Zabaleta et al. (2026) Changing rivers: Hydrological shifts in the Pyrenees revealed by daily streamflow indicators (1950–2019)

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2026
• Date: 2026-03-27
• Authors: Ane Zabaleta, Leticia Palazón, Iñaki Antigüedad, Santiago Beguería
• DOI: 10.1016/j.ejrh.2026.103374

Research Groups

• Hezkuntza, Filosofia eta Antropologia Fakultatea, Euskal Herriko Unibertsitatea (EHU), Donostia, Spain
• Estación Experimental de Aula Dei (EEAD), Consejo Superior de Investigaciones Científicas (CSIC), Zaragoza, Spain
• Zientzia eta Teknologia Fakultatea, Euskal Herriko Unibertsitatea (EHU), Leioa, Spain

Short Summary

This study provides the first region-wide assessment of streamflow trends across the Pyrenees using daily flow records from 93 gauging stations (1950–2019). Results reveal a robust long-term decline in Pyrenean river flows, with a seasonal redistribution showing declines in spring, summer, and autumn, and emerging increases in winter high flows, reflecting a transition from snow- to rain-dominated regimes.

Objective

• Provide the first region-wide assessment of streamflow trends across the Pyrenees using daily discharge records from natural reference gauging stations in Spain, France, and Andorra.
• Characterise long-term changes in the full distribution of daily flows (mean, low, high, and variability indices) at both annual and monthly scales.
• Assess the temporal evolution of these trends across multiple analysis windows (1950–2019 through 2000–2019) and their spatial coherence across the mountain range.

Study Configuration

• Spatial Scale: The Pyrenees mountain range, spanning Spain, France, and Andorra, with a total surface area of approximately 21,000 square kilometres. It extends for about 430 kilometres in longitude and 150 kilometres in latitude, with elevations ranging from near sea level to 3,404 metres.
• Temporal Scale: Daily streamflow records analysed over multiple periods from 1950 to 2019, including specific windows: 1950–2019, 1960–2019, 1970–2019, 1980–2019, 1990–2019, and 2000–2019.

Methodology and Data

• Models used:
  • Non-parametric Mann–Kendall test for trend detection.
  • Sen’s slope estimator for trend magnitude quantification.
  • Yue–Pilon method for pre-whitening to reduce serial autocorrelation.
  • Non-stationary Generalised Extreme Value (GEV) distribution for estimating the 20-year return period flood index (rl20).
  • s-mode and t-mode Principal Component Analysis (PCA) for spatial and seasonal pattern identification.
• Data sources:
  • Daily streamflow records from 93 natural or quasi-natural gauging stations within the Pyrenean region.
  • Data compiled from official hydrological networks of various water agencies: Confederación Hidrográfica del Cantábrico (CHC), Confederación Hidrográfica del Ebro (CHE), Agència Catalana de l’Aigua (ACA), Agencia Vasca del Agua / Ur Agentzia (URA), Diputación Foral de Gipuzkoa (DFG), Diputación Foral de Bizkaia (DFB), Gobierno de Navarra (GN), and Eaufrance.

Main Results

• Long-term Decline (1950–2019, 1960–2019): A robust, region-wide decline in Pyrenean river flows. Median daily discharge (q50) decreased by 4–13% per decade, low flows (q10, q25) by 5–14% per decade, and high flows (q75, q90) by 3–11% per decade. Baseflows (vcn3) also showed significant reductions.
• Weakening Negative Trends (post-1970): The percentage of significant negative trends decreased in shorter analysis windows, with some positive trends emerging, though not regionally consistent.
• Emerging Winter High Flows: The flood index (rl20) showed robust and significant increases in recent periods (1990–2019: +35% per decade; 2000–2019: +71% per decade), particularly in winter (January–February), where some high-flow indicators increased by over 100% per decade.
• Seasonal Redistribution: Declines were most pronounced in spring, summer, and autumn, especially in October, where most indices (q10–q90, vcn3) consistently decreased. Mean, low, and high flows dropped by 8–13% per decade from May to November, with baseflows decreasing by 9–11% per decade.
• Spatial Heterogeneity: While a homogeneous Pyrenean-wide decline is the dominant signal, secondary north–south and east–west contrasts emerged. Winter increases in high flows were stronger on northern slopes, while summer declines were more evident in the southern Pyrenees.

Contributions

• Provides the first region-wide assessment of streamflow trends in the Pyrenees using daily discharge records, moving beyond traditional monthly or annual analyses.
• Reveals not only the magnitude but also the temporal reorganisation of hydrological regimes by capturing changes across the full flow duration curve, from low-flow percentiles to flood indicators.
• Offers a detailed view of how the full distribution of daily flows (mean, low, high, and variability) has evolved over time, detecting long-term shifts in total runoff and changes in flow regime, variability, and extremes.
• Systematically identifies dominant common patterns of spatial variability across the Pyrenean river network using Principal Component Analysis, distinguishing region-wide trends from secondary north–south and east–west contrasts.

Funding

• Project EFA210/16/PIRAGUA, co-funded by the European Regional Development Fund (ERDF) through the Interreg V Spain-France-Andorre Programme (POCTEFA 2014–2020) of the European Union.

Citation

@article{Zabaleta2026Changing,
  author = {Zabaleta, Ane and Palazón, Leticia and Antigüedad, Iñaki and Beguería, Santiago},
  title = {Changing rivers: Hydrological shifts in the Pyrenees revealed by daily streamflow indicators (1950–2019)},
  journal = {Journal of Hydrology Regional Studies},
  year = {2026},
  doi = {10.1016/j.ejrh.2026.103374},
  url = {https://doi.org/10.1016/j.ejrh.2026.103374}
}