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