RADAR MAPPING OF SURFACE SOIL-MOISTURE

Intended as an overview aimed at potential users of remotely sensed sp atial distributions and temporal variations of soil moisture, this pap er begins with an introductory section on the fundamentals of radar im aging and associated attributes. To place the soil moisture sensing ta sk in proper perspective, the prerequisite step of classifying terrain into four basic types-bare surfaces, short vegetation, tall vegetatio n, and urban-is addressed by demonstrating how a dual-frequency polari metric radar can correctly classify terrain with an accuracy greater t han 90%. Over 5000 image pixels with known terrain identity were invol ved in the evaluation of the radar image classifier. For bare soil (wi th vegetation cover shorter than 15 cm), radar can estimate the volume tric moisture content (expressed in per cent) of the top 5 cm soil lay er with an r.m.s. error of 3.5%. Based on theoretical model prediction s as well as experimental observations, strong evidence exists in supp ort of radar's potential for sensing soil moisture under vegetation co ver, but no operational algorithm exists at present.