Challenges to Defining Sediment Concentration-Discharge Relationships in the Ethiopian Highlands

Food systems around the world are being threatened by soil degradation while perpetuating the conditions that foster erosion. The challenge in investigating the factors that cause soil erosion is that estimates can vary largely depending on the scale being used. Small scale experimental plot results can give a more precise view of the processes responsible for detachment on a field scale, yet watershed estimates of soil loss will give a better representation of detachment and deposition that occurs as discharge leaves the catchment. We studied three small catchments in the Ethiopian Highlands for which long term runoff and sediment data are available. Watershed sediment yield ranged from 5 to 23 t*ha-1y-1 using estimates on a watershed scale, yet, as our investigation shows, it is the variety of small scale factors that affect the determination of sediment concentration-discharge relationships. The generation of sediment rating curves is a common, simple method for investigating erosion patterns and the relationship between sediment concentration and discharge, however in these three watersheds, their efficacy in explaining sediment concentration variations was limited. Stratification and time scale adjustments were implemented to enhance the rating curves, yet this improvement was limited. In analyzing the data in relation to cumulative effective precipitation, we point to consistent time periods in which tillage activity, vegetation growth and sediment supply have an effect of interfering with the generation of sediment rating curves. Thus, although the sediment rating curves vary through the growing season, they were consistent from year to year when plotted in stratified cumulative effective precipitation ranges after a dry monsoon season ended.