Relationships among soil moisture at various depths under varying climate conditions and heterogeneous land cover and soil types

Authors: LI, NAN - University Of California, Riverside; Skaggs, Todd; ELLEGAARD, PETER - Aquaspy, Inc; BERNAL, ANDRES - Aquaspy, Inc; SCUDIERO, ELIA - University Of California, Riverside

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2024
Publication Date: 7/7/2024
Citation: Li, N., Skaggs, T.H., Ellegaard, P., Bernal, A., Scudiero, E. 2024. Relationships among soil moisture at various depths under varying climate conditions and heterogeneous land cover and soil types. Science of the Total Environment. 947,174583. https://doi.org/10.1016/j.scitotenv.2024.174583.
DOI: https://doi.org/10.1016/j.scitotenv.2024.174583

Interpretive Summary: Satellite remote sensing is increasingly used to monitor soil moisture in agricultural lands. However, the penetration depth of remote sensors is relatively shallow (a few inches) which greatly limits their utility as a tool in agricultural management and other applications where the amount of moisture in the whole plant root zone is of interest. In this study, the possibility of extrapolating surface soil moisture measurements to deeper depths was investigated using daily soil moisture measurements from 5,104 locations in agricultural lands across the contiguous United States. Based on statistical correlation analyses of the moisture contents in different soil layers, it was found that extrapolation of surface moisture to depths of less than 16 inches is most promising. However, the influence of climate, land cover, and soil types on the strength of relationships between surface and lower layers makes the prediction difficult. This work will be of interest to water resource managers and researchers seeking to extract the maximum possible information from satellite remote sensors.

Technical Abstract: Soil moisture is an important component of the hydrological cycle and a key mediator between land surface and atmospheric interactions. Although substantial progress has been made in remote sensing of soil moisture at different spatial scales, the shallow penetration depth of remote sensors greatly limits their utility for applications in meteorological modeling and hydrological studies where the critical variable of interest is the root-zone soil moisture content. Therefore, this study assesses the relationship between soil moisture at the surface (10 cm) and in lower soil layers (20, 40, 60, 80, 100, and 120 cm) under varying climates, soils, and vegetation types. Cross-correlation analysis is applied to daily in-situ soil moisture measurements from 5,104 locations in agricultural lands across the contiguous United States. Our analysis demonstrates that zero-day lag always produced the highest correlation between 10 cm soil moisture and soil moisture in the lower layers. In addition, a positive and strong relationship between 10 and 20 cm soil moisture (r = 0.84) was observed, while the relationships between 10 and 40 cm soil moisture were moderate (0.52). The decline in cross-correlation continued to the deeper soil layers, which indicated that, on a daily timescale, the surface soil moisture gradually becomes decoupled with soil moisture at greater depths. Therefore, our research suggests that the estimation of soil moisture in the soil layers < 40 cm based on surface soil moisture is most promising. However, the influence of climate, land cover, and soil types on the strength of relationships between surface and lower layers makes the prediction difficult. Comparatively, both the relationship between precipitation and soil moisture (0.09 – 0.32) and the relationship between evapotranspiration and soil moisture (-0.19 – 0.18) were very weak for all soil layers.