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

Evidence of hydrological control of Sr behavior in stream water (Strengbach catchment, Vosges mountains, France)

Strontium and particularly 87Sr/86Sr ratios in stream water have often been used to calculate weathering rates in catchments. Nevertheless, in the literature, discharge variation effects on the geochemical behavior of Sr are often omitted or considered as negligible. A regular survey of both Sr concentrations and Sr isotope ratios of the Strengbach stream water draining a granite (Vosges mountains, France) has been performed during one year. The results indicate that during low water flow periods, waters contain lower Sr concentrations and less radiogenic Sr isotope ratios (Sr=11.6 ppb and 87Sr/86Sr=0.7246 as an average, respectively) than during high water flow periods (Sr= 13 ppb and 87Sr/86Sr=0.7252 as an average, respectively). This is contrary to expected dilution processes by meteoric waters which have comparatively lower Sr isotopic ratios and lower Sr concentrations. Furthermore, 87Sr/86Sr ratios in stream water behave in 3 different ways depending on moisture and on hydrological conditions prevailing in the catchment. During low water flow periods (discharge < 9 l/s), a positive linear relationship exists between Sr isotope ratio and discharge, indicating the influence of radiogenic waters draining the saturated area during storm events. During high water flow conditions, rising discharges are characterized by significantly less radiogenic waters than the recession stages of discharge. This suggests a large contribution of radiogenic waters draining the deep layers of the hillslopes during the recession stages, particularly those from the more radiogenic north-facing slopes. These results allow one to confirm the negligible instantaneous incidence of rainwater on stream water chemistry during flood events, as well as the existence in the catchment of distinct contributive areas and reservoirs. The influence of these areas or reservoirs on the fluctuations of Sr concentrations and on Sr isotopic variations in stream water depends on both moisture and hydrological conditions. Hence, on a same bedrock type, 87Sr/86Sr ratios in surface waters can be related to flow rate. Consequently, discharge variations must be considered as a pre-requisite when using Sr isotopes for calculating weathering rates in catchments, particularly to define the range of variations of the end-members

Temporal dynamics in dominant runoff sources and flow paths in the Andean Páramo

Funded by Central Research Office of the Universidad de Cuenca (DIUC) Secretaría de Educación Superior, Ciencia, Tecnología e Innovación (SENESCYT 112–2012) . Grant Number: SENESCYT 112-2012 German Research Foundation (DFG) . Grant Number: BR2238/14-1Peer reviewedPublisher PD

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

Application of multi‑method approach to assess groundwater–surface water interactions, for catchment management

Globally, the dependence of river systems to delayed discharge of subsurface water to augment flows during dry seasons is well documented. Discharge of fresh subsurface water can dilute concentrated river flow quality during reduced flow. Observed and reported results on the Berg River’s declining water quantity and quality are a concern to the regions socio-economic growth and environmental integrity. Understanding the role of subsurface water discharges on the quantity and quality of receiving surface water courses can improve their management during dry periods. A case study was designed and implemented in the upper Berg River catchment in the Western Cape Province of South Africa to assess the influence of groundwater–surface water interaction on water quantity and quality. This study aimed to quantify and characterize the quality of subsurface water available in the upper catchment to improve observed declining water quality downstream. Hydrograph separation provided estimates of water fluxes during 2012–2014 low and high flow periods, while hydrochemical analysis provided insights on impacts of major land use activity in this catchment on water resources. Hydrograph separation analysis indicated that the Berg River is 37.9% dependent on subsurface water discharges annually. Dominant Na–Cl-type water indicates the quality of water from the upper Berg River is largely affected by natural processes including short residence times of aquifer water, rock–water interactions and atmospheric deposition of NaCl ions. These results provide insights for suggesting management options to be implemented to protect subsurface water for continued dilution and water resources management in the lower catchments