Combining the Penman-Monteith equation with measurements of surface temperature and reflectance to estimate evaporation rates of semiarid grassland

The Penman-Monteith equation is useful for computing evaporation rates of uniform surfaces, such as dense vegetation or bare soil. This equation becomes less useful for evaluation of evaporation rates at the regional scale, where surfaces are generally characterized by a patchy combination of vegetation and soil. This is particularly true in the arid and semi-arid regions of the world. The approach proposed here is an attempt to use remotely-sensed measurements of surface reflectance and temperature to allow application of the Penman-Monteith theory to partially-vegetated fields without a-priori knowledge of the percent vegetation cover and canopy resistance. Basically, the Penman-Monteith equation was combined with the energy balance equation to estimate the surface temperature (T_s) associated with four states: surfaces characterized by full-cover vegetation and bare soil with evaporation rates at potential and zero. Then, linear interpolations between T_s values computed for full-cover and bare soil conditions were used to provide information at intermediate states based on measurements of actual surface reflectance and temperature. The approach was first tested using ground-based measurements of surface reflectance and temperature at a rangeland site; the results compared well with on-site measurements of surface evaporation rate (RMSE = 29 W m^-2). Then, the approach was tested based on a set of four Landsat Thematic Mapper (TM) images acquired in southeast Arizona during 1992. Maps of surface air temperature and wind speed were combined with maps of surface temperature and spectral vegetation index to produce regional estimates of evaporation rates for the grassland biome.