1

Evapotranspiration• Interception – precipitation abstracted by vegetation and

other forms of surface cover.

• Evaporation: conversion of a substance from the liquid or solid state into vapor (Brutsaert, 1982. Evaporation into the Atmosphere. D. Reidel Publishing Company, Boston, MA). If a substance is converted from its solid state directly into vapor, the process is called sublimation. e.g. ice on a hot stove. Evaporation is a physical process.

• Transpiration: conversion of water into vapor through the stomata of living plants. Transpiration is a physiological process that links biosphere and hydrosphere together.

• Evapotranspiration (ET): the evaporation from soil and open water surfaces and transpiration from vegetation together as the two are difficult to separate in computations.

ET continuedET is also called consumptive water use, meaning that water is lost to the atmosphere and can't be recovered for other uses. The Hydrology Handbook of the ASCE in 1949 defines consumptive water use as " the quantity of water transpired by plants during their growth or retained in the plant tissue, plus the moisture evaporated from the surface of the soil and the vegetation, expressed in feet or inches depth of water lost or used in a specified time." (Jensen, Burman and Allen, 1990. Evapotranspiration and Irrigation Water Requirements, p.6).

ET continued

• Potential evapotranspiration (ETp): the amount of water evaporated from soil surface and transpired from the plant surface under the optimal plant growth conditions, that is, there is no restriction to plant in nutrient and water supply.

• Reference crop evapotranspiration (ETo): the amount of water evapotranspirated from a reference crop under optimal growth conditions.

ET Factors• Energy: solar radiation, reflectance, thermal

emission, net radiation

• Atmosphere: temperature, vapor pressure, topography, wind, storms

• Vegetation: type, height, density of canopy, number of vegetative layers, land cover of the area.

• Soils: soil texture, structure, depth, moisture content, organic and mineral content.

Energy Balance Equation

• Rn +LE + G + H + PS + M = 0

• Rn = net radiation (cal. cm.-2 min.-1 ) • LE = latent heat to and from the surface through

vaporization (evaporation) of water or condensation.• G=soil heat flux• H = sensible heat (energy transferred between the surface

of the earth and the air by the process of convection. This transfer determines air temperature.)

• PS= energy stored in plants by photosynthesis.• M= energy involved in miscellaneous processes such as

respiration and heat storage in the plant canopy.Source: http://www.fao.org/docrep

2

Energy Unit• 1 calorie is defined as the quantity of heat needed to raise the

temperature of 1 gram of water 1 Celsius degree (from 14.5 to 15.5).

The common unit of energy today is joule (J).

• 1 cal =4.1868 J.

• British thermal unit (Btu): the amount of heat required to raise the temperature of 1 lb of water for one Fahrenheit degree (from 62 to 63).

• 1 Btu=252 cal and 1055 J.

• Solar constant: the rate at which solar radiation falls on a surface located at the top of the atmosphere and positioned perpendicular to the sun's rays when Earth is at its mean distance from the sun. It is about 1.97 cal/cm2, or 1372 w/m2

• Convection: mass motions of fluid (air, in this case) resulting in transport and mixing of the properties of that fluid in vertical direction (Rosenberg et al., 1983. Microclimate: The Biological Environment. 2nd Edition. John Wiley & Sons. New York.). Energy transfer by the physical movement of some energized medium from one location to another. Convection occurs within the atmosphere due to differences in air density. Warmer air is less dense than cold air so the warm air rises and cooler, dense air sinks.

• Advection: horizontal transfer of heat within the atmosphere. Sensible heat is transported by the combined processes of conduction and convection.

• Conduction: the direct transfer of energy between adjacent molecules (touching) or objects.

• Radiation: energy transfer from one body to another in the absence of an intervening material medium.

Measuring ET

Water Balance Method• P ± ∆SW - RO - D - ET = 0• P = precipitation or irrigation • RO = runoff • ∆SW = the change in soil moisture• D = percolation or deep drainage

Source: http://www.fao.org/docrep

• Evaporation Pan: Class-A evaporation pan is 47.5" inside diameter and 10" deep. It is mounted on a wooden platform above the surrounding soil, allowing heat transfer from the air through the bottom and side of the pan. The pan is thus provided a heat source not available to the water in a lake or soil.

• Pan evaporation data could be used to estimate potential evapotranspiration. The value of ET/E-Pan varies with the type of vegetation:

• 1.0 for alfalfa• 0.7-0.85 for grass;

0.8 for corn, grapes, and peaches with no cover crop ; 0.9 for sugarbeets, soybeans, red beans, late potatoes, and wheat;1.05 for apple trees with a grass cover crop.

Estimating Potential evapotranspiration using the pan

Equation;

PET = CeEp

Where;Ce Pan coefficient ranging between 0.5 to 0.8Ep Pan evaporation

3

PAN EVAPORIMETER • Pan evaporimeter gives an indication of the

integrated effect of radiation, wind, temperature and humidity on evapotranspiration from an open field.

• The most widely is the Class A pan, introduced by the United States Weather Bureau.

• The Class A Evaporation pan is circular, 120.7 cm in diameter and 25 cm deep.

• It is made of galvanized iron (22 gauge) or Monel metal (0.8 mm)

• The pan is mounted on a wooden open frame platform, which is 15 cm above ground level.

• The soil is built up to within 5 cm of the bottom of the pan and the pan must be level.

• It is filled with water to 5 cm below the rim, and the water level should not be allowed to drop to more than 7.5 cm below the rim.

• The water should be regularly renewed, at least weekly, to eliminate extreme turbidity.

• The site should preferably be under grass, 20 by 20 m, open on all sides to permit free circulation of the air.

Source: http://www.fao.org/docrep

Evaporation Pan

4

Colorado sunken pan

• The Colorado sunken pan is 92 cm (3 ft) square and 46 cm (18 in) deep, made of 3 mm thick iron

• Placed in the ground with the rim 5 cm (2 in) above the soil level.

• Dimensions 1 m square and 0.5 m deep are frequently used.

• The water level is maintained at or slightly below ground level, i.e., 5-7.5 cm below the rim.

• Measurements are taken similarly to those for the Class A pan.

• Environment requirements are also similar to those for the Class A pan.

• . The disadvantage is that maintenance is more difficult and leaks are not visible.

Source: http://www.fao.org/docrep

Problems with the pan methods:

• Container itself has significant effects on radiation absorption and the production of turbulence.

• Not representative of the field conditions. The water available for evaporation is not identical to that of the upwind area while the temperature and humidity of the overpassingair are in equilibrium with the evaporation regime of the upwind area (Morton, 1994).

• Weighing Lysimeter: A container filled with soil is buried in the ground. ET is measured by measuring the changes in weight of the container. Useful for the study of small scale processes but the results cannot be extrapolated to environmentally significant areas. Same problem as the pan methods.

• Experimental Plot

• In a well confined area, in-flow and out-flow are measured to calculate ET over a period of time (weekly, monthly or yearly). The results can be extrapolated to environmentally significant areas if the plot is located in the middle of identical surroundings.

• P - Ro ±∆ S = ET

5

Drainage Lysimeter Suction type drainage Lysimeter

Non weighing lysimeters

Source: http://www.fao.org/docrep

• Excess water is collected from below• A tube with a small diameter is placed

through the soil to the layer of pebbles. • Excess water is removed at frequent

intervals by using a thin metal tube open at the bottom which is connected to a receiver bottle in which suction can be applied using a reversed hand pump.

Weighing Lysimeters• Provide precise information on soil moisture

changes for daily or even hourly periods.• Lysimeter is placed inside another tank, which is in

contact with the surrounding soil.• The inside container is free for weighing by scales. • Lysimeter tank can be floated in water; a suitable

heavy liquid (ZnCl2) is used whereby the change in liquid displacement is a measure for the water gain or loss to or from the lysimeter tank.

Disadvantages

• High cost• Restricted root growth• disturbed soil structure in the lysimeter

causing changes in water movement and possibly the tank temperature regime,resulting in condensation of water on the walls of the container.

• The 'bouquet effect' whereby the canopy of the plants grown in the lysimeter is above and extends over the surrounding crop, resulting in a higher evapotranspiration rate.

• Assumption: exchanges of water and water vapor with the underlying layers of soil are insignificant. This is only for convenience and ignores the evidence that the exchange of vapor fluxes does exist.

• Problem: almost impossible to measure the surface runoff and the vertical exchanges of water and water vapor with the layers of soil beneath the limits of soil moisture measurement. Studies (Abramova, 1968) in the Precaspian Lowland of Russia have shown that there were significant net downward vapor fluxes during the early summer when the soil temperature decreased with depth, and significant net upward vapor fluxes during the later summer when the soil temperature increased with depth (Morton, 1994).

6

Estimating ETAir Temperature-Based Formulas

Lj = actual day length (h)N = number of days in a monthTa = mean monthly air temperature ( oC )

a1 = 6.75 * 10-7 I-3 - 7.71 * 10-5 I2 + 1.79 * 10-2 I + 0.49I = heat index derived from the sum of 12 monthly index values.

514.112

1 5∑=

⎥⎦

⎤⎢⎣

⎡=

j

ajTI

1)10(*30

)12

(16 aajp I

TNLET =

Thornthwaite Method (1948):

Thornthwaite Method

• This method is suitable for long term estimation of ET.

• The calculated ET often underestimates ET at the time of annual maximum radiation reception during summer and is consequently out of phase in fall as well (Rosenberg et al. 1983).

FAO-24 Penman

• ∆= slope of the saturation water vapor curve versus temperature

• ETo = potential ET in mm/day• c= adjustment factor • Rn= net radiation in mm/day• u2 = wind speed in m/s at 2-m height

⎥⎦

⎤⎢⎣

⎡−+

+∆+−

+∆∆

= ))(864.01(7.2)( 20 ssasn eeuGRcETγ

γγ

)()(

dewamean

ssas

TTee−−

=∆

dsn

dd

n

dds URRH

UUU

UUURRHc ***10*3.4*0097.0013.0068.0018.00028.068.0 max

4max

−+++−++=

FAO-24 Method

• eas = saturated vapor pressure at the mean air temperature in kPa.

• ea = actual vapor pressure at the mean air temperature in kPa.

⎥⎦

⎤⎢⎣

⎡+

−=

)3.237()9.116*78.16(exp

a

aas T

Te

⎥⎦

⎤⎢⎣

⎡+

−=

)3.237()9.116*78.16(exp

dew

dewss T

Te

7

if RH is known.

)()( )()( dewa TdewTasssas eeee −=−

)100

1)(()( )(RHeee

ameanTasssas −=−

If dew point temperature is known:

Vapor pressure deficit can be calculated by a number of methods:

)(21)( )()( minmax TssTasssas eeee

a−=−

• RHmax = maximum daily relative humidity in percentage (e.g. 87% here should be 87).

• Rs in mm/day• Ud/Un = ratio of daytime to night time wind

speed• Ud=daytime wind speed (0700-1900 hours) in

m/s• Limits: o.5≤ Ud/Un ≤4.0, 3.0≤ Rs ≤12.0,

30 ≤RHmax ≤90, Ud ≤9.0 m/s.

Source: Jensen et al. 1990. Evapotranspirationand Irrigation Water Requirements, American Society of Civil Engineers, New York.

Calculation ExamplesLocation: Kimberly, Idaho; latitude = 42.4°N, elevation = 3922 ft and 1195 m above sea levelAtmospheric pressure P=87.7 kPaJuly mean max. air temperature = 85.1 F = 29.5 °Cmean min. air temperature = 53.3 F = 11.8 Cmean air temperature = 69.2 F = 20.7 Cmean dew-point temperature = 49.4 F = 9.7 Cmean vapor pressure = 12.0 mb =1.2 kPamean wind speed at a height of 3.66 m = 2.48 m /s =214 km /dmean wind speed adjusted to a height of 2 m =2.20 m/s =190 km /daymean extraterrestrial solar radiation = 40.24 MJ m-2 /d=16.42 mm/dayNet radiation Rn=13.94 MJ m-2 /d=5.69 mm/daymean percentage sunshine = 82.5mean day length = 14.8 hEstimated soil heat flux =0.21 MJ m-2 /dMean day/night wind speed ratio = 1.72Mean measured evaporation (USWB Class A pan) = 0.33 in./ d = 8.4 mm /day Mean measured evapotranspiration (Alfalfa, 30 to 80 cm growth) = 0.310 in /d = 7.87 mm /dayCompute the mean daily ET rate using the FAO-24 method

Reference

Brutsaert, W. 1982. Evaporation into the Atmosphere. D. Reidel Publishing Company, Boston, MA.

Doorenbos, I. and W.O. Pruitt, 1977 Guidelines for Predicting Crop WaterRequirements. FAO Irrigation and Drainage Paper 24, Rome.

Fetter, C.W. (1994), 3rd ed. Applied Hydrology. Prentice Hall USA.Hornberger, G. M et al (1998). Elements of Physical Hydrology. John HopkinUniversity Press, USA. Jensen, M.E., R.D. Burman and R.G. Allen, 1990. Evapotranspiration and Irrigation

Water Requirements. American Society of Civil Engineers, New York, New York. Morton, F.I., 1994. Evaporation Research - A Critical Review and Its Lessons for the

Environmental Sciences. Envir. Sci. & Tech. 24(3):237-280.Ponce, M V. (1989). Engineering Hydrology: Principles and practices. Prentice Hall, USA.Rosenberg, N.J., B.L. Blad, and S.B. Verma. 1983. Microclimate: The Biological

Environment. 2nd Edition. John Wiley & Sons. New York. http://wmo.ch/web/gcos/terre/variable/evapot.htmlhttp://fao.org/docrep

8

9

10

DroughtDrought is a pervasive natural hazard.

• Since 1895, the U.S. has experienced “extreme’ or ‘severe”drought in some part of the country every year.

• A 1995 report by the Federal Emergency Management Agency indicates that the annual cost of U.S droughts ranged between $6 and $8 billion.

• The 1988 drought cost almost $62 B in 2002 dollars. In comparison, the 1993 Mississippi River floods cost about $27 B.

• Droughts are responsible for about 40% of the total losses due to natural hazards.

Andreadis, K.M., D.P. Lettenmaier, J. Sheffield, and E.F. Wood. 2005. U.S. Historic Drought Characteristics Estimated using N-LDAS. GEWEX Newsletter of May, 2005.

Drought I. Definitions• It is difficult to give a universe definition of drought. • Meteorological Drought: annual precipitation

significantly deviates from the long term average annual precipitation in an area.

• Hydrological Drought: shortage of surface water and/or groundwater supplies due to the occurrence and/or persistence of meteorological drought.

• Agricultural Drought: Significant crop yield reduction and financial losses due to reduction of soil moisture supply to crops.

Common variables used to study drought include:

• Precipitation• Temperature • Evaporation• ET• Soil moisture• Streamflow, • Reservoir levels and storage, and• Groundwater levels.

11

• Methods for evaluating droughts are not well developed. • A key problem is the spatial dimension of droughts

(Andreadis et al. 2005). Drought intensity and duration can be measured at a point but its spatial characteristics vary over time.

• Andreadis et al. (2005) used gridded land surface model in the context of the North American Land Data Assimilation project (N-LDAS) to analyze the spatial dimension (Severity Area Duration) of droughts for the period of 1915-2003. They report that the 1930s and 1950s were the most severe for large areas. The 1988 Midwest drought, the 1977 drought over the western U.S., and the mid 1960s drought over the eastern U.S. were also notable. The 2000 western U.S drought was among the most severe in the period, especially for the short duration.

Andreadis, K.M., E.A. Clark, A.W. Wood, A.F. Hamlet, and D.P. Lettenmaier. 2005. 20th Century Drought in the Conterminous United States. J. Hydrometerology.

World Meteorological Organization (WMO) classified definitions of drought into:

• 1) definitions based on rainfall alone;• 2) definitions based on rainfall and mean

temperature;• 3) definitions based on soil moisture and

crop parameters;• 4) definitions based on climatic indices

and estimates of ET.

The Palmer Meteorological Drought Index (the Palmer Index)

• It provides a relative scale by which the severity of the general moisture conditions can be assessed against a “normal” condition. The PI takes into account the monthly “normal” (i.e. average) temperature, precipitation and soil moisture conditions over a given period of time. The ET, antecedent climatological and soil moisture condition, recharge, and runoff can also incorporated into the PI.

The PDSI is defined as

110.8973

( )

i i i

i i

PDSI PDSI Z

Z Kd

−= +

=The subscripts i and i - 1 indicate current and previous months at some arbitrary time, respectively, and PDSIi = 0.

( )i i i i

ii

i

ii

i

ii

i

ii

i

d P PE PR PRO PL

ETPE

RPR

ROPRO

LPL

α β γ δ

α

β

γ

δ

= − + + −

=

=

=

=

P =actual monthly precipitation. PE =potential evapotranspirationR=soil water rechargeRO=runoff L=available soil water contentPR= potential water recharge to soilPRO =potential runoffPL= the sum of soil water of the two layers available

for evapotranspiration. Palmer called this term'potential loss of soil water to evapotranspiration'.

α, β, γ, and δ are coefficients.

Extreme drought -4.00 or lessSevere drought-3.00 to -3.99 Moderate drought-2.00 to -2.99 Mild drought-1.00 to -1.99 Incipient dry spell-0.50 to -0.99 Near normal0.49 to -0.49 Incipient wet spell0.50 to 0.99 Slightly wet1.00 to 1.99Moderately wet2.00 to 2.99 Very wet3.00 to 3.99 Extremely wet4.00 or more

PDSI Classifications for Dry and Wet Periods

12

Limitations of the PI. • 1) use monthly values, usually too late for

crop relief• 2) ignore timeliness of precipitation• 3) use too coarse soil data to reflect local

soil moisture conditions• 4) Fail to characterize drought extent• 5) Lack of cold land processes

representation.

• Crop Moisture Index (CMI), a modification of PI. CMI usually uses weekly values and more soil layers

(1 ) )W P ICMINσ θ+ − − +

=

W=available soil moisture within 1 m deep of soilP=precipitationσ =runoff coefficient I=infiltration coefficientθ= available soil water in deep layersN=crop water demand

Hydrologic Drought

ii

i i

EPdP R

=−

Subscript i represents the ithperiodd=hydrologic drought indexEP=potential evapotranspirationP=precipitationR=runoff

References• Hu, Qi, and Gary D. Willson. 2000. Effects of temperature anomalies

on the Palmer Drought Severity Index in the central United States. International Journal of Climatology 20:1899-1911. Jamestown, ND: Northern Prairie Wildlife Research Center Online. http://www.npwrc.usgs.gov/resource/2001/tempanom/tempanom.htm (Version 20AUG2001).

• Kumar, V. and U. Panu, 1997. Predictive Assessment of Severity of Agricultural Droughts Based on Agro-Climatic Factors. J. of AWRA 33(6):1255-1264.

• Lohani, V.K. and G.V. Loganathan, 1997. An Early Warning System for Drought Management Using the Palmer Drought Index. J. of AWRA 33(6):1375-1386.

• Mather, J.R., 1984. Water Resources: Distribution, Use, and Management. John Wiley & Sons, Inc. New York, pp. 362- 382.

• Nurnberger, F.V., 1980. Meteorological Drought in Michigan: A Review of the Past 50 Years. Michigan Department of Agriculture/WeatherService, East Lansing, MI.

• Palmer, Wayne C., 1965. Meteorological Drought, U.S. Weather Bureau Research Paper No.45, Washington, D.C.