Rodríguez-Castellanos et al. (2026) Disentangling the hydrological implications of the climate 80s effect and water transfers in a large Mediterranean river

Identification

• Journal: Environmental Earth Sciences
• Year: 2026
• Date: 2026-01-07
• Authors: José Manuel Rodríguez-Castellanos, Silvia Janeth Martínez-Pérez, Alejandro Sanchez-Gomez, Eugenio Molina-Navarro
• DOI: 10.1007/s12665-025-12762-8

Research Groups

• Department of Geology, Geography and Environment, Faculty of Sciences, University of Alcalá, Alcalá de Henares, Madrid, Spain
• Department of Civil & Environmental Engineering, University of California, Davis, CA, USA

Short Summary

This study disentangles the hydrological impacts of the "climate 80s effect" and water transfers on the Tagus River, revealing that climate change-driven reductions in precipitation and increased temperature, combined with the Tagus-Segura Water Transfer, have led to a 76% decrease in middle Tagus streamflow and severe water quality degradation.

Objective

• To disentangle the hydrological implications of the "climate 80s effect" and the Tagus-Segura Water Transfer (TSWT) on water availability and quality in the middle reaches of the Tagus River.

Study Configuration

• Spatial Scale: Tagus River headwaters (approximately 7,300 km²) and its middle reaches, including the Entrepeñas-Buendía reservoir system and downstream areas affected by the Jarama and Guadarrama river confluences (Madrid metropolitan area).
• Temporal Scale: Hydrological years from 1951/52 to 2021/22 for climate data, 1954/55 to 2019/20 for reservoir inflows and streamflow, and 2013–2020 for water quality data.

Methodology and Data

• Models used: Not explicitly stated for this study's analysis, primarily data-driven statistical analysis.
• Data sources:
  • Meteorological data: 5 x 5 km resolution gridded data interpolated by the State Meteorological Agency (AEMET) for temperature and precipitation.
  • Hydrological data: Monthly inflows to Entrepeñas and Buendía reservoirs from the Public Works Studies and Experimentation Centre (CEDEX) gauging yearbook. Daily streamflow records from CEDEX database (Embocador and Aranjuez gauging stations).
  • Water transfer data: Volumes transferred through the TSWT from Confederación Hidrográfica del Segura (CHS) and CEDEX.
  • Water quality data: Total nitrogen (TN) and total phosphorus (TP) from the Tagus River Basin Authority (CHT) and a field survey by the research team. Nitrate (NO₃⁻) and phosphate (PO₄³⁻) data from the ecological status control (CEMAS) network of the Tagus River Basin Authority.

Main Results

• Since the 1980s, the Tagus headwaters have experienced a 16% reduction in average annual precipitation (from 716 mm to 602 mm) and a 1.0 °C increase in average annual temperature (from 10.5 °C to 11.5 °C).
• These climatic changes have led to a 48% decline in annual inflows to the Entrepeñas-Buendía reservoir system, decreasing from an average of 1435 x 10^6 m³/yr (pre-1980) to 746 x 10^6 m³/yr (post-1980).
• The Tagus-Segura Water Transfer (TSWT) has diverted an average of 332.3 x 10^6 m³/yr, representing 45% of the average inflow to the headwaters reservoir system during its first 40 years of operation.
• The combined effect of climate change and water transfers has resulted in a 76.2% reduction in streamflow in the middle reaches of the Tagus River (Aranjuez-Embocador), dropping from 1160 x 10^6 m³/yr (pre-1980) to 317 x 10^6 m³/yr (post-1980).
• Average daily flow at Aranjuez has decreased from 35 m³/s to 8.5 m³/s, leading to a near-complete loss of the river's natural interannual variability.
• Water quality in the middle Tagus has severely degraded, with total nitrogen and total phosphorus concentrations quadrupling downstream of the Jarama River confluence, as the reduced flows cannot dilute pollutants from the Madrid metropolitan area's treated wastewater.

Contributions

• Provides a novel and holistic perspective by combining a detailed analysis of climatic trends, reservoir inflows, transferred volumes, and downstream flow dynamics over an extended temporal horizon (1954/55–2019/20).
• Analyzes the evolution of nutrient concentration along the Tagus River, with special emphasis on the middle reaches affected by the Madrid metropolitan area, contributing to the knowledge on water quantity-quality interactions.
• Identifies the main drivers of current unfavorable hydrological conditions and highlights the urgent need for robust adaptation measures and efficient water management strategies.

Funding

• Department of Education, Culture and Sports of Castilla-La Mancha Government and the European Union (IMPACT Project, SBPLY/21/180501/000221).
• Department of Economy, Business and Employment of Castilla-La Mancha Government (CEAGU, 2023/00029/001).
• University of Alcalá (UAH) PhD Fellowships Programme.
• Community of Madrid PhD Fellowships Programme.

Citation

@article{RodríguezCastellanos2026Disentangling,
  author = {Rodríguez-Castellanos, José Manuel and Martínez-Pérez, Silvia Janeth and Sanchez-Gomez, Alejandro and Molina-Navarro, Eugenio},
  title = {Disentangling the hydrological implications of the climate 80s effect and water transfers in a large Mediterranean river},
  journal = {Environmental Earth Sciences},
  year = {2026},
  doi = {10.1007/s12665-025-12762-8},
  url = {https://doi.org/10.1007/s12665-025-12762-8}
}