Rootzone Soil Moisture Dynamics Using Terrestrial Water-Energy Coupling

A lack of high-density rootzone soil moisture ( θ ) observations limits the estimation of continental-scale, space-time contiguous θ dynamics. We derive a proxy of daily θ dynamics ― active rootzone degree of saturation ( S ) ― by recursive low-pass (LP) filtering of surface soil moisture ( θ ) within a terrestrial water-energy coupling (WEC) framework. We estimate the LP filter parameters and WEC thresholds for the piecewise-linear coupling between S and evaporative fraction (EF) at remote sensing and field scale over Contiguous U.S. We use θ from Soil Moisture Active-Passive (SMAP) satellite and 218 in-situ stations, with EF from Moderate Resolution Imaging Spectroradiometer. The estimated S shows high validation skill against SMAP Level-4 estimates and in-situ θ , at the corresponding scale. The instantaneous hydrologic state of S vis-à-vis the WEC thresholds is proposed as a rootzone soil moisture stress index (SMS) for near-real-time operational agricultural drought monitoring― demonstrating high agreement with established drought metrics.