Groundwater management of a highly dynamic karst by assessing baseflow and quickflow with a rainfall-discharge model (Dardennes springs, SE France)

Abstract

-This paper aims at characterizing the groundwater flow in a highly dynamic karst aquifer using a global modeling approach based on rainfall and spring discharge time series. The Dardennes aquifer (SE France) was studied as it is used for drinking water supply and it also produces karst flash floods that increase the flood hazard downstream in urban areas. Three years of data were available, including a normal rainy year, a wet year and a dry year. Modeling was performed with the new platform KarstMod, a rainfalldischarge model with calibration tools. The Dardennes aquifer model was structured with three interconnected reservoirs: Epikarst, Matrix, and Conduit. Using this modeling approach, we were able to determine the groundwater hydrograph separation of the karst spring discharge, at the annual scale and at the event scale (flood). This gives insight into the low flow (Matrix) available for the drinking water demand and the fast flow (Conduit) that generates flash floods. In such a dynamic aquifer, part of the water budget cannot be accounted for by water resources as fast flow is not stored within the aquifer and is not available for the drinking water demand. The results were compared with the current groundwater management to determine whether the withdrawal is sustainable. Depending on whether it is a wet or a dry year, the proportion of slow flow ranges from 27 to 61% of the total discharge, respectively. During floods in high water periods, the proportion of quickflow increases drastically up to more than 90% of the spring discharge. In the case of a 300 mm/d simulated Mediterranean rainfall event, the mean daily peak value may reach 74 m 3 /s. This discharge can be reduced if the aquifer is previously depleted, which increases the storage within the aquifer. Coupling the geological context and the model results opens up future perspectives for the active management of the karst aquifer. disponible pour les besoins en eau potable et l'écoulement rapide (Conduit) qui génère des crues éclairs. Dans les systèmes hyper-dynamiques, une partie de l'eau qui transite ne peut pas être comptabilisée comme ressource en eau car l'écoulement rapide n'est pas stocké dans l'aquifère et n'est donc pas disponible pour alimenter la demande en eau potable. Les résultats sont comparés avec la gestion actuelle des eaux souterraines afin de montrer si l'exploitation de l'eau est durable. Selon l'année, la proportion de débit de base varie entre 27 et 61 % du débit total, respectivement selon une année pluvieuse ou sèche. Lors des crues en période de hautes-eaux, la proportion du débit rapide augmente considérablement jusqu'à plus de 90 % du débit total des sources. Dans le cas d'un événement pluvieux de type Méditerranéen avec 300 mm/j de pluie, le débit moyen maximum simulé avec le modèle atteint 74 m 3 /s. Ce débit peut être réduit si l'aquifère est déjà déprimé, ce qui augmente le stockage de l'eau. Le couplage du contexte géologique et des résultats du modèle ouvre une perspective future pour une gestion active ...

Dynamics of the Flow Exchanges between Matrix and Conduits in Karstified Watersheds at Multiple Temporal Scales

“…The rainfall, noted as P, and the evapotranspiration, noted as ET, only impact the reservoir E. Then the water flows towards the deep reservoirs C, M, and L or is lost to the system. The KarstMod model has been successfully applied to the Norville chalk aquifer [33], Dardenne's spring [34] and Fontaine de Vaucluse [28]. Also, it constitutes a valuable tool for hydrodynamic analysis in a context of intrinsic vulnerability and pollution risk mapping [35] or even for the assessment of the vadose zone contribution in the groundwater recharge [36].…”

“…where q sim is the discretized time series simulated by the model, and q obs is the discretized experimental time series measured in the field. The NSE criterion is fluently used to judge the quality of a rainfall-runoff model [28,34,42,43]. Nonetheless, this criterion tends to favor the conformity of the model for the strong discharge values to the detriment of the weakest ones.…”

“…So, the warm-up period is long enough to ensure the calibration independence from initial conditions. Also, this allows a good balance between the number of calibration and validation data with a validation period at least two times longer than the calibration period [26,34].…”

Karst recharge-discharge semi distributed model to assess spatial variability of flows

“…KaRaMel structure is made of n lumped model units running in parallel. All lumped model units have the same three-reservoir structure (Figure 2-a and Table 1), which is commonly used to model karst aquifers (Pinault et al, 2001;Moussu, 2011;Schmidt et al, 2014;Baudement et al, 2018;Schmidt et al, 2014). The upper reservoir (S) represents soil behaviour.…”

“…• Spatial parallelization of a lumped model whose structure is based on a widely accepted karst conceptual model and that is commonly used to model karst aquifers (e.g. Pinault et al, 2001;Fleury et al, 2007;Moussu, 2011;Mazzilli et al, 2017;Baudement et al, 2018);…”