Evaluating Stream Depletions in Small Watersheds and Headwaters of California stream Depletion Risk Assessment Framework & Tools (sDRAFT)

Intro/AbstractIt is widely recognized that groundwater extraction has the potential to deplete surface water in nearby streams. Passage of the Sustainable Groundwater Management Act (SGMA) in 2014 established a framework to address the groundwater extraction impacts; however, it focuses on lowland alluvial basins that constitute less than 40 percent of California. WhileSGMA will address the majority of the groundwater volume extracted in California, it will not address many of the localized impacts to aquatic species in areas tributary to or distant from SGMA basins. This includes impacts in small, rural watersheds and headwater streams throughout California where streamflow depletions associated with groundwater pumping may negatively impact aquatic species. In order to consistently evaluate the potential impacts of groundwater pumping on surface water, State Water Resources Control Board (State Water Board) staff have developed the stream Depletion Risk Assessment Framework & Tools(sDRAFT). sDRAFT was developed to: qualitatively identify areas of probable surface water-groundwater connectivity; and quantitatively estimate the timing and magnitude of potential stream depletion. Groundwater located in alluvial valley bottom settings is most likely to be connected to surface waters, and groundwater pumping in these settings would present the highest potential for near-term streamflow depletion. sDRAFT uses the Valley Bottom Extraction Tool (V-BET) to identify valley floor portions of the landscape that are most likely to contain recent alluvial deposits that have rapid connectivity between surface water and groundwater. Several analytical models with existing calculators were evaluated to determine which methods best approximate numerical modeling results. The Jenkins and Butler equations were identified as the best overall equations to estimate streamflow depletion. The main benefits of the sDRAFT approach include that it: can be applied statewide, with or without knowledge of specific groundwater well locations; relies on publicly available datasets including Digital Elevation Models (DEMs) and the National Hydrography Dataset (NHD); and provides a consistent approach to address the potential impacts of groundwater well pumping.

MethodsHigh resolution geologic mapping is not available for the entire state of California, and alternative methods must be used to identify areas where alluvial deposits lie adjacent to surface waters. These alluvial deposits are likely to be unconsolidated and have high hydraulic conductivity leading to rapid depletion of adjacent surface waters from well pumping. The Valley Bottom Extraction Tool (V-BET) was identified as the best tool to spatially identify these alluvial features. Results of V-BET were compared to maps of Quaternary alluvium and to maps of stream channel and nearby alluvial deposits to validate that the tool identified alluvium in close proximity to surface water features. The V-BET identifies the contemporary floodplain but often excludes historic terraces and alluvial fan components which would otherwise be considered High Risk areas for streamflow depletion.

Numerical modeling is often considered the standard in environmental modeling due to the ability to simulate a wide variety of real-world conditions and processes. However, lengthy and costly development, parameter estimation and calibration periods limit the use of numerical modeling in many applications. Analytical models are simplifications of real-world conditions which can be implemented quickly and cost effectively while still providing reliable results. Numerous analytical equations exist to estimate streamflow depletion from pumping wells, however only eight equations were identified which are accompanied by calculators {Jenkins(1965), Hantush(1969), Hunt(1999), Butler(2001), Hunt(2003), Hunt(2008), Hunt(2009), Ward & Laugh(2001)}. These models represent a wide range of well-aquifer-stream configurations including varying degrees of stream and well penetration, varying well and valley wall setbacks, varying number of layers and varying boundary conditions.

Thirty scenarios were developed and streamflow depletion calculated using numerical techniques within the MODFLOW software (3 scenarios didn’t produce complete results and were discarded). Scenarios included differing numbers and types of layers, well setbacks, valley wall setbacks, and boundary conditions. Each scenario was then translated into the required input parameters for up to eight of the analytical equations and the calculators run to estimate streamflow depletion. Not all of the scenarios could be properly translated into all eight of the analytical equations (i.e. some multi layer scenarios could not be replicated in the analytical equations). Time series for each scenario from each calculator were plotted along with the numerical streamflow depletion estimate, and the analytical method which best represented the numerical solution was identified. When more than one analytical calculator produced identical or similar results which were identified as the best match to the numerical solution, the analytical method with the least number of input variables was chosen as the “best fit”.

Two analytical methods emerged as the best fit for most of the scenarios, and the methods represent opposing assumptions regarding the lateral extent of an aquifer. The Butler method which assumes a laterally confined aquifer, provides the best comparison to numerical methods when low permeability valley walls confine an alluvial valley floor, as well as conditions where the width of the watershed is less than approximately 1,000 ft. When valley walls are greater than about 1,000 feet wide, or the watershed is greater than about 1,000 feet wide, the Jenkins method (assuming a laterally unconfined aquifer) provides the best comparison to numerical methods. The Recommended Analytical Models Table provides guidance on which analytical methods should be used under various conditions.

Tools Inputs Processing Outputs Validation Framework Example Application

ConclusionsLandscape analysis can be used to spatially identify areas which likely consist of unconsolidated alluvial deposits that are hydrologically connected to adjacent surface waters, and considers pumping of wells in these areas to be a High Risk to streamflow. Well Depth and stream elevation could be used to separate wells outside of the unconsolidated alluvium into Medium and Low Risk to streamflow depletion. The Valley Bottom Extraction Tool is currently the best landscape analysis tool which identifies likely alluvial deposits, but does not specifically identify the alluvium-bedrock contact.

Current modeling suggests that two analytical methods of estimating streamflow depletion can be used in most well-aquifer configurations. Existing calculators for the Jenkins and Butler methods allow for rapid and re-producible estimation of streamflow depletion, however only the Jenkins calculator allows for estimation of streamflow depletion from intermittent pumping and aquifer recovery following pumping cessation. Combining results from multiple wells can provide insight on cumulative depletions from multiple wells including estimation of legacy depletions from pumping in previous years, and can be used to assess impacts from existing conditions as well as from potential management alternatives.

References• Butler, James J. Jr.; Zlotnik, Vitaly A.; and Tsou, Ming-Shu, "Drawdown and Stream Depletion Produced by Pumping in the Vicinity of a Partially Penetrating Stream"

(2001). Papers in the Earth and Atmospheric Sciences. 272. http://digitalcommons.unl.edu/geosciencefacpub/272• Gilbert J.T., William W. Macfarlane, Joseph M. Wheaton, The Valley Bottom Extraction Tool (V-BET): A GIS tool for delineating valley bottoms across entire drainage

networks, Computers & Geosciences, 97 (2016) 1–14.• Jenkins, C.T., 1968, Computation of Rate and Volume of Stream Depletion by Wells. U.S. Geological Survey, Techniques of Water-Resources Investigations, Book 4,

Chapter D1.• Stetson, 2008 : Stetson Engineers Inc., Approach to Delineate Subterranean Streams and Determine Potential Streamflow Depletion Areas, Policy For Maintaining

Instream Flows In Northern California Coastal Streams, February 28, 2008.• Bedrossian, T.L., Roffers, P., Hayhurst, C.A., Lancaster, J.T., Short, W.R., McCrea, S., Wanish, B., Thompson, J., Carney, A., Meyers, M.A., and Utley, S., 2012, Geologic

Compilation of Quaternary surficial deposits in southern California: California Geological Survey, Special Report 217 (Revised 2012), scale 1:100,000.

Jeffrey Sanchez, M.S., P.G., P.H. and Behrooz Etebari, M.S., P.G.SWRCB Division of Water Rights*

Link to landing page for V-BET Codehttps://bitbucket.org/jtgilbert/riparian-condition-assessment-tools/wiki/Tool_Documentation/Version_1.0/VBET

Link to USGS Calculatorshttps://mi.water.usgs.gov/software/groundwater/CalculateWell/index.html

Link to landing page for Butler Codehttp://www.kgs.ku.edu/StreamAq/Software/strp.html

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Intermittent Pumping &Recovery

Continuous Pumping

Values in these tables represent the percentage of the total unit in a watershed that falls within the Valley Bottom polygon. • Single set of V-BET parameters used for 4 watersheds• V-BET designed to identify the stream channel and

contemporary floodplain, but not all alluvial fans and terraces (V-BET margin ≠ alluvium: bedrock contact)

• V-BET does well at identifying the more recent alluvial deposits and excluding older terrace and fan deposits

• V-BET provides conservative (smaller) estimate of alluvial areas compared to other methods

• V-BET provides more precise results than traditional large-area geology mapping efforts

Stetson Maps Green Valley Mark West

Stream Channel Alluvial Deposits (not adjacent to bedrock)

88.5% 95.0%

Recent near-stream alluvial deposits (adjacent to bedrock)

70.4% 81.6%

Potential Stream Depletion Area (older alluvial deposits)

25.4% 34.2%

Quaternary Deposits in Southern California

Type Unit Age Sespe Ck Ventura River

Wash Qw Youngest 88% 94%

Qyw Younger NA NA

Qow Older NA NA

Qvow Oldest NA NA

Fan Qf Youngest 75.9% 63.1%

Qyf Younger 71.4% NA

Qof Older 37.8% 27.5%

Qvof Oldest NA NA

Valley Qa Youngest 80% 62%

Qya Younger 77.2% 45.8%

Qoa Older 38% 44.2%

Qvoa Oldest NA NA

Terrace Qt Youngest NA 92.7%

Qyt Younger 66% NA

Qot Older 25% NA

Qvot Oldest 6.4% NA

Risk determinations based on well location and depth reported during the 2015 Russian River Tributaries Emergency Regulations. Some pumping data was simulated for illustrative purposes.

Legacy depletions from previous pumping

Three tools are used to qualitatively identify zones of streamflow depletion risk and quantitatively estimate the timing and magnitude of the streamflow depletion. Focus was placed on streamflow depletion methods with existing streamflow calculators, rather than developing new calculators for existing equations. The links lead to landing pages for each tool from which the necessary code can be downloaded, or calculations can be made directly through the website.

Input data comes from public sources, or is determined using Geographic Information Systems (GIS). Digital Elevation Models (DEMs) and the National Hydrography Dataset (NHD) are needed to identify the extent of recently deposited and unconsolidated alluvium, which define the High Risk Zone of streamflow depletion.

GIS is used to measure spatial field attributes while a variety of sources can be used to determine aquifer properties including previous studies and well completion reports. The Jenkins and Butler equations represent opposing assumptions regarding the lateral extent of an aquifer with the geology and valley width being used to determine which equations best represents the system under investigation.

A polygon of recent unconsolidated alluvium is created within the ESRI ArcMap software, with the extent of the zone controlled by user selected parameters. The Valley Bottom Extraction Tool (V-BET) can process 1 to 10-meter DEMs, and NHD or LiDAR derived streamflow lines. User controlled parameters may vary by hydrologic or geologic region to best reflect local conditions.

The quantitative streamflow depletion calculators developed by the USGS (Jenkins, Hantush, Hunt ‘99 and Hunt ‘03 equations) can be run in two ways to generate different types of results. The web based calculators can only simulate a single period of continuous pumping, and only calculate depletions during the pumping period. The same webpage for the online USGS calculators provides executable code which allows for the simulation of continuous or intermittent pumping, as well as a recovery period following cessation of pumping. Executable code is available to run the Butler equation, however the code is limited to simulation of continuous pumping with no recovery period. Estimation of the shape of streamflow depletion curve during the recovery period is important to understanding ongoing legacy depletions from previous pumping, so while the Butler and online USGS calculators are useful in providing estimation of initial depletion from a single well, the lack of simulation ability for the recovery period suggests that the Butler calculator and online USGS calculators in their current configuration are of limited use in cumulative multi-year streamflow depletion analysis.

To validate the hypothesis that the V-BET can be used to identify recent alluvial deposits, four watersheds were selected to run the V-BET and compare to existing maps of near stream alluvial deposits (Stetson Maps) and to Quaternary alluvium (CGS Southern California Qal maps). The Stetson maps identified 3 types of deposits: recent alluvial deposits adjacent to bedrock, stream channel alluvial deposits not adjacent to bedrock, and older alluvial deposits. As expected, the V-BET did very well at identifying recent alluvial deposits not adjacent to bedrock and pretty well at identifying recent alluvial deposits that are adjacent to bedrock. Older alluvial deposits were identified within the Valley Bottom at a low rate as expected. When comparing the V-BET output to mapping of Quaternary alluvium in two southern California watersheds, the majority of the recent alluvial deposits were identified as within the valley bottom, while only small portions of the older alluvium including alluvial fans and terraces were included. The V-BET tool was designed to only identify the contemporary channel and floodplain, and within the four watersheds tested, appears to do this quite well. While the V-BET will exclude some Quaternary alluvial deposits which could also have direct hydrologic connectivity to surface waters, it does well at identifying the areas most likely to have the highest conductivity and therefore most immediate and substantial impact to streamflow.

In order to identify the best analytical methods to quickly estimate streamflow depletion from pumping wells, local scale numerical models were developed then approximated by up to eight analytical equation calculators (Jenkins, Hantush, Hunt ‘99, Hunt ‘03, Butler, Hunt ‘08, Hunt ’09, Ward & Laugh) which represent differing aquifer configurations. The analytical streamflow depletion curve which had the best overall match to the numerical streamflow depletion curve was identified for each of the scenarios. In cases where multiple analytical calculators return similar results, the calculator requiring the least input variables was identified as the best calculator for that aquifer configuration.

The High Risk Zone is narratively defined as the extend of unconsolidated alluvium deposited on the floor of a valley, and spatially approximated using the V-BET. The High Risk zone can be defined without knowledge of specific well locations or depths. Low Risk wells are those whose base is located above the elevation of the streambed for the nearest stream channel, are physically unable to directly dewater a stream, and would be the first wells to go dry during dry periods. Medium Risk wells are located outside of the unconsolidated alluvium but have a well base below the elevation of the streambed and are physically capable of directly dewatering a stream. Exact well location and depth is needed to separate Medium from Low Risk wells. Wells without depth information which are located outside the High Risk Zone would be considered Medium Risk until depth information is available.

The table of Recommended Analytical Models summarizes the results of the individual scenario comparisons. The Jenkins (aka Glover) calculator provides the best approximation of stream depletion associated with pumping from a laterally infinite aquifer when compared to numerical estimation methods. The Jenkins calculator requires estimation of only 2 aquifer parameters whereas the other analytical calculators assuming laterally infinite aquifers require estimation of up to 10 aquifer parameters. In situations where aquifer parameters and configuration are well known, more complex analytical calculators may produce reasonable results, however increasing the number of parameters requiring estimation also increases the chances of incorrectly estimating one of those parameters and obtaining inaccurate streamflow depletion results. The Butler calculator provides the best approximation of stream depletion associated with pumping from a laterally confined aquifer when compared to numerical estimation methods. However, the Butler calculator does not allow for the estimation of streamflow depletion associated with intermittent pumping, nor does it allow for simulation of aquifer recovery following cessation of pumping. This limits the applicability of the calculator for analysis of cumulative pumping impacts, or estimation of legacy depletions from previous pumping.

Using well information provided to the State Water Board through previous regulatory efforts, the Risk Zone framework was applied to the Mark West Creek watershed (tributary to the Russian River in Sonoma County, CA). One potential V-BET output was used to identify the wells within the High Risk Zone, and the well depths were used to separate wells within the Medium and Low Risk Zones. Wells without depth information and located outside the High Risk zone were categorized as Medium Risk. Proposed wells can be placed into Risk Zones, and Risk Zones could be used to investigate management alternatives for groups of wells. Approximately 49% of wells fall into the High Risk Zone, 44% in the Medium Risk Zone and 7% in the Low Risk Zone in this example.

In order to investigate cumulative streamflow depletion from a specific area or group of wells, streamflow depletion and recovery for each well is estimated using the depletion calculators, then plotted with the other individual depletions. Stacking or summing depletions at specific times or locations can be used to estimate existing impacts, or forecast how streamflow depletion impacts may change as a result of management actions. Wells can be grouped by Risk category, use type, or other parameter based on the needs of the analysis. Plotting of streamflow corresponding to the location of investigation helps illustrate periods of highest impact to overall streamflow and can assist in the development of management alternatives such as for the protection of instream beneficial uses. It is important to understand the impact of pumping during the pumping period, however it is equally important to understand what legacy stream depletions may occur in the future as a result of current or previous pumping. Unfortunately a streamflow depletion calculator representing laterally confined aquifer configurations and capable of simulating intermittent pumping and post pumping recovery has not been identified, but would be important to more accurately simulating streamflow depletions in those conditions than the existing calculators which assume laterally infinite aquifers.

1- Recommendations may be refined based on additional modeling, and are likely to vary regionally and with geology2- Width thresholds may be refined based on additional modeling3- Analytical modeling ignored upper aquifer and aquitard4- Width of sub-watershed draining to watercourse where depletion is occurringAnalytical models only use aquifer parameters for the unit screened by the well

The Withdrawal Impact Continuum illustrates several important concepts to understanding and evaluating streamflow depletion by well pumping. In the Legal and Policy context, some wells fall under traditional Water Rights permitting authority while many do not. Near stream wells are most likely to directly deplete streamflow through the mechanism of Streamflow Capture or Induced Infiltration where as distant and topographically elevated wells are more likely to deplete streamflow by intercepting or diverting groundwater discharge that would otherwise contribute to streamflow. Wells located nearest to the stream are likely to have high magnitude and short duration impacts to adjacent streams whereas distant wells are likely to have low magnitude and long duration impacts which appear similar to baseflow reductions. While streamflow depletion will often approach the volume of water pumped from a well over a lengthy timeframe, the shape of the depletion curve varies significantly as distance to the stream increases.

*Views presented in this poster are those of the authors, and do not necessarily reflect the views of the State Water Board, nor do they represent any formal Policy, proposal or position under consideration by the State Water Board. This poster is for discussion purposes only.

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