Publications

Sehgal, V; Mohanty, BP; Reichle, RH (2024). Rootzone Soil Moisture Dynamics Using Terrestrial Water-Energy Coupling. GEOPHYSICAL RESEARCH LETTERS, 51(19), e2024GL110342.

Abstract
A lack of high-density rootzone soil moisture (theta RZ) observations limits the estimation of continental-scale, space-time contiguous theta RZ dynamics. We derive a proxy of daily theta RZ dynamics - active rootzone degree of saturation (SRZ) - by recursive low-pass (LP) filtering of surface soil moisture (theta S) within a terrestrial water-energy coupling (WEC) framework. We estimate the LP filter parameters and WEC thresholds for the piecewise-linear coupling between SRZ and evaporative fraction (EF) at remote sensing and field scale over the Contiguous U.S. We use theta S from the Soil Moisture Active-Passive (SMAP) satellite and 218 in-situ stations, with EF from the Moderate Resolution Imaging Spectroradiometer. The estimated SRZ compares well against SMAP Level-4 estimates and in-situ theta RZ, at the corresponding scale. The instantaneous hydrologic state (SRZ) vis-& agrave;-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. Rootzone soil moisture plays a vital role in agricultural, hydrological, and ecosystem processes. The available spaceborne satellites for monitoring soil moisture can only capture variability in a shallow soil layer at the surface, typically limited to the top 5 cm. Hence, spatiotemporally continuous estimation of rootzone soil moisture dynamics typically relies on soil moisture estimates from land-surface models, which are subject to errors in the surface meteorological forcing data, process formulations, and model parameters. Some studies suggest that the rootzone soil moisture dynamics can be estimated by filtering the high-frequency variability in the surface soil moisture. However, such filters require observed rootzone data (often unavailable at high spatial density) for calibration. This study uses the relationship between surface soil moisture and evaporative fraction derived using spaceborne observations from the Soil Moisture Active Passive mission and the Moderate Resolution Imaging Spectroradiometer to estimate rootzone soil moisture dynamics for the Contiguous U.S. at 9 km grid resolution. We further demonstrate that this approach can be extended into a near-real-time agricultural drought monitor to assess drought impacts on vegetation using surface soil moisture observations. Terrestrial water-energy coupling is used to parameterize low-pass filter to estimate rootzone dynamics from surface soil moisture Rootzone degree of saturation and water-energy coupling thresholds are estimated using evaporative fraction and surface soil moisture SMAP-based rootzone degree of saturation can used for operational, near-real-time agricultural drought monitoring over Contiguous U.S

DOI:
10.1029/2024GL110342

ISSN:
1944-8007