Hydrology and Climate Impacts on Streamflow and Sediment Yield in the Nyando River Basin, Kenya

This study seeks to establish the extent to which climate change and/or variation is affecting streamflow variability and sediment transport in the Nyando River Basin in Kenya. The basin is one of the country’s most important, because it is a source of water for domestic use and irrigation. The river also discharges water and sediment into Lake Victoria, one of the largest freshwater lakes in the world. Data used in this study were obtained from the Water Resources Management Authority (WARMA) and Kenya Meteorological Department (KMD), and include river discharge, rainfall, and total suspended sediment for stations located in the basin. Statistical methods and application of various hydrological data analyses of time-series, flow duration, and Gumbel Distribution for flood flow analysis were used. Land-use change was determined through analysis of Landsat satellite images and use of overlay GIS techniques. Results show that there is significant variability in rainfall and river discharge of the Nyando River, which can be attributed to climate variability. There has also been major change in land use in the basin, which is evidenced by heavy deforestation in the catchment areas for agriculture and settlement of the rapidly increasing population. This increase in degraded areas, coupled with an increase in runoff, has led to increased peak flood flows. The increase in frequency of flood flows is attributed to rapid flow of surface runoff in degraded areas with low water infiltration and percolation rates. However, the relationship between rainfall and river discharge is strongly positive, reflecting the rapid hydrologic response to rainfall input in the basin. There is also a strong positive relationship between sediment discharge and river discharge. The increase or decrease of river discharge leads to a corresponding change in sediment yield. As a result of land degradation and high hydrologic response, the basin experiences high rates of soil erosion and sediment yield. High hydrologic response also leads to flooding in the low-lying Kano plains. Extreme streamflow variability in the plains has both direct and indirect impacts that are ultimately increasing poverty levels in the basin. This study shows that the effects of climate change and/or variability on the hydrologic systems in tropical Africa are being compounded by population growth that has driven land-use change. Therefore, it is unlikely that efforts to reduce sediment load and control floods in the basin can be successful without a deliberate effort to mitigate the root causes of land degradation in the basin: rapid population growth and land-use changes.