Fine-Resolution Measurement of Soil Moisture From Cumulative InSAR Closure Phase

Soil moisture can vary spatially at the scale of agricultural fields (~10–100 m), which is generally too fine to resolve using passive radiometric methods. Active radar provides an opportunity for finer resolution measurements; in particular, the interferometric synthetic aperture radar (InSAR) closure phase parameter is sensitive to changing soil moisture. We have developed a model showing that a systematic nonzero closure phase can result from scattering from objects at different depths in a medium of time-varying dielectric, such as from changes in soil moisture. The model predicts that interference between surface and subsurface reflections is needed for the closure phase to be nonzero. We find that, under certain circumstances, we can estimate soil moisture from the closure phase using a data reduction approach that includes a cumulative sum of the closure phase over time and the removal of a trend. The correlation between the cumulative closure phase and soil moisture suggests that the closure phase is related to the change in soil moisture. We examine a large test region in Oklahoma, where the detrended cumulative closure phase from Sentinel-1 data demonstrates some agreement with in situ soil moisture measurements. In other areas, the match is weaker, implying a terrain dependence on the quality of fit. The cumulative InSAR closure phase promises to provide a valuable new method to remotely estimate soil moisture.