Soil Erosion and Sedimentation in Four Catchments Near Dodoma, Tanzania

Abstract

The catchments studied are located in semi-arid central Tanzania. The mean annual rainfall at Dodoma is 567 mm and the dry season is 7–8 months long. All four catchments are underlain by granitic rocks of Precambrian age. Pediment slopes and rocky inselbergs are the predominant landforms. The land use is a combination of cattle grazing and cultivation.The rate of sedimentation in four reservoirs was measured by periodic re-levelling of known cross- profiles on the floors. Inventories of the erosion features in the catchment areas were made through air photo interpretation and field checking.Rainsplash and sheet wash are probably the most important types of erosion. Gullies appear in distinct zones on the upper pediment slopes near the foot of the inselbergs, but gullying is probably of less quantitative importance in these areas as compared to splash and wash.The reservoirs have high rates of sedimentation and two of them (Ikowa and Matumbulu) have very short expected total life-lengths of 30 years and still shorter economic life-lengths. The annual sediment yield corresponding to the sedimentation in the four reservoirs is 195, 406. 601 and 729 m3/km2 (mean values for longest period of available data). In addition large volumes of sediment have been deposited upstream of the reservoirs as thin sandy sheets on lower pediments, as sand fans along stream channels and silty-clayey mbuga floodplains, the latter particularly in the Ikowa catchment. The reservoir sedimentation corresponds to a soil denudation rate of 0.2–0.73 mm per year.Erosion control and increased life-length of the reservoirs is mainly a question of better management of grass cover in the catchments and protection of cultivated fields against splash erosion. Reduction of stock numbers, of overgrazing and of excessive burning of grass and mulch is necessary in order to combat erosion and increase the life-lengths of the reservoirs. Three of these have considerable potential as future groundwater storages as they are being filled mainly with sandy sediments. This potential should be further investigated.