Hydrology of the Upper-Garonne basin (Valle de Arán, Spain)

The runoff in the Valle de Arán, which is a steep and high mountain basin in the Pyrenees, consists essentially of baseflow, and fast runoff is only a minor phenomenon. The baseflow is related to areas of scree and forest, and the percentage of the precipitation which contributes to the fast runoff, is related to the area of barren rocks. The orographic precipitation pattern has been statistically determined for eight valleys. It was found, that orographic precipitation increases linearly with altitude, while the maximum increase occurs perpendicular to the slopes of the steep mountain ranges. The expected errors, due to measurements of rainfall at 1.50 m above ground level, showed as a 10—15 percent error in the water balance. The Penman evaporation values were corrected for snow evaporation and they subsequently gave reliable results.