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Hillslope-Scale Controls on Remote Sensing of Soil Moisture with Microwave Radiometry

Microwave radiometry is emerging as an important tool for global remote sensing of near-surface soil moisture in the coming decade. In a modeling study, we find that hillslope-scale topography (tens of meters) influences predicted microwave brightness temperatures at significantly coarser scales (kilometers). Through the physics of microwave remote sensing, topography is understood to effect brightness temperature observations in two important ways: (1) modulating the spatial distribution of factors affecting emission like vegetation biomass, moisture, and surface and canopy temperatures, and (2) determining the incidence angle and polarization rotation the observing sensor makes with the local land surface. Local incidence and polarization rotation angles can be explicitly computed knowing local terrain slope and aspect and sky position of the sensor. In an analysis of two synthetic domains, predicted hillslope-scale brightness temperatures within a less rugged landscape that is presented here can vary from approximately 224 to 302 K in the horizontal polarization and from approximately 298 to 320 K in the vertical polarization. Impacts of hillslope-scale heterogeneity in factors effecting emission account for at most approximately 2 K in predicted watershed-scale brightness temperature, while impacts of hillslope-scale topography on observing geometry can account for up to 28 K in predicted watershed-scale brightness temperatures in a topographically rugged area.