Changing rivers: Hydrological shifts in the Pyrenees revealed by daily streamflow indicators (1950–2019)

The Pyrenees mountain range, spanning Spain, France, and Andorra, constitute a critical freshwater source for southwestern Europe. Its rivers are highly sensitive to climate and land-use change, and their monitoring is essential for water resource management. This study provides the first region-wide assessment of streamflow trends across the Pyrenees based on daily flow records and derived hydrological indicators, moving beyond the traditional focus on monthly or annual discharge. Using daily data from 93 gauging stations, we analyse long-term changes in mean, low-, and high-flow indices at both annual and monthly scales over periods from 1950 to 2019. Results reveal a robust long-term decline in Pyrenean river flows. Since 1960, median daily discharge has decreased by 4–13% per decade, low flows by 5–14%, and high flows by 3–11%. Declines are strongest in spring, summer, and autumn, and include significant reductions in baseflows. In contrast, high-flow indicators show emerging increases in winter (January–February) since the 1990s, reflecting a transition from snow- to rain-dominated regimes. While the dominant signal is homogeneous across the mountain range, secondary north–south and east–west contrasts emerge seasonally. By capturing changes across the full flow duration curve, from low-flow percentiles to flood indicators, this study reveals not only the magnitude but also the temporal reorganisation of hydrological regimes in the Pyrenees. These trends are consistent with regional warming, snowpack loss, and reforestation, and imply reduced summer water availability, greater drought risk, and potential intensification of winter floods, underscoring the need for adaptive water management. Seasonal evolution of streamflow trends across the Pyrenees: monthly trend magnitude of quartiles (q25, q50, q75), inter-quartile range (iqr), 3-days baseflow (vcn3), and 20-years return period exceedance flow (rl20). Rings, from inner to outer, represent different epochs: 1950–2019, 1960–2019, 1970–2019, 1980–2019, 1990–2019, 2000–2019. Colors represent the sign and magnitude of median monthly Sen’s slopes (% change per decade). Widespread summer–autumn decreases that weaken toward the present, while February and January show consistent recent increases linked to warmer winters. • First region-wide assessment of streamflow trends in the Pyrenees (1950–2019) • Long-term declines in mean (–4 to –13%/decade), low (–5 to –14%), and high flows (–3 to –11%) • Strongest decreases in spring–autumn, especially September–October low flows • Emerging increases in winter high flows (January–February) since the 1990s • Trends linked to warming, reduced snowpack, and reforestation, with major water resource impacts