[1] This paper investigates the effect of a drainage base level drop on the groundwater system in its vicinity, using theoretical analysis, simulations, and field data. We present a simple and novel method for analyzing the effect of a base level drop by defining two characteristic times that describe the response of the water table and the transition zone between the fresh and saline water. The Dead Sea was chosen as a case study for this process because of the lake's rapid level drop rate. During a continuous lake level drop, the discharge attains a constant value and the hydraulic gradient remains constant. We describe this new dynamic equilibrium and support it by theoretical analysis, simulation, and field data. Using theoretical analysis and sensitivity tests, we demonstrate how different hydrological parameters control the response rate of the transition zone to the base level drop. In some cases, the response of the transition zone may be very rapid and in equilibrium with the water table or, alternatively, it can be much slower than the water table response, as is the case in the study area.

[1] The present study examines the response of groundwater systems to expected changes in the Mediterranean Sea (rise of <1cm/yr) and Dead Sea levels (decline of ∼1 m/yr). A fast response is observed in the Dead Sea coastal aquifer, exhibited both in the drop of the water levels and in the location of the fresh-saline water interface. No such effect is yet observed in the Mediterranean coastal aquifer, as expected. Numerical simulations, using the FeFlow software, show that the effect of global sea level rise depends on the coastal topography next to the shoreline. A slope of 2.5‰ is expected to yield a shift of the interface by 400 m, after a rise of 1m (∼100 years), whereas a vertical slope will yield no shift. Reduced recharge due to climate change or overexploitation of groundwater also enhances the inland shift of the interface.

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