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 less than or equal to 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.