Using Satellite Remote Sensing to Extrapolate Evapotranspiration Estimates in Time and Space over a Semiarid Rangeland Basin

R e m o t e sensing data from the NOAA-11 A VHRR satellite were collected over the USDA-Agricultural Research Service Walnut Gulch Experimental Watershed in southeastern Arizona during the MONSOON 90 field campaigns. An energy balance model which relies primarily on remotely sensed inputs was used to extrapolate evapotranspiration (ET) estimates from one location containing near-surface meteorological data to other areas in the basin. Satisfactory results were obtained under a wide range of environmental conditions. However, the ET values are essentially instantaneous and therefore do not necessarily provide reliable estimates of daytime or daily ET fluxes required for many hydrological and resource management applications. An operational technique was developed to extrapolate one time of day ET estimates to daytime averages using the evaporative fraction concept and empirical methods for converting midday available energy to daytime average values. Model derived daytime average ET fluxes were in reasonable agreement with local ground-based measurements. The technique also was used to estimate daily ET at the basin scale.