Abstract Accurate quantification of the rootzone soil moisture ( θ ) dynamics and hydrologic thresholds is important to advance sustainable agricultural water management and drought monitoring. Various low‐pass filters are popularly used to generate a first‐order approximation of θ , referred to here as , from temporally smoothed and lagged values of surface (5 cm) soil moisture ( θ 5 ). However, this method does not identify the critical thresholds and governing controls of θ needed to physically interpret values. We introduce a state‐based low‐pass filter, governed by a pseudo‐diffusivity () characteristics function, to represent surface–rootzone hydrologic connectivity and capture the nonlinear redistribution of soil moisture as a function of the prevailing hydrologic state of the rootzone. We propose a non‐linear (sigmoidal) − relationship as an emergent signature of soil and climate controls on surface‐rootzone soil hydrological connectivity, represented with three parameters − saturation (proxy) moisture content ( s ), residual (proxy) moisture content ( r ), and a critical pseudo‐diffusivity coefficient ( c ). We observe that s and r increase with clay fraction, while c is predominantly influenced by climate, with humid conditions showing higher values of c . We further demonstrate the estimation of rootzone plant water stress using θ 5 by leveraging s and r as critical soil hydrologic thresholds. The proposed approach is tested on in situ observations from 200 US Climate Reference Network and the Soil Climate Analysis Network sites. The physical basis and flexibility of this framework establish it as a scalable, data‐driven approach for improving predictions of the space‐time contiguous dynamics of θ , with applications in irrigation scheduling and agricultural drought monitoring.
Adekilae et al. (2026) studied this question.