Abstract

Abstract. Erosion modeling has been generally scaling up from plot scale but not based on landscape topographic position, which is a main variable in saturation excess runoff. In addition, predicting sediment loss in Africa has been hampered by using models developed in western countries and do not perform as well in the monsoon climate prevailing in most of the continent. The objective of this paper is to develop a simple erosion model that can be used in the Ethiopian Highlands in Africa. We base our sediment prediction on a simple distributed saturated excess hydrology model that predicts surface runoff from severely degraded lands and from bottom lands that become saturated during the rainy season and estimates interflow and baseflow from the remaining portions of the landscape. By developing an equation that relates surface runoff to sediment concentration generated from runoff source areas, assuming that baseflow and interflow are sediment-free, we were able to predict daily sediment concentrations from the Anjeni watershed with a Nash–Sutcliffe efficiency ranging from 0.64 to 0.78 using only two calibrated sediment parameters. Anjeni is a 113 ha watershed in the 17.4 million ha Blue Nile Basin in the Ethiopian Highlands. The discharge of the two watersheds was predicted with Nash–Sutcliffe efficiency values ranging from 0.80 to 0.93. The calibrated values in Anjeni for degraded (14%) and saturated (2%) runoff source area were in agreement with field evidence. The analysis suggests that identifying the runoff source areas and predicting the surface runoff correctly is an important step in predicting the sediment concentration.

Highlights

  • In the African highlands, erosion has occurred for a long time (Hudson, 1957, 1963; Lal, 1985; Nyssen et al, 2004)

  • A comparison of predicted and observed daily stream flow for the Anjeni watershed is shown in Fig. 5 and in the Supplement Fig. S1 and predicted and measured sediment concentration in Fig. 6 and Fig. S2 in the Supplement

  • The model calibration suggests (Table 2) that 14 % of the Anjeni watershed and 20 % of the Blue Nile Basin consist of degraded area with shallow soil or exposed hardpan, which requires only a little rain to generate direct runoff

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Summary

Introduction

In the African highlands, erosion has occurred for a long time (Hudson, 1957, 1963; Lal, 1985; Nyssen et al, 2004). Despite large investments in soil and water conservation practices, sediment yields have been increasing in Africa (Lal, 1985; Fleitmann et al, 2007). For the Blue Nile Basin, a part of the Ethiopian Highlands, reported soil losses vary from 1 to over 400 t ha−1 yr−1 (Hurni, 1988; Mitiku et al, 2006; Tebebu et al, 2010) with an average of 7 t ha−1 yr−1, or equivalent to a depth of 0.5 mm yr−1 (Garzanti et al, 2006). At the same time several large dams are planned in the Blue Nile Basin; these future developments urgently need better ways to reduce soil loss in order to sustain the efficient operation of the dams well into the future

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