Using hydrodynamic modeling for an assessment of climate change impact on a covered karst system. Example of the Moulineaux spring (Dordogne, France)

Undercover karst are characterized by limestone formations underneath a variability thick, low-permeability cover. Karst landforms such as sinkholes or swallow holes are thus not very frequent in these environments. This leads to a high inertia of the environment. In the current context of climate change, there is a growing interest in the impact on water resources in karst systems. While several scientific studies have modeled these impacts, most of them focus on outcropping karst system. Research on undercover karst remains limited, primarily due to a high level of inertia of these environments. It represents a challenge for the application of conventional tools and methods usually employed to characterize a system and complicating the interpretation of usual chemical methods to understand the role of the cover karst system

The Moulineaux spring is an example of a covered system. It is a key resource for the urban area of Perigueux (France) by ensuring the supply of drinking water to more than 60,000 inhabitants. It’s average flow rate is 820 L.s^-1 and can range between 118 L.s^-1 and 4 000 L.s^-1. The karstic system is mostly covered by a thick semi-permeable layer of alternating marly limestone, alterite rocks and sediments dating from the Campanian period (Upper Cretaceous). Its sizeable catchment area spans more than 80 km² more than 50% of which is occupied by agricultural activities.

Following continuous monitoring of spring flows and rainfall since 2011, modelling of the Moulineaux karstic system was carried out using various existing modelling software packages, such as KarstID and KartsMod. A conceptual three-reservoir model was used to represent the karst system of the Moulineaux. The model consists of an epikarst, a cover, a matrix reservoir, and a bypass. Achieving a correlation of over 85% according to the Nash coefficient, this model appears to be the most representative of the real system. Based on this model, several scenarios of climate change set up by the “DRIAS portal” in the coming years were applied. The results obtained with the most likely future scenario (RCP 8.5) show a stabilization of the average flow over 100 years, but greater variability in the flows throughout the year. The results enable better management and protection of these karstic hydrosystems. In the future, the goal is to apply this approach to hydrochemical modeling.