Evaluation of Class A pan coefficients for estimating reference evapotranspiration in humid location

Evaporation pans [Class A pan, U.S. Weather Bureau (USWB)] are used extensively throughout the world to measure free-water evaporation and to estimate reference evapotranspiration (ET_o). However, reliable estimation of ET_o using pan evaporation (E_pan) depends on the accurate determination of pan coefficients (K_pan). Two equations developed by Frevert et al. in 1983 and Snyder in 1992 to estimate daily K_pan values were evaluated using a 23-year climate dataset in a humid location (Gainesville, Florida). The ET_o data, calculated using daily K_pan values from these equations, were compared to the Food and Agricultural Organization (FAO)-Penman-Monteith (FAO56-PM) method. The two equations resulted in significantly different daily K_pan values that produced different daily, monthly, and annual total ET_o estimates. The ET_o values calculated using Frevert et al.'s 1983 K_pan coefficients were in very good agreement with the FAO56-PM method with daily, monthly, and annual mean percent errors (PE) of 5.8, 5.5, and 5.7%, respectively. The daily and annual mean-root-mean-square error (RMSE) of the estimates using this method were as low as 0.33 and 7.3 mm, respectively. Snyder's 1992 equation overestimated FAO56-PM ET_o with daily, monthly, and annual mean PEs of 16.3, 13.8, and 13.2%, respectively. The daily and annual mean RMSEs for this method were higher (0.6 and 18 mm) than those obtained with Frevert et al.'s 1983 coefficients. The overestimations with Snyder's 1992 method were highest in the peak ET_o month of May and in summer months. The performances of the K_pan equations were also evaluated using randomly selected individual years (1979, 1988, 1990, and 1994) of climate data that had different climate characteristics than the 23-year average dataset. Frevert et al.'s 1983 coefficients resulted in good ET₀ estimates with lower annual mean PEs of 7.0, 0.1, 15.7, and 1.3% for 1979, 1988, 1990, and 1994, respectively, compared to Snyder's 1992 equation, which resulted in considerably higher PEs of 17.6, 9.1, 26.2, and 14.3% in 1979, 1988, 1990, and 1994, respectively. It was concluded that using Frevert et al.'s 1983 equation to calculate daily K_pan provided more accurate ET_o estimates, relative to the FAO56-PM method, from E_pan data compared to Snyder's 1992 equation under the humid-region climatic conditions in this study. The method is very useful in computer calculations of ET_o since it does not require "table lookup" for K_pan values.