A Model of Surface Roughness for Use in Passive Remote Sensing of Bare Soil Moisture

Spaceborne radiometers operating near 1.4 GHz are the primary instrument for recent efforts to remotely sense nearsurface soil moisture around the globe. Generally, these instruments must contend with the effects of vegetation growing in the soil. However, an important first step is to model the measurements made by a radiometer that is viewing bare (vegetation-free) soil. The proposed model uses a matching layer and a random depolarizer to describe bare soil surface roughness and some aspects of antenna beamwidth. The model suggests that the effects of nearsurface soil moisture and roughness upon the radiometer measurement are more distinct than is currently thought. Furthermore, it appears that both moisture and roughness can be retrieved from a single set of radiometer measurements made at orthogonal linear polarizations. This retrieval precision is predicted to be poor at soil observation angles near nadir but improves for larger angles. At observation angles near 50°, the vertically polarized radiometer measurements are predicted to be nearly insensitive to roughness. A convenient parameterization of the model is provided and permits quick implementation.