Effect of Rainfall Variability on Groundwater Levels in Nzoia River Basin, Kenya

Abstract

Groundwater is a critical supply of freshwater in Africa, providing safe drinking water near to where it is needed for homes, agricultural, and industrial uses. Due to the high vulnerability of surface water resources to anthropogenic activities and climate change, groundwater consumption has increased dramatically over the last 50 years and is likely to continue rising in future. This study examines the effect of rainfall variability on groundwater levels using rainfall and groundwater level fluctuation data from Nzoia River Basin, Kenya. A cross-sectional survey design was used. Three counties were randomly selected from the basin for study with Busia representing the lower catchment, Kakamega middle catchment and Trans Nzoia upper catchment. Groundwater is the main drinking water resource in the basin supplying 78.8% of the population. Knowledge of long-term rainfall variability and the associated response of the basin’s groundwater resources are essential for efficient and sustainable groundwater management.  Despite the relatively short period for which water level records are available, these records contain valuable information that can be used beneficially in the development of strategies to assist in the responsible management of our limited groundwater resources. Rainfall and groundwater levels were analysed using the parametric test of linear regression and the non-parametric Mann–Kendall statistical test. Rainfal data for this study was obtained from the Kenya Meteorological Department (KMD) and the Groundwater levels from Kenya Water Resources Management Agency (WRMA). Pearson moment correlation was used to check the relationship between monthly rainfall and monthly groundwater levels. Correlations between groundwater levels and rainfall will help assess aquifer vulnerability to climate change in the basin. Our results indicate that rainfall variability induced by climate change has greatly impacted groundwater levels in the basin. Annual groundwater levels have been steadly falling at the rate between 0.03 ft/year (Kakamega Tande School) and 0.49 ft/year (Kitale Golf Club) in the basin. Annual rainfall over the basin has shown stations recording both decreasing and increasing trends. The falling groundwater level trends indicate a cause for concern and provide a scientific basis for the national and county governments in the basin to strategize on ground water development and management for sustainable water use.