González-Cao et al. (2026) Impact of dams on river regime and extreme flow events in MIÑO–SIL river basin (NW of the IBERIAN peninsula)

Identification

• Journal: CATENA
• Year: 2026
• Date: 2026-01-20
• Authors: J. González-Cao, D. Fernández-Nóvoa, N.G. deCastro, H. Barreiro-Fonta, M. deCastro, M. Gómez-Gesteira
• DOI: 10.1016/j.catena.2026.109836

Research Groups

• Centro de Investigación Mariña, Universidade de Vigo, Environmental Physics Laboratory (EphysLab), Campus da Auga, Ourense, Spain

Short Summary

This study evaluates the long-term impacts of dam regulation, precipitation variability, and land-use change on river flow regimes and extreme flood events in the Miño–Sil River Basin (NW Iberian Peninsula) from 1944 to 2023. It concludes that dam regulation is the primary driver of hydrological alterations, reshaping seasonal flows, decoupling streamflow from precipitation, and conditionally attenuating floods based on reservoir storage.

Objective

• To evaluate the respective roles of precipitation variability, land-use change, and dam regulation on river flow regimes and extreme flood events in the Miño–Sil River Basin (NW Iberian Peninsula).

Study Configuration

• Spatial Scale: Miño–Sil River Basin, specifically the upper reach from headwaters to the city of Ourense (NW Iberian Peninsula), with elevations ranging from 97 meters to approximately 2100 meters above sea level.
• Temporal Scale: Hydro-meteorological records spanning 1944–2023, analyzed across a quasi-natural phase (1944–1959) and a fully regulated phase (1995–2023).

Methodology and Data

• Models used: Not applicable (no specific hydrological or climate simulation models were run by the authors; ERA5 is a reanalysis dataset).
• Data sources:
  • Streamflow and Reservoir Storage: Automatic Hydrological Information System (SAIH) of the Confederación Hidrográfica Miño–Sil, and Anuario de Aforos of CEDEX.
  • Monthly Precipitation and Evapotranspiration: ERA5 reanalysis dataset (European Centre for Medium-Range Weather Forecasts - ECMWF, Copernicus Climate Change Service - C3S) at 0.25° x 0.25° spatial resolution.
  • Rain Gauge Precipitation (for extreme events): AEMET rain gauge stations.
  • Land-use data: Instituto Nacional de Estadística (INE), Instituto Galego de Estatística (IGE), and HILDA+ (Historic Land Dynamics Assessment+) dataset.
  • Data Completion: Streamflow at San Pedro was reconstructed by summing discharge from Monforte de Lemos (Cabe River) and Santo Estevo (Sil River), validated with Nash–Sutcliffe Efficiency (NSE), Root Mean Square Error–Standard Deviation Ratio (RSR), and Percent Bias (PBIAS).
  • Statistical Analysis: Kolmogorov–Smirnov (K–S) test for comparing monthly distributions, linear regression for trend analysis, and percentile-based thresholds for extreme event classification.

Main Results

• Land-use patterns in the Miño–Sil basin remained remarkably stable from the mid-20th century to the early 21st century, with Forest & Pastures & Grass consistently dominating (~93% of total area).
• Dam regulation was identified as the primary driver of long-term streamflow changes, overriding the relatively stable basin-scale precipitation and land-use patterns.
• In regulated river sections (San Pedro, Ourense), reservoirs significantly altered the seasonal flow regime, reducing winter discharges and enhancing summer flows, leading to a clear seasonal redistribution of streamflow and a marked decoupling between discharge and precipitation.
• Unregulated sections (Lugo) maintained a natural flow regime closely linked to rainfall variability.
• Mean annual streamflow showed a statistically significant positive trend at Lugo (7.79 x 10⁶ m³/year, p ≈ 3.40 x 10⁻⁴), while regulated stations (San Pedro, Ourense) exhibited non-significant negative trends, suggesting regulation dampens long-term signals.
• Annual precipitation trends were positive but not statistically significant at any site, with implied changes less than 1% of mean annual precipitation.
• The Kolmogorov–Smirnov test revealed statistically significant shifts in streamflow distributions during the dry season (July–October) at regulated stations, indicating a strong impact of dam operations under low-flow conditions.
• The effectiveness of flood mitigation by dams is strongly conditional on reservoir storage levels prior to an event.
  • The December 1959 flood, occurring under quasi-natural conditions, reached a peak discharge of 5700 m³/s, exceeding the "historical" threshold.
  • The 1978–1979 episode, despite being the second-highest accumulated precipitation event (approximately 1.080 meters), did not reach the severe streamflow threshold because major reservoirs (Belesar and Bárcena) collectively stored over 3.00 x 10⁸ m³ of water, attenuating the peak discharge.
  • The December 2000–March 2001 event, associated with the highest accumulated precipitation (approximately 1.150 meters), saw Belesar rapidly fill to near capacity, limiting its regulation potential during the peak.

Contributions

• Provides a detailed, long-term, event-based assessment of dam impacts on river flow regimes and extreme floods in the Miño–Sil River Basin, explicitly disentangling the roles of dam regulation, precipitation variability, and land-use change.
• Offers a unique case-by-case analysis of historical high-flow events, emphasizing the critical role of reservoir storage dynamics in flood attenuation, which has received limited attention in previous basin-scale assessments.
• Informs water resource management and supports climate resilience and flood mitigation strategies in similarly regulated river basins by highlighting the need to incorporate reservoir storage and operational constraints into flood-risk frameworks.

Funding

• Xunta de Galicia under project ED431C 2021/44 (Grupos de Referencia Competitiva).
• Interreg POCTEP program under project RISCPLUS (0031RISCPLUS6_E).
• D.F.-N. was supported by Xunta de Galicia through a postdoctoral grant (ED481D-2024-004).

Citation

@article{GonzálezCao2026Impact,
  author = {González-Cao, J. and Fernández-Nóvoa, D. and deCastro, N.G. and Barreiro-Fonta, H. and deCastro, M. and Gómez-Gesteira, M.},
  title = {Impact of dams on river regime and extreme flow events in MIÑO–SIL river basin (NW of the IBERIAN peninsula)},
  journal = {CATENA},
  year = {2026},
  doi = {10.1016/j.catena.2026.109836},
  url = {https://doi.org/10.1016/j.catena.2026.109836}
}