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Impact of karstic aquifer on water balance in Mediterranean region

Plagnes Valérie

There are numerous karstic zones in the Mediterranean region. The functioning of karst aquifers is characterised by non linearity caused by physical heterogeneities in space and by dynamic variability. Karstic springs are characterized by a high infiltration, a rapid discharge variability and an important storage of water that contributes to support the summerflow.
A rainfall-discharge lumped model was developed at the scale of the karstic system and applied over 3 areas South of France. It was calibrated at the local scale, and gives very satisfactory results in terms of the simulation of the karstic spring flow. At larger scale, the SIM hydrometeorological model is applied over the whole France in order to monitor the water budget at a 8km resolution. At this scale, the karstic systems are badly represented.
The aim of this study is to compare the water balance of the karstic aquifer estimated by the large and fine scales models. The first comparison is made on 3 karstic systems that are part of 2 larger basins (Rhone and Garonne).
The comparison of the water budget of the 2 models shows that the large scale model tends to overestimate the evaporation of the karstic systems. This is consistent with the fact that evaporation processes are often lower on carbonate rocks because there are fast and direct infiltrations to the phreatic zone through swallow holes and fractures. However, it is not consistent with a tendency of the SIM model to overestimate the discharge of some Mediteranean basins. This can be due to the fact that karstic systems can have: 1) diffuse discharge towards a deeper aquifer or a sedimentary aquifer; or 2) non negligible "loss" of water into the sea for coastal aquifers (concentrated or diffuse submarine springs).
The generalisation of such approach should help to infer a subgrid scale parametrisation of the karstic area in the large scale modelling approach, and to better understand the impact of the karstic system at larger scale.