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Abstract:
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 ( ) ― 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 and evaporative fraction (EF) at remote sensing and field scale over the Contiguous U.S. We use θ_S from the Soil Moisture Active-Passive (SMAP) satellite and 218 in-situ stations, with EF from the Moderate Resolution Imaging Spectroradiometer. The estimated compares well against SMAP Level-4 estimates and in-situ θ_RZ, at the corresponding scale. The instantaneous hydrologic state (SRZ) vis-à-vis the WEC thresholds is proposed as a rootzone soil moisture stress index (SMSRZ) for near-real-time operational agricultural drought monitoring and agrees well with established drought metrics.