New insights from an enhancement of hydrogeological archives of a French karst watershed over the last 50 years

Karst aquifers, like most hydrological systems, are sensitive to the influence of changes in land use as well as current and future climate change. To be able to identify the effects of these changes, in particular on the variability of groundwater flows, we have endeavored to produce one of the longest hourly spring discharge time series available for a French karst watershed. This reconstitution is based on a digitisation of the limnigraphic archives available for the Baget karst watershed. Correlation, spectral and multifractal analyses were then performed based on the spring discharge time series to confirm the multiscale nature of the hydrological response of this basin and to highlight the non-stationarity of this response

New insights from an enhancement of hydrogeological archives of a French karst watershed over the last 50 years

Karst aquifers, like most hydrological systems, are sensitive to the influence of changes in land use as well as current and future climate change. To be able to identify the effects of these changes, in particular on the variability of groundwater flows, we have endeavored to produce one of the longest hourly spring discharge time series available for a French karst watershed. This reconstitution is based on a digitisation of the limnigraphic archives available for the Baget karst watershed. Correlation, spectral and multifractal analyses were then performed based on the spring discharge time series to confirm the multiscale nature of the hydrological response of this basin and to highlight the non-stationarity of this response

Couplage d'approches conceptuelles, systémiques et distribuées pour l'interprétation de traçages artificiels en domaine karstique : implications pour la détermination de la strucure interne des aquifères karstiques

The main objective of these thesis is to improve the understanding of the internal structure of karstic aquifers by relying on the interpretation of artificial tracer tests. To meet these objectives, this thesis is based on the karst watersheds of Baget and Aliou (Ariège, France) as well as other karst hydrosystems (the Norville, Val de Loire and Foubanne systems) included in the National Karts Observation Service (SNO Karst, INSU-CNRS) and the tracing database of the BRGM including the Ouysse karst system. The overall approach adopted for this thesis is to couple the results of different approaches. [1] Conceptual reservoirs modeling with the dedicated KarstMod model allows the partitioning of flows at the scale of the watershed and to understand the dynamics of the exchanges between the matrix and the conduits, in the short and long term. It is estimated that the annual contribution of the matrix to the total outflow is about 3% and can increase up to 25% during periods of low precipitation. [2] A systemic approach of the "transfer function" type has been developed and applied to various hydrosystems (the Norville, Ouysse, Val de Loire and Foubanne systems). This approach consists of studying the dynamics of solute mass transport by interpreting artificial tracer tests data and partitioning the flows by estimating the residence times of the different components that contribute to solute transfer between the injection and recovery points. [3] A distributed approach integrating a coupling of geophysical data and pseudo-genetic simulations makes it possible to simulate drainage networks. The associated solute flows and mass transport are then simulated and compared with field data. This is an approach to deduce the structuring of underground flows from the data of artificial tracer tests.L'objectif principal de ces travaux de thèse consiste à améliorer la compréhension de la structure interne des aquifères karstiques en s'appuyant sur l'interprétation de traçages artificiels. Pour répondre à cet objectif, cette thèse s'appuie sur les bassins versants karstiques du Baget et d'Aliou (Ariège, France) ainsi que d'autres hydrosystèmes karstiques (les systèmes de Norville, du Val de Loire et de Foubanne) inclus dans le Service National d'Observation des Karsts (SNO Karst, INSU-CNRS) et sur la base de données BD traçage du BRGM incluant notamment le système karstique de l'Ouysse. L'approche globale retenue pour ces travaux de thèse consiste à coupler les résultats de différentes approches. [1] La modélisation conceptuelle à réservoirs avec l'outil dédié KarstMod permet de partitionner les écoulements à l'échelle du bassin versant et d'appréhender la dynamique des échanges entre la matrice et les conduits, à court et long terme. On estime que la contribution annuelle de la matrice au débit à l'exutoire est d'environ 3% et qu'elle peut augmenter jusqu'à 25% en période de faible précipitation. [2] Une approche systémique de type "fonction de transfert" a été développée et appliquée à divers hydrosystèmes (dont les systèmes de Norville, de l'Ouysse, du Val de Loire et de Foubanne). Cette approche consiste à étudier la dynamique du transport de masse en soluté en interprétant des données de traçages artificiels et permet de partitionner les écoulements en estimant les temps de résidence des différentes composantes qui contribuent en transfert de solutés entre les points d'injection et de restitution. [3] Une approche distribuée intégrant un couplage de données géophysiques et de simulations pseudo-génétique permet de simuler des réseaux de drainage. Les écoulements et le transport de masse en soluté associé sont ensuite simulés et confrontés à des données de terrain. Cela constitue une approche originale pour déduire la structuration des écoulements souterrains à partir des données de traçages artificiels

Coupling of conceptual, systemic and distributed approaches for the interpretation of artificial tracings in the karst field : implications for determining the internal structure of karst aquifers

L'objectif principal de ces travaux de thèse consiste à améliorer la compréhension de la structure interne des aquifères karstiques en s'appuyant sur l'interprétation de traçages artificiels. Pour répondre à cet objectif, cette thèse s'appuie sur les bassins versants karstiques du Baget et d'Aliou (Ariège, France) ainsi que d'autres hydrosystèmes karstiques (les systèmes de Norville, du Val de Loire et de Foubanne) inclus dans le Service National d'Observation des Karsts (SNO Karst, INSU-CNRS) et sur la base de données BD traçage du BRGM incluant notamment le système karstique de l'Ouysse. L'approche globale retenue pour ces travaux de thèse consiste à coupler les résultats de différentes approches. [1] La modélisation conceptuelle à réservoirs avec l'outil dédié KarstMod permet de partitionner les écoulements à l'échelle du bassin versant et d'appréhender la dynamique des échanges entre la matrice et les conduits, à court et long terme. On estime que la contribution annuelle de la matrice au débit à l'exutoire est d'environ 3% et qu'elle peut augmenter jusqu'à 25% en période de faible précipitation. [2] Une approche systémique de type "fonction de transfert" a été développée et appliquée à divers hydrosystèmes (dont les systèmes de Norville, de l'Ouysse, du Val de Loire et de Foubanne). Cette approche consiste à étudier la dynamique du transport de masse en soluté en interprétant des données de traçages artificiels et permet de partitionner les écoulements en estimant les temps de résidence des différentes composantes qui contribuent en transfert de solutés entre les points d'injection et de restitution. [3] Une approche distribuée intégrant un couplage de données géophysiques et de simulations pseudo-génétique permet de simuler des réseaux de drainage. Les écoulements et le transport de masse en soluté associé sont ensuite simulés et confrontés à des données de terrain. Cela constitue une approche originale pour déduire la structuration des écoulements souterrains à partir des données de traçages artificiels.The main objective of these thesis is to improve the understanding of the internal structure of karstic aquifers by relying on the interpretation of artificial tracer tests. To meet these objectives, this thesis is based on the karst watersheds of Baget and Aliou (Ariège, France) as well as other karst hydrosystems (the Norville, Val de Loire and Foubanne systems) included in the National Karts Observation Service (SNO Karst, INSU-CNRS) and the tracing database of the BRGM including the Ouysse karst system. The overall approach adopted for this thesis is to couple the results of different approaches. [1] Conceptual reservoirs modeling with the dedicated KarstMod model allows the partitioning of flows at the scale of the watershed and to understand the dynamics of the exchanges between the matrix and the conduits, in the short and long term. It is estimated that the annual contribution of the matrix to the total outflow is about 3% and can increase up to 25% during periods of low precipitation. [2] A systemic approach of the "transfer function" type has been developed and applied to various hydrosystems (the Norville, Ouysse, Val de Loire and Foubanne systems). This approach consists of studying the dynamics of solute mass transport by interpreting artificial tracer tests data and partitioning the flows by estimating the residence times of the different components that contribute to solute transfer between the injection and recovery points. [3] A distributed approach integrating a coupling of geophysical data and pseudo-genetic simulations makes it possible to simulate drainage networks. The associated solute flows and mass transport are then simulated and compared with field data. This is an approach to deduce the structuring of underground flows from the data of artificial tracer tests

Variations à court terme de la réponse d’essais de traçage dans un bassin d’alimentation très karstifié

International audienceModelling non-reactive solute transport based on artificial tracer tests have been widely used in the past decades. The dependence of solute transport from boundary conditions have been investigated across different hydrological conditions (low and high-water level) but still not investigated on short-term scale (i.e. hourly and daily scale). In this study, a campaign of several tracer tests is performed on a few days to investigate the short-term variations of tracer tests responses in a conduit dominated kart system during a recession without the influence of rainfall. Also, an improved artificial tracer test interpretation using a process engineering tool is introduced. It consists of a Laplace-transform transfer function approach of the residence time distribution curve. Considering the karstic system as a chemical reactor, the introduction of a transfer function approach appears to be an efficient way to describe the solute transport. Moreover, the transfer function is parametrized depending on the spring discharge. Finally, the model is extended to deal with source pollution scenario testing

Coupling SKS and SWMM to Solve the Inverse Problem Based on Artificial Tracer Tests in Karstic Aquifers

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The Effects of Exchange Flow on the Karst Spring Hydrograph under the Different Flow Regimes: A Synthetic Modeling Approach

International audienceIn this study, a synthetic modeling approach is proposed to quantify the effect of the amount and direction of the exchange flow on the karstic spring discharge fluctuations under different hydrologic conditions corresponding to high and low flow conditions. We hypothesis that the spring discharge fluctuations constitute a valuable proxy to understand the internal processes of the karst system. An ensemble of spring hydrographs was synthetically produced to highlight the effect of exchange flow by exploring the plausible range of variability of coefficients of exchange flow, conduit diameter, and matrix hydraulic conductivity. Moreover, the change of the rate of point recharge through the karst conduit allows for the quantifying of the sensibility of the spring hydrograph to the directions of exchange flow. We show that increasing the point recharge lies to a remarkable linear recession coefficient (β) as an indication of the conduit flow regime. However, a reduction in and/or lack of the point recharge caused the recession coefficient to change to exponential (α) due to the dominant effect of the matrix restrained flow regime and/or conduit-influenced flow regime. The simulations highlight that the exchange flow process from the conduit to the matrix occurred in a short period and over a restricted part of the conduit flow regime (CFR). Conversely, the exchange flow dumped from the matrix to the conduit occurs as a long-term process. A conceptual model is introduced to compare spring hydrographs’ characteristics (i.e., the peak discharge, the volume of baseflow, and the slope of the recession curve) under the various flow conditions with the directions of the exchange flow between the conduit and the matrix

Interprétation de traçages artificiels en domaine karstique par approche de type fonction de transfert : Application à l'hydrosystème karstique de l'Ouysse (Lot, France)

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Impact of Withdrawals on Karst Watershed Water Supply

International audienceKarst 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