Linking soil erosion dynamics with identification of source areas and stratigraphic-geochemical sediment core analysis for deciphering the impact of land-use change in the watershed of a drinking water reservoir in Central Kenya

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Land-use change and agricultural intensification frequently lead to increased soil erosion and sediment loads in tropical regions such as East Africa. This soil loss results in degraded agricultural fertility, as well as reduced storage volume in reservoirs, which is critical for water supply and/or hydropower generation. Despite the severe environmental problems caused by erosion, to date only limited knowledge regarding the magnitude and dynamics of erosion and transport of suspended sediments in rivers of East Africa exists. This project follows a novel ‘source-to-sink’ approach combining plot experiments, river monitoring, and detailed stratigraphic-geochemical analyses of dated sediment cores with the reconstruction of historical land-use change based on the analysis of maps and satellite images. The investigations will be carried out in the catchment of River Ruiru in the central Kenyan highlands. The river drains into the reservoir that since its construction in 1949 has continuously been operated for the drinking water supply of the capital city Nairobi. The project focuses on a detailed analysis of short heavy rainfall events that are expected to cause most of the erosion and river sediment load. Eroded particles are traced from source to sink by means of phosphorus fractions and selected biomarkers. This allows for the identification of sediment source areas, and by this to connect the different spatial scales ranging from plot to catchment. The combination of detailed process studies and historic reconstruction will improve the understanding of the impact of land-use change on soil erosion and associated matter transport during the last seven decades and short heavy rainfall events in a typical rural landscape of the East African highlands.