Oduor et al. (2026) Future of Water Security in Mediterranean Reservoirs: Advancing SWAT + Modeling of Hydrological Response To Climate Change in Central Spain

Identification

• Journal: Earth Systems and Environment
• Year: 2026
• Date: 2026-02-01
• Authors: Brian Omondi Oduor, Silvia Martínez-Pérez, José Manuel Rodríguez-Castellanos, Alejandro Sánchez-Gómez, Eugenio Molina-Navarro
• DOI: 10.1007/s41748-026-01042-5

Research Groups

• Department of Geology, Geography, and Environment, Faculty of Science, University of Alcalá, Madrid, Spain

Short Summary

This study applied the SWAT+ model with an innovative multi-criteria calibration to simulate hydrological behavior and assess climate change impacts in two reservoir catchments in central Spain. Projections indicate significant declines in precipitation and water availability, coupled with increased potential evapotranspiration, leading to an accelerating transition towards an arid hydrological regime by the end of the century.

Objective

• To precisely set up and configure detailed, process-based hydrological models for the El Torcón and Picadas reservoir catchments using advanced SWAT+ capabilities such as reservoir operation and water transfer modules.
• To assess the potential impacts of climate change on their hydrological regimes using an ensemble of 11 regionally downscaled General Circulation Models (GCMs) under moderate (SSP2–4.5) and high (SSP5–8.5) emission scenarios.

Study Configuration

• Spatial Scale: El Torcón catchment (203 km²), Picadas catchment (approximately 2,000 km²), both located within the Tagus River basin in central Spain.
• Temporal Scale:
  • Historical baseline: 1981–2010
  • Model calibration: 2011–2015 (with a 5-year warm-up period)
  • Model validation: 2016–2020
  • Future projections: 2021–2100, subdivided into medium-term (2041–2070) and long-term (2071–2100)

Methodology and Data

• Models used:
  • Hydrological model: Soil Water Assessment Tool (SWAT+)
  • Climate models: Ensemble of 11 General Circulation Models (GCMs) from CMIP6 (ACCESS-CM2, CMCC-CM2-SR5, CNRM-ESM2-1, EC-Earth3-Veg, IITM-ESM, KACE-1-0-G, MIROC6, MPI-ESM1-2-HR, MRI-ESM2-0, NorESM2-MM, UKESM1-0-LL).
  • Calibration/validation package: R package SWATRunR.
• Data sources:
  • Topography (DEM): 25 m resolution, National Geographic Information Center (CNIG) Download Center.
  • Land use: 25 m resolution (2017), Spanish Land Use Information System (SIOSE) from CNIG.
  • Soil type: 250 m resolution, OpenLandMap database.
  • Meteorological (precipitation, temperature): Daily, 5 km grid resolution (1951–2022), Spanish Meteorological Agency (AEMET).
  • Weather generator: Monthly (1979–2013), Catholic University of Murcia (UCAM).
  • Reservoir data (inflow, outflow, storage): Daily (2004–2023), Integrated Streamflow Monitoring Network (SAIH-ROEA), CEH-CEDEX platform, and Tagus River Basin Authority.
  • Streamflow: Daily (2011–2020), Integrated Streamflow Monitoring Network (SAIH-ROEA), CEH-CEDEX platform.
  • Climate change (precipitation, temperature): Daily (1951–2100), AEMET, derived from IPCC AR6, regionally downscaled and bias-corrected using seasonal Quantile Delta Mapping (QDMs).
  • Calibration approach: Multi-criteria (combining soft/process-based metrics like runoff coefficient and groundwater contribution, and hard/statistical metrics like NSE, R², PBIAS, RMSE) and multi-variable (reservoir inflow, storage, streamflow).

Main Results

• Climate projections indicate a consistent trend of declining annual precipitation (6% to 23%) and increasing potential evapotranspiration (9.5% to 25%) across both catchments and emission scenarios.
• Actual evapotranspiration is projected to decline by 3% to 15% in El Torcón and 2% to 11% in Picadas due to reduced moisture availability.
• Long-term (2071–2100) projections under the high-emission SSP5–8.5 scenario show substantial reductions in water availability:
  • Reservoir inflows decline by 53% in El Torcón and 41% in Picadas.
  • El Torcón Reservoir storage decreases by 11% to 52%.
  • Streamflow in Picadas declines by 13% to 40% at Navaluenga and 17% to 49% at San Martín de Valdeiglesias.
  • Direct flow reduces by 20% to 51% in El Torcón and 14% to 44% in Picadas.
  • Groundwater flow reduces by 31% to 81% in El Torcón (though absolute changes are small) and 13% to 45% in Picadas.
• These changes signal an accelerating transition towards an arid hydrological regime, with El Torcón showing greater hydrological stress due to its arid baseline conditions.

Contributions

• First application of the SWAT+ model in Spain to simulate dynamic reservoir behavior while explicitly coupling it with future climate change projections, offering a novel modeling framework for managed river basins.
• Development and application of an innovative multi-criteria, multi-variable calibration technique that integrates soft (process-based) and hard (statistical) metrics, along with reservoir operations and water allocation modules, to ensure robust and realistic simulations of both natural and regulated flows.
• Provided site-specific, high-resolution insights into climate change impacts on water resources in vulnerable Mediterranean catchments, highlighting the amplifying role of baseline aridity and the necessity of localized assessments for adaptive water management.
• Demonstrated the effectiveness of SWAT+ as a tool for evidence-based decision-making and long-term water resource planning amidst climate uncertainty, particularly in water-stressed regions.

Funding

• Schweppes S.A. (ref. 2023/167)
• University of Alcalá PhD fellowship programmes (Alejandro Sánchez-Gómez)
• Community of Madrid PhD fellowship programmes (José Manuel Rodríguez-Castellanos)
• CRUE-CSIC agreement with Springer Nature (Open Access funding)

Citation

@article{Oduor2026Future,
  author = {Oduor, Brian Omondi and Martínez-Pérez, Silvia and Rodríguez-Castellanos, José Manuel and Sánchez-Gómez, Alejandro and Molina-Navarro, Eugenio},
  title = {Future of Water Security in Mediterranean Reservoirs: Advancing SWAT + Modeling of Hydrological Response To Climate Change in Central Spain},
  journal = {Earth Systems and Environment},
  year = {2026},
  doi = {10.1007/s41748-026-01042-5},
  url = {https://doi.org/10.1007/s41748-026-01042-5}
}