Hillslope Hydrology in a Deeply Weathered Saprolite and Associated Nitrate Transport to a Valley Bottom Wetland in Central Uganda

While interflow from the slopes can be crucial for water and nutrient availability in low-input farming systems in wetlands in East Africa, very little data exist on hillslope hydrology and associated nutrient transport in deeply weathered saprolites over crystalline rocks. This study aims for a better process understanding of interflow generation and routing in this environment and its contribution to water and nitrate availability at the wetland fringe of a valley bottom wetland in central Uganda. The study was set up as a plot study following a multi-method approach, including interflow trenches, soil analysis, and geo-electrical measurements. We found that interflow generation was related to the undulating subsurface topography and the conductivity of the upper saprolite, while interflow was conducted to the slope toe via small, perched aquifers and preferential flow paths within the saprolite, which are connected during the rainy season. Interflow volumes and nitrate transport were strongly related to the land-use type and rainfall characteristics. As the nitrate delivered from the slopes was quickly lost in the anaerobic environment of the wetland fringe, sustainable agricultural management should focus on the slope toe and the upland positions.