Assessing the Effects of Deforestation on Surface Water Yields Using a Modelling Approach: The Case of Sondu River Basin
Abstract
Climatic factors determine the amount and distribution of atmospheric water received at the land surface while the land cover conditions determine partitioning of this water into different hydrological components and ultimately the catchment surface water yields. This study assessed the effects of deforestation of a tropical catchment on surface water yields with a view to addressing fluctuating flows of the rivers emanating from Mau Forest, the largest water tower in Kenya. Sondu basin traverses South West Mau Forest covering an area of 3500 km2. The main channel in the basin flows in a south west direction into Lake Victoria in an altitudinal range of 2900 to 1130 m above sea level over a length of 173 km. Different deforestation scenarios over the basin were integrated with climate data to form inputs to a hydrologic model, Soil and Water Assessment Tool. Using model outputs, effects of deforestation on annual and seasonal surface water yields, represented by changes in streamflow volumes under different deforestation scenarios, were evaluated. Deforestation scenarios were derived from a supervised classification scheme of time series of Satellite images to show deforestation trends. Effects of deforestation on the catchment water yielding capacity were estimated as the ratio of the difference between simulated yields under different deforestation scenarios and those simulated under the pre-deforestation scenario of 1973. Results show that forest cover declined by 18.2% and a corresponding growth in land under agriculture by 18.2% in the period between 1973 and 2010. The decline in forest coverage resulted in an increase in the annual surface water yields of about 23% over the period of study. This is possibly as a result of limited groundwater recharge due to reduced infiltration capacity leading enhanced flow fluctuations and subsequently to lower flows during the dry seasons and a higher frequency of flood incidents during the wet seasons. The study has therefore, demonstrated that deforestation has reduced the stability of Mau Forest as a water tower as evidenced by fluctuations in streamflow. Conservation of the forest will enhance the catchment’s water holding capacity thereby ensuring a stable water supply to rivers emanating from it as a way of combating floods and low flows in the basin.
