Groundwater recharge of confined aquifers in the upper Ewaso Ng'iro North River Basin, Kenya
Groundwater has been, and will continue to be an important water supply for industrial, agricultural and residential use in Kenya. Rainfall is the main source of groundwater recharge. In order to estimate the groundwater recharge to confined aquifers of the upper Ewaso Ng'iro North River Basin, quantitative information on the response of groundwater levels to rainfall variability have been used. Recently a number of boreholes have dried up and some have dropped their water levels. Therefore, it became matter of concern to relate rainfall and fluctuations in water rest levels (WRL). This work therefore is focused on the study area located in the Northwest of Mt. Kenya and to the North of Aberdare Ranges whose altitude varies between 1600 -5200 m above sea level. The area is composed of confined aquifers and extends from Ngusishi to Suguroi Rivers of the basin, the study was carried out between May to October 2009. Objectives of the study were: i) Determination of groundwater flow directions in order to determine the recharge and discharge areas within the basin, ii), To determine the relationship between rainfall and groundwater level fluctuations; iii) Estimate groundwater recharge in six sub-regions of Upper Ewaso Ng'iro North River Basin. Both primary and secondary data were utilized, rainfall data from 1983 to 2008 and monitored water rest levels from 2005 to 2009. Where data gaps existed, methods of spatial correlation and simple liner regression were used to fill the gaps. Data from rainfall stations close to boreholes and those far of in the highlands were correlated and their correlation coefficient (r2) was compared. Time lag and moving averages methods were used to mimics the graph correctly. The results showed that rainfall is directly related to groundwater recharge; however fluctuations are steady due to confined aquifer conditions, Drillage time series showed that boreholes tapping from deeper aquifers have better yields than shallow ones. Groundwater Recharge occurs in the forest area while below the forest water discharge occurs in form of springs. The recharge that goes deeper gets into the aquifer and creates pressure that makes water to rise whenever a borehole is drilled. The water rise after drilling in the area confirms a confined aquifers characteristics. The study, recommends longer groundwater monitoring period.