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)

Using IDPR to characterize recharge area of karst aquifers from catchment to regional scales

International audienceThe shape and the nature of hydrographic networks on karst environments are closely linked to the infiltration capacity of geological formations. Thus, the presence or absence of a river in a favorable topographic context can be interpreted in terms of karstic aquifers recharge potential. We present the GIS built index of infiltration and runoff properties of landscape (called IDPR) as an interesting tool for the characterization of the karst extension and for the further recharge estimation of such aquifers. A new version of the IDPR tool has been released over France in 2017 which spatial resolution (25 m) becomes very useful for the detection of contributive zones for karst aquifers recharge. Examples of the IDPR application at the river basin scale (Fontaine de Vaucluse) and at the regional scale (Rhone river basin) will be presented. Base flow estimation methods are used for the calibration of the IDPR index in terms of infiltration capacity, leading the IDPR index to be useful for recharge estimation

Uma ferramenta simples de balanço de água subterrânea para avaliar o rendimento específico tridimensional e a recarga bidimensional: aplicação a um aqüífero cristalino profundamente intemperizado no sul da Índia

International audienceCrystalline aquifers are among the most complex groundwater systems, requiring adequate methods for realistic characterization and suitable techniques for improving the long-term management of groundwater resources. A tool is needed that can assess the aquifer hydrodynamic parameters cost-effectively. A model is presented, based on a groundwater- budget equation and water-table fluctuation method, which combines the upscaling and the regionalization of aquifer parameters, in particular specific yield (S y) in three dimensions (3D) and the recharge in two dimensions (2-D) from rainfall at watershed scale. The tool was tested and validated on the 53-km 2 Maheshwaram watershed, southern India, at a 685 m × 685 m cell scale, and was calibrated on seasonal groundwater levels from 2011 to 2016. Comparison between computed and observed levels shows an absolute residual mean and a root mean square error of 1.17 m and 1.8 m, respectively, showing the robustness of the model. S y ranges from 0.3 to 5% (mean 1.4%), which is in good agreement with previous studies. The annual recharge from rainfall is also in good agreement with earlier studies and, despite its strong annual variability (16 to 199 mm/y) at watershed scale, it shows that spatial recharge is clearly controlled by spatial structure, from one year to another. Groundwater levels were also forecasted from 2020 to 2039 based on the climate and groundwater abstraction scenarios. The results show severe water-level depletion around 2024-2026 but it would be more stable in the future (after 2030) because of a lower frequency of low-rainfall monsoons

Introducing economy into suitability mapping of MAR scheme

International audienceDuring phase 1 (desktop study based on available information and data collection) of the course of a MAR project, an analysis of suitability maps is generally done in order to identify the most suitable location for a given MAR scheme. Most of the approaches found in the literature rely on the construction of suitability maps using spatial multi-criteria analysis (SMCA) and focus on the aptitude of the aquifer to store water, the infiltration capacity of soils, the distance from the targeted surface resource or the available space (derived from landuse) necessary for building such a MAR scheme. At this stage of the project, only physical parameters are considered and no economic analysis is ever carried out. The objective of this study is to introduce economy in this first phase by producing a distributed map of total costs of an infiltration basin scheme. Both capital costs (water abstraction, transfer, land acquisition, basin construction) and operational costs (energy, maintenance, monitoring, water pre-treatment) are taken into account. In a first step, an objective of volume to be recharged is defined. A cells grid is applied on the study area and, using a GIS tool the following distributed data are extracted for each cell: (i) Distance D between cell and the nearest surface water point; (ii) Head change H between cell and the nearest surface water point (using DEM); (iii) the soil infiltration rate on the cell (value obtained from permeability maps) and (iv) Land value. Other parameters are not distributed and are then fixed. In a second step, from these parameters and data, a distributed cost function is developed to map the levelised costs of recharged water for a given operating life duration of MAR scheme and discount rate. The cost function is applied for all the surface streams that can be used for recharge purpose. Afterwards, the minimum cost is kept in order to build a map of levelised costs (in €/m3 recharged). The methodology has been applied to a case study (500 km2 area). The cost function can be used in order to illustrate how the various parameters (distance from the surface stream, head difference, pre-treatment cost…) impact the levelised costs. The costs map can be mixed with other types of suitability maps in order to identify the most suitable location for a MAR scheme taking into account economic and financial aspects

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

Recharge des aquifères à l'échelle de la France : estimation, évolution et incertitudes associées

International audienceEvaluating future groundwater recharge with associated uncertainty at the scale of France Among aquifer recharge processes, effective precipitation infiltration is the most directly impacted by climate. We propose a methodology for estimating this recharge at the scale of large territories, based on global hydrological models requiring few data and parameters. The originality of our work lies in the use of the IDPR (Network Development and Persistence Index) which is a qualitative cartographic index of infiltration potentiality. The study of more than 350 hydrological basins made it possible to highlight a linear relation between the BaseFlow Index and the IDPR for the sedimentary basins. Based on this relationship and assuming that the effective rainfall infiltration ratio could be assimilated to the BFI, an map of this ratio was generated all over France, allowing the effective precipitation map to be converted into a recharge map. The methodology was then applied to estimate future recharge using the outputs of climate models for two contrasted radiative concentration pathway scenarios. By 2030, whatever the scenario, the modeling results predict a moderate decrease in the direct recharge of aquifers everywhere in France except in Paris basin and around the Mediterranean sea. By 2080, the optimistic scenario allows to recover recharge values close to the current ones, outside the north of the Paris basin. At the opposite, within the pessimistic scenario, a majority of aquifers would undergo a recharge decrease by 10 to 30%, with stress zones in the Paris basin, around the Mediterranean sea and in Corsica. Nonetheless, these first results should be considered carefully as the associated uncertainties are significant and can reach 50% for the most pessimistic forecasts.Parmi les processus qui participent à la recharge des aquifères, la recharge par infiltration des précipitations efficaces est la plus directement impactée par le climat. Nous proposons une méthodologie qui permet d'estimer cette recharge à l'échelle de grands territoires, sur la base de modèles hydrologiques globaux nécessitant peu de données et de paramètres. L'étude de plus de 350 bassins hydrologiques a permis de mettre en évidence pour les bassins sédimentaires, une relation entre le BaseFlow Index et l'IDPR (Indice de Développement et de Persistance des Réseaux), qui est un indice cartographique qualitatif de potentialité d'infiltration. Sur la base de cette relation, une carte du ratio d'infiltration de la pluie efficace a pu être générée pour toute la France métropolitaine, permettant de transformer la carte de pluie efficace en carte de recharge. La méthodologie a été appliquée pour estimer la recharge future en utilisant les sorties de modèles climatiques pour deux scenarios de forçage radiatif. A l'horizon 2030, quel que soit le scénario, les résultats de la modélisation prévoient une baisse modérée de la recharge directe des aquifères sur la majorité du territoire, en dehors du bassin parisien et du pourtour méditerranéen. A l'horizon 2080, le scénario optimiste permet de revenir à des valeurs de recharge proches de l'actuel, en dehors du nord du bassin parisien. En revanche, dans le scénario pessimiste, une majorité des aquifères voit sa recharge diminuer de 10 à 30%, avec des zones plus tendues dans le bassin parisien, le pourtour méditerranéen et la Corse. Les incertitudes associées à ces résultats sont néanmoins importantes et peuvent atteindre 50% sur les prévisions les plus pessimistes

Developing a pan-European high-resolution groundwater recharge map – Combining satellite data and national survey data using machine learning

Groundwater recharge quantification is essential for sustainable groundwater resources management, but typically limited to local and regional scale estimates. A high-resolution (1 km × 1 km) dataset consisting of long-term average actual evapotranspiration, effective precipitation, a groundwater recharge coefficient, and the resulting groundwater recharge map has been created for all of Europe using a variety of pan-European and seven national gridded datasets. As an initial step, the approach developed for continental scale mapping consists of a merged estimate of actual evapotranspiration originating from satellite data and the vegetation controlled Budyko approach to subsequently estimate effective precipitation. Secondly, a machine learning model based on the Random Forest regressor was developed for mapping groundwater recharge coefficients, using a range of covariates related to geology, soil, topography and climate. A common feature of the approach is the validation and training against effective precipitation, recharge coefficients and groundwater recharge from seven national gridded datasets covering the UK, Ireland, Finland, Denmark, the Netherlands, France and Spain, representing a wide range of climatic and hydrogeological conditions across Europe. The groundwater recharge map provides harmonised high-resolution estimates across Europe and locally relevant estimates for areas where this information is otherwise not available, while being consistent with the existing national gridded datasets. The Pan-European groundwater recharge pattern compares well with results from the global hydrological model PCR-GLOBWB 2. At country scale, the results were compared to a German recharge map showing great similarity. The full dataset of long-term average actual evapotranspiration, effective precipitation, recharge coefficients and groundwater recharge is available through the EuroGeoSurveys' open access European Geological Data Infrastructure (EGDI)

Use of expert elicitation to assign weights to climate and hydrological models in climate impact studies

Various methods are available for assessing uncertainties in climate impact studies. Among such methods, model weighting by expert elicitation is a practical way to provide a weighted ensemble of models for specific real-world impacts. The aim is to decrease the influence of improbable models in the results and easing the decision-making process. In this study both climate and hydrological models are analysed, and the result of a research experiment is presented using model weighting with the participation of six climate model experts and six hydrological model experts. For the experiment, seven climate models are a priori selected from a larger EURO-CORDEX (Coordinated Regional Downscaling Experiment - European Domain) ensemble of climate models, and three different hydrological models are chosen for each of the three European river basins. The model weighting is based on qualitative evaluation by the experts for each of the selected models based on a training material that describes the overall model structure and literature about climate models and the performance of hydrological models for the present period. The expert elicitation process follows a three-stage approach, with two individual rounds of elicitation of probabilities and a final group consensus, where the experts are separated into two different community groups: a climate and a hydrological modeller group. The dialogue reveals that under the conditions of the study, most climate modellers prefer the equal weighting of ensemble members, whereas hydrological-impact modellers in general are more open for assigning weights to different models in a multi-model ensemble, based on model performance and model structure. Climate experts are more open to exclude models, if obviously flawed, than to put weights on selected models in a relatively small ensemble. The study shows that expert elicitation can be an efficient way to assign weights to different hydrological models and thereby reduce the uncertainty in climate impact. However, for the climate model ensemble, comprising seven models, the elicitation in the format of this study could only re-establish a uniform weight between climate models.This work was funded by the project AQUA-CLEW, which is part of ERA4CS (European Research Area for Climate Services), an ERANET (European Research Area Net-work) initiated by JPI Climate (Joint Programming Initiative) andfunded by Formas (Sweden); German Aerospace Center (DLR, Germany); Ministry of Education, Science and Research (BMBWF,Austria); Innovation Fund Denmark; Ministry of Economic Affairs and Digital Transformation (MINECO, Spain); and French National Research Agency with co-funding by the European Commission (grant no. 69046). The contribution of Philippe Lucas-Picher was supported by the French National Research Agency (future investment programme no. ANR-18-MPGA-0005). Rafael Pimentel acknowledges funding by the Modality 5.2 of the Programa Propio 2018 of the University of Córdoba and the Juan de la Cierva Incorporación programme of the Ministry of Science and Innovation (grant no. IJC2018-038093-I). Rafael Pimentel and María J. Polo are members of DAUCO (Unit of Excellence reference no. CEX2019-000968-M), with financial support from the Spanish Ministry of Science and Innovation and the Spanish State Research Agency, through the Severo Ochoa Centre of Excellence and María de Maeztu Unit of Excellence in research and development (R&D)

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