Analysis of L-Band Radar Signatures of Surface Topography, Soil Moisture, and Vegetation Water Content

This paper presents a comprehensive analysis of L-band SAR backscatter (σ) from the Soil Moisture Active Passive (SMAP) mission with respect to soil moisture in terms of the real part of soil dielectric constant, vegetation water content (VWC), and surface topography represented by the standard deviation of slope (STD slope). We examine backscatter response across seven landcover types and four major regions globally, employing a two-dimensional (2D) binned framework to isolate the individual impacts of soil moisture and surface roughness under a range of VWC. Results show that radar backscatter increases substantially with the STD slope and soil moisture, with the strongest response under low VWC conditions. The radar backscatter also shows a general increase with VWC for VWC below 5 kg/m2. A piecewise linear function is used to model the relationship between STD slope and backscatter, as the rate of change is higher at lower STD slope ranges. Under low vegetation conditions (VWC < 0.25 kg/m²), the backscatter sensitivity to STD slope (∂σ/∂(STD slope)) is approximately 1 dB/° for STD slopes less than 8° and about 0.3 dB/° for slopes between 8° and 16°, while the sensitivity becomes negligible when VWC approaches 5 kg/m². We also utilize the semi-empirical water cloud model (WCM) of the vegetated soil surfaces to quantify the attenuation and volume scattering by vegetation. The results reveal an exponential decay of vegetation attenuation with increasing VWC and a nonlinear increase in volume scattering, consistent with prior theoretical and experimental studies.