Modeling transpiration and water stress in vegetation from satellite and ground measurements

A method is developed to estimate transpiration (T) by combining satellite and ground measurements. The method follows the energy balance approach where transpiration is estimated as a residual when net radiation (Rn), sensible (H) and soil heat (G) fluxes are known. The model estimates T from the vegetation layer only, and does not require a complex two layer model to separate the fluxes from the canopy and the ground layers. The method was validated using data collected during the First ISLSCP Field Experiment (FIFE), and the modeled values of evapotranspiration (ET) were within 9% of ground observations. The model was applied to data collected from four crop reporting districts in Iowa during 1988 to derive T and a crop water stress index. Model results successfully monitored the incidence, degree and duration of water stress among the different crop reporting districts