ABSTRACTWithin the scope of the DUSER project (1968–74) research on land degradation and related problems was carried out in four catchments with reservoirs in Dodoma district, Tanzania. In this paper are presented findings from one of the catchments. Imagi. The results include quantitative indications of the seriousness of soil erosion and its implications on range conditions, surface water storage and water supply in central Tanzanian drylands.The catchment area covers 2.2 km2. It is situated in semi- arid environment with inselbergs and pediment slopes as predominant landforms. Mean annual rainfall is 528 mm and annual potential evaporation is well above 2000 mm. Rainfall is strongly seasonal and confined to the months December-April. The vegetation is dense thicket and miombo woodland on the inselberg slopes and shrub and sparse grass on the pediment. The soils are of low fertility. Present dominating land use is grazing by livestock.For a period of more than 20 years, beginning in 1929, when the reservoir was built, the area was reserved as catchment for the Dodoma town water supply. In that period hydro-meteorological research was carried out in the catchment. Much of the rainfall was found to be lost by evaporation from interception storage and transpiration from the catchment vegetation. Annual runoff from the catchment varied between 3 and 17 % of the rainfall. Losses through leakage and evaporation from the reservoir amounted in average to 50 % of the max. volume stored in the respective years. The small size of the catchment, the low rainfall and the great water losses all contribute to the reservoir being filled in one year out of five only. During the period 1928–55 the responsible authorities in Dodoma had to enforce rationing of water in altogether 10 seasons.The reservoir is gradually losing capacity through sedimentation due to soil erosion in the catchment. As part of the DUSER programme the volume of sediment was assessed through levelling and sounding of profiles across the reservoir. Inventories of vegetation and erosion features in the catchment were made through air photo interpretation and field studies.Erosion was found to be slight on the densely vegetated inselberg slopes while on the pediment surfaces runoff and soil loss are more prominent. Splash, sheet wash and rilling are the dominating types of erosion. Gullying occurs along cattle tracks.The observed variations in sediment yield are shown to be directly related to changes in land use. When the earlier protected catchment was opened for cattle grazing the sediment yield rose by 50 %. The average annual sediment yield for the period 1934–71 was 602 m3or 900 ton/km2which is comparatively low considering the steep slopes and the small catchment size. The rate of soil denudation distributed over the whole catchment varied during the same period between 0.50 and 0.83 mm p.a. which means a denuding effect measured on the pediment only between 1 and 3 mm p.a.The reservoir has on an average lost 0.78 % p.a. of its original volume and the maximum depth has decreased from 10.8 to 5.7 m. Provided the average rate of sedimentation remains the same in the future the total life of the reservoir will be 120–130 years.As the ground water resources of the Dodoma area are unsufficiently known and permanent water for domestic purposes and watering of stock is scarce it is recommended not to discontinue the construction of surface water reservoirs but each project should include a prognosis of expected rate of sedimentation and a plan for soil and water conservation measures in the catchment. And unless control can be obtained over cultivation practices and the number of animals grazing in the area little can be done to prevent land degradation.