Accuracy of Estimated Reference Crop Evapotranspiration

Evapotranspiration from cool‐season grasses is used as the reference for ET0. The requirements for local calibration for three equations for ET0 recommended by the Food and Agriculture Organization of the United Nations (FAO) are shown for different climatic regions of California. Data from measured coolseason‐grass evapotranspiration and Class A pan evaporation from sites located in large well‐watered, irrigated grass pastures in three distinct climatic conditions are used to evaluate a method for estimating ET0 that requires only measured values of maximum and minimum air temperatures. Mean temperature, minimum relative humidity, and solar radiation can all be estimated from maximum and minimum air temperatures. The equation for ET0 that requires only temperature measurements may seem to be quite simple, but it is in reality a very comprehensive procedure incorporating most of the climatic interactions that influence the evaporative potential. The analysis of data from four lysimeter and Class A pan sites in California indicates that the temperature‐range equation estimates ET0 reasonably well in a large diversity of climates. The corrected Penman and other equations recommended by FAO are shown to usually overestimate ET0 in various climatic regions of California.