Analysis of ERS-1 SAR data to study soil moisture content in rocky soils

Abstract

The objective of this research was to analyze the potential of ERS-1 SAR data to study the surface soil moisture content of rocky soils in semi-arid regions. The study took place at the Walnut Gulch Experimental Watershed, Tombstone, AZ. Field soil moisture contents and dry bulk densities in the top 5 cm were obtained from 51 different sampling sites in the watershed during a satellite overpass on July 25, 1994. One set of 22 points was characterized by a sandy soil, 3-15% slope, and vegetation dominated by shrubs. Another set of 29 sampling sites was characterized by a clayey soil, 0-3% slope, and covered by grasses and forbes. Soil roughness was measured at eight of these sites. The SAR image was georeferenced in the Universal Transverse Mercator (UTM) coordinate system and calibrated using a digital elevation model (DEM). In general, the correlation between volumetric soil moisture content and the radar backscatter signal was poor, but improved (r/sup 2/=0.78 and 0.41 for the first and second sets, respectively) when limited to sites with volumetric moisture contents higher than 10%. The low correlation was due to the dominant influence of soil roughness which was directly related to the proportion and size of rock fragments. Although roughness seems to explain much of the variance from the radar backscatter signal, it was not clear how best to parameterize the roughness properties. Continued research includes modeling and separating moisture and roughness influences.