Recursive and explicit combination methods for calculating reference evapotranspiration

Efficient use of water in agroecosystems requires the accurate quantification of evapotranspiration (ET). Explicit combination method (ECM) equations (i.e., the Penman equation and its derivatives) have been widely used and tested. These equations rely on an assumption to remove the surface temperature from the calculation, and the validity of this assumption has been questioned as likely underestimating ET, particularly in semiarid and arid climates. Information on the validity of this assumption in subhumid regions is rare. Recursive combination methods (RCM), based on an equation proposed by Budyko, make no assumptions for the surface temperature, but they require iteration to arrive at a solution. The objective of this research was to compare daily ET calculated by the ECM and RCM and compare those values to Bowen ratio energy balance system (BREBS)-measured ET for a well-watered and maintained grassland near Central City, Nebraska. The results indicated that both equations were sensitive to the value chosen for the surface resistance (r_s). For equivalent values of r_s, ET predicted by the ECM was greater than ET predicted from the RCM. Using r_s values of 40sm^-1 for the ECM and 20sm^-1 for the RCM resulted in the best predictive performance for the two methods, and using these values, the ECM performed slightly better. While the RCM may potentially provide more accuracy under some conditions, it was no more accurate than the ECM under these experimental conditions.