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 (