Evaporation pans provide a measurement of the integrated effect of radiation, wind, temperature, and humidity on evaporation from a specific open water surface. In a similar fashion, the plant responds to the same climatic variables, but several major factors may produce significant differences in loss of water. Reflection of solar radiation from a water surface is only 5%-8%, but from most vegetative surfaces is 20%-25%. Storage of heat within the pan can be appreciable and may cause high evaporation during the night; most crops transpire only during the daytime. Also, the difference in water losses from pans and from crops can be affected by differences in turbulence, temperature, and humidity of the air immediately above the surfaces. Heat transfer through the sides of the pan can occur, which may be severe for sunken pans. Also the color of the pan and the use of screens will affect water losses. The siting of the pan and the pan environment influence the measured results, especially when the pan is placed in fallow rather than cropped fields.
Notwithstanding these deficiencies, with proper siting the use of pans to predict CWR for periods of 10 days or longer is still warranted. The use of the U.S. Class A pan and the Colorado sunken is presented by Doorenbos and Pruitt . To relate pan evaporation (Epan) to reference crop ET (ET0), empirically derived coefficients (Kp) are used to account for climate and pan environment. ET0 can be obtained from
where Epan is the pan evaporation (mm day-1) and represents the mean daily value for the period considered, and Kp is the pan coefficient ( ).
Values for Kp are given by Doorenbos and Pruitt . Allen and Pruitt  developed polynomial equations to replace tables for the determination of the pan coefficients Kp according to the type of evaporation pan and upwind fetch. In general, local calibration of Kp coefficients can be performed as indicated for Eqs. (5.48) and (5.49) making Kp = b in the linear regression through the origin. Polynominals for Kp can be obtained through stepwise multiregression analysis between the ratios ET0/Epan and the variables U2 and RHmean observed for the same period.
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