Regional Estimating Methods and Methods Not Commonly Used in the United States

Several historical methods for estimating evapotranspiration (ET) are no longer commonly used. However, because some studies and litigations may involve historical ET data calculated with uncommon methods, this appendix briefly summarizes these methods to preserve the information and to identify general references. Other methods, such as the 1982 Kimberly Penman method and the CIMIS Penman method, were used in specific regions of the United States. Several solar radiation-based reference ET methods were developed in the 1960s to reduce the need for humidity and wind speed data. Relationships between reference ET and air temperature or between some other form of ET and air temperature are usually empirical and developed using some form of statistical correlation. Pan evaporation data can be used to estimate reference ET using the simple proportional relationship.