Combined effect of climate change and groundwater abstraction on multi-layer and alluvial aquifers in France
Résumé
Although nowadays groundwater resources are mostly affected by increasing water abstraction, they will have to
face the impact of climate change. Even in a temperate climate as the one in northern France these two drivers
will have strong impact on the aquifers. Evolution of groundwater resource is most often studied based on
evolution of the rainfall infiltration that is assumed to be the aquifer recharge, although river-aquifer and multilayer
aquifer exchanges are also part of the recharge and can only be estimated using a hydrogeological model.
Moreover, providing a change of the piezometric level is usually not found to be meaningful by stakeholder as a 1
meter variation can have different impact depending on the hydrogeological context. Therefore, in this study, to
provide greater insight into the impact of groundwater, two new indicators are used: the evolution of the extension
of groundwater-fed wetlands and the frequency with which a crisis piezometric head is exceeded. The crisis
piezometric levels are used locally by stakeholders and as soon as the observed piezometric heads fall below these
levels, actions are taken to preserve the water resource.
For the present study, the local crisis piezometric heads were extended to the whole studied basins, the Seine and
Upper Rhine Graben basins.
The results show that the two basins under study are not affected in the same way by climate and groundwater
abstraction changes. The Seine multi-layer aquifers are projected to suffer from a substantial decrease in rainfall infiltration (about 20%), while the decrease is lower in the Upper Rhine Graben aquifer (about 6%).
In 2050, the extension of groundwater-fed wetlands is expected to lose about 900m2/km2, while the crisis
piezometric heads are expected to be exceeded about 40% and 26% of the time on average on the Seine and Upper
Rhine Graben aquifers, respectively. The reduction of the groundwater abstraction has a large impact locally but is
not efficient enough to overcome the impact of climate change, although the impact is more sensitive on the Upper Rhine Graben than on the Seine, and particularly on the extension of wetlands and low flow.
Sensitivity to the parameters that drive the river-aquifer exchange was found to be second order compared to the
uncertainty due to the climate model in the Upper Rhine Graben, although these parameters have a considerable impact on present-day estimation of the aquifer balance. The two indicators are found to be highly useful to discuss the impact of climate change on groundwater with stakeholders.
In order to be able to extent such studies to the whole aquifers of France, and to have a survey of such impact as soon as new downscaled projections are available, the AquiFR project is bringing together hydrogeological
applications used by stakeholder in a common plateform that will be based at Météo-France