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

Nouveaux éléments sur la structure et le fonctionnement hydrogéologique du plateau basaltique de l'Aubrac (Massif central, France). Première évaluation des potentialités en eau souterraine

International audienceIn volcanic areas, the quality of the geological data at the various scales is of paramount importance for the estimation of aquifer potentialities and thus for groundwater exploitation. Geological and hydrogeological investigation (synthesis of existing data, field geological observations, reconstruction of the infra-volcanic morphology, stream gauging, field hydrogeology, etc.) have been performed on the 450-km2 Aubrac basaltic plateau (Massif Central, France). These results allow us to specify the hydrological structure and functioning of this volcanic plateau that were quite unknown before. It is mainly constituted by the piling up of lava flows resting on a plutonic and metamorphic substratum. This volcanic unit constitutes an aquifer of regional importance whose groundwater resources are currently underexploited. These results allow us to delineate zones and strategies for priority hydrogeological prospectingEn milieu volcanique, la qualité de la connaissance géologique aux différentes échelles est de première importance pour l'évaluation des potentialités en eau souterraine des aquifères et pour leur mise en valeur. Des investigations géologiques et hydrogéologiques (synthèse des données existantes, observations géologiques de terrain, reconstitution de la morphologie du substratum infra-volcanique, jaugeages, hydrogéologie de terrain, etc.) ont été mises en œuvre au sein du plateau basaltique de l'Aubrac (Massif central, France). Elles permettent de préciser la structure et le fonctionnement hydrogéologique, très peu connu jusqu'à présent, de cet empilement de formations volcaniques, à dominante lavique, d'une superficie d'environ 450 km2, reposant sur un substratum principalement plutonique et métamorphique. Cet ensemble se révèle constituer un aquifère aux potentialités d'importance régionale, dont la ressource en eau souterraine est actuellement très largement sous-exploitée. Les résultats obtenus permettent de proposer des zones et stratégies de prospection prioritair

Paramétrisation du potentiel de ruissellement des bassins versants au moyen de la Télédétection et des systèmes d'Informations Géographiques. Application à des bassins versants du Pays de Caux

International audienceThe increasing number of run-off damages occurring in Pays de Caux is a serious concern for regional authorities. The suggested method aims the assessmenl of run-off parameters using Remote Sensing and GIS. This method allows an objective comparison of the intrinsic and anthropic pressures of cultivated catchment from 1000 to 5000 ha. The potenliaI ron-off characteristics are analysed using land use components increasing or limiting run-off. and their spoliaI distrihulion on three different functional units. The selected parameters are the grassland proportion and compacity around concentrated surficial water channels (80 m wide), the proportion and compacity of the soil surfaces contributive to run-off on the top of catchments and, finally, the simple proportion of grassland and forest upon high sloped areas. The temporal evolution of these indices and the comparison between calchments show the signiflcant recorded changes whÎch favour the increase of surfaces contributing la mn-off between 1990 and /997. The interest of Ihese indices is to identify areas sensitive to run-off and to act as a decision supporl tool for a lerritorial management policy aimed al run-off reduclion on catchments. The outlook of this study is to generalize these parameters for regional applications

Gully erosion prediction in a Sahelian context

In Sahelian region, concentrated overland flow often leads to the formation of gullies. Although this phenomenon is widespread in those regions, research efforts are still needed to be able to model their spatial distribution and the role of the different parameters involved in this process. In this context, the objectives of this study are twofold. The first step is to investigate to what extent the role of Sahelian soil surface crusts (biological and/or physical) on soil surface infiltrability and detachment affect the formation and development of gullies. The second step is to integrate the results of these investigations in a simple geomorphological model to predict gully location at the watershed scale. The evaluation of the resulting model on two test catchments demonstrated that the integration of soil crusting is a key parameter to insure the quality and relevance of gully prediction. The model is able to distinguish between two types of gullies, those whose width range between 0.5m and 4m and those whose width exceeds 4m. The application of the model at the regional scale is however limited by the resolution of available regional digital elevation model (i.e. the 90m resolution SRTM DEM) which only permits the prediction of large gullies

Cartographie de la perméabilité des sols pour l’aide à la prévision des crues ; cas d’étude sur le Gardon d’Anduze

Sur le bassin versant du Gardon d’Anduze, choisi par le SCHAPI comme bassin expérimental en ce qui concerne la modélisation des crues rapides, des mesures de perméabilité ont été réalisées depuis 2002 par le BRGM. Après analyse statistique, une valeur de perméabilité a été affectée à chacune des unités pédo paysagère du bassin versant, permettant ainsi la réalisation d’une cartographie spatialisée de la perméabilité des sols. Ces valeurs de perméabilité ont été intégrées dans le modèle d’écoulement hortonien STREAM. Le coefficient de corrélation obtenu (r2 = 0,83) démontre la bonne relation entre les débits observés et simulés, que ce soit sur le bassin du gardon d’Anduze ou bien les sous bassins versants de Mialet et de Saumane pour lesquels des données limnigraphiques sont disponibles. La corrélation baisse significativement lorsqu’on prend une perméabilité homogène de 30 mm/h sur l’ensemble du bassin, perméabilité moyenne calculée en fonction de la surface des différentes entités pédo paysagères. On démontre ainsi l’intérêt de la prise en compte d’une perméabilité des sols spatialisée pour la modélisation, avec un modèle hortonien tel STREAM, des écoulements sur un bassin versant méditerranéen tel que le Gardon d’Anduze

Recent advances on the characterization of karst-river interactions during floods

International audienceFlood hazard and groundwater resource management in karst catchments require a better understanding of hydrogeological processes and notably groundwater/surface water interactions. This is a great challenge due to the heterogeneity of karst aquifers and due to the diversity of karst types (small perched units, wide sedimentary basins, barren/covered karst, etc.). The aim of this communication is to present recent developments on the characterization of karst-river interactions through two main approaches: a spatial approach aiming at localizing karst areas promoting surface flows, and a temporal approach aiming at modelling lateral flows from karst units in rivers. The first spatial approach is based on the GIS index IDPR (Index of Development and Persistency of River networks, developed by BRGM©), quantifying the hydrological connectivity to the hydrographic network. From the standard version of the IDPR over France (25 m resolution), we propose to compare IDPR calculations differentiating intermittent and perennial reaches of rivers in order to detect infiltrations zones that contributive temporary to rivers, as many karst units. The second approach is a modelling framework based on the inverse problem for the diffusive wave model, to simulate lateral flow during floods on a reach between two stations. Knowing the upstream and downstream hydrographs, we can model the lateral one, given information on the hydrological processes involved in the intermediate catchment. Applying such approach on river reaches crossed by karst units is a new way to quantify river losses and gains, characterizing localized recharge and aquifer drainage to rivers, respectively. We propose to illustrate these two approaches through several case studies in France, where a better characterization of karst-river interactions and flood risk management are critical issues

Avaliação dos recursos hídricos subterrâneos do aquífero Saq em Al Ula, Arábia Saudita

International audienceRegional sandstone aquifers in arid areas provide prolific water resources that can meet human water demands, but their sustainability is at risk where recharge is very low and significant withdrawals occur. The Saq sandstone aquifer in the Al Ula area (Saudi Arabia) has been investigated through a field survey that included groundwater level measurement, groundwater sampling and analysis, crop inventory mapping and irrigation practice enquiries to assess groundwater abstractions. A groundwater model has been built to assimilate the collected data and investigate the aquifer response under various abstractions scenarios to meet future agricultural and domestic water demand. Since the 1960’s, water levels have dropped by 5–20 m within the Al Ula Valley, with maximum water level drawdowns reaching 30–40 m in Al Ula City. Present water level drawdown rates range from 0.5 to 2 m year–1 in the agricultural areas of the valley. Less than half of the sampled groundwater is in compliance with regulatory guidelines for drinking water purpose, and high groundwater nitrate concentrations are observed in agricultural areas. Predictive scenarios of decreasing abstractions suggest a reduction in the water level decline by 2050 but do not reverse the current decreasing trend. The estimated recharge is much lower than groundwater abstraction rates and current withdrawals are effectively mining the groundwater reserves. Further challenges arise from a water quality standpoint, due to the presence of radionuclide activities (mostly 228Ra) in the aquifer above the WHO guidelines.Los acuíferos regionales de arenisca de las zonas áridas proporcionan recursos hídricos considerables que pueden satisfacer la demanda humana de agua, pero su sostenibilidad está en peligro cuando la recarga es muy baja y se producen extracciones importantes. El acuífero de arenisca de Saq, en la zona de Al Ula (Arabia Saudí), se ha investigado mediante un estudio de campo que incluye la medición del nivel freático, el muestreo y análisis de aguas subterráneas, la cartografía del inventario de cultivos y la investigación de las prácticas de riego para evaluar las extracciones de agua subterránea. Se ha construido un modelo de aguas subterráneas para asimilar los datos recogidos e investigar la respuesta del acuífero en distintos escenarios de extracción para satisfacer la futura demanda de agua para uso agrícola y doméstico. Desde la década de 1960, los niveles de agua han descendido entre 5 y 20 m en el valle de Al Ula, con máximos de 30–40 m en la ciudad de Al Ula. Las tasas actuales de descenso del nivel del agua oscilan entre 0.5 y 2 m year–1 en las zonas agrícolas del valle. Menos de la mitad de las aguas subterráneas muestreadas cumplen las directrices reglamentarias para el agua potable, y se observan altas concentraciones de nitratos en las zonas agrícolas. Los escenarios predictivos de disminución de las extracciones sugieren una reducción del descenso del nivel del agua para 2050, pero no invierten la tendencia actual a la baja. La recarga estimada es muy inferior a las tasas de extracción de aguas subterráneas y las actuales cantidades extraídas están minando las reservas existentes. Desde el punto de vista de la calidad del agua se plantean otros problemas, debido a la presencia de actividades de radionucleidos (principalmente 228Ra) en el acuífero por encima de las directrices de la OMS.Les aquifères régionaux de grès dans les zones arides fournissent d’importantes ressources en eau qui peuvent satisfaire les demandes en eau, mais leur durabilité est menacée là où la recharge est très faible et où des prélèvements importantes prennent place. L’aquifère des grès du Saq dans la région d’Al Ula (Arabie Saoudite) a fait l’objet d’une étude de terrain comprenant des mesures de niveaux d’eaux souterraines, échantillonnage et analyse d’eaux souterraines, cartographie des cultures et enquêtes des pratiques d’irrigation afin d’évaluer les prélèvements en eaux souterraines. Un modèle hydrogéologique a été établi pour intégrer les données collectées et pour étudier la réponse de l’aquifère pour différents scénarios de prélèvements pour satisfaire la demande en eau agricole et domestique. Depuis les années 1960, les niveaux d’eau ont chuté de 5–20 m au sein de la vallée d’Al Ula, avec des rabattements maximums atteignant 30–40 m dans la ville d’Al Ula. Les taux de rabattement actuellement des eaux souterraines sont compris entre 0.5 et 2 m an–1 dans les zones agricoles de la vallée. Moins de la moitié des eaux souterraines échantillonnées sont conformes aux directives réglementaires relatives à l’eau potable, et des concentrations élevées de nitrates dans les eaux souterraines sont observées dans les zones agricoles. Les scénarios prédictifs de diminution des prélèvements suggèrent une réduction de la baisse du niveau d’eau d’ici 2050, mais n’inversent pas la tendance actuelle baissière. La recharge estimée est bien inférieure aux taux de prélèvements d’eaux souterraines et les prélèvements actuels exploitent effectivement comme une mine les réserves d’eaux souterraines. D’autres défis surviennent du point de vue de la qualité de l’eau, en raison de la présence de radionucléides (principalement 228Ra) dans l’aquifère dans des concentrations au-dessus des directives de l’OMS.Os aquíferos regionais de arenito em zonas áridas fornecem recursos hídricos prolíficos que podem satisfazer as necessidades humanas de água, mas a sua sustentabilidade está em risco quando a recarga é muito baixa e ocorrem retiradas significativas. O aquífero de arenito Saq na área de Al Ula (Arábia Saudita) foi investigado através de um levantamento de campo que incluiu medição do nível das águas subterrâneas, amostragem e análise de águas subterrâneas, mapeamento de inventário de culturas e questionários sobre práticas de irrigação para avaliar as captações de águas subterrâneas. Foi construído um modelo de águas subterrâneas para assimilar os dados recolhidos e investigar a resposta do aquífero sob vários cenários de captação para satisfazer a futura procura de água agrícola e doméstica. Desde a década de 1960, os níveis da água caíram 5–20 m no Vale de Al Ula, com rebaixamentos máximos do nível da água atingindo 30–40 m na cidade de Al Ula. As atuais taxas de redução do nível da água variam entre 0.5 e 2 m ano–1 nas áreas agrícolas do vale. Menos de metade das águas subterrâneas amostradas estão em conformidade com as diretrizes regulamentares para fins de água potável, e são observadas concentrações elevadas de nitrato nas águas subterrâneas em áreas agrícolas. Cenários preditivos de diminuição das captações sugerem uma redução no declínio do nível da água até 2050, mas não revertem a atual tendência decrescente. A recarga estimada é muito inferior às taxas de captação de águas subterrâneas e as atuais extrações estão efetivamente a minar as reservas de águas subterrâneas. Outros desafios surgem do ponto de vista da qualidade da água, devido à presença de atividades de radionuclídeos (principalmente 228Ra) no aquífero acima das diretrizes da OMS

Quantifying peakflow attenuation/amplification in a karst river using the diffusive wave model with lateral flow

International audienceThe aim of this paper is to quantify peakflow attenuation and/or amplification in a river, investigating lateral flow from the intermediate catchment during floods. This is a challenge for the study of the hydrological response of permeable/intermittent streams, and our contribution refers to a modelling framework based on the inverse problem for the diffusive wave model applied in a karst catchment. Knowing the upstream and downstream hydrographs on a reach between two stations, we can model the lateral one, given information on the hydrological processes involved in the intermediate catchment. The model is applied to 33 flood events in the karst reach of the Iton River in French Normandy where peakflow attenuation is observed. The monitored zone consists of a succession of losing and gaining reaches controlled by strong surface-water/groundwater (SW/GW) interactions. Our results show that despite a high baseflow increase in the reach, peakflow is attenuated. Model application shows that the intensity of lateral outflow for the flood component is linked to upstream discharge. A combination of river loss and overbank flow for highest floods is proposed for explaining the relationships. Our approach differentiates the role of outflow (river loss and overbank flow) and that of wave diffusion on peakflow attenuation. Based on several sets of model parameterization, diffusion is the main attenuation process for most cases, despite high river losses of up to several m(3)/s (half of peakflow for some parameterization strategies). Finally, this framework gives new insight into the SW/GW interactions during floods in karst basins, and more globally in basins characterized by disconnected river-aquifer systems

Cartographie de la conductivité hydraulique à saturation du sol de surface à l'aide de points de mesures obtenus par différentes méthodes

International audienceTopsoil field-saturated hydraulic conductivity, Kfs, is a parameter that controls the partition of rainfall between infiltration and runoff and is a key parameter in most distributed hydrological models. There is a mismatch between the scale of local in situ Kfs measurements and the scale at which the parameter is required in models for regional mapping. Therefore methods for extrapolating local Kfs values to larger mapping units are required. The paper explores the feasibility of mapping Kfs in the Cévennes-Vivarais region, in south-east France, using more easily available GIS data concerning geology and land cover. Our analysis makes uses of a data set from infiltration measurements performed in the area and its vicinity for more than ten years. The data set is composed of Kfs derived from infiltration measurements performed using various methods: Guelph permeameters, double ring and single ring infiltrotrometers and tension infiltrometers. The different methods resulted in a large variation in Kfs up to several orders of magnitude. A method is proposed to pool the data from the different infiltration methods to create an equivalent set of Kfs. Statistical tests showed significant differences in Kfs distributions in function of different geological formations and land cover. Thus the mapping of Kfs at regional scale was based on geological formations and land cover. This map was compared to a map based on the Rawls and Brakensiek (RB) pedotransfer function (mainly based on texture) and the two maps showed very different patterns. The RB values did not fit observed equivalent Kfs at the local scale, highlighting that soil texture alone is not a good predictor of Kfs