COMPARISON OF PENMAN-MONTEITH, SHUTTLEWORTH-WALLACE, AND MODIFIED PRIESTLEY-TAYLOR EVAPOTRANSPIRATION MODELS FOR WILDLAND VEGETATION IN SEMIARID RANGELAND

Eddy correlation measurements of sensible and latent heat flux are use d with measurements of net radiation, soil heat flux, and other microm eteorological variables to develop the Penman-Monteith, Shuttleworth-W allace, and modified Priestley-Taylor evapotranspiration models for us e in a sparsely vegetated, semiarid rangeland. The Penman-Monteith mod el, a one-component model designed for use with dense crops, is not su fficiently accurate (r2 = 0.56 for hourly data and r2 = 0. 60 for dail y data). The Shuttleworth-Wallace model, a two-component logical exten sion of the Penman-Monteith model for use with sparse crops, performs significantly better (r2 = 0. 7 8 for hourly data and r2 = 0. 85 for d aily data). The modified Priestley-Taylor model, a one-component simpl ified form of the Penman potential evapotranspiration model, surprisin gly performs as well as the Shuttleworth-Wallace model. The rigorous S huttleworth-Wallace model predicts that about one quarter of the vapor flux to the atmosphere is from bare-soil evaporation. Further, during daylight hours, the small leaves are sinks for sensible heat produced at the hot soil surface.