Groundwater level and withdrawable volume forecasts in the Adour-Garonne basin (France) to enable sustainable groundwater management

The SUDOE AQUIFER project (http://www.igme.es/aquifer/) aims at capitalizing, testing, diffusing and transferring innovative practices for groundwater monitoring and integrated management.BRGM has developped the « MétéEAU Nappes » web platform (https://meteeaunappes.brgm.fr/fr) for several years. It enables to visualize the current and future behavior of groundwater bodies in France and to forecast groundwater availability in many monitoring wells which have been modeled using a lumped hydrological model [1].Although more than 500 wells are monitoring groundwater level in real time in unconfined aquifers in the Adour-Garonne basin (France) (https://ades.eaufrance.fr/), none of these monitoring points have been modeled to enable 6 months groundwater levels forecast. The SUDOE AQUIFER project enables to model ten monitoring points in 2022 and 2023 to forecast groundwater levels using different climatic scenarios. These forecasts are updated on a monthly basis and can be compared to groundwater levels thresholds (piezometric drought thresholds from local authority use-restriction orders [2]).These groundwater level forecasts are further used to predict groundwater withdrawable volume using a three-dimensional groundwater flow model in the Garonne, Tarn and Aveyron alluvial plain [3]. The main activity of this region is agriculture and the main groundwater use is crop’s irrigation. Groundwater withdrawal is especially important in the summer, and can impact the volume of groundwater reaching the rivers and sustaining their baseflow. This competition in use creates the need to accurately define potential withdrawable volumes.Combining the lumped hydrological models with a three-dimensional groundwater flow model enables to define the potential withdrawable volume based on (1) the summer climatic scenario chosen by the decision maker, (2) the forecasted groundwater level at the end of the low-water season and (3) the status of the groundwater body (critical, balanced, conservative) to achieve at the end of the low-water season. This decision support tool is developed as a web platform and will be accessible to groundwater managers and decision makers. After choosing the groundwater level forecasted at the start of the irrigation period within 6 scenarios based on different climatic conditions, three potential withdrawable volumes will be defined depending on the status of the groundwater body considered acceptable to obtain at the end of the low-water season. This information can then be communicated to groundwater users.These innovative practices will be extended to other regions where increase groundwater pressure forces local authority to develop methods and tools to sustainably manage groundwater bodies.Références bibliographiques : [1] Mougin B., Nicolas J., Vigier Y., Bessière H., Loigerot S. (2020). « MétéEAU Nappes » : un site Internet contenant des services utiles à la gestion des étiages. La Houille Blanche, numéro 5, p. 28-36. https://doi.org/10.1051/lhb/2020045[2] Surdyk N., Thiéry D., Nicolas J., Gutierrez A., Vigier Y., Mougin B. (2022). MétéEAU Nappes: a real-time water-resource-management tool and its application to a sandy aquifer in a high-demand irrigation context. Hydrogeology Journal. https://doi.org/10.1007/s10040-022-02509-1[3] Le Cointe, P., Nuttinck, V., Rinaudo, JD. (2020). A Tool to Determine Annual Ground-Water Allocations in the Tarn-et-Garonne Alluvial Aquifer (France). In: Rinaudo, JD., Holley, C., Barnett, S., Montginoul, M. (eds) Sustainable Groundwater Management. Global Issues in Water Policy, vol 24. Springer, Cham. https://doi.org/10.1007/978-3-030-32766-8_13