Evaluation of changes in groundwater recarge in Arusha town basin.

Groundwater levels are affected by many variables e.g. precipitation, pumping, river flow, and aquifer characteristics. Evaluation of changes in groundwater recharge is valuable because it provides current information against which future water levels can be compared, it helps to determine naturally occurring seasonal variations in groundwater levels (due to changes in precipitation), and it provides data that can be used to develop a groundwater flow model. Excessive groundwater abstraction may lead to salt intrusion and depletion of the aquifer. It is therefore necessary to estimate the amount of water that must be replenished to the aquifer in order to balance the inputs to and outputs from the aquifer. This research is aimed at assessing the annual variability of groundwater recharge in Arusha aquifer. Two approaches have been used in this study. The first one is to use the numerical model, based on Poisson's equation for unsteady flow, to generate heads distribution in the aquifer and then amount of recharge contributed by rainfall is obtained by simple optimization technique. The second approach is the use of the cumulative rainfall departure (CRD) method to determine the response of groundwater table to rainfall input. Again the fraction of rainfall that contributes to recharge was obtained by optimization. The result using the two approaches were compared and it was found that the method of cumulative rainfall departure (CRD) gave better results and the CRD method is an outstanding technique of estimating the groundwater recharge.