MAPPING SURFACE SOIL-MOISTURE WITH MICROWAVE RADIOMETERS

Water stored in the soil serves as a reservoir for the evapotranspirat ion (ET) process on land surfaces, therefore knowledge of the soil moi sture content is important for partitioning the incoming solar radiati on into latent and sensible heat components. There is no remote sensin g technique which directly observes the amount of water in this reserv oir, however microwave remote sensing at long wavelengths (lambda > 10 cm) can give estimates of the moisture stored in the surface 5-cm lay er of the soil. This approach is based on the large dielectric contras t between water and dry soil, resulting in emissivity changes from 0.9 6 for a dry smooth soil to less than 0.6. In this paper, basic relatio nships between soil moisture and emissivity are described using both t heory and observations from various platforms. The ability of the appr oach to be extended to large regions has been demonstrated in several aircraft mapping experiments, e.g., FIFE, Monsoon 90, Washita 92 and H APEX Sahel. Some results from Monsoon 90 are presented here. Applicati ons of these soil moisture maps in runoff prediction, rainfall estimat ion, determining the direct evaporation from the soil surface and serv ing as a boundary condition for soil profile models are presented.