Sensitivity Analysis of C- and Ku-Band Synthetic Aperture Radar Data to Soil Moisture Content in a Semiarid Region.

Abstract

In this study, the sensitivity of the C-band (5.3 GHz) with a 23° incidence angle and the Ku-band (14.85 GHz) with 35°, 55°, and 75° incidence angles to surface soil moisture content from a semiarid region were evaluated. To obtain an improved soil moisture estimation, a practical technique to reduce the influence of soil roughness and vegetation in the SAR data was developed in a study area located in the Walnut Gulch Experimental Watershed, a representative site of shrub- and grass-dominated rangelands of the southwestern part of the United States. To correct for soil roughness effects, the C-band radar backscattering coefficients σ° from a wet season image were subtracted from a° derived from a dry season image. The assumption was that, in semiarid regions, the SAR data from the dry season was dependent only on the soil roughness effects. To correct for vegetation effects, an empirical relation between σ° and leaf area index was used, the latter derived from Landsat Thematic Mapper data. The results showed that when both soil roughness and vegetation effects were corrected for, the sensitivity of a° to soil moisture improved substantially. The sensitivity of σ° to soil moisture was also evaluated in agricultural fields with bare soil and periodic roughness components (planting row and furrow structures). Four types of SAR system configurations were analyzed: C-band with a 23° incidence angle and Ku-band with 35°, 55°, and 75° incidence angles. The test sites were located at the University of Arizona's Maricopa Agricultural Center, south of Phoenix, Arizona. The results showed that the sensitivity of σ° to soil moisture was strongly dependent upon the field conditions. The SAR signals were nearly insensitive to soil moisture for furrowed fields (furrow spacing ∼ 95 cm; amplitude ∼ 22 cm), but for fields with planting row structures (row spacing ∼ 24 cm; amplitude ∼ 2 cm), the SAR data were sensitive to soil moisture, particularly with the C-band at a 23° incidence angle and the Ku-band with a 35° incidence angle, regardless of the row direction.