GIS-based soil erosion modeling and sediment yield of the N’djili River basin, Democratic Republic of Congo

Abstract

In the Democratic Republic of Congo, the N’djili River and its tributaries are the most important potable source of water to the capital, Kinshasa, satisfying almost 70% of its demand. Due to increasing watershed degradation from agricultural practices, informal settlements and vegetation clearance, the suspended sediment load in the N’djili River has largely increased in the last three decades. With an area of 2,097 km², the N’djili River basin delivers high suspended sediment concentration, and turbidity levels that cause considerable economic losses, particularly by disrupting the operation in the N’djili and Lukaya water treatment plants, and increasing dramatically the cost of chemical water treatment. The objectives of this study are to: (1) determine the change in the land cover/use of the N’djili River basin for 1995, 2005 and 2013; (2) predict and map the annual average soil losses at the basin scale and determine the effects of land cover/use change on the soil erosion; (3) estimate the sediment yield and the sediment delivery ratio at the water intake of the N’djili water treatment plant; and (4) quantify the effects of ash concentration on water turbidity in order to understand the high turbidity observed at the beginning of the rainy season. The Revised Universal Soil Loss Equation (RUSLE) model was implemented in a Geographic Information System (GIS) to estimate the spatially distributed soil loss rates in the N’djili basin under different land uses. RUSLE model parameters were derived from digital elevation model (DEM), average annual precipitation, soil type map and land cover maps (1995, 2005, 2013) obtained from Landsat images. The land cover/use change analysis shows that bare land/burned grass/agricultural land cover represented almost 22% of the N’djili basin area in 2013 whereas it was covering only 6% of the basin area in 1995. Settlements, which covered about 8% of the basin area in 1995, represented about 18% of the N’djili Basin area in 2013. The expansion of settlements, bare land, burned areas and agricultural lands was realized at the expense of the forest, grass, and shrubs cover. The annual average soil loss rate of the N’djili River Basin is estimated to be 7 tons/acre/year for 1995, 8.7 tons/acre/year for 2005 and 16 tons/acre/year for 2013. In 2013, bare land, burned areas and rainfed crops produced about 60% of the soil loss. The analysis of the relationship between probability of soil erosion and annual average soil loss rates indicated that up to 82, 79, and 73% of the basin area are in the range of tolerable soil erosion (0 – 5 tons/acre /year) in 1995, 2005 and 2013 respectively. Based on the gross erosion and sediment yield observed in 2005 and 2013, the sediment delivery ratio of 4.6% and 4.1% were predicted in 2005 and 2013, suggesting that most of the soil eroded from upland areas of the basin is trapped on flood plains covered by grass, shrubs and trees. Regarding the effects of ash concentration on turbidity, this study found that turbidity increased as a power function of ash concentration.