Xue et al. (2025) A study of the dependence between soil moisture and precipitation in different ecoregions of the Northern Hemisphere

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-10-22
• Authors: Shouye Xue, Guocan Wu
• DOI: 10.5194/hess-29-5575-2025

Research Groups

• State Key Laboratory of Earth Surface Processes and Disaster Risk Reduction, Faculty of Geographical Science, Beijing Normal University, Beijing, China

Short Summary

This study quantifies the spatiotemporal distribution and driving mechanisms of the nonlinear dependence between soil moisture and precipitation across different ecoregions of the Northern Hemisphere. It reveals that negative dependence is widespread, particularly in surface soil, and is primarily driven by land surface temperature and air temperature–gross primary production interactions, with distinct seasonal and annual patterns influenced by freeze–thaw cycles and long-term climate variability.

Objective

• To quantify the spatiotemporal distribution of the nonlinear dependence of soil moisture to precipitation.
• To identify the dominant factors influencing this dependence in different ecoregions of the Northern Hemisphere.
• To explore the driving mechanisms and regional patterns of the soil moisture-precipitation relationship.

Study Configuration

• Spatial Scale: Northern Hemisphere, encompassing 16 major terrestrial ecoregions. Data analyzed on a 0.25° × 0.25° grid resolution for three soil layers: 0 to 7 cm, 7 to 28 cm, and 28 to 100 cm.
• Temporal Scale: 2000 to 2019 (20 years), with analyses conducted at monthly, seasonal (spring, summer, autumn, winter), and annual scales.

Methodology and Data

• Models used:
  • Copula function: To establish the joint distribution between precipitation and soil moisture, capturing nonlinear and asymmetric dependence structures (e.g., Kendall’s tau, upper/lower tail dependence).
  • Ridge regression models: To quantify the relative influence of precipitation volume, precipitation frequency, and evapotranspiration on soil moisture, addressing multicollinearity.
  • Bayesian generalized non-linear multivariate multilevel models: To identify key driving factors (Gross Primary Production, Land Surface Temperature, Near-surface Air Temperature) influencing the precipitation–soil moisture coupling relationship.
• Data sources:
  • Soil moisture: ERA5-Land reanalysis data (European Centre for Medium-Range Weather Forecasts).
  • Precipitation: Global Precipitation Climatology Project (GPCP) data (integrating satellite and meteorological station observations).
  • Gross Primary Production (GPP): Vegetation Optical Depth Climate Archive v2 (microwave remote sensing estimates).
  • Near-surface air temperature (Ta): Climatic Research Unit gridded Time Series (CRU TS v4.07) (interpolated weather station observations).
  • Land Surface Temperature (LST): European Space Agency Climate Change Initiative (ESA CCI) (satellite data records).
  • Evapotranspiration (ET): Global Land Evaporation Amsterdam Model (GLEAM v3.8a).
  • Terrestrial Ecoregions: Conservation Biology Institute (global dataset based on biogeographical studies).

Main Results

• Nonlinear Negative Dependence:
  • Monthly scale: Negative dependence observed in 19.2 % (0-7 cm), 0.7 % (7-28 cm), and 2.3 % (28-100 cm) of grid cells. Most prominent in surface soil, distributed across tundra, montane grasslands, shrublands, deserts, xeric shrublands, and tropical/subtropical moist broadleaf forests.
  • Annual scale: Negative dependence observed in 3.0 % (0-7 cm), 4.0 % (7-28 cm), and 8.6 % (28-100 cm) of grid cells, primarily in arid regions like the southern Sahara Desert, Mongolia, and the Elizabeth Islands, expanding into montane grasslands and shrublands.
• Control of Soil Moisture:
  • Monthly scale: Precipitation volume exerted the strongest control (>40 % variation) in boreal forest/taiga, temperate grasslands, savannas, shrublands, and eastern North America. Evapotranspiration predominated in Alaska–Northwest Canada, western United States, Sahara Desert, and Middle East. Precipitation frequency influenced high-latitude regions (northern Canada).
  • Seasonal scale: Evapotranspiration dominated soil moisture control in spring and summer (>40 % of grid cells, nearly 80 % in middle soil layers). Precipitation volume had greater influence during autumn and winter.
  • Annual scale: Precipitation amount dominated across all three soil layers (>40 % of total area), consistent with monthly patterns. Precipitation frequency remained dominant in high-latitude areas (Greenland, northern Canada).
• Drivers of Negative Dependence:
  • Monthly scale: Primarily driven by interactions between Gross Primary Production and Land Surface Temperature (GPP:LST) and Air Temperature and Gross Primary Production (Ta:GPP). Also linked to freeze–thaw cycles, precipitation seasonality, and temperature fluctuations causing asynchronous changes.
  • Annual scale: Mainly observed in montane grasslands and shrublands, driven by the GPP:LST interaction across all three soil layers. Associated with long-term changes in precipitation and temperature affecting vegetation and soil water capacity.

Contributions

• Provides a comprehensive spatiotemporal quantification of the nonlinear dependence between soil moisture and precipitation across diverse ecoregions and soil depths in the Northern Hemisphere, moving beyond traditional linear analyses.
• Identifies the dominant biophysical and climatic factors (LST, Ta:GPP interactions, freeze-thaw processes, precipitation seasonality) driving negative soil moisture-precipitation dependence at both monthly and annual scales.
• Enhances the understanding of complex land–atmosphere interactions and the mechanisms underlying counterintuitive negative correlations, offering a valuable basis for future research on drought, hydrometeorology, and ecological conservation.
• Utilizes advanced statistical methods (copula functions for nonlinear dependence and Bayesian models for attribution) to address existing gaps in understanding the heterogeneity and driving forces of these relationships.

Funding

• National Key Research and Development Program of China (grant no. 2022YFF0801302)
• National Natural Science Foundation of China (grant nos. 41930970 and 42077421)

Citation

@article{Xue2025study,
  author = {Xue, Shouye and Wu, Guocan},
  title = {A study of the dependence between soil moisture and precipitation in different ecoregions of the Northern Hemisphere},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-5575-2025},
  url = {https://doi.org/10.5194/hess-29-5575-2025}
}