The Djeffara plain, extending over Tunisia and Libya (48,000 km 2 ), experiences unsustainable exploitation of groundwater, the severity of which needs to be determined. Here, we used data from Gravity Recovery and Climate Experiment (GRACE) and Global Land Data Assimilation System (GLDAS) to assess the groundwater storage variation (∆GWS) for the period 2002-2016. ∆GWS values, downscaled over a 0.5°× 0.5°grid using pumping wells spatial distribution and rates, were introduced in a discrete set of groundwater and salt budget equations to ascertain the recharge and the coastal fluxes. An average recharge of 4.3 ± 2.7 mm•yr −1 representing 2.2 ± 1.4% of the domain-averaged annual rainfall was obtained. A saline intrusion of 147.3 × 10 6 and 46.9 × 10 6 m 3 •yr −1 was calculated for the Tripoli (Libya) and Djerba (Tunisia) areas, respectively. The overall Submarine Groundwater Discharge (SGD) of freshwater along the 450 km long Djeffara coast represents about 339.6 × 10 6 m 3 •yr −1 for the 2002-2016 period.

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We formulate a hydro-economic model of the North-Western Sahara Aquifer System (NWSAS) to assess the effects of intensive pumping on the groundwater stock and examine the subsequent consequences of aquifer depletion. This large system comprises multi-layer reservoirs with vertical exchanges, all exploited under open access properties. We first develop a theoretical model to account for relevant features of the NWSAS by introducing, in the standard Gisser-Sanchez model, a non-stationary demand and quadratic stock-dependent cost functions. In the second step, we calibrate parameters values using data from the NWSAS over 1955-2000. We finally simulate the time evolution of the aquifer system with exploitation under an open-access regime. We specifically examine time trajectories of the piezometric levels in the two reservoirs, the natural outlets, and the modification of water balances. We find that natural outlets of the two reservoirs might be totally dried before 2050.

Determining groundwater recharge is fundamental for the understanding and management of groundwater systems, especially for renewability analysis. Assessing the long-term recharge rate is pivotal to understand aquifers' behavior and predict potential effects of recent climate and land use changes on groundwater resources. Recharge rate can be defined as the amount of water that effectively flows across the unsaturated zone and reaches the water table of an aquifer over a given time. Among the different methods to identify the recharge, such as numerical modeling (unsaturated or saturated zone) and water table fluctuation, the use of geochemical tracers is one of the most appropriate approaches especially in arid environments where recharge is low and sporadic (