ABSTRACT
ABSTRACT: Groundwater is the perennial source of supply in the southwestern sector of the Chad basin. Thus, quantifying the rate of recharge is fundamental to the sustainable management of this resource. Chloride concentrations in rainfall, the unsaturated zone and groundwater were used to estimate the diffuse recharge flux using a steady-state chloride mass balance approach. Average Cl concentrations in rainfall measured from three stations over eight years is 1.63 mg/l (although modelling gives a lower long term value of 0.65 mg/l). Eight unsaturated zone Cl profiles were obtained and Cl concentrations in the regional groundwater were measured for over 400 samples from wells and boreholes. An average recharge rate of 40 mm/a was estimated from the unsaturated zone profiles. The regional rate obtained using Cl concentrations in groundwater is slightly higher, estimated at 50 mm/a, which also takes into account other mechanisms of recharge such as from river channels, pools, depressions and regional flow that bypass the unsaturated zone. The estimated rate of recharge reduces to 20 mm/a when the modelled Cl in rainfall is used as input data, this is considered a long term minimum recharge rate to the region. Therefore, estimated recharge rate to the groundwater in this region from the Cl data range from 20-50 mm/a. Analysis of abstraction in the area indicates that this rate can sustain present day abstractions for domestic use in villages via dug wells. This implies that the observed decline in the water table in recent years is largely due to a concurrent reduction in rainfall due to drought and consequent reduction in recharge.
