An inexorable depletion of groundwater occurs where groundwater abstraction exceeds the natural recharge, a typical state of (semi-)arid regions, which calls for sustainable management of groundwater resources. This study aims to assess the available storage and recharge rates on a national scale in time and space by modelling the natural recharge in combination with a method to evaluate changing groundwater volumes, which revealed measures to quantify the overdraft of the observed national groundwater resources in Jordan. Applying the combination of hydrological model and method to evaluate changing groundwater volumes, a climate-driven systematic decline of groundwater recharge was eliminated as responsible process, while overdraft leads to dropping groundwater tables.

The major findings are, the intensity of groundwater abstraction from a basin becomes visible through the fact, that simulated baseflow exceeds significantly the observed baseflow. About 75% of Jordan's groundwater basins are subject to intense groundwater depletion, reaching annual rates of up to 1 m in some basins. The most affected areas are the basins Zarka, Azraq and the predominantly fossil groundwater reservoirs in Southern Jordan.

Contrasting the past, when variable annual precipitation patterns did not negatively influence groundwater recharge, simulations show significantly reduced annual groundwater recharge all over Jordan. Particularly affected is the agricultural backbone in the Jordan Mountains, where recharge rates are predicted to vary between −30 mm/yr and +10 mm/yr in the coming decades, being reflected in the disappearance of freshwater springs and ascending saltwater. The applied methodology is relevant and transferable to other data- and water scarce areas worldwide, allowing (i) a fast estimation of groundwater reservoir development on a national scale and (ii) an investigation of long-term effects of overdraft.