Runoff Response To Climate Variability: An Analysis Of Thika River Basin In Kenya

Abstract

Changes in climatic conditions have greatly affected surface runoff and stream flows both at local and global scale. This has led to adverse effects on surface run off and climatic system as a whole. Research on these hydrological changes at basin scale is of great importance to the water managers for the future planning and management of water resources. The Thika River catchment is of great importance to Kenya and plays host to Ndakaini Dam which provides about 84% of Nairobi’s water supply to a population of over 3 million residents, whose contribution to Kenya’s Gross Product is 60%. Observed climatic variability and trends for Thika catchment were assessed for significance with Mann Kendall’s trend test and discussed in light of future climate variability scenarios. The results indicate that the catchment has become relatively warmer over the last four decades. The annual precipitation and means of daily mean temperatures over the past 30 years has increased by about 7.8 mm (although not statistically significant), and 2.14°C respectively. The trend for the annual mean of daily temperatures was statistically significant. Hydrological simulation model was used to simulate runoff and quantify the effects of climate variability on runoff within the area of study. The model was calibrated and validated giving a coefficient of determination (R2) of 0.923, an RMSE of 0.56 and a BIAS of 1.697. The future climate of the catchment is projected to be warmer and, with less confidence, wetter. However, stream flow could increase by between 1.2% on the lower case to 4.5% on the higher case under these projections. There is therefore need to prepare for the increased runoff as it would affect the agricultural sector, industry, urban communities, as well as the environment.