COMBINING OPTICAL AND MICROWAVE REMOTE-SENSING FOR MAPPING ENERGY FLUXES IN A SEMIARID WATERSHED

A dual-source model treating the energy balance of the soil/substrate and vegetation that was developed to use radiometric diometric surface temperature observations is revised to use remotely sensed near-surfa ce moisture from a passive microwave sensor for estimating the soil su rface energy balance. With remotely sensed images of near-surface soil moisture, land cover classification, and leaf area index, the model i s applied over a semiarid area in the Walnut Gulch Watershed in southe rn Arizona. The spatial and temporal variation of the Bowen ratio (i.e ., the ratio of the turbulent fluxes, sensible, and Intent heat) ''map s'' generated by the model ere similar to the changes in near-surface moisture fields caused by recent precipitation events in the study are a. The estimated fluxes at the time of the microwave observations (i.e ., ''instantaneous'' estimates) and those simulated over the daytime p eriod are compared with the ground observations within the watershed. Differences between predicted and observed ''instantaneous'' fluxes we re usually comparable to the measurement uncertainties, namely, 5% for net radiation and 20-30% for soil, sensible and latent heat fluxes, e xcept when there was large temporal and spatial variations in solar ra diation across the study area. However by running the model over the d aytime period, this variability in solar radiation proved to have a mi nor effect on computed daytime totals. In fact, differences with obser ved heat fluxes were significantly less (i.e., around 15%) than when c omparing ''instantaneous'' values. Model predictions of the total soil heat flux over the daytime period were generally higher than measured . An empirical model was developed to reduce this bias, but it is not known how generally applicable it will be. Model sensitivity to typica l uncertainties in remotely sensed leaf area index (LAI) and near-surf ace (0-5 cm) water col?tent, W was quantified. The variation in flux p redictions caused by errors in prescribing leaf area index and W was l ess than 30%. More tests with this model over different landscapes are necessary to evaluate its potential for predicting,a regional fluxes, lit particular, microwave and radiometric surface temperature observa tions are needed under drought conditions for evaluating if the model formulation of vegetation transpiration can properly adjust to this ex treme and very important environmental condition. Published by Elsevie r Science Inc., 1998.