In irrigated agroecosystems, a large part of the irrigation water applied to agricultural lands is consumed by evaporation and transpiration. In practice, in field measurements, it is hard to separate evaporation from transpiration, and the two processes are usually considered as one component. Crop ET can be measured directly using precision weighing lysimeters, Eddy correlation system, Bowen ratio energy balance system, atmometers, including evaporation pans, soil water balance by measuring soil water status continuously, etc. However, because direct measurement of crop ET (ETc) is difficult, time consuming, and costly, the most common procedure is to estimate ETc using climatic data. Currently, most commonly practiced way of estimating the crop ET rate (or crop water use rate) for a specific crop or vegetation surface requires first calculating reference ET (ETref) and then applying the crop coefficients (Kc) to estimate actual crop ET (ETc) as
where ETc is the crop ET (crop water use) in units of water depth (inches d-1, cm d-1, or mm d-1), ETref (ETo or ETr) is the reference ET in unit ofwater depth (inches d-1, cmd-1, or mmd-1) as calculated from the basic weather variables (solar radiation, air temperature, wind speed, and relative humidity) measured with a weather station in reference conditions.
Although the first equation by Penman for potential ET, (ETp), was introduced almost 60 years ago; it still provides fundamental principles for the calculation and/or modification of ET models today. Numerous methods have been introduced for computing ETref causing confusion among users, decision- and policymakers as to which method to select for ETref estimation. Recently, the American Society of Civil Engineers (ASCE) Evapotranspiration in Irrigation and Hydrology Committee established a Task Committee on 'Standardization of Reference Evapotranspiration Calculation'. Based on extensive research and data analyses and comparison of lysimeter-measured reference ET across various climates and Task Committee experience, the Task Committee recommended the use ofthe ASCE-Penman-Monteith (PM) method as the representation for reference ET. A reduced form of the ASCE-PM was used as the basis for 'standardized' ETref computation. Equation parameters differ for hourly and 24-h data. Coefficients and parameters for a taller, rougher crop surface (0.5 m tall, like alfalfa) were also developed. The ASCE standardized ETref equation based on a surface resistance of 50 sm-1 during daytime and 200 sm-1 during nighttime provided the best agreement with the full form of the ASCE-PM method applied on a daily basis. The advantages of adapting a specific procedure as a standardized method are (1) it provides commonality to computing ETref, and (2) the use of a standardized method enhances the transferability of crop coefficients.
The standardized ASCE-PM equation is intended to simplify and clarify the application of the method and associated equations for computing aerodynamic and bulk surface resistance (ra and rs, respectively). Equations were combined into a single expression for both grass and alfalfa reference surfaces and for a 24-h or an hourly time step by varying coefficients. Computation of standardized short grass ETo with a 24-h time step uses a grass height of 0.12 m and an rs value of 70 s m-1, which is the same as for the FAO56-PM equation. For hourly time steps, rs is set to 50 s m-1 for daytime hours and to 200 s m-1 for nighttime hours. The standardized ASCE-PM equation is
0.408A(Rn - G)+7(Cn/(T + 273)) U2(es - ea; [A + 7(1 + CdU2)]
where ETref is the standardized reference ET (mm d-1 or mmh- ), A is the slope of saturation vapor pressure versus air temperature curve (kPa °C- ), Rn is the calculated net radiation at the crop surface (MJm-2d-1 for 24-h time steps or MJ m-2 h-1 for hourly time steps), G is the heat flux density at the soil surface (zero for 24-h time steps or MJm-2h-1 for hourly time steps), Tis the mean daily or hourly air temperature at 1.5-2.5 m height (°C), U2 is the mean daily or hourly wind speed at 2 m height (ms-1), es is the saturation vapor pressure (kPa), ea is the actual vapor pressure (kPa), es - ea is the vapor pressure deficit (kPa), 7 is the psychrometric constant (kPa °C- ), Cn is the numerator constant that changes with reference surface and calculation time step (Cn = 900 °C mm s3 Mg-1 d-1 for 24-h time steps, and Cn = 37 °C mm s3 Mg-1 h-1 for hourly time steps for the grass reference surface), Cd is the denominator constant that changes with reference surface and calculation time step (Cd = 0.34 s m- for 24-h time steps, Cd = 0.24 s m-for hourly time steps during daytime, and Cd = 0.96 s m-1 for hourly nighttime for the grass reference surface), and 0.408 is the coefficient having units of m2 mm MJ-1. The values of Cn and Cd for the grass and alfalfa reference surfaces for daily and hourly time steps are given in Table 1.
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