Why, when and how we need to apply conjunctive water management of surface and groundwater: The case of the Charente basin, France

In France, the development of irrigation since the 1970s is an important cause of imbalance between available water resources and demands. In the Charente river basin, dry climatic conditions during summer and intensive water uses create severe low flow period, which could be prolonged very late in the season (until December). In this context of water scarcity, managers need to deal with both surface and groundwater resources and not as there are two separate entities. Through the Charente basin example, we try to explain how a coupled surface-groundwater modelling can improve water management for low flows. Moreover, management tools, as decision support system (DSS), could provide essential information to managers for their decisions on allocation of water resources. Indeed, managers are faced with the task to identify optimal or near-optimal water management solutions in highly complex hydrosystems. The model TBR used in Charente, a water resources dashboard (Tableau de Bord de la Ressource en eau), is a DSS model with hydrologic and irrigation water demand modules. However, the TBR model need to be improved for low flow forecasting while considering the interrelationships between surface and groundwater component. In the Charente basin, the exchange between the aquifer and the river and the influence of irrigation withdrawals on the rivers flows are dominating and need to be included in the TBR management tools.