Rootzone Soil Moisture Dynamics Using Terrestrial Water‐Energy Coupling

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 ( 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 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 S RZ compares well against SMAP Level‐4 estimates and in‐situ θ RZ , at the corresponding scale. The instantaneous hydrologic state ( 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 and agrees well with established drought metrics.