Estimating Potential Evapotranspiration

Methods of computing potential evapotranspiration by analytical procedures have been based on the application of the turbulent-transport and energy balance concepts. Empirical formulas, correlating some temperature function and adjusting for daytime hours, have proved valuable in practical utilization. This latter approach has been used to formulate a simple computational procedure whereby average daily potential evapotranspiration is represented as proportional to the product of daytime hours squared and the saturated water vapor concentration (absolute humidity) at the mean temperature. The day time factor was determined from a consideration of the disparity between net radiation and temperature, latitudinally, and the fact that transpiration is restricted during darkness since the leaf stomata are closed. Computed values of potential evapotranspiration obtained by the new procedure are compared with those obtained by the more complex Thornthwaite method (24)2 and other methods currently employed. General applicability seems justified from comparisons between observed and computed values of potential evapotranspiration, both on a yearly and seasonal basis.