Landsat-based maps of actual ET to: 1) estimate water budgets for hydrologic modeling of river basins, 2) monitor compliance of irrigated farms having stipulated caps on water use, 3) support water resources planning where quantification of total ET from water sources is otherwise unavailable, 4) estimate net depletions of water from aquifers by thousands of unmetered irrigation pumps, 5) support groundwater model calibration/operation, 6) estimate water use by irrigated agriculture, 7) estimate historical water use for valuating agricultural water use for transfer of water rights, 8) develop new ET curves for agricultural crops, and 9) evaluate relative performance of large irrigation projects by comparing ET with diversions (Allen and others, 2007b). In the Rio Grande Valley of New Mexico, Landsat-


A close-up of ET within center-pivot irrigated fields in central Spain during 2003, where the actual ET rate is expressed as a fraction (Kc) of ET from well-watered grass, derived by Allen and others (2007b) using Landsat 5, including the thermal band, with 30-meter (about 100 feet) resolution.


continued from previous page

scale ET maps helped estimate water consumption by invasive species along riparian corridors. In the Imperial Valley of California, ET maps have been used to assess irrigation and salinity management.

NASA has partnered with the University of Montana to explore the development of a global product for actual ET using data from the MODIS satellite. The process uses land cover-based parameterization of resistance parameters in the Penman-Monteith method coupled with regionally gridded weather data. (www.ntsg.umt.edu/modis/).

The USGS EROS center has developed a relatively simple technology that uses thermal and short-wave imagery from MODIS to estimate relative ET for major portions of the world, most commonly in underdeveloped countries where ET data can be used to assess food security or drought conditions. Bastiaanssen and others (2005) routinely estimate crop yields and ET from satellite data in developed and developing countries for the World Bank and other clients.

Rodell and others (2004a) have experimented with deriving ET for large areas using the GRACE gravimetric satellite using a regional water balance where ET = precipitation - stream outflow + change in soil water storage. The “change in soil water storage” component

is derived from the change in local gravity detected by GRACE, primarily due to variations in the water table. These observations have very low resolution, on the order of hundreds of kilometers.

The Future Looks… Bleak?Over the past thirty years, engineers and scientists have advanced the development of energy balance principles and relationships between vegetation amounts and ET to produce valuable maps of ET. Remote sensing surface energy balance estimates require the measurement of surface temperature via satellite or airplane, and thermal sensors are expensive and rare, especially at resolutions needed to estimate ET at scales of thin ribbons of riparian vegetation and irrigated fields. Unfortunately, the U.S. government has expressed reluctance to fund a thermal sensor on the next Landsat satellite, scheduled for launch in 2011. This effectively discontinues a 25-year archive of extremely valuable, high-resolution (200-400 feet) thermal imaging at the global scale. No other satellite platform routinely provides affordable, readily applied thermal imagery at this critical resolution, allowing discrimination between moisture conditions in adjacent agricultural fields. This occurs just when Landsat thermal data are beginning to be put to hard use in managing water resources and supporting hydrologic studies including impacts of climate change.

ReferencesAllen, R.G., M. Tasumi, and R. Trezza, 2007a.

Satellite-based energy balance for mapping evapotranspiration with internalized calibration (METRIC) – Model, ASCE J. Irrig. & Drainage Eng., 133: 380-394.

Allen, R.G., M. Tasumi, A. Morse, and others, 2007b. Satellite-based energy balance for mapping evapotranspiration with internalized calibration (METRIC) – Applications, ASCE J. Irrig. & Drainage Eng., 133: 395-406, www.idwr.idaho.gov/gisdata/et.htm..

Anderson, M.C., W.P. Kustas, and J.M. Norman, 2007a. Upscaling flux observations from local to continental scales using thermal remote sensing, Agronomy J., 99: 240-254.

Anderson, M.C., J.M. Norman, J.R. Mecikalski, and others, 2007b. A climatological study of evapotranspiration and moisture stress across the continental U.S. based on thermal remote sensing, II: Surface Moisture Climatology, J. Geophys. Res., 112, D11112, doi:10.1029/2006JD007507.

Bastiaanssen, W.G.M., E.J.M. Noordman, H. Pelgrum, and others, 2005. SEBAL model with remotely sensed data to improve water-resources management under actual field conditions, ASCE J. Irrig. & Drainage Eng., 131(1): 85-93.

Cosgrove, B.A., D. Lohmann, K.E. Mitchell, and others, 2003. Real-time and retrospective forcing in the North American Land Data Assimilation System (NLDAS) project, J. Geophys. Res., 108(D22): 8842, doi:10.1029/2002JD003118.

Hendrickx, J.M.H., J. Kleissl, J.D. Gómez Vélez, and others, 2007. Scintillometer networks for calibration and validation of energy balance and soil moisture remote sensing algorthms, Proceedings of the SPIE, 6565: 65650W.

Loescher, H.W., T. Ocheltree, B. Tanner, and others, 2005. Comparison of temperature and wind statistics in contrasting environments among different sonic anemometer-thermometers. Agric. For. Meteorol., 133: 119-139.

Rodell, M., J.S. Famiglietti, J. Chen, and others, 2004a. Basin scale estimates of evapotranspiration using GRACE and other observations, Geophys. Res. Let., 31(20), L20504, doi:10.1029/2004GL020873.

Rodell, M., P.R. Houser, U. Jambor, and others, 2004b. The Global Land Data Assimilation System, Bull. Amer. Meteor. Soc., 85(3): 381-394.


heat transfer, which can be substantial. Finally, recall that the goal is to measure fluctuations in humidity and wind speed at the same point; in a closed-path system it is possible to pull the sample air from the same volume being measured by the anemometer. In an open-path system there is a gap of at least 10 centimeters between sensors that causes an underestimation of the covariance. Corrections can be applied, but inevitably introduce additional uncertainty. Both types of IRGAs require periodic calibration, usually with a dew-point generator, adding several thousand dollars to the $25,000 to $30,000 cost of the basic eddy-covariance instrumentation.

Data analysis can be the most intimidating aspect of eddy covariance. Data collected at 10 Hertz accumulate quickly and require numerous steps to process. Software is available to perform these processes, but it requires a knowledgeable user. Detailed mathematical descriptions can be found in the listed references, but the first step, data screening, deserves a few words.

Not all data collected with an eddy-covariance system are usable. Instrument failures and environmental factors—particularly precipitation, winds from an unfavorable direction, or extremely calm conditions—can cause erratic, nonsensical results. Algorithms must be used to flag these gaps and suspicious values and replace them with informed estimates.

Site Requirements for BREB and ECTo accurately measure the ET of a given surface type, the entire “flux footprint,” or area over which surface exchange is being measured, should be uniform. The surface should extend upwind for a distance approximately 100 times as great as the height of measurement. Thus, a two-meter tower requires placement at a site with 400-meter study area on each side, or around 40 acres, so that ET can be measured from any wind direction. Tower height can be lowered over shorter crops to reduce the flux footprint, but eddy size decreases and eddy frequency increases near the surface, factors which can cause systematic underestimation of the flux.

Thus sensors should be at least one meter above a relatively smooth surface like grass or a row crop, and higher over rough surfaces. Additional complications arise if the measured surface is not level.

Eddy covariance and BREB are powerful methods for measuring ET, but neither is a routine, turnkey technique with universal application. Potential users should evaluate the characteristics of their site, their ability to periodically calibrate gas analyzers, and their willingness to learn and apply the necessary data processing procedures before investing the money and effort required to install either system.

Contact John Baker at [email protected].

ResourcesMassman, W.J., and X. Lee, 2002. Eddy covariance

flux corrections and uncertainties in long-term studies of carbon and energy exchanges, Agric. and Forest Meteorology, 113, 121-144.

Meyers, T.P., and D.D. Baldocchi, 2005. Current micrometeorological flux methodologies with applications in agriculture. In Micrometeorology in Agricultural Systems ed. by J.L. Hatfield and J.M. Baker, 381-396. Amer. Soc. Agronomy, Madison, WI.

Challenges, continued from page 25

The initial rate ranges were assessed and refined using ET rates measured for one near-average precipitation year at six eddy-covariance ET sites established specifically for this study (Moreo and others, 2007). Five of these sites were located in shrubland to evaluate the effect of vegetation density on ET rates, and to better understand the relation between ET and groundwater discharge in the dominant (greater than 80 percent) vegetation type of the study area. One site, established near a boundary between the grassland and meadowland ET units, was located in a mixed-grass riparian area to represent an environment indicative of greater ET (part C on figure). Annual ET rates based on a combination of reported and measured ET data vary slightly between basins, and range from 0.8 feet to 5.0 feet for the ET units in Spring Valley (part E on figure).

Estimating Groundwater DischargeAverage annual ET from a discharge area is estimated as the sum of the ET (the product of the ET rate and its acreage)

of all the component ET units. In Spring Valley, total annual ET was estimated to be 200,000 acre-feet.Regional groundwater discharge is estimated from total ET by subtracting the volume of local precipitation falling directly on the discharge area (part F on figure). In Spring Valley, about 0.69 feet or 124,000 acre-feet of local precipitation annually falls on the 180,000-acre discharge area, thus annual regional groundwater discharge from the valley is estimated to be 76,000 acre-feet.

As the population of the Southwest increases, so will the competition and need for additional water supplies. Agencies responsible for water-resources management must be prepared to tap the limited water supply in the most efficient manner and will require more thorough quantification of the water budget beyond our current reconnaissance-level understanding. The accuracy of these estimates will rely, to a large degree, on a representative basin- or region-wide coverage of spatial and temporal ET measurements. And just as importantly, accurate estimates of ET

and groundwater discharge will improve confidence in the results of modeling efforts, particularly those directed at predicting the effects of increased groundwater pumping.

Contact Michael Moreo at [email protected].

ReferencesLaczniak, R.J., J.L. Smith, P.E. Elliott, and others,

2001. Ground-water discharge determined from estimates of evapotranspiration, Death Valley regional flow system, Nevada and California: USGS WRI Report 01-4195, pubs.usgs.gov/wri/wri014195.

Moreo, M.T., R.J. Laczniak, and D.I. Stannard, 2007. Evapotranspiration rate measurements of vegetation typical of ground water discharge areas in the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent areas in Nevada and Utah, Sept. 2005-Aug. 2006: USGS Sci. Invest. Report 2007-5078, pubs.usgs.gov/sir/2007/5078/.

Smith, J.L., R.J. Laczniak, M.T. Moreo, and T.L. Welborn, 2007. Mapping evapotranspiration units in the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent parts of Nevada and Utah: USGS Sci. Invest. Report 2007-5087, pubs.usgs.gov/sir/2007/5087/.

Welch, A.H., D.J. Bright, and L.A. Knochenmus, eds., 2007. Water resources of the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent areas in Nevada and Utah: USGS Sci. Invest. Report 2007-5261, pubs.usgs.gov/sir/2007/5261/.

Budget, continued from page 29


groundwater use from 2001 to 2005 within the subwatershed was nearly constant over the study period despite an ongoing drought, indicating that the vegetation’s access to groundwater has so far buffered it against meteorological drought (see figure, right). For the same reach of the San Pedro, the annual amounts determined by this new approach range from 7,800 to 9,400 acre-feet (9.6 million to 11.6 million cubic meters), within the range of values determined by Scott and others (2006a) for 2003. However, because of the larger estimates for groundwater use for the main tributary of the San Pedro, the Babocomari, watershed totals were close to or exceeded the upper end of the range of previous estimates.

This work has been supported by the USDA-ARS, SAHRA, and the Upper San Pedro Partnership. Our scope has also broadened to involve researchers of the Rio Grande and Colorado River basins so that generalized methods can be developed with broad application.

Contact Russell Scott at [email protected].

ReferencesGazal, R.M., R.L. Scott, D.C. Goodrich, and D.G.

Williams, 2006. Controls on transpiration in a desert riparian cottonwood forest, Agric. For. Meteorol., 137: 56-67.

Goodrich, D.C., R. Scott, J. Qi, and others, 2000. Seasonal estimates of riparian evapotranspiration using remote and in situ measurements, Agric. For. Meteorol., 105: 281-309.

Scott, R.L., D. Goodrich, L. Levick, and others, 2006a. Determining the riparian groundwater use within the San Pedro Riparian National Conservation Area and the Sierra Vista Subwatershed, Arizona. In Hydrologic Requirements of and Consumptive Ground-Water Use by Riparian Vegetation along the San Pedro River, Arizona, ed. by J.M. Leenhouts, J.C. Stromberg, and R.L. Scott, USGS Sci. Invest. Report 2005-5163, 107-152.

Scott, R.L., T.E. Huxman, D. G. Williams, and D.C. Goodrich, 2006b. Ecohydrological impacts of woody plant encroachment: Seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment, Global Change Biology, 12: 311–324.

Scott, R.L., 2007. Multiyear riparian evapotranspiration and groundwater use for the Upper San Pedro Basin, presented at the 2007 Southwest Hydrology/Arizona Hydrological Society Regional Water Symposium, Aug. 29-30, 2007, Tucson, AZ.

Williams, D.G., and R.L. Scott, in press. Vegetation-hydrology interactions: Dynamics of riparian plant water use along the San Pedro River, in Arizona Ecology of Desert Riparian Ecosystems: The San Pedro River Example, ed. by J. Stromberg and B. Tellman.

Riparian ET, continued from page 27 2001-2005 reach-level riparian groundwater use (ET in excess of precipitation) along the San Pedro and Babocomari rivers in the Sierra Vista subwatershed. The ranges of estimates determined by Scott and others (2006) for 2003 are indicated by the shaded regions.

2001 2002 2003 2004 2005Year

Rea

ch G

roun

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ater

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[ac

re-f

eet/

year

] 11000

10000

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San PedroBabocomari

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R & DPolonium-210 Found in Nevada WellsIn August, the U.S. Geological Survey released data to state and local officials in Nevada documenting the occurrence of elevated polonium-210 (Po-210) levels in 17 wells in rural Lahontan Valley, near the town of Fallon in west-central Nevada. Po-210 is a radioactive element derived from the decay of uranium. According to the USGS, Po-210 concentrations in Lahontan Valley groundwater appear to result entirely from the decay of naturally occurring uranium in alluvial sediments derived from Sierra Nevada granites. Scientists found no indication that human activity was in any way responsible.

Po-210 concentrations in untreated, unfiltered water collected from 25 wells in the valley ranged from less than 0.1 to 67.7 picocuries per liter (pCi/L). Thirteen of the wells had concentrations greater than 15 pCi/L, the U.S. Environmental Protection Agency’s maximum contaminant level for gross alpha radioactivity in public-supply wells.

According to the USGS, no systematic surveys of the occurrence of Po-210 in the United States have been completed, but based on the literature, the Nevada data were unexpectedly elevated (greater than about 1 pCi/L). Except for a well in Louisiana, one in Virginia, and about 35 in Florida, Po-210 concentrations that exceed the values in the Lahontan Valley wells have not been documented elsewhere in the country. Only under an uncommon set of geologic and groundwater conditions does the compound become mobile and available for transport in water; the compound typically binds strongly to sediment.

Most of the wells included in the USGS study provide water for human or domestic animal consumption. The public water supply for Fallon and the Fallon Naval Air Station were known to be safe and were not included in the study. Water filters and treatment technologies, such as

in-home reverse osmosis systems, have been shown to remove Po-210 from water.

Visit nevada.usgs.gov/polonium/faq.pdf.

Water Managers: Prepare for DroughtThe drought and dry conditions currently gripping much of the country are a taste of things to come, according to a July 2007 report by the Natural Resources Defense Council (NRDC) that assessed the effects of global warming on water supplies in the West. The researchers say that as the hotter, drier weather already afflicting the region becomes the norm, water managers will have to take bold measures to improve conservation and efficiency.

The NRDC report analyzes the effects of global warming on a full range of water management tools and offers comprehensive recommendations to help meet the challenge. Conservation tops the list of proven water supply solutions. For example, water use in Los Angeles has remained steady for 30 years despite dramatic population growth thanks to investments such as low-flow showerheads and toilets. The city can save even more water through programs promoting drought-tolerant landscaping.

The report calls on regions to work much more closely together, developing cooperative solutions to meet their water needs and providing other important benefits. For example, groundwater desalters in California’s Chino Basin produce water supplies while cleaning up contaminated underground aquifers. Urban stormwater retention programs designed to reduce flooding and pollution can also provide water supplies. Wastewater recycling is another promising source of water, especially because it will not be affected by global warming.

The report suggests that traditional approaches—dams, diversions, and groundwater pumping—are likely to perform worse in a warmer, drier climate.

“Increasingly, traditional dams are no longer realistic or financially feasible solutions,” said Barry Nelson, the study’s co-author and co-director of NRDC’s western water project. “The thousands of dams across the West have already captured most of the water. There are so few rivers left, and the cost of building dams is so high that the result is very expensive water. And global warming is likely to reduce the potential water supplies from new dams even further.”

The 90-page report is available at www.nrdc.org/globalwarming/hotwater/contents.asp.

SNWA Seeks Automated Irrigation DeviceLast fall, the Southern Nevada Water Authority (SNWA) solicited bids to develop an irrigation scheduler that will automatically change seasonal watering schedules based on a customer’s assigned watering group. SNWA appropriated up to $250,000 for the initial purchase of such a device that could be attached to existing landscape irrigation controllers to help ensure compliance with the region’s mandatory watering schedules. In theory, the customer would input the current date, time, and assigned watering group; once programmed, the device would comply with the mandated watering schedules without further intervention from the customer.

“We currently use an extensive public outreach campaign to remind people to make the seasonal changes,” said Doug Bennett, SNWA’s conservation manager. “The device would provide a carefree way to assure compliance with watering restrictions while maintaining the health of their landscapes, which are prone to overwatering.” Research has shown that landscapes are most over-watered in the late summer, fall, and early winter, according to Bennett.

Under SNWA watering restrictions, landscape irrigation is limited to three days a week in the spring and fall and


one day a week in winter. Watering is permitted any day of the week in summer, although lawn sprinkler irrigation is prohibited from 11 a.m. to 7 p.m. While many people comply with the regulations, water-waste enforcement personnel still issue hundreds of violations.

The device would allow the controller’s signal to activate the valves only during allowable days and times. Although it is specifically intended to be programmed for SNWA watering schedules, the manufacturer may be able to adapt it for use in other communities.

“With complete, year-around compliance from residents, we estimate the additional savings could be as much as 30,000 acre feet of water each year,” Bennett said.

Proposals were due at the end of October.

Visit www.snwa.com/html/news_conservation_proposal.html.

Algae: A Promising Biofuel SourceLast summer, Diversified Energy Corporation, a privately held alternative and renewable energy company, announced a new partnership and licensing arrangement for an algae production system invented by XL Renewables Inc., a biorefinery project developer. The approach, called Simgae (for simple algae), utilizes common agriculture and irrigation components to produce algae at relatively low cost. According to the press release, the new system is expected to offer the biofuels industry access to cheap and readily available oils and starches for the production of biodiesel, ethanol, and other renewable fuels with significantly lower water requirements than other biofuels crops.

Widespread algae production has thus far been hindered by high costs to build and maintain the systems. Typical architecture consists of a series of rigid frameworks that incur capital costs on the order of hundreds of thousands of dollars to over $1 million per acre.

Simgae, however, uses thin-walled polyethylene tubing called Algae Biotape, similar to conventional drip irrigation tubes, laid out in parallel across a field. Under pressure, water that contains nutrients and a small fraction of algae is slowly introduced into the biotape. Carbon dioxide is injected periodically, and after about 24 hours the flow leaves the biotape with a markedly greater concentration of algae than at the start. All supporting hardware components and processes involved in Simgae are direct applications from the agriculture industry.

The Simgae design is expected to provide an annual algae yield of 100 to 200 dry tons per acre. Capital costs are expected to be approximately $45,000 to $60,000 and profitable oil production costs are estimated at $0.08 to $0.12 per pound. These oil costs compare to recent market prices of feedstock oils of $0.25 to $0.44 per pound, said Diversified Energy.

The team is currently conducting a demonstration of the technology in Casa Grande, Arizona. Continued testing and system optimization is expected to occur through 2008.

In another very promising test in Arizona last summer, Arizona Public Service Co. (APS) and GreenFuel Technologies Corp. of Cambridge, Massachusetts, successfully grew algae with biomass production levels 37 times higher than corn and 140 times higher than soybeans, according to APS. The algae was grown using carbon dioxide emissions from the Redhawk natural gas power plant near the Palo Verde Nuclear Generating Station. Further testing is being undertaken at APS’s Four Corners coal power plant in Farmington, New Mexico.

Visit www.diversified-energy.com/simgae, www.xlrenewables.com, and www.aps.com.

Update: Trees on Levees OKAfter nearly a year of discussion about whether trees on California levees could stay or must go, the California Reclamation Board, Sacramento Area

Flood Control Agency (SAFCA), U.S. Army Corps of Engineers (ACE), U.S. Fish and Wildlife Service, National Marine Fisheries Service, local levee districts, and the state’s Water Resources and Fish and Game departments seem to have reached an agreement that, in California at least, trees on levees are acceptable, maybe even useful.

Six months ago, Southwest Hydrology (July/Aug 2007) reported that ACE had announced its intention to enforce a national policy requiring all trees and vegetation to be cleared from levees. This directive was prompted largely by levee failures in New Orleans following Hurricane Katrina and the subsequent inspection of levees nationwide. Californians objected to the policy for reasons including expense, loss of aesthetic appeal, and the fact that tree-lined levees are often the only riparian habitat available.

After 32 levee districts in the Central Valley failed a maintenance inspection last winter, largely due to having vegetation greater than ACE’s 2-inch-diameter maximum requirement, they faced loss of federal levee rebuilding funds and Federal Emergency Management Agency decertification unless they removed the vegetation by March 2008, reported the Sacramento Bee. Yet many experts—including some ACE scientists—claimed the trees actually helped maintain levee integrity.

To attempt to resolve the issue, in August SAFCA held a symposium on levee vegetation. Much evidence was presented in favor of trees, and little could show their harm, according to the Bee. Following the symposium and another meeting among the agencies several weeks later, the paper reported, ACE agreed to drop the March 2008 compliance deadline and develop a new policy more suited to local conditions.

Visit www.sacbee.com.



Business DirectoryR & D (continued)

Municipal Wastewater Provides Clues to Community Drug UseA team of researchers from Oregon State University and the University of Washington has developed an automated monitoring method that makes it possible to detect traces of drugs, from cocaine to caffeine, in municipal wastewater and monitor the patterns of drug use in entire communities. Their findings were reported last August at the American Chemical Society meeting in Boston.

The presence of both pharmaceutical and illicit drugs in municipal wastewater has been known for several years, beginning with European studies that tracked drugs in sewage and river water. Oregon State University chemist Jennifer Field and her colleagues have developed new methods of analysis so that detection is possible from very small samples taken automatically over a 24-hour period from wastewater as it enters a treatment plant.

The analyses can detect the presence of a long list of illicit drugs, from methamphetamine to Ecstasy, and other markers of human presence such as caffeine and cotinine, a product of nicotine from cigarette smoke.

Although wastewater is often tested for contaminants after it is treated as a measure of potential environmental impact, this new approach tests sewage as it enters a wastewater treatment plant and before it is treated, to get a profile of the drugs being used in the community.

Finding patterns of drug consumption in the wastewater can alert municipalities to problems that occur in particular communities or at particular times. This may be useful for tracking such things as the geographic patterns of methamphetamine use.

The researchers tested wastewater from ten midsized (unnamed) municipalities, calculating the concentrations of individual drugs and using the volume of wastewater flowing into the treatment plant and the municipal population in order to estimate the community load of each drug.

Even in their preliminary study, the researchers found patterns over time of drug occurrence in wastewater, with higher concentrations of recreational drugs such as cocaine on weekends. They found no change in concentrations of either prescription drugs or methamphetamines in their samples over time, which suggests more consistent use of both.

The researchers’ wastewater analysis demonstrates that the new methodology can be applied cost-effectively on a larger scale to collect data from communities across a region or state. And because the data can be collected daily, weekly, or monthly, they represent a real-time measure that provides communities with more opportunity for prevention and intervention.

Visit oregonstate.edu/dept/ncs/newsarch/2007/Aug07/drugsinwastewater.html.

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SOCIETY PAGESWateReuse Releases New Case Studies ReportUnder pressure from such factors as population growth, climate change, depletion of groundwater resources, and impacts from salt, many communities are struggling to find enough water to meet their needs. A new publication from the WateReuse Association, “Innovative Applications in Water Reuse and Desalination: Case Studies Two” profiles ten communities that have faced such problems and found innovative solutions that combine conservation, water reuse, and sometimes desalination. The case studies demonstrate how communities have found success by examining a range of options for alternative water supplies.

Highlighted projects and sites include Cary, North Carolina; Denver Water Recycling Project; Dunedin, Florida; El Paso, Texas; Inland Empire Utilities Agency; Las Vegas, Nevada; Long Beach, California; Santa Rosa Subregional Recycled Water Program; Singapore NE Water Project; and Scottsdale, Arizona.

The report costs $10 for members, $20 for nonmembers and is available at www.watereuse.org/publications.htm.

New Guidebooks Offer Help for NM Water SystemsThe New Mexico Rural Water Association, New Mexico Environmental Finance Center, Rural Community Assistance Corporation, and the State of New Mexico have teamed up to develop three guidebooks to help water and wastewater systems better manage their water resources and plan for their future. The guidebooks address core issues regarding water system sustainability, auditing water use to reduce water losses and increase system efficiency, financial planning and management to ensure sufficient revenues to sustain operations, and asset management. They are designed for water and wastewater system owners, operators, managers, and board members to assess the current status of their operations and develop strategic plans for sustainable water and wastewater service.

“Water Use Auditing: A Guide to Accurately Measure Water Use and Water Loss” is a 26-page document intended to provide a broad overview of water-use auditing concepts and a specific method for categorizing all water use into a standard water balance.

The 56-page “Financial Planning and Rate Setting Guidebook” is designed to help system owners ensure sufficient revenues to sustain operations.

“Asset Management: A Guide for Water and Wastewater Systems” is a 91-page document published in 2006 with the assistance of New Mexico Tech. It aims to help operators determine how to operate their systems to provide a sustained level of service at the lowest life cycle cost.

The guidebooks are intended to be used together as integrated tools for efficient management to enable water systems to meet future service demands and regulatory requirements and to provide long-term sustainability. Asset management, for example, is a fundamental step in determining financial resources needed to operate the system and pay for improvements, expansions, or replacements. The water auditing program can tie to asset management by providing information about the condition of the buried assets. In turn, the auditing process also relates to water conservation and rate setting.

The three documents are available at www.nmrwa.org/wateraudit.php

CA Groundwater MeetingNearly 350 people met in Sacramento last September for the joint 16th Groundwater Resources Association (GRA) of California Annual Meeting and 26th Biennial Groundwater Conference sponsored by the University of California Center for Water Resources. While the theme of the meeting related to expanding the role of groundwater in the state, issues concerning management and operation of the Sacramento-San Joaquin Delta were hot topics as well. Featured topics included desalination, groundwater recharge, assessing California’s water quality, salt water intrusion, preparing for climate change, groundwater



SOCIETY PAGES (continued)management and land use planning, data management, and infrastructure.

GRA presented two awards at the meeting. The Kevin J. Neese Award was given to the University of California Groundwater Cooperative Extension Program for its investigation of groundwater quality in cooperation with the dairy community. Thomas Harter of the University of California at Davis, the program leader, accepted the award. It recognizes significant accomplishment by a person or entity within the most recent 12-month period that fosters the understanding, development, protection and management of groundwater.

The Lifetime Achievement Award was presented to Herman Bouwer (see photo), now retired from a 42-year career the U.S. Water Conservation Laboratory in Phoenix, where he made many advances in the fields of artificial recharge, soil aquifer treatment, and surface water-groundwater interactions. Bouwer was unable to accept the award in person, but it was presented to him on behalf of GRA at the Sixth Biennial International Symposium on Managed Aquifer Recharge (see next article), where he helped teach a workshop. GRA’s Lifetime Achievement Award is presented to individuals for their exemplary contributions to the groundwater industry

and for contributions that have been in the spirit of GRA’s mission and objectives.

Visit www.grac.org.

ISMAR is MarvelousHosted by the Arizona Hydrological Society, the 6th Biennial International Symposium on Managed Aquifer Recharge (ISMAR) was held in Phoenix Oct. 28-Nov. 2. The event drew a crowd of nearly 300 people representing 26 countries (all

continents but Antarctica!) and 27 U.S. states, with more than half the attendees from outside Arizona. The program included three days of technical and poster sessions, four workshops, and field trips to aquifer storage and recovery sites both near (the East Salt River Valley in Phoenix) and far (the Las Vegas Valley).

Edward J. Bouwer of Johns Hopkins University presented the keynote address. Bouwer is recognized for his research in microbial process engineering and bioremediation processes, and as chairman of the National Research Council Committee on Managed Underground Storage of Recoverable Water. His talk focused his committee’s recent report on the the biogeochemical, engineering, and institutional factors that may affect the performance of managed aquifer recharge technology.

Technical sessions at ISMAR covered the role of artificial recharge in integrated water management, groundwater hydraulics and storage, regulations and economics, geochemistry, the fate of pathogens and other organics, regional and Arizona-specific issues, basin recharge, subsurface water quality changes, operations and management.

Hard-bound copies of the proceedings are available for $90 at www.acaciapublishing.com. Also visit www.ismar2007.org.

Herman Bouwer (left) receives the Groundwater Resources Association of California’s Lifetime Achievement Award from Doug Bartlett, co-chair of the ISMAR conference, in Phoenix.

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�0 • January/February 2008 • Southwest Hydrology

PEOPLEReclamation Names New Regional DirectorsIn September, U.S. Bureau of Reclamation Commissioner Robert W. Johnson announced the selection of two new regional directors. Lorri Gray is now heading Reclamation’s Lower Colorado Region, based in Boulder City, Nevada, and Larry Walkoviak is in charge of the Upper Colorado Region, working out of Salt Lake City.

Gray has worked in the Lower Colorado Region for 24 years, most recently as manager of the Lower Colorado River Multi-Species Conservation Program. Prior to that, she was the region’s deputy director for more than three years. She has also managed the region’s financial and security programs, the Salton Sea restoration study program, and the Coachella Canal lining program, among others.

Gray’s position had been unfilled since the appointment of Johnson to his position as Commissioner in September 2006.

With Reclamation for more than 32 years, Walkoviak had been deputy regional director of Reclamation’s Lower Colorado Region since 2005. Prior to that, he held numerous other management positions, including in Reclamation’s Oklahoma-Texas Area Office in the Great Plains region and in the Upper Colorado region for 17 years.

Walkoviak filled the position of Rick Gold, who retired.

Visit www.usbr.gov.

New ADWR Chief Hydrologist NamedAfter many years with the Arizona Department of Water Resources (ADWR), the agency’s chief hydrologist, Frank Putman, resigned from the agency at the end of September to pursue other interests. Putman managed the Hydrology Division of ADWR, the technical arm of the agency, which collects and analyzes statewide water

resource data and maintains the state’s groundwater site inventory database, assists with the scientific components of specific research projects, and assists in making determinations on permit applications.

In announcing Putman’s departure, ADWR Director Herb Guenther stated that Frank Corkhill, a hydrologist with the agency for many years, would assume Putman’s position.

Visit www.azwater.gov.

Climate Scientists Recognized for Water Planning AssistanceCalifornia Department of Water Resources (DWR) Director Lester Snow presented his agency’s first-ever awards for Climate Science Services at the Climate Change Water Adaptation Summit last fall. The awards recognize ongoing assistance provided by members of the academic community who have been working closely with DWR to plan for climate variability and change.

“As water managers, we must take the initiative to clearly communicate our needs for applied science to the research

community,” said Snow. “I am pleased to acknowledge the exemplary assistance that these scientists have provided to us, and look forward to a productive long-term working relationship with them and with their institutions as we move forward to develop water management strategies for adapting to climate variability and change.”

The award recipients are:• Daniel Cayan and Michael Dettinger,

California Applications Program, Scripps Institution of Oceanography, University of California, San Diego

• Gregg Garfin, Climate Assessment for the Southwest, University of Arizona

• Bradley Udall, Western Water Assessment, University of Colorado

• Connie Woodhouse, Geography and Regional Development, University of Arizona

They were cited for their contributions to DWR climate change and variability efforts, including providing advice, data, analyses, publications, and presentations, and for their willingness to engage with resource managers and support information transfer from academia to those managers.

Visit www.water.ca.gov.

Dissolved solids in basin-fill aquifers and streams in the southwestern United States, by D.W. Anning, N.J. Bauch, S.J. Gerner, M.E. Flynn, S.N. Hamlin, S.J. Moore, D.H. Schaefer, S.K. Anderholm, and L.E. Spangler.http://pubs.usgs.gov/sir/2006/5315/

Hydrogeology of the Coconino Plateau and adjacent areas, Coconino and Yavapai Counties, Arizona, by D.J. Bills, M.E. Flynn, and S.A. Monroe.http://pubs.usgs.gov/sir/2005/5222/

Land subsidence and aquifer-system compaction in the Tucson Active Management Area, south-central Arizona, 1987-2005, by R.L. Carruth, D.R. Pool, and C.E. Anderson.http://pubs.usgs.gov/sir/2007/5190/

Modeling the spatial and temporal variation of monthly and seasonal precipitation on the Nevada Test Site and vicinity, 1960-2006, by J.B. Blainey, R.H. Webb, and C.S. Magirl.http://pubs.usgs.gov/of/2007/1269

January/February 2008 • Southwest Hydrology • �1

T H E C A L E N D A R

JANUARY2008

FEBRUARY2008

January 14-15 CLE International. Nevada Water Law. Reno, NV. www.cle.com/product.php?proid=917&page=Nevada_Water_Law

January 22-23 Groundwater Resources Association of California. Intro to Groundwater and Watershed Hydrology Monitoring, Assessement, and Protection. Davis, CA. www.grac.org

January 22-25 Texas Ground Water Association. 200� Annual Convention and Trade Show. Lubbock, TX. www.tgwa.org/meetings/2008annual/

January 27-29 American Water Works Association. Inorganic Contaminants Workshop. Albuquerque, NM. www.awwa.org/Conferences/Content.cfm?ItemNumber=3529&navItemNumber=29842

January 31-February 1 Texas Water Conservation Association and Texas Rural Water Association. TWCA/TRWA Water Law Seminar. Austin, TX. www.trwa.org/08%20Law%20Seminar%20Save%20the%20Date.pdf

February 3- 7 USDA - CSREES. National Water Conference: Research, Extension, and Education for Water Quality and Quantity. Sparks, NV. www.soil.ncsu.edu/swetc/waterconf/2008/home08.htm

February 4- 8 Princeton Groundwater Inc. The Groundwater Pollution and Hydrology Course. San Francisco, CA. www.princeton-groundwater.com

February 10-13 American Water Works Association. Sustainable Water Sources: Conservation and Resources Planning. Reno, NV. www.awwa.org/Conferences/Content.cfm?ItemNumber=3540&navItemNumber=1545

February 20-21 Groundwater Resources Association of California. Site Closure Strategies. Concord, CA. www.grac.org

February 27-29 Midwest Geosciences Group. Advanced Aquifer Testing Techniques Featuring AQTESOLV: New Concepts, Field Methods, and Data Analysis Techniques. San Antonio, TX. www.midwestgeo.com/sanantonio2008.htm

February 28-29 CLE International. Climate Change Law. Los Angeles, CA. www.cle.com/product.php?proid=972&page=Climate_Change_Law

March 4- 6 Nevada Water Resources Association. 200� Annual Conference. Mesquite, NV. www.nvwra.org/events.asp

March 11-14 Nielsen Environmental Field School. Environmental Sampling Field Course. Las Cruces, NM. www.envirofieldschool.com

March 12-14 Water Education Foundation. Lower Colorado River Tour. Lake Mead to Salton Sea. www.water-ed.org/tours.asp#tourdates

March 17-19 American Water Resources Association. 200� AWRA Spring Specialty Conference: GIS and Water Resources V. San Mateo, CA. www.awra.org/meetings/San_Mateo2008/

March 17-20 New Mexico Rural Water Association. 200� Annual Conference. Albuquerque, NM. www.nmrwa.org/2008conference.php

March 24-26 WateReuse Association. 200� California Section Annual Conference: California’s Recycled Water: Sailing into the Future. Newport Beach, CA. www.watereuse.org/ca/2008conf/

March 25-28 Midwest Geosciences Group. Improving Hydrogeologic Analysis of Fracture Bedrock Systems with a Field Trip Inside the Proposed Yucca Mountain Repository Tunnel. Las Vegas and Nevada Test Site, NV. www.midwestgeo.com/upcomingcourses.htm

March 25-28 Water Environment Association of Texas and AWWA. Texas Water ‘0�. San Antonio, TX. www.texas-water.com/

March 30-April 3 National Ground Water Association. 200� Ground Water Summit. Memphis, TN. www.ngwa.org/2008summit/

April 6-10 Environmental and Engineering Geophysical Society. 21st Symposium on the Application of Geophysics to Engineering and Environmental Problems. Philadelphia, PA. www.eegs.org/sageep/

April 7-15 Nielsen Environmental Field School. Ground-Water Monitoring Well Design, Construction and Development (April 7-8), Complete Ground-Water Monitoring Field Course (April 7-11), Complete Ground-Water Sampling Field Course (April 9-11), and Complete Surface Water and Sediment Sampling Field Course (April 14-15). San Diego, CA. www.envirofieldschool.com

April 16-18 Water Education Foundation. Central Valley Tour. San Joaquin Valley, CA. www.water-ed.org/tours.asp#tourdates

April 28-May 2 Princeton Groundwater Inc. The Remediation Course. Las Vegas, NV. www.princeton-groundwater.com

APRIL2008

MARCH2008

�2 • January/February 2008 • Southwest Hydrology

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