Evaporation, Evapotranspiration, and Irrigation Water Requirements

This chapter summarizes the development of the first edition and preparation of the revised edition of this manual by the Irrigation Water Requirements Committee of the Irrigation and Drainage Division. As competition for available water resources increases, transfers of water rights and associated litigation require background in historical water-use measurements and previously used methods of estimating crop water requirements. Consumptive use of water, or evapotranspiration (ET), is one of the most basic components of the hydrologic cycle. Evaporation and transpiration have been studied for centuries, but the term “consumptive use” probably was not applied to water consumption prior to 1900 in the United States. Methods for estimating ET using standard weather measurements of air temperature, humidity, wind, and solar radiation are based on the Penman equation.

Evapotranspiration (ET) is the sum of vaporization by the combined processes of evaporation (E) and transpiration. Reference crop ET, or reference evapotranspiration (${\mathit{E}}_{\mathit{ref}}$), addresses some of the ambiguities associated with definitions for potential ET and serves as a consistent climatic index for ET. The most common method of estimating E or ET is estimating the energy available for E or ET. The most accurate means of estimating ET is by applying methods that consider all primary factors affecting the ET rate, such as the Penman-Monteith equation. Those factors include weather, which supplies both the energy to evaporate water and the means to transport water vapor from the surface. Most current estimates of E and ET are based on some form of the energy balance equation, or a combination equation containing energy balance and aerodynamic components.

This chapter describes the soil-plant-atmosphere (SPA) system to support the development and calculation of evapotranspiration (ET). The SPA system involves absorption and storage of energy in the soil; storage of water in the soil; evaporation from soil and plant surfaces; transpiration from leaves of plants; and transfer of water vapor, sensible heat, and radiation from the soil and plants to the atmosphere. The chapter focuses on items of concern to engineers in solving problems involving evaporation (E) and ET from plant and soil communities, conducting research in E and ET, and managing water supplies and delivery systems. The physical basis of the evaporation process comprises physical properties of liquid water, vapor, and air. Most soils literature uses the terms capillary potential, matric potential, and soil-water suction or tension when referring to the pressure potential of water in soils.

Studies of evaporation (E) and evapotranspiration (ET) clearly show that, where soil water is not limiting, the primary variable controlling the rate of E and ET is solar radiation impinging on the evaporating surfaces (irradiance). This chapter explains underlying principles to enable readers to estimate hourly and daily net radiation and soil heat flux as needed to solve E and ET problems. The examples provided reveal that the actual fraction of net radiation converted into E or ET can vary markedly depending upon air mass conditions above the surface and the availability of surface water. The main component of net radiation is short-wave radiation, or solar radiation. Net radiation also has a long-wave radiation component. Net radiation flux can be measured directly using hemispherical net radiometers, or estimated from net short-wave and long-wave components.

The conservation of energy at the evaporating surface controls the close linkages among solar or net radiation, the primary energy source; sensible heat flux to and from the air and to and from the soil; and the resulting conversion of energy into latent heat flux. This chapter describes the general interactions of the available surface energy and air-mass along with common approaches used to obtain evapotranspiration (ET) estimates. The combination energy balance–aerodynamic equations, such as those proposed by Penman or Slatyer and McIlroy, model how various weather parameters interact to affect energy partitioning at evaporating surfaces. The chapter explains why the term reference ET has replaced the term potential evaporation in irrigation design, planning, and management. Reference ET is a visual and practical means to establish a near-maximum index of climatic demand for water and is straightforward to measure.

This chapter describes some U.S. studies on estimating evaporation from water bodies. Particular emphasis is on practical procedures and techniques to estimate evaporation from water surfaces using pan evaporation, available weather data, or new data collected specifically for estimating daily evaporation. This chapter describes several methods commonly used to estimate evaporation from water surfaces and discusses their strengths, weaknesses, data requirements, and relative accuracies. The methods include pan evaporation, water budget procedure, aerodynamic method, energy balance method, and a combination of the aerodynamic and energy balance procedures. The chapter then presents results of example applications of the combination method and the aerodynamic and Bowen ratio methods for a shallow lake, a deep lake, a river-reservoir system, and a reservoir where advection of energy is small. The examples illustrate various means of independent checks and assessment.

This chapter describes evapotranspiration (ET) measurement approaches. The challenge with these approaches is in extrapolating point or localized measurements to large spatial scales and in extrapolating measurements to other time periods. Energy balance methods based on satellite imagery can be applied successfully to estimate actual ET over large areas. Mass balance involves measuring the change in mass caused by evaporation (E) or ET and determining the E or ET rate or volume as a residual of a balance. The Bowen ratio energy balance method rearranges the energy balance equation to cancel aerodynamic transport terms. ET can also be measured directly using eddy covariance sampling of the boundary layer according to the relationship described in the chapter. The chapter also describes the combined use of aerodynamic expressions for sensible heat transfer and energy balance methods to estimate ET as a residual of the surface energy balance.

Sensible and latent heat transfer equations combine with the energy balance equation, following simplifying assumptions by Penman, to result in the well-known Penman equation, which produces evapotranspiration (ET) for a defined reference crop. The common Penman-Monteith equation for estimating ET involves a two-step process: (1) determining a reference ${\mathit{ET}}_{\mathit{ref}}$ which has been for either clipped grass or alfalfa, and (2) by multiplying ${\mathit{ET}}_{\mathit{ref}}$ with a crop coefficient, ${\mathit{K}}_{\mathit{c}}$, to obtain an estimate of actual ET for a particular crop or surface condition. The chapter provides procedures for estimating solar radiation, vapor pressure, and wind speed data when they are missing or of questionable quality. Estimates of reference ET are produced by measured evaporation from atmometers and from evaporation pans. These measurement systems can represent conditions within a local environment, which may be useful for some applications.

This chapter focuses on evaporation from bare or nearly bare soils and includes procedures for adjusting estimated evaporation for partial plant cover. Evaporation estimates from bare soil are important in early and late crop-growing periods and in nongrowing periods. They are also important for estimating effects of wet soil within vegetation on total evapotranspiration (ET). Evaporation from bare soil is often divided into two or three stages that characterize the form or nature of control on the evaporation process and rate. Several models are used to estimate evaporation from soils. When used in conjunction with total estimates of ET and on an operational basis, simplified models are often applied that do not require extensive soil hydraulic data. The examples show how the simple square root of time and FAO56 models compare with absolute measurements of evaporation taken by a precision-weighing lysimeter system.

This chapter focuses on using the two-step approach to estimate evapotranspiration (ET) for farm crops, landscapes, and some natural vegetation. Two families of crop coefficient ($K_c$) curves for agricultural crops have been developed for the two commonly used reference crops (tall and short): Pruitt and others' grass-based curves and Wright and others’ alfalfa-based $K_c$ curves. Although several crop coefficient models use a curvilinear shape, the Food and Agricultural Organization’s (FAO) linear segment model is widely used and easy to formulate. FAO24 and FAO56 present the procedure for constructing crop (cover) coefficients. Many of the principles and procedures described for using the FAO grass-based crop coefficients also apply when using alfalfa-based crop coefficients. The dual $K_c$ method provides means to estimate effects of wetting frequency on total water consumption and to estimate ET year round.

The main advantage of the Penman-Monteith (PM) equation and other combination equations over direct aerodynamic approaches is that air temperature and humidity measurements are required at only one reference height. However, specifying these parameters over time can be challenging. Many land surfaces have vegetation canopies that do not completely cover the ground surface or understory, including forests, cultivated row crops, orchards, vineyards, and natural systems. The chapter describes three resistance-based methods for estimating evapotranspiration (ET) from sparse canopies: the standard PM method, the multilayer method, and the two-source method. Aerodynamic roughness length and displacement of the zero plane of the wind profile are common parameters of many physically based models. Finally, the chapter summarizes several example applications of the single-layer PM method for estimating ET directly, without the use of crop coefficients.

Estimates of regional evapotranspiration (ET) are necessary inputs to world climate models, regional water use and supply planning and assessment, estimates of potential food production capacities and supplies, and hydrologic studies. The chapter cites methods for determining ET at specific locations that are regionally valid when applied with gridded or spatially representative data. The most readily used methods for regional application are the water balance method, lysimeters, energy balance methods, and mass transfer or modeling methods. The chapter describes applications of the complementary approach and energy balance models for regional estimates of ET from forests, grasslands, and agricultural areas. It also describes data sources to support regional ET estimates, including gridded weather data. Satellite imagery combined with surface climate measurements has been used to estimate ET from large-scale cultivated lands and determine the productivity of water in these areas.

All methods of calculating evaporation and evapotranspiration (ET) that are based on climatological data produce only estimates of ET. The estimates’ accuracy depends substantially on the quality of data and on the completeness and physical basis of the method. Considerations in both method selection and resource allocation include the time required to collect and screen climate data for quality and to process the climate data. The time required to obtain associated relevant data such as soil type, crops being grown, or landscape vegetation, should also be considered. The major weather variables to review are solar radiation, temperature, humidity, and wind speed. In general, the crop coefficient–reference crop coefficient method has proven to be accurate, robust, consistent, and straightforward to apply. The direct ET methods have the potential for more precise measurements, but generally require more detailed parameterization and crop-specific information.

The major component of the irrigation water requirement is the replacement of soil water depleted by evaporation and crop or landscape evapotranspiration (ET). Estimating total irrigation water requirements includes estimating various system “losses” associated with delivering water to a farm or field and the associated hydraulics and uniformity of water application to the field. The net irrigation requirement per unit area is the water that irrigation must supply to satisfy ET, leaching, and miscellaneous water requirements that water stored in the soil and precipitation that enters the soil do not provide. Depletions of the water resource caused by irrigation are equivalent to the net irrigation water requirement calculated when ET is at the potential value, which will occur when irrigation water is fully applied.

This appendix includes five tables with conversion and weather-related information. These tables cover conversion factors, values for the saturation vapor pressure over water at different temperatures, basic thermodynamic constants and values for specific heats of air and water, calculations of the density of air at various temperatures and pressures, and a method for including the day of the year in energy balance equations.

This appendix includes a table of the mean crop coefficients, ${\mathit{K}}_{\mathit{c}}$, for various crops cultivated in subhumid climates, including vegetables, fiber crops, oil crops, cereals, forages, sugar cane, and many different types of fruits.

This appendix provides a table of the number of days needed for different crops to develop at various planting dates and in various climatic regions. Specific crops listed include vegetables, fiber and oil crops, cereals, forages, sugar cane, and many different types of fruits.

The included table provides basal crop coefficients, ${\mathit{K}}_{\mathit{ch}}$, for various crops cultivated in subhumid climates. Specific crops include vegetables, fiber and oil crops, cereals, forages, sugar cane, and many different types of fruits.

This appendix presents conversion of mean and basal crop coefficients, ${\mathit{K}}_{\mathit{c}}$ and ${\mathit{K}}_{\mathit{cb}}$, for crops common to temperate and continental climates for use with alfalfa reference evapotranspiration (${\mathit{ET}}_{\mathit{r}}$), as computed by the American Society of Civil Engineers Standardized Penman-Monteith Reference Method. The mean and basal crop coefficient curves are traceable to time-based curves based on weighing lysimeter measurements at Kimberly, Idaho. During the conversion work, standardized ${\mathit{ET}}_{\mathit{r}}$, denoted ${\mathit{ET}}_{\mathit{rs}}$, was computed daily using air temperature, humidity, and wind speed data collected by the National Weather Service at the USDA-ARS Center at Kimberly and using solar radiation data collected by the USDA-ARS at Kimberly. Root mean square error between cumulative ${\mathit{ET}}_{\mathit{c}}$ by the original and converted curves was generally less than 1 mm per decadal period. Alfalfa ${\mathit{K}}_{\mathit{c}}$ curves are converted for the four growth cycles.

This appendix presents the basal crop coefficients, ${\mathit{K}}_{\mathit{cb}}$, for crops common to temperate and continental climates for use with alfalfa reference ${\mathit{ET}}_{\mathit{r}}$, as computed by the American Society of Civil Engineers Standardized Penman-Monteith Reference Method using a growing-degree-day basis. Cumulative growing degree days (CGDD) provides automated flexure, expansion, or compression of the ${\mathit{K}}_{\mathit{c}}$ curve basis under varying weather conditions and eliminates the need to specify the time of effective full cover. The appendix describes the conversion and basis for applying CGDD. Automating the estimation of planting and greenup dates for crops is required to enable the ${\mathit{K}}_{\mathit{c}}$ computation process to operate for multiyear periods without manual intervention or specification of starting and ending dates for ${\mathit{K}}_{\mathit{c}}$ curves. Killing frosts can terminate growing seasons prematurely for crops that grow late into fall or for crops that are sensitive to light frost.

Increasing numbers of publications report measured evapotranspiration (ET) and crop coefficients (${\mathit{K}}_{\mathit{c}}$). This appendix discusses the upper limits for crop coefficients and summarizes precautions, recommendations, and literature associated with sap flow and remote-sensing-based determination of ET and ${\mathit{K}}_{\mathit{c}}$. The upper limit on potential crop evapotranspiration is readily approximated by comparing against the widely used reference ET through a crop coefficient. Sap flow methods may require scaling from branches or whole plants to stands of plants to provide wide-area ET estimates. Satellite- or ground-based energy balance methods generally require extensive time investment and require learned skill sets. The appendix lists detailed types of documentation that should accompany publication of ET and ${\mathit{K}}_{\mathit{c}}$ data that are useful when applying or interpreting reported values.

This appendix reviews general procedures for assessing the integrity and representativeness of weather data used for reference evapotranspiration (${\mathit{ET}}_{\mathit{ref}}$) calculation. Most modern automated weather stations measure the primary variables affecting ET—solar radiation, air temperature, wind speed, and humidity—and therefore provide somewhat complete data for estimating ET as compared with the older, manually operated weather stations that measured only air temperature. Meteorological data acquired to estimate ${\mathit{ET}}_{\mathit{ref}}$ should be subjected to several quality control checks prior to use. The first and most important quality control check involves gathering information about the origin of the weather data and conditions affecting measurements. The appendix describes procedures recommended by the American Society of Civil Engineers for assessing the integrity of meteorological data sets used in the computation of ${\mathit{ET}}_{\mathit{ref}}$.

This appendix describes an analytical procedure and uses software simulations to estimate the contribution of water from a shallow water table to the ground surface for direct evaporation or into a root zone for extraction by plant roots. The analytical procedure provides a somewhat simple approximation of potential upward flux of water from a water table. In reality, soil layering, soil disturbance, soil cracking, soil temperature, and tillage can complicate the actual flow and evaporation process. Therefore, the simplified analytical approach provides only an approximation of upward flow. The software model UPFLOW's estimates for a wet clay soil follow the Brutsaert-Gardner function closely and consider effects of soil layering, root density, soil moisture distribution, and anaerobiosis effects on upward flux.

Deriving the Penman-Monteith (PM) equation is useful in understanding how it functions and some of the assumptions made in its development. Two fundamental approaches to estimating evaporation and evapotranspiration are the surface energy balance equation and the aerodynamic equation. This appendix combines the two approaches. It gives the aerodynamic-based expression for latent heat flux based on the surface-to-air-vapor-pressure gradient. Coincident measurements of relative humidity and air temperature, which are commonly available, or measurement of dewpoint temperature, also commonly available, can yield vapor pressure of air. To create the PM equation, one utilizes the Bowen ratio, which is simply the ratio between latent heat flux and sensible heat exchange from the surface to the air.

Several historical methods for estimating evapotranspiration (ET) are no longer commonly used. However, because some studies and litigations may involve historical ET data calculated with uncommon methods, this appendix briefly summarizes these methods to preserve the information and to identify general references. Other methods, such as the 1982 Kimberly Penman method and the CIMIS Penman method, were used in specific regions of the United States. Several solar radiation-based reference ET methods were developed in the 1960s to reduce the need for humidity and wind speed data. Relationships between reference ET and air temperature or between some other form of ET and air temperature are usually empirical and developed using some form of statistical correlation. Pan evaporation data can be used to estimate reference ET using the simple proportional relationship.

This appendix compares and ranks various methods of estimating evapotranspiration (ET). The methods include the Penman method, the Priestley-Taylor and Turc methods, the FAO Blaney-Criddle temperature method, the Hargreaves-Samani method, and pan evaporation methods. Data used to evaluate the various methods were selected from publications or obtained by personal communication with investigators working where lysimeter data for well-watered grass or alfalfa were available along with adequate supporting climatic data. Most methods of estimating evapotranspiration require the computation of parameters such as saturation vapor pressure, latent heat of vaporization, the psychrometric constant, etc., from climatic data. This appendix presents ET estimates for 16 methods expressed as percentages of the corresponding lysimeter-measured ET for peak ET months for 11 lysimeter locations. The analyses and comparisons of methods were made over a broad range of climates and geographic locations.

This appendix contains a glossary.