Rasheed et al. (2026) Assessing Spatiotemporal Variability in Soil Moisture under a Changing Climate Using Hydrological Models in Hilly Landscapes

Identification

• Journal: Water Resources Management
• Year: 2026
• Date: 2026-01-01
• Authors: Muhammad Waseem Rasheed, Junfang Cui, Jialiang Tang, Abid Sarwar, Mostafa Moradzadeh, Muhammad Faheem
• DOI: 10.1007/s11269-025-04469-z

Research Groups

• Key Laboratory of Mountain Surface Processes and Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, China
• State Key Laboratory for Regional and Urban Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China
• University of Chinese Academy of Sciences (UCAS), Beijing, China
• Department of Civil and Environmental Engineering, University of California Merced (UCM), Merced, CA, USA
• Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), EFNO, Nogent-sur-Vernisson, France
• Department of Farm Machinery and Power, University of Agriculture, Faisalabad, Pakistan

Short Summary

This study utilized the HYDRUS-1D model, calibrated with field observations, to assess spatiotemporal soil moisture variability in a hilly agroforestry watershed in southwestern China under current and future climate scenarios (RCP 4.5 and RCP 8.5). The findings indicate that hilltop croplands are projected to experience more optimal soil moisture conditions, suggesting increased suitability for crop production in a changing climate, while lower slopes show higher variability.

Objective

• To quantify soil moisture variations at different depths and slope positions (hilltop, middle slope, lower slope) using field observations under natural rainfall conditions.
• To simulate and project soil moisture dynamics under current climate conditions and future scenarios (RCP 4.5 and RCP 8.5) from 2020 to 2050.
• To evaluate multi-year soil moisture responses in purple-soil croplands to inform sustainable agroforestry and water management in hilly landscapes.

Study Configuration

• Spatial Scale: Yanting agroforestry watershed, central Sichuan Basin, southwestern China (31° 16' N, 105° 28' E). Three cropland sites (hilltop, middle slope, lower slope) were studied, with plots measuring 20 m × 4 m on a 7° slope, with soil depths up to 0.8 m. Soil moisture was measured at 0.2 m, 0.4 m, and 0.6 m depths for hilltop and middle slopes, and at 0.17 m, 0.28 m, and 0.4 m for the lower slope.
• Temporal Scale: Field measurements were conducted from January to August 2015 and January to July 2017. The HYDRUS-1D model simulated soil moisture dynamics at five-year intervals from 2020 to 2050. Climatic data for the study site spanned 2005 to 2019.

Methodology and Data

• Models used: HYDRUS-1D (Simunek et al. 2008) was used to simulate vertical water flow and soil moisture dynamics in unsaturated soils, based on the Richards equation and van Genuchten-Mualem constitutive relationships. The Rosetta model (Schaap et al. 2001) was integrated to predict soil hydraulic parameters using pedo-transfer functions.
• Data sources:
  • Field Observations: Volumetric soil moisture content (SMC) was measured every 10 minutes using Decagon 5TM probes at various depths. Rainfall was measured with a tipping bucket rain gauge. Air temperature, relative humidity, wind speed, and solar radiation were collected from an automated meteorological station.
  • Soil Characteristics: The study site features purple soil (Entisol/Regosol) with an average texture of 21% clay, 52% silt, and 27% sand. Average soil pH was 8.3, organic matter 8.75 g/kg, and bulk density 1330 kg/m³. Saturated water permeability coefficient ranged from 1.67 × 10⁻⁶ to 1.68 × 10⁻⁵ m/s.
  • Climate Change Scenarios: Daily meteorological data (rainfall, maximum/minimum temperatures, wind speed, relative humidity, solar radiation) for RCP 4.5 and RCP 8.5 were obtained from the CMA-downscaled CMIP5 multi-model ensemble (including HadGEM2-ES, MPI-ESM-LR, GFDL-ESM2M, NorESM1-M, and BCC-CSM1.1).

Main Results

• Model Performance: The HYDRUS-1D model accurately simulated soil moisture dynamics. Calibration yielded R² values ranging from 0.87 to 0.94 and RMSE values up to 0.033 cm³/cm³. Validation showed R² values from 0.87 to 0.90 and RMSE values up to 0.017 cm³/cm³.
• Observed Soil Moisture Dynamics: Soil moisture variability generally followed the pattern: lower slope > middle slope > hilltop. Rainfall events exceeding 20 mm significantly increased soil moisture, while smaller events had minimal impact.
• Future Projections (2020-2050):
  • Both RCP 4.5 and RCP 8.5 scenarios predict an annual increase in soil moisture content (SMC) of 9% to 11% from 2030 to 2050 compared to the reference period.
  • Under RCP 4.5, projected annual SMC increases were 32% for hilltop, 5% for middle slope, and 0.1% for lower slope. Under RCP 8.5, these increases were 33%, 7%, and 1.2%, respectively.
  • Hilltop cropland is projected to have the highest number of days with optimal SMCs (e.g., 357 ± 1 days/year under RCP 8.5 in 2050), suggesting increased suitability for crop production.
  • Between 2020 and 2050, croplands on the hilltop, middle slope, and lower slope experienced 46, 140, and 217 days per year, respectively, when average SMCs exceeded field capacity.
  • Soil moisture content varied with depth (0.2 m to 0.6 m), exhibiting higher spectral and temporal variability at the surface compared to deeper soil layers.

Contributions

• This study provides a comprehensive assessment of spatiotemporal soil moisture variability in a subtropical mountain agroforestry watershed, specifically focusing on purple soils, under projected climate change scenarios (RCP 4.5 and RCP 8.5) using the HYDRUS-1D model.
• It extends the application of HYDRUS-1D to analyze rainfall-driven soil moisture dynamics across different slope positions (hilltop, middle slope, lower slope), addressing a gap in existing literature for such complex terrains.
• The findings offer crucial insights for irrigation water managers and hydrologists, highlighting the potential for hilltop croplands to become more suitable for crop production in the future and informing the development of sustainable agroforestry systems and water resource protection strategies in similar hilly landscapes.

Funding

• Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) under Grant No. 2023YFF0806002.
• Belt and Road Scholarship Council, China.

Citation

@article{Rasheed2026Assessing,
  author = {Rasheed, Muhammad Waseem and Cui, Junfang and Tang, Jialiang and Sarwar, Abid and Moradzadeh, Mostafa and Faheem, Muhammad},
  title = {Assessing Spatiotemporal Variability in Soil Moisture under a Changing Climate Using Hydrological Models in Hilly Landscapes},
  journal = {Water Resources Management},
  year = {2026},
  doi = {10.1007/s11269-025-04469-z},
  url = {https://doi.org/10.1007/s11269-025-04469-z}
}