Effect of vegetation cover and land slope on runoff and soil losses from the watersheds of Burundi

Abstract

Soil loss and runoff data were obtained in two consecutive seasons (1986–1987) from 17 runoff-erosion plots which represent different conditions of vegetation cover, slope gradient and slope length. Results revealed that soil loss from bare fallow soil per unit of rainfall and also per unit of runoff increased as the slope gradient increased. The associated runoff-rainfall ratios were always high under the steep slopes. As slope length increased from 5 to 20 m, soil loss per unit of rainfall was doubled and sediment concentration increased, five-fold, while runoff-rainfall ratio decreased to one-half. C u (crop-factor calculated from the Universal Soil Loss Equation (USLE)) values ranged from 0.013 to 0.104, C u, (crop-factor calculated as soil loss ratio from vegetative and fallow plots) from 0.064 to 0.353 and C p from 0.004 to 0.114. The highest values were obtained under cultivated soils and the lowest values under forest and grass soils. Statistical analysis showed that C u or C p (crop-factor related to plant cover characteristics) is recommended in the calculation of the C-factor in the USLE rather than C r which over-estimates the C-factor and thus the total soil loss. Also, cultivation contributed to significant losses of sediment and runoff due to the removal of vegetation cover. The relationship between runoff and soil loss was linear and significant under various conditions of slope and vegetation cover.