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Abstract
Abstract. Loss of top soil and subsequent filling up of reservoirs in much of the lands with variable relief in developing countries degrades environmental resources necessary for subsistence. In the Ethiopia highlands, sediment mobilization from rain-fed agricultural fields is one of the leading factors causing land degradation. Sediment rating curves, produced from long-term sediment concentration and discharge data, attempt to predict suspended sediment concentration variations, which exhibit a distinct shift with the progression of the rainy season. In this paper, we calculate sediment rating curves and examine this shift in concentration for three watersheds in which rain-fed agriculture is practiced to differing extents. High sediment concentrations with low flows are found at the beginning of the rainy season of the semi-monsoonal climate, while high flows and low sediment concentrations occur at the end of the rainy season. Results show that a reasonably unique set of rating curves were obtained by separating biweekly data into early, mid, and late rainfall periods and by making adjustments for the ratio of plowed cropland. The shift from high to low concentrations suggests that diminishing sediment supply and dilution from greater base flow during the end of the rainfall period play important roles in characterizing changing sediment concentrations during the rainy season.
Highlights
Soil erosion in the Ethiopian highlands, a natural phenomenon due to erosive rainfall and steep and undulating topography, is enhanced under agricultural systems that reduce protective soil cover (Vanmaercke et al, 2010; Haile et al, 2006; Hurni et al, 2005)
The sediment load is greatest for the watersheds in either July or August, sediment concentrations are decreasing, showing that the increase in load is offset by greater increases in discharge (Fig. 2)
Higher concentrations for low flows and lower concentrations for high flows in the whole data set made the use of a single rating curve impractical
Summary
Soil erosion in the Ethiopian highlands, a natural phenomenon due to erosive rainfall and steep and undulating topography, is enhanced under agricultural systems that reduce protective soil cover (Vanmaercke et al, 2010; Haile et al, 2006; Hurni et al, 2005). A large number of soil and water conservation (SWC) practices have been installed attempting to reduce soil loss (Hurni, 1988; Nyssen et al, 2008; Herweg and Ludi, 1999) It is not clear what the effectiveness of these practices is beyond the immediate locations of where they have been tested (Vanmaercke et al, 2010). It is imperative in order to prevent siltation of the reservoirs of the dams planned on the major rivers, that the relationship between soil loss, discharge, and sediment concentration in rivers is understood. One of the obstacles is that most of our knowledge on erosion is based on empirical evidence for temperate climates while the Ethiopian highlands have a monsoonal climate with a long dry period and either one or two rainy periods
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