Rainfall erosivity and sediment yield in Northeast Algeria: K’sob watershed case study

Abstract

In semiarid areas, rainfall is often intense on dry soils with poor vegetation which might lead to high soil erosion susceptibility. Therefore, an appropriate assessment of the rainfall erosivity is of particular importance due to negative effects caused by top soil depletion and excessive sediment loading to receiving waters of reservoirs. The present study was conducted on the Wadi K’sob watershed (1480 km2) in northeast Algeria with an aim to examine the rainfall erosivity. The calculation of such an index is based on rains exceeding a specific threshold and requires rainfall data with a fine temporal resolution, which, often, are rare or difficult to acquire. The examination of daily rains occurring before a flood event carrying sediment load showed a spatial variability of the thresholds of rainfall erosivity. The seasonal values of the thresholds are low and lying between 2 mm in summer and 6 mm in winter, highlighting an erosivity process characteristically high in semi-arid regions. Empirical relationships, established at seasonal scale, were proposed as an alternative solution to the R-index calculation derived from the Revised Soil Loss Equation. The determined models allowed us to simulate the erosivity of rainfall events as a function of daily rain. Between 68 and 78% of the variance of rainfall erosivity is explained by the daily rainfall giving rise to an erosive rainfall event. Then, the spatial mean of the annual erosivity index fluctuated between 228 and 386 MJ mm ha−1 h−1 year−1 with an interannual average of 302 MJ mm ha−1 h−1 year−1, which if underestimated by 6%, the erosivity index quantified according to the Revised Universal Soil Loss Equation. Rainfall erosivity is the determining factor in sediment yield with a different degree of binding during the year. In autumn, 68% of the variance in sediment production is explained by rainfall erosivity, compared with only 42% in spring due to changes in soil conditions, including the presence of a vegetation cover that protects the soil against rainfall erosivity.