Impact of the storm Alex on water exchanges between the Roya River and its alluvial aquifer

The alluvial aquifer of the transnational Roya River watershed is an important water resource for drinking water supply. Through successive European projects, a monitoring network has been implemented over the alluvial plain to improve the understanding of the functioning of this aquifer. For instance, studies highlighted the predominant role of surface water in the recharge of the aquifer. Following the storm Alex and the resulting exceptional flood event in the Roya valley in October 2020, a general decrease of the piezometric levels was observed in the alluvial aquifer. Changes in the river morphology and in the granulometry of the hyporheic zone have impacted surface water – groundwater exchanges and reduced the aquifer recharge.</p

Karst–River Interaction, Elaboration of an Indicator of the Karst Hydrological Conditions Applied to the Cèze River (Gard, France)

International audienceIn the interest of creating a point of reference allowing to know the hydrological conditions of the karst system and for the purpose of using it as a parameter in a modeling procedure, a Hydric Indicator of the Karst (HIK) was established from the knowledge acquired on the functioning of the karstic hydrosystem of the Cèze. This indicator makes it possible to identify and qualify if at a specific moment “t” the karst is more able to contribute to the flow of the river or on the contrary to infiltrate precipitation water. The HIK is constructed from the data collected at the Ussel spring and the MétéoFrance rain gauge at the karst plateau of Méjannes-le-Clap (Gard, France). Chronic rainfall and discharge measurements from the source include two hydrological cycles. The discharge of the spring is calculated from the recordings of water amounts from autonomous water level probes. These water levels are converted into discharge through a calibration curve drawn for this study. For rain events with at least 4 mm of precipitation, ten parameters on rainfall characteristics and the flow rate of the Ussel spring are analyzed to obtain the HIK. In total, 74 events were analyzed. Finally, this indicator is a relevant entry for the simulation of flow variations in the river within its karst canyon. It is also a relevant tool for institutions which monitor the condition of the karst aquifer and thus can be a useful tool for the management of the aquifer. Our goal was to develop an approach, with which we can calculate and predict, what will be the response at karst spring, if we know current discharge of the Ussel spring and expected amount of rainfall

Neural Networks Model as Transparent Box: Toward Extraction of Proxies to Better Assess Karst/River Interactions (Coulazou Catchment, South of France)

International audienceKarst catchments frequently exhibit complex exchanges between surface and subterranean flow. While the swing between surface flood and underground flood is complex, the ability to predict such behavior would be of great interest for flood forecasting and water recharge assessment. To this end an innovative methodology is proposed to visualize internal variables of a neural network model. It proves to be efficient to extract internal variables highly correlated to measured signals previously identified as proxy of the karst-river exchanges. The study focuses on a small Mediterranean catchment where karst/river interactions control the dynamic and genesis of surface floods. But the methodology is generic and can be applied to any catchment provided the availability of a sufficient database

Conceptualization and classification of groundwater–surface water hydrodynamic interactions in karst watersheds: Case of the karst watershed of the Coulazou River (Southern France)

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A conceptual semidistributed model of the Coulazou River as a tool for assessing surface water-karst groundwater interactions during flood in Mediterranean ephemeral rivers

International audienceThis study aims to assess surface water-karst groundwater interactions during floods in the Mediterranean karst watershed of the Coulazou River (southern France) using a conceptual semidistributed model at 5 min time steps. The kinematic wave routing approximation is used for the transfer of surface flow, while overflows from a linear underground reservoir account for karst flows along the riverbed. After calibration, values of parameters and simulated time series are compared to independent physical measurements. Results show that direct runoff can be neglected on the karst terrains. In addition, this study demonstrates that, in some cases, karst watersheds can be considered as relatively poor systems of regulation but strong systems of amplification or generation of floods and flash floods, depending on rainfall characteristics and also on groundwater level conditions prior to the flood event. Considering that the flood peak is the most important factor defining flash flood hazard, it is shown that the flood hazard regulation effect of the karst is relatively limited for low water table conditions prior to the flood, while the aggravating effect for high water table conditions may be higher than 80% with respect to expected values from surface runoff only. These results show that understanding groundwater-surface water interactions is crucial for describing the flash flood dynamics in karst terrains

Dataset on onshore groundwaters and offshore submarine spring of a Mediterranean karst aquifer during flow reversal and saltwater intrusion

Groundwater from various shallow and deep reservoirs converges in interaction with marine waters into the limestone aquifer of the Balaruc peninsula (Thau lagoon, southern France). This aquifer faces temporary phenomena of marine water intrusion through the Vise submarine spring located at −29.5 m below the lagoon level. Since the 1960s, seven flow reversal phenomena have occurred, the last one occurring between 11/28/2020 and 03/14/2022. During these phenomena, which can last from a few weeks to several months, the salty water is absorbed from the lagoon to the conduit of the submarine spring, which leads to the salinization of the underlying karst aquifer. The monitoring of flow, water specific conductivity and water temperature data from the karst submarine spring is a key element of the research project to understand the hydrogeological functioning of the karst aquifer under normal conditions or during flow reversal periods. This monitoring allows the characterization of the (in- or out-) flows at the submarine spring, the evaluation of the volume or mass balances, the identification of the hydrogeological and physico-chemical responses (water temperature, specific conductivity) observed within the karstic aquifer. Here, we present the means implemented offshore to acquire data at the submarine spring over the 06/25/2019 - 12/31/2022 time period together with lagoon water's physico-chemical parameters and levels and onshore groundwater's physico-chemical parameters and levels acquired at springs and boreholes from the karst aquifer

Monitoramento in situ de teste de traçadores: como distinguir a recuperação do traçador da composição natural

International audienceHydrogeological tracer tests are primarily conducted with fluorescent tracers. Field fluorometers make it possible to monitor tracers at very low concentrations (<1 ppb) and at high frequency. However, changes in natural fluorescence at a site resulting from variations of dissolved and suspended inorganic and organic material may compromise the measurement of useful signals, thereby limiting the chances of identifying or quantifying the real tracer recovery. An elevated natural signal can mask small concentrations of the tracer while its variability can give the impression of a false recovery. This article shows how the use of a combination of several continuous measurements at different wavelengths allows a better extraction of the natural signal. Field multispectral fluorometers were installed at two Mediterranean karst outlets; both drain carbonate systems but have different environmental conditions. The fluorometers functioned over several hydrologic cycles, in periods affected or not by artificial tracers, making it possible to observe natural signal variations at these sites. The optical properties of this type of field fluorometer were used to calculate the spectral response of the different optics of the measuring probe. These responses, superimposed on three-dimensional excitation/emission matrices produced from laboratory fluorescence measurements, allowed an understanding of what the fluorometer sees under natural flow conditions. The result is an innovative method for correcting artificial tracer results. This type of correction makes it possible to fine-tune the effect of natural background variation on tracer recovery curves for a clear identification of the tracer presence and a more precise quantification of its recovery

Time series analyses for Karst/River interactions assessment: Case of the Coulazou river (southern France)

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XLKarst, uma ferramenta em Excel para análise de series temporais, análise de curva de recessão da fonte e classificação de aquíferos cársticos

Karst aquifers are complex hydrogeological systems that require numerous in-situ measurements of hydrological and physico-chemical parameters to characterize transfer processes from the recharge area to the karst spring. Numerous graphical, statistical or signal processing methods have been developed for decades to interpret these measurements, but there is no simple and standardized tool that can be used for this purpose, which is necessary for a rigorous comparison of results between case studies. This Technical Note presents XLKarst, which has been developed to provide a simple and easy-to-use tool to process a selection of proven methods that characterize the functioning of karst systems. This tool allows (i) time series analysis based on correlation and spectral analysis and, for flow measurements, the use of other statistics and base flow separation, (ii) calculation of the cumulative distribution function to build a spring flow probability plot, and (iii) analysis of spring flow recession and expression of the results in a karst system classification scheme. These methods are first described by providing the key elements of their use and interpretation in the scientific literature. Then, an application to the Fontaine de Nîmes karst system (southern France) is used to highlight the complementarity of the methods proposed by XLKarst to describe the hydrodynamic behavior of a karst system based on daily data of rainfall and discharge over 22 years.Resumen Los acuíferos kársticos son sistemas hidrogeológicos complejos que requieren numerosas mediciones in situ de parámetros hidrológicos y fisicoquímicos para caracterizar los procesos de transferencia desde la zona de recarga hasta el manantial kárstico. Durante décadas se han desarrollado numerosos métodos gráficos, estadísticos o de procesamiento de señales para interpretar estas mediciones, pero no existe ninguna herramienta sencilla y estandarizada que pueda utilizarse con este fin, lo cual es necesario para una comparación rigurosa de los resultados entre casos de estudio. Esta Nota Técnica presenta XLKarst, que se ha desarrollado para proporcionar una herramienta sencilla y fácil de usar para procesar una selección de métodos probados que caracterizan el funcionamiento de los sistemas kársticos. Esta herramienta permite (i) el análisis de series temporales basado en la correlación y el análisis espectral y, para las mediciones de caudal, el uso de otros estadísticos y la separación del caudal base, (ii) el cálculo de la función de distribución acumulativa para construir un gráfico de probabilidad del caudal de manantial, y (iii) el análisis de la recesión del caudal de manantial y la expresión de los resultados en un esquema de clasificación de sistemas kársticos. Estos métodos se describen primero proporcionando los elementos clave de su uso e interpretación en la literatura científica. A continuación, se utiliza una aplicación al sistema kárstico de Fontaine de Nîmes (sur de Francia) para poner de relieve la complementariedad de los métodos propuestos por XLKarst para describir el comportamiento hidrodinámico de un sistema kárstico a partir de datos diarios de precipitación y descarga a lo largo de 22 años.Résumé Les aquifères karstiques sont des systèmes hydrogéologiques complexes qui nécessitent de nombreuses mesures in-situ de paramètres hydrologiques et physico-chimiques pour caractériser les processus de transfert de la zone de recharge à la source karstique. De nombreuses méthodes graphiques, statistiques ou de traitement du signal ont été développées depuis des décennies pour interpréter ces mesures, mais il n'existe pas d'outil simple et standardisé pouvant être utilisé à cette fin, ce qui est nécessaire pour une comparaison rigoureuse des résultats entre les études de cas. Cette note technique présente XLKarst, qui a été développé pour fournir un outil simple et facile à utiliser pour traiter une sélection de méthodes éprouvées qui caractérisent le fonctionnement des systèmes karstiques. Cet outil permet (i) l'analyse de séries temporelles basée sur la corrélation et l'analyse spectrale et, pour les mesures de débit, l'utilisation d'autres statistiques et la séparation du débit de base, (ii) le calcul de la fonction de distribution cumulative pour construire un graphique de probabilité de débit de source, et (iii) l'analyse de la récession du débit de source et l'expression des résultats dans un schéma de classification des systèmes karstiques. Ces méthodes sont d'abord décrites en fournissant les éléments clés de leur utilisation et de leur interprétation dans la littérature scientifique. Ensuite, une application au système karstique de la Fontaine de Nîmes (sud de la France) est utilisée pour mettre en évidence la complémentarité des méthodes proposées par XLKarst pour décrire le comportement hydrodynamique d'un système karstique basé sur des données journalières de précipitations et de débits sur une période de 22 ans.Resumo Aquíferos cárstico são sistemas hidrogeológicos complexos que requerem inúmeras medições in-situ de parâmetros hidrológicos e físico-químicos para caracterizar os processos de transferência da área de recarga para a fonte cárstica. Inúmeros métodos de processamento gráfico, estatístico ou de sinal foram desenvolvidos por décadas para interpretar estas medições, mas não há uma ferramenta simples e padronizada que pode ser utilizada para este propósito, que é necessário para a comparação rigorosa dos resultados entre estudos de caso. Esta Nota Técnica apresenta XLKarst, que tem sido desenvolvido para fornecer uma ferramenta simples e fácil de utilizar para processar uma seleção de métodos comprovados que caracterizam o funcionamento de um sistema cárstico. Essa ferramenta permite (i) Análise de séries temporais baseada na correlação e análise espectral e, para medidas de fluxo, o uso de outros estatísticas baseadas na separação de fluxo, (ii) cálculo da função de distribuição cumulativa para construção de gráfico de probabilidade de fluxo da fonte e (iii) análise do fluxo de recessão da fonte e expressão dos resultados em um esquema de classificação de sistemas cárstico. Estes métodos são primeiramente descritos fornecendo os elementos chave de seu uso e interpretação na literatura científica. Em seguida, a aplicação no sistema cárstico de Fontaine de Nimes (sul da França) é usada para exaltar a complementariedade dos métodos propostos pelo XLKarst para descrever o comportamento hidrodinâmico do sistema cárstico baseado em dados diários de precipitação e descarga ao longo de 22 anos