Measurement of Soil Moisture with Synthetic Aperture Radar1

An experiment was performed in Northern Maine recently to evaluate the use of Synthetic Aperture Radar (SAR) data in the measurement of soil moisture content in small area plots with varying surface cover and at various subsurface depths. Ground preparation efforts involved defining six contiguous 30 m x 30 m plots with three surface cover conditions: bare tilled soil, short grass and forest. For each surface cover we irrigated one of the two plots to obtain areas of contrasting soil moisture content. The NASNJPL DC-8 and its three frequency (P, L, and C) SAR made passes over the area on two days and at three different depression angles collecting a multifrequency and multiangle data set. Near the time of the SAR overflights, soil samples were taken at 2, 5, 10, and 25 cm depths and gravimetric moisture content (mg) determined. While significant spatial variability was observed in the moisture samples, and variations in the SAR backscatter data induced by speckle due to surface roughness caused problems, a proportional relationship was observed between the backscatter coefficient and moisture content for the tilled and grass areas, especially near the surface. L band data seemed to be the most sensitive to mg in the open areas, while P band data showed sensitivity to the surface moisture in the forest area. Inconclusive results were obtained in correlating the SAR data with moisture measurements at the 10 and 25 cm depths. Also, the variation in backscatter due to speckle seemed to be reduced through a Polarimetric Whitening Filter (PWF) transformation.