Passive microwave emission from smooth bare soils: developing a simple model to predict near surface water content

MICRO-SWEAT is a coupled microwave emission and soil-vegetation-atmosphere transfer scheme. It can predict, among other things, the L band (21 cm wavelength) microwave brightness temperature (T-B) for a smooth bare soil from knowledge of the soil hydraulic properties and local meteorological data. T-B is the product of the apparent emissivity (e(app)) and the effective physical temperature of the soil profile (T-eff). The modelled relationship between e(app) and average 0-2 cm soil water content was used to develop a simple semi-empirical model, which predicts the near surface water content from knowledge of the particle size distribution and e(app). T-eff was found to be comparable to the soil temperature at 11 cm depth, and was estimated using a simple sinusoidal function based on thermal diffusivity to extrapolate the temperature at 11 cm depth from knowledge of the diurnal variation in air temperature. The dependence of T-B on incidence angle was predicted by MICRO-SWEAT and also incorporated. This simple model was then successfully used to predict the 0-2 cm water content from knowledge of T-B, the air temperature regime and the particle size distribution, for data collected at various look angles over three different soils using a truck-based radiometer.