Suspended Sediment Concentration And Discharge Relationships In The Ethiopian Highlands

Abstract

This study presents a basic investigation on the influence of discharge on the variable sediment concentrations in the Andit Tid, Anjeni, and Maybar watersheds of northern Ethiopia. With valuable sediment concentration and stage measurements recorded by the Soil Conservation Research Programme at monitoring sites starting in the 1980's , it is possible to gain knowledge about the important processes that affect changing sediment concentrations. For the years of observation, sediment yield estimates were 5.4 t[MIDDLE DOT]ha-1y-1, 22.5 t[MIDDLE DOT]ha-1y-1, and 8.8 t[MIDDLE DOT]ha-1y-1 for Andit Tid, Anjeni, and Maybar respectively. Although catchments tend to exhibit positive correlation between runoff and sediment concentration, albeit with a degree of scatter, these catchments exhibit a peculiar scatter of observations with high concentrations for low flows and low concentrations for high flows. Due to this behavior, a sediment rating curve for all observations was unsatisfactory and subsequently, using several cumulative precipitation divisions to group related observations, non-linear regression showed a weak correlation for the Andit Tid basin, for most of the Anjeni basin and during the early portion of the rainy season in the Maybar basin. Stratification of points into cumulative precipitation ranges served to bring some order to the scatter plots of sediment concentration versus discharge to demonstrate that higher concentrations are observed during the beginning of the rainy season and lower concentrations are observed towards the end of the rainy season. Still, the exhaustion of available soil for transport may prove to have an important role in limiting the ability of discharge to account for the variability of sediment concentration throughout the season.