Water Balance Evaluation for Flood Risk Reduction in the Yala River Basin, Western Kenya

The Yala River Basin (YRB) in Western region of Kenya has in the past experienced climate and weather extreme events that include floods. Floods have triggered loss of lives, destruction of property, outbreak of water borne diseases and siltation of arable land. This study utilized the Nedbør-Afstrømnings-Model (NAM) hydrologic model (available in the NAM Module of the MIKE 11 hydrodynamic model) on the Yala River Basin (YRB) to generate flood flows for water balance evaluation. The study utilized satellite imagery data for the period 1984-2010 sourced from the Regional Centre for Mapping of Resources for Development, rainfall (1980-2012) and river discharge (1947-2012) data sets from the Kenya Meteorological Department (KMD) and the Water Resources Management Authority (WARMA), respectively. Data quality control was statistically checked before sensitivity analysis, calibration, validation, and simulation of the flood flows. Daily water balance estimates for the Yala River Basin (YRB) over the period 1980-2010 were developed using the NAM hydrologic model. The results indicate that the mapped flood area extent varied by a value of 34.23 km2 over the period 1980-2010. The Yala River Basin (YRB) indicated an estimated inflow value of 4,814.72 MCM and outflow value of 4,578.23 MCM, with a coefficient of determination of 0.867. The difference between the inflow and outflow values was 236.49 MCM, that formed the flood flow or the water balance. This study concluded that the water balance value of 236.49 MCM was responsible for the floods recorded in the Yala River Basin (YRB) for the period 1980-2010, and it should be taken care of through flood risk reduction initiatives.