Hydrological effects of the increasing vegetation in the headwaters of the Spanish Pyrenees

Mediterranean mountains are key areas for providing water to their semiarid surrounding lowlands. Land abandonment and intense reforestation activities have caused a marked increase of vegetation in the headwaters of the Pyrenees during the 20th century. This research shows evidences on how this increase of vegetation has caused a severe reduction in the runoff generation that may not simply be explained by the recent climatic evolution. The reduction in water yield is already severely affecting to water resources availability and water management in the region. Scenarios for the future suggest that afforestation may continue or even be accelerated in the Pyrenees affecting mostly to the subalpine belt. Increasing vegetation around the current tree line is expected to continue enhancing evapotranspiration rates, but also to alter the accumulation and melting of the snowpack by increasing interception and producing an earlier melting onset. Such changes are likely to interact with the projected temperature warming for this region. Implications of the climate and land cover scenarios on water availability for the next decades are examined and discussed in depth.