Radar mapping of surface soil moisture

Intended as an overview aimed at potential users of remotely sensed spatial distributions and temporal variations of soil moisture, this paper begins with an introductory section on the fundamentals of radar imaging and associated attributes. To place the soil moisture sensing task in proper perspective, the prerequisite step of classifying terrain into four basic types—bare surfaces, short vegetation, tall vegetation, and urban—is addressed by demonstrating how a dual-frequency polarimetric radar can correctly classify terrain with an accuracy greater than 90%. Over 5000 image pixels with known terrain identity were involved in the evaluation of the radar image classifier. For bare soil (with vegetation cover shorter than 15 cm), radar can estimate the volumetric moisture content (expressed in per cent) of the top 5 cm soil layer with an r.m.s. error of 3.5%. Based on theoretical model predictions as well as experimental observations, strong evidence exists in support of radar's potential for sensing soil moisture under vegetation cover, but no operational algorithm exists at present.