A Comparison of Common Pan Evaporation to Reference Evapotranspiration Equations

Reference evapotranspiration(ETo) is commonly used to obtain a measure of atmospheric, evaporative demand to help estimate the water use of crops and landscape plants. ETo is an estimate of the maximum evapotranspiration of a large field of 0.12 m tall, cool-season grass. For many years, evaporation pans have been used to estimate ETo; however, the pan evaporation(Epan) data are usually corrected for the upwind distance of vegetation around the pan, humidity, and wind speed. A table presented in the UN-FAO Irrigation and Drainage Paper 24 was commonly used to obtain a pan coefficient(Kpan) that is used to estimate of ETo from Epan data as ETo=Epan x Kpan. However, this table does not lend itself well for use in computer programs. Consequently, many attempts have been made to fit empirical equations to the coefficients in the FAO Paper 24 table. In this paper, several of the published equations for Kpan values to convert Epan to ETo were compared. Over a 7-12 year period, the California Department of Water Resources collected pan evaporation data from large irrigated grass fields in a wide variety of climates. These data were compared with ETo estimates from the California Irrigation Management Information System(CIMIS). Daily ETo was estimated as the 24-hour sum of hourly calculations using a Penman-type equation with calibrated daytime and nighttime wind functions(Pruitt and Doorenbos, 1977). Four non-linear regression equations to match the pan coefficient(Kpan) values in FAO Paper 24 were tested. The four Kpan equations were reported by Cuenca(1989), Snyder(1992), Allen-Pruitt(1989) and Orang(1998). Wind speed and humidity data from nearby CIMIS weather stations were used to estimate the Kpan values. All of the Kpan equations performed well at estimating ETo=Epan x Kpan and none stood out as greatly superior to the others.