Assessing the water balace in the high mountain mediterranean karst aquifer of Ordesa and Monte Perdido (Central Pyrenees)

High mountain karst aquifers generate important natural water resources that are used in the low zones to satisfy the demand of both the users depending on this resource and the existing downstream ecosystems. These hydrological systems are highly vulnerable, especially those located in the drought-prone Mediterranean area, where climate change is expected to have a significant negative effect on water availability. Despite of their importance, most of these hydrogeological systems are still vaguely characterized. The Ordesa and Monte Perdido National Park (PNOMP) is located in the Southern Pyrenees and it constitutes the highest karst system in Western Europe. It is formed by upper Cretaceous and lower Paleocene-Eocene fissured and karstified limestones. The recharge zones are mainly located between 2500 m and 3200 m a.s.l., where snow accumulates and remains almost half of the year. During the spring, snow-melt water infiltrates though the well-developed karst system and recharges the aquifer. Groundwater discharges through a large number of springs that naturally drain the aquifer. This work is twofold: (1) to estimate the terms of a first water mass balance of the Ordesa and Monte Perdido hydrogeological system, highlighting the uncertainties and weaknesses of the estimated terms of the mass balance, and (2) to delineate the conceptual model of the aquifer system behavior