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

在东非等热带地区，土地利用变化和农业集约化经常导致土壤侵蚀和泥沙负荷增加。这种土壤流失导致农业肥力下降，以及水库储水量减少，而水库储水量对供水和/或水力发电至关重要。尽管侵蚀造成了严重的环境问题，但迄今为止，对东非河流中悬浮沉积物的侵蚀和运移的规模和动态只有有限的了解。该项目采用了一种新颖的“源到汇”方法，结合了地块实验、河流监测和对年代沉积物岩心的详细地层-地球化学分析，以及基于地图和卫星图像分析的历史土地利用变化重建。调查将在肯尼亚中部高地的瑞鲁河集水区进行。这条河汇入水库，自1949年建成以来，一直为首都内罗毕提供饮用水。该项目侧重于对短期强降雨事件的详细分析，这些事件预计会导致大部分侵蚀和河流泥沙负荷。利用磷组分和选定的生物标志物对侵蚀颗粒进行了从源头到汇的追踪。这样就可以识别沉积物来源区域，并以此连接从地块到集水区的不同空间尺度。详细的过程研究和历史重建相结合，将提高对过去七十年来土地利用变化对土壤侵蚀和相关物质运输的影响以及对东非高地典型农村景观的短期强降雨事件的了解。

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.