Analysis of Potential Impacts of Climate Change and Deforestation on Surface Water Yields from the Mau Forest Complex Catchments in Kenya
This study focuses on an in-depth understanding of the extent to which climate change and deforestation impact on the surface water yields in the Mau forest complex, the largest of the five water towers in Kenya. The forest complex forms the source of important national and international rivers including Sondu, Nyando, the Nile, Mara and Ewaso Ng‟iro which are economic life lines for three sectors of the country‟s economy: Tourism, Agriculture, and Energy. Demand for more arable land to support the area‟s growing population has led to large tracts previously preserved as gazetted forests being excised since late 1940s which has affected the hydrology of the water tower. The study analysed the past climate and forest cover changes and modelled future changes in climate and the extent to which these changes impact on the surface water yields in this region. Potential impacts of climate change and deforestation on surface water yields were analysed using a modelling approach in which observed and projected climate outputs from a regional climate model, commonly referred to as PRECIS, which stands for Providing Regional Climate for Impacts Studies, and changes in forest cover were used to drive a hydrologic model, the Soil and Water Assessment Tool (SWAT). Outputs from SWAT were used to assess the impacts of changes in climate and forest cover on the surface water yields from the Mau forest complex water tower as exemplified by changes in river flow volumes. The study analysed historical climate, forest cover and streamflow changes that have taken place within these catchments with a focus on South West Mau forest block located within Sondu River basin. Projections of future climates under the Special Report on Emissions Scenarios (SRES) A2 emissions scenario were obtained from the third generation Hadley Centre Regional Climate model (HadRM3) using PRECIS regional climate model while Landuse/Landcover (LULC) changes were obtained from LANDSAT satellite image analysis using supervised classification methods. It was shown from the analyses of historical data that the climate of the area has progressively become warmer and wetter since the 1970s. Analysis of temperature and rainfall indicated increasing trends while streamflow indicated a decreasing trend. Analysis of forest cover indicated increasing deforestation trends over the Mau forest complex of about 27% between 1973 and 2010. Analyses of mean daily maximum and minimum temperatures indicate that days and nights in this area have become warmer since the 1961-1990 baseline period by about 0.5˚C and 0.4˚C respectively. In the same period monthly rainfall distribution has shown increasing trends in the relatively dry DJF and SON seasons which have become wetter by about 7.5% and 9.2 respectively, and decreasing trends in the relatively wet MAM and JJA seasons which have become drier by about 2.2% and 4.5% respectively. The changes in the distribution of monthly rainfall translate into redistribution of seasonal water yields from the catchments. Analysis of projected temperature and rainfall shows strong indications that the climate of the area will significantly change in future under the SRES A2 emissions scenario with warmer and wetter climates being experienced by 2030 and beyond. The annual average temperatures and rainfall are expected to change by about 2.7˚C and 4.7% respectively by 2030, and by 4.7˚C and 18.9% respectively by 2050, relative to the baseline. The projected monthly rainfall distribution show increasing trends in the relatively dry DJF and SON seasons while showing decreasing trends in the relatively wet MAM and JJA seasons. Simulated water yields under climate change at the baseline forest cover scenario show an increasing trend but show decreasing trends as forest cover over South West Mau forest block diminishes. Projected water yields in 2010s and 2030s indicate a decreasing trend in potential water yields of about 0.69 MCM/yr in 2010s and 0.71 MCM/ yr in 2030s Results of the study indicate that indeed deforestation of the Mau forest catchment has notable impacts on water yields from the water tower. The study has shown that the overall impacts of climate change coupled with deforestation on the water yielding capacity of the Mau forest catchments will be a reduction in the potential annual water yields in the range of 15% and 16% per decade of the baseline yields to between 28% and 45% in 2010s and between 31% and 50% in 2030s respectively. Results of this study have provided useful insights into the impacts of climate change and deforestation on surface water yields which can be used to inform short, medium to long term planning of water resources. The results, methods and products of the study should be incorporated in the mainstream economic development strategies especially in the development of the national water resources master plan. To ensure adequate flow in rivers from this important water tower to sustain both the socio-economic and environmental uses, it is recommended that efforts to rehabilitate the Mau forest complex be stepped up and sustained.