Sediment budgeting: A case study in the Katiorin drainage basin, Kenya

Abstract

AbstractThe primary objective of this study was to compute a detailed budget for a small semiarid tropical drainage basin in Kenya. Results indicated that transfer of sediments (‘inputs’) from primary source areas was minor in comparison to changes in storage. The major sediment source area within the Katiorin drainage basin was the colluvial hillslope zone. The net change in storage within this zone was approximately 2100 Mg yr−1. Surface wash and rilling were the dominant transport processes responsible for the remobilization of colluvial sediments. Sediment storage within the in‐channel reservoir increased by 60 Mg yr−1, which was minor when compared to the total store of sediment in this reservoir. During 1986, the channel network stored only a small fraction ( < 3 per cent) of the sediment delivered from the hillslope subsystem. Therefore, the in‐channel reservoir had limited influence on sediment conveyance to the basin outlet. These data indicate that a static equilibrium condition cannot be assumed within the Katiorin drainage basin. Such an assumption would result in erosion estimates of approximately 5.5 mm yr−1 for the entire basin (based on a sediment output of 7430 Mg km−2 yr−1 and a measured bulk density of 1.35 Mg m−3). However, this masked the actual rates of 1.2 to 7.1 mm yr−1 in subbasin primary source areas, and rates of 0.6 to 17 mm yr−1 for colluvial material in the various subbasins. The extreme accelerated erosion rates resulted from minimal ground vegetation, steep slopes, soil crust formation, an erodible substrate, and a well‐integrated drainage network for rapid conveyance of sediments from the hillslope subsystem to the basin outlet.