The ASCE Standardized Reference Evapotranspiration Equation

This chapter discusses evapotranspiration (ET), which is the loss of water from the earth’s surface through the combined processes of evaporation and plant transpiration. The chapter then defines reference evapotranspiration (ET_ref) as the ET rate from a uniform surface of dense, actively growing vegetation that has a specified height and surface resistance, is not short of soil water, and expands across at least 100 m. Two ET_ref surfaces are needed for standardized computational procedures. The two adopted ET_ref surfaces are a short crop and a tall crop. In addition, these procedures need an equation capable of calculating both hourly and daily E_ref. As a part of the standardization, the ASCE Penman-Monteith (ASCE-PM) equation and associated equations for calculating aerodynamic and bulk surface resistance were combined and condensed into a single equation that applies to both surfaces.

Based on intensive review of reference evapotranspiration calculated for 49 sites throughout the United States, the ASCE-ET found the standardized reference evapotranspiration equation to be reliable. The ASCE-ET recommends its use for calculating reference evapotranspiration and in turn crop evapotranspiration (ET_C), developing new crop coefficients, and facilitating transfer of existing crop coefficients.

This chapter describes data requirements, equations, and procedures for calculating standardized reference evapotranspiration equations (ET_SZ) daily and hourly. Calculating reference evapotranspiration (ET_ref) historically has used a daily time step. Calculating ET_SZ requires measuring or estimating air temperature, humidity, solar radiation, and wind speed. These parameters are the minimum requirements for estimating ET_0S and ET_rs. Any evapotranspiration calculation’s accuracy depends on the quality of the weather data, which in turn requires good quality control and quality assurance procedures. The calculation of ET_SZ uses the constant value of 0.23 for albedo for daily and hourly periods, although the albedo is known to vary somewhat with time of day and with time of season and latitude due to changes in the angle of the sun.

This chapter discusses the application of ET_SZ and K_c in nongrowing periods. Calculating crop evapotranspiration (ET_C) requires selecting the appropriate crop coefficient to use with the standardized evapotranspiration reference (ET_0S or ET_rs). Crop coefficients and landscape coefficients available in the literature are referenced to either clipped, cool-season grass or full-cover alfalfa. Establishing the differences between ET equations is important because some equations developed to estimate grass or alfalfa reference ETs may not agree exactly with ET_0S or ET_rs at all time periods or under all climatic conditions. Mean crop coefficients incorporate the effects of irrigation, rainfall, and soil type at the development site. Users of these mean crop coefficients should recognize that differences in irrigation frequency, rainfall patterns, and/or soil-drying characteristics between the development site and the study site could cause error in the ET_C estimate.

Evapotranspiration (ET) from inactive vegetation in nongrowing periods is generally less than reference ET because plants may be dormant and therefore may have substantially increased surface resistance. Besides surface resistance, albedo or reflectance of dormant or dead vegetation is generally greater than that of green vegetation. Both characteristics reduce the potential rate of ET from plant residue. Although the reference ET equations do not represent measurable quantities during nongrowing periods, the standardized reference evapotranspiration equation (ET_SZ) can still be useful as an evaporative index. However, the user must be aware that conditions for the reference surfaces for ET_0S and ET_rs may not exist in nongrowing periods. Under many nongrowing conditions, incorporating the differences between dormant or dead vegetation ET and ET_SZ into the crop coefficient (K_c) value is possible.