Performance of Evapotranspiration Models for Maize Bare Soil to Closed Canopy

Simulating evapotranspiration (ET) processes in agricultural crop systems requires dynamic models capable of describing the entire range of crop cover. The Shuttleworth and Wallace (S-W) ET model has the potential to fulfill this requirement, but it has not been rigorously tested on crops over the season. A field study assessed the seasonal performance of the Penman-Monteith (P-M) and the S-W ET models against Bowen ratio energy balance (BREB) measurements for irrigated maize during 1993 in Fort Collins, Colorado. The P-M performed poorly at low Leaf Area Index (LAI < 2) because soil evaporation was neglected by the chosen means of calculating surface resistance. Potential model users are warned of possible misapplication of the P-M to partial canopy with the surface resistance defined solely by the canopy resistance. Model performance at higher LAI was satisfactory. Results of the S-W model are encouraging; it performed satisfactorily for the entire range of canopy cover as judged by the simulation of cumulative ET. It deviated by 6, 3, and 4% from cumulative BREB ET for periods of LAI = 2, LAI > 2, and over the season, respectively. To facilitate the practical use of the S-W model, simplified and effective methods for determining soil and canopy resistances are needed.