Improved Models of Soil Emission for Use in Remote Sensing of Soil Moisture

Microwave radiometer measurements of the Planck emission from soil can be used to estimate the near-surface soil moisture. A more straightforward and consistent model of this emission results if nonuniform, rather than uniform, plane waves are used. Adaptation of this new model to a layered medium representation for the soil is improved using a normalization that is based on the isothermal soil limit. The separate concepts of radiometer sensing depth and in situ sampling depth for soil moisture are examined and theory for the sensing depth is presented. Improved approximations (at 1413 MHz) to the full model for soil emission are developed since they are needed to construct algorithms that retrieve an estimate of soil moisture from the radiometer raw measurement. The error associated with the comparison of these remotely sense values against in situ measurements is calculated. Results suggest that this comparison error could be lowered if the soil moisture sampling depth was reduced to values less than 0.01 m from the currently used values near 0.02 m, although the effect of surface roughness has not yet been analyzed.