Evaluation de l’impact du changement climatique sur la ressource en eau du système karstique du Lez au moyen d’un modèle global semi-distribué

International audienceDans cette étude, l’impact du changement climatique sur la ressource en eau du système karstique du Lez pompé pour l’AEP de l’Agglomération de Montpellier a été évalué à l’aide d’un modèle hydrogéologique global semi-distribué. Une modélisation inverse a été menée au moyen de fonctions de transfert de façon à prendre en compte l’existence de plu-sieurs compartiments hydrogéologiques et la sollicitation du système karstique par les pompages. 9 scénarios climatiques issus de modèles climatiques forcés avec le scénario d’émission ́n A1B ̇z, et désagrégées par la méthode « type de temps » du CERFACS ont été utilisés. Les scénarios climatiques choisis couvrent deux périodes temporelles, l’une pour le présent (1971-2000) et l’autre pour le futur (2045-2065). Ces scénarios projettent une augmentation de la température moyenne mensuelle comprise entre +1,5 ̊C et +2,3 ̊C (±1 ̊C) suivant la période de l’année (moyenne multi-modèle). Pour la pluie, une tendance à la diminution des cumuls pourrait se dessiner de l’ordre de 10 % à l’échelle annuelle. La recharge par la pluie efficace calculée par le modèle hydrogéologique pourrait fortement diminuer dans le futur, pour atteindre-30% du cumul annuel moyen de pluie efficace. La diminution de la recharge se traduirait par une augmentation de la durée des assecs de la source du Lez ainsi que par une diminution du débit moyen de déborde-ment de la source, en période de hautes eaux. Lors des périodes estivales (assecs), les niveaux piézométriques seraient plus fréquemment situés sous les niveaux actuels. Toutefois, en maintenant le taux de pompage actuel, le niveau piézométrique retrouverait chaque année le niveau de débordement de la source (à 65 m NGF)

Hydrological behaviour of the granitic Strengbach catchment (Vosges massif, Eastern France) during a flood event

A field campaign combining monitoring devices and determination of isotopes and chemical elements has been performed during a summer thunderstorm in the small granitic Strengbach catchment (Vosges, France). The collected ground data were used in a hydrological modelling exercise including two conceptual rainfallrunoff models (GR4, TOPMODEL). The predominant role in flood generation of pre-event water coming from the superficial layers of the water saturated area has been shown and a conceptual scheme has been proposed derived from the field observations. The two tested modelling structures and assumptions are not able to take into account fully the complexity of the physical processes involved in flood generation

Hydrograph separation using isotopic, chemical and hydrological approaches (Strengbach catchment, France)

The streamflow components were determined in a small catchment located in Eastern France for a 40 mm rain event using isotopic and chemical tracing with particular focus on the spatial and temporal variations of catchment sources. Precipitation, soil solution, springwater and streamwaters were sampled and analysed for stable water isotopes (18O and 2H), major chemical parameters (SO4, NO3, Cl2, Na1, K1, Ca21, Mg21, NH4, H1, H4SiO4, alkalinity and conductivity), dissolved organic carbon (DOC) and trace elements (Al, Rb, Sr, Ba, Pb and U). 18O, Si, DOC, Ba and U were finally selected to assess the different contributing sources using mass balance equations and end-member mixing diagrams. Isotopic hydrograph separation shows that the pre-event water only contributes to 2% at the beginning of the stormflow to 13% at the main peak flow. DOC associated to Si and U to Ba allow to identify the different contributing areas (upper layers of the saturated areas, deep layers of the hillslope and rainwater). The streamflow (70%) originates from the deep layers of the hillslope, the remaining being supplied by the small saturated areas. The combination of chemical (both trace and major elements) and isotopic tracers allows to identify the origin of water pathways. During the first stage of the storm event, a significant part of the runoff (30±39%) comes from the small extended saturated areas located down part of the basin (overland runoff then groundwater ridging). During the second stage, the contribution of waters from the deep layers of the hillslope in the upper subcatchment becomes more significant. The final state is characterised by a balanced contribution between aquifers located in moraine and downslopes. Indeed, this study demonstrates the interest of combining a variety of hydrometric data, geochemical and isotopic tracers to identify the components of the streamwater in such conditions

Impact des changements globaux sur l’alimentation en eau potable de Montpellier Méditerranée Métropole

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Comparison of climate change impacts on the recharge of two karst systems computing different modelling approaches

International audienceKarst systems constitute aquifers in which infiltration and groundwater flows are generally complex processes and are characterized by limited knowledge in terms of geometry and structure. Nonetheless, they often represent interesting groundwater resources, some of them being subjected to intensive exploitation and others non exploited due to their poor understanding. In the future, it is likely that climate change impact on water resources will increase the interest for such a kind of aquifers due to their strong infiltration and storage capacity, in a broad context of higher water scarcity.The Lez and the Lison karst systems in Southern and Eastern France, respectively, provide 2 examples of such systems of several km² under two contrasted climate conditions, the first one being heavily exploited. This study presents a comparative climate change assessment onboth karst systems. Nine climate scenarios corresponding to the Fourth assessment report of the IPCC (SRES A1B scenario), downscaled using weather-type methods by the CERFACS, have been applied to various recharge modelling approaches, as standard analytical solutions of recharge estimation and soil-water balance models. Results are compared and discussed in order to assess the influence on climate change impacts of i) the climate conditions(geographic location), ii) the groundwater exploitation and iii) the modelling approach

Socio-economic assessement of farmers' vulnerability as water users subject to global change stressors in the hard rock area of southern India. The SHIVA ANR project

International audienceDemand for vulnerability assessments is growing in policy-making circles, to support the choice of appropriate measures and policies to reduce the vulnerability of water users and resources. Through the SHIVA ANR project, we are seeking a method to assess and map the vulnerability of farmers in southern India to both climate and socioeconomic changes, and secondly, to assess the costs and benefits associated with trends farmers' vulnerability in the medium and long-term. The project is focusing on southern India 's hard rock area, as in the geological context, both surface and ground water resources are naturally limited. We are also focusing on farming populations as these are the main water users in the area and rely exclusively on groundwater. The area covers southern India's semi-arid zone, where the rainfall gradient ranges from 600 mm to 1100 mm. Vulnerability is expected to vary according to local climatic conditions but also the socioeconomic characteristics of farming households. The SHIVA research team has been divided into six thematic groups in order to address the different scientific issues : downscaling the regional climate scenario, farm area projections, vulnerability assessments and quantification, vulnerability mapping, hydrological modelling and upscaling, and vulnerability impact assessements. Our approach is multidisciplinary to cater for for numerous inherent themes, and integrated to cater for vulnerability as a dynamic and multidimensional concept. The project 's first results after 10 months of research are presented below

Recharge processes in karstic systems investigated through the correlation of chemical and isotopic composition of rain and spring-waters

Four karstic springs in southern France, along with rainwater in the same area, were monitored during two hydrological cycles. Stable isotopic ratios (d18O and dD) allow the contribution of the rain waters from the previous periods (discretised as winter or summer period) to the spring water to be estimated. These computations indicate that heavy rainfall events during the autumn season contribute for 56 ± 7% and 68 ± 9% of the spring water discharge during the following winter and summer seasons, respectively. During the low flow period, residence time does not exceed 1 hydrological cycle, for a large part of the water. Stable isotopic ratios (d18O and dD) combined with Cl concentration allow the evapotranspiration coefficients to be estimated although the recharge surface is not known and hydrological balance can not be computed. Except for one spring, the evapotranspiration coefficients range from 0.54 to 0.38 (46­62% volume reduction), even during the high flow period. The short residence times, and the strong evapotranspiration coefficients whatever the period (winter or summer) are interpreted as the result of the major role of the epikarst reservoir in the karst recharge functioning over direct or diffuse infiltration

Suivi de l'inversion du fonctionnement de la source sous-marine de la Vise (près de Sète) du 28 novembre 2020 : apports de la Plateforme expérimentale DEM'EAUX Thau

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Interactions de fluides (eaux douces, marines, thermales) dans les réservoirs karstiques sous couverture de la marge proximale du Golfe du Lion : le projet DEM'EAUX Thau

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Isotope hydrological study of mean transit time in the granitic Strengbach catchment (Vosges Massif, France). Application of the FlowPC model with modified input function. ; .

International audienceMeasurements of 18O concentrations in precipitation, soil solution, spring and runoff are used to determine water transit time in the small granitic Strengbach catchment (0.8 km2; 883-1146 m above sea level) located in the Vosges Mountains of north-eastern France. Water transit times were calculated by applying the exponential, exponential piston and dispersion models of the FlowPC program, developed by Maloszewski and Zuber (1994), to isotopic input (rainfall) and output (spring and stream water) data sets during the period 1989-1995. The input function of the model was modified compared to the former version of the model and estimated by a deterministic approach based on a simplified hydrological balance. The fit between observed and calculated output data showed marked improvements compared with results obtained using the initial version of the model. An exponential piston version of the model applied to spring water indicates a 38.5 month mean transit time which suggests that the volume in the aquifer, expressed in water depth, is 2.4 m. A considerable thickness (> 45 m) of fractured bedrock may be involved for such a volume of water to be stored in the aquifer