Thonon-les-Bains is located in the French Alps, on the shores of Lake Geneva. The town has several groundwater resource tapping facilities at elevations ranging from 390 m to about 600 m (with a watershed elevation of up to about 1400 m). These resources differ in terms of their yield, their vulnerability to pollution, their ability to supply by gravity the whole town or only certain districts, and their long-term investment requirements.
To ensure a sustainable water supply for the town, a master plan has been developed. One of its objectives was to define an optimum groundwater resources operating scheme (defining the resources to be kept, developed or abandoned).
Within this framework, and as amortization of long-term investments has to be taken into consideration, the impact of climate change on the groundwater resources had to be assessed.
While significant developments have been made in assessing the impacts of various global economic growth scenarios on climate change, fewer works have examined the impacts of climate change on groundwater resources. This is due notably to the fact that the link between climate and groundwater resources depends on local parameters. Consequently, a long-term decrease in rainfall (depending on the global economic growth scenario considered) cannot be carried over to groundwater resources.
In the present case, a lumped groundwater model was developed using the Gardenia simulation code (BRGM) for the Blaves catchment supplying Thonon-les-bains. The model was calibrated on groundwater abstraction flow rates measured over a 6-year period using measured rainfall and potential evapotranspiration time series as input parameters. The gap between measured and simulated values was assessed in order to evaluate the reliability of the groundwater model. The IPCC Fifth Assessment Report RCP 8.5 scenario was then considered. Local monthly long-term (2070-2100) rainfall and evapotranspiration values were then used in the groundwater model. While the results showed that long-term rainfall was 21% lower on average compared to the present state, groundwater resources were estimated to be 25% lower on average (annual volume) and up to 45% lower during low-water periods. These two values (long-term average and low-water groundwater flows) were then used to develop an appropriate groundwater resources operating scheme.