Water balance components of the Pyrenees: A 30-year modelling study in a transboundary context

The Pyrenees, stretching from the Atlantic to the Mediterranean, constitute a major water tower for southwestern Europe. Steep climatic and altitudinal gradients generate heterogeneous hydrological regimes that sustain downstream rivers, reservoirs, and irrigation systems across France and Spain. We reconstructed the regional water balance for the historical baseline 1981–2010 using two contrasting hydrological models: SASER, a fully distributed physically based model, and SWAT, a semi-distributed conceptual model. Both were forced with a shared high-resolution meteorological dataset, enabling consistent comparison of water balance components. Outputs were evaluated against MODIS-based evapotranspiration and snow products to assess spatial and temporal performance beyond discharge calibration. reveal strong hydroclimatic gradients. Precipitation exceeds 2000 mm/yr in Atlantic headwaters but declines below 1000 mm/yr in southern and eastern foothills. Snowfall and snowmelt dominate high central and western ranges, sustaining summer flows. Evapotranspiration (400–800 mm/yr) is the largest loss flux and shows a northwest–southeast gradient; rising potential evapotranspiration and declining ET/PET ratios indicate a shift toward water-limited conditions. Groundwater recharge is modest (<150 mm/yr) and declining, suggesting reduced baseflow resilience. Water yield peaks near 1500 mm/yr in the humid northwest but falls below 200 mm/yr in the arid southeast, with a slight downward trend. Notably, the relative contribution of snow to runoff has increased, reflecting declining rainfall-driven runoff rather than enhanced snowmelt. Broad-scale hydrological controls emerge robustly across models, yet discrepancies with MODIS highlight structural and observational uncertainties. Together, the findings establish the first integrated, transboundary hydrological baseline for the Pyrenees, providing a foundation for cooperative water management under global change. The Pyrenees act as a water tower for southwestern Europe, with strong west–east precipitation gradients, snowmelt sustaining spring and early summer flows, and evapotranspiration as the dominant water loss. Over 1981–2010, potential evapotranspiration increased and the ET/PET ratio declined, indicating a shift toward more water-limited conditions. While both SWAT and SASER reproduce the main climatic patterns, they diverge in flux partitioning, highlighting robust hydrological signals but also persistent structural uncertainty. • Basin-wide Pyrenean water balance reconstructed for 1981–2010. • Strong west–east and north–south gradients in precipitation and water yield. • Snowmelt sustains spring flows; its relative share has increased. • Evapotranspiration is the largest flux, with growing water limitation. • Models agree on patterns but diverge in flux partitioning.