An Assessment of Soil Moisture Effects on C- and L-band InSAR Observables

The use and availability of interferometric synthetic aperture radar (InSAR) data to characterize surface displacements has grown tremendously over the past decade. With the publicly accessible data, low-latency, and large regional coverage of ESA's Sentinel-1 and the upcoming NASA-ISRO SAR (NISAR) missions, the utility of InSAR for investigating a diverse range of geophysical processes will only expand. However, in order to resolve subtle changes due to actual surface motion, there is a need to correct for non-deformation components of phase. While atmospheric phase delays and orbital errors are well documented, the effects of changing soil moisture on InSAR observables are poorly studied and rarely accounted for. Using independent datasets, such as NASA's Soil Moisture Active Passive (SMAP) mission and the North American Land Data Assimilation System (NLDAS) products, we investigate systematic relationships in time and space between soil moisture and InSAR phase differentials. Specifically, we estimate surface displacements over southern California using both C-band Sentinel-1 and L-band ALOS-2 data, each corrected to minimize nuisance signals. We then evaluate the dissimilarities between the different wavelength missions and with independent continuous Global Positioning System (cGPS) daily position data. We assess spatiotemporal correlations between soil moisture estimates and interferometric displacement error residuals to develop empirical relationships between changes in soil moisture and observed phase change specific to radar wavelength.