Numerical modelling of groundwater vulnerability: the example Namibia

The UNSAT-H, HELP3 and MACRO4.3 computer codes, which have been developed for simulating the water balance of the unsaturated zone of soils and unconsolidated sediments, are also capable simulating water flow in low-porosity media such as fractured rock. The codes can be used to model the ability of rocks and overlying soils in the vadose zone to protect the groundwater in the uppermost aquifer. The net infiltration rates simulated by the different codes are compared against recharge determined by the chloride-balance method in Namibia. The dual-permeability code MACRO4.3 was found to produce more realistic estimates of net infiltration than the UNSAT-H and HELP3 codes, which are based on a single-permeability or effective-continuum method. The net infiltration rate together with the water storage in the unsaturated zone and the groundwater depth are used to calculate the residence time of pore water in the unsaturated zone. This parameter determines the intrinsic vulnerability of the aquifer.