Impact of Withdrawals on Karst Watershed Water Supply

Abstract

Karst systems consist of heterogeneous aquifers characterized by non-linear hydrogeological behavior. This is intrinsically linked to the coexistence of saturated versus unsaturated, open-channel versus closed-conduit flow and laminar versus turbulent flows within these aquifers. These multiple dualities together with a lack of knowledge of their internal structure lead to increasing difficulties in the management of groundwater resources related to karst aquifers. However, karst aquifers constitute strategic fresh water resources and many stakeholders carry out withdrawals sometimes continuously or centered on the period of low water level as for the irrigation. These withdrawals generate discharge decreases that can constitute sources of conflicts between upstream and downstream users of the resource. In this study, we propose a methodology to assess the impact of withdrawals on the spring discharge of a karst aquifer based on a conceptual non-linear reservoirs model. This methodology is applied to the second largest karst system in France: The Touvre karst system (La Rochefoucauld). The simulated influence of the total withdrawals on runoff deficit is larger than the total withdrawals in relationship with the non-linearity of the conceptual models. Globally, this impact is about one and a half of the total withdrawal depending of the year under consideration. Moreover, we show that it also implies a decrease from 10% to 20% of the low-flow annual discharge. The next step will be to also address the issues raised by the climate change projections.

Improvement of the KarstMod modeling platform for a better assessment of karst groundwater resources

“…These lasts are surface stream flow within metamorphic rocks that partly infiltrate to subterranean at the contact with sedimentary formations, mainly composed of Middle to Upper Jurrasic limestones. The springs of the Touvre, located 7 km east of Angoulême (western France), have three main outlets (the Bouillant, the Dormant and the Font de Lussac) and a secondary outlet (the Lèche) (Labat et al, 2022). In the following, the discharge of the four outlets are accumulated and named Touvre spring.…”

“…The area is characterized by an ocean influenced climate with a mean annual precipitation around 800 mm/year distributed over 255 rainy days. The estimation is performed with Thiessen polygon methods based on eleven meteorological stations over the area (Labat et al, 2022). The mean annual potential evapotranspiration is around 770 mm/year according to the Penman-Monteith estimation provided by the french meteorological survey (Météo-France).…”

“…The mean annual potential evapotranspiration is around 770 mm/year according to the Penman-Monteith estimation provided by the french meteorological survey (Météo-France). The Touvre spring discharge shows a significant variability ranging from 3 m 3 /s to 49 m 3 /s with a coefficient of variation around 0.46 (Figure 3 Figure 2 shows the model structure for the Touvre karst system that consists of three compartments organized in two levels 270 (Labat et al, 2022). The upper level corresponds to reservoir E and represents both the unsaturated part of the system and a temporary aquifer.…”

“…2 Background and motivations 2.1 Challenges in karst groundwater resources Karst aquifers are affected by the combination of different components of global change such as (i) effects of climate change which are particularly pronounced in the Mediterranean area (Dubois et al, 2020;Nerantzaki and Nikolaidis, 2020), (ii) increasing groundwater abstraction (Labat et al, 2022), as well as (iii) changes in land cover land use (Bittner et al, 2018;Sarrazin et al, 2018). Therefore, the assessment of karst groundwater resources vulnerability in the present context requires operational tools for estimating the sustainable yield of karst aquifers but also to predict the impacts of climatic or anthropogenic forcing on groundwater resources in the long term (Sivelle et al, 2021).…”

“…In order to address these issues, different modeling approaches have been developed (Jeannin et al, 2021) such as, among others, fully-distributed models (Chen and Goldscheider, 2014), semi-distributed models (Doummar et al, 2012;Dubois et al, 2020;Ollivier et al, 2020), and lumped parameter models (Mazzilli et al, 2019) including semi-distributed recharge (Bittner et al, 2018;Sivelle et al, 2022a). Among these, lumped parameter models are recognized as major tools to explore the ability of conceptual representations to explain observations in karst systems (Poulain et al, 2018;Sivelle et al, 2019;Duran et al, 2020;Frank et al, 2021) and for managing karst groundwater resources (Sivelle and Jourde, 2020;Sivelle et al, 2021;Labat et al, 2022;Cousquer and Jourde, 2022).…”