Abstract
Renewable freshwater resources in drylands are rare with the exception of inland freshwater lenses (IFLs), which form atop regional saline groundwater and are recharged by rare, albeit extreme rainfall events. A three-dimensional, numerical model of IFL formation and transient evolution is presented. The SEAWAT computer package generated finite difference solutions to solve the spatial and temporal variation of hydraulic heads and solute concentrations over varying recharge rates. IFL geometry (i.e. thickness, length) and position were calibrated against observed heads from simulated IFLs using a laboratory-scale porous media tank (2.0 X 1.0 X 0.10 m). Parameters employed in the physical model including hydraulic conductivity (0.0015 m/s), aquifer thickness (0.41 m), salinity (35,000 mg/l), and freshwater recharge rate ranges (1.85 X 10 (super -5) -7.9 X 10 (super -5) m/s) were assigned to the numerical model. The domain of the numerical model was designed with an irregular, telescopic finite difference grid to provide finer nodal spacing in areas where recharge was simulated and where the vertical head was expected to change significantly. In addition, cells ranging between 0.01-0.0005 m (super 2) in the horizontal plane and 18 layers in the vertical plane were chosen. Results indicate that the method successfully simulates the freshwater accumulation mechanism, and that IFLs are highly sensitive to the spatiotemporal dynamics of focused recharge. Increases in recharge rate are positively correlated with IFL geometry, negatively correlated with degradation, and uncorrelated with the rate of change in position. The model results can be used for the testing of IFL sustainability and their viability as exploitable water resources in drylands and elsewhere with similar environmental conditions.