Specific sediment yield in Tigray-Northern Ethiopia: Assessment and semi-quantitative modelling

Sediment deposition in reservoirs is a serious off-site consequence of soil erosion in Tigray (Northern Ethiopia). So far insufficient and less reliable sediment yield data have been collected for Northern Ethiopia. Nor are there any adaptable methodologies for sediment yield assessment in the country as a whole, which could be used when designing new reservoirs. This study addresses those problems by (1) undertaking reservoir sediment deposition measurements and (2) by calibrating and adapting the Pacific Southwest Inter Agency Committee (PSIAC) and the Factorial Scoring Model (FSM) sediment yield assessment models to Ethiopian conditions. Field rating of catchment characteristics and the sediment yield data from the reservoir survey were used for calibration and validation of the models. Our reservoir survey indicates that specific sediment yield (SSY) varies significantly between catchments: i.e. 487 t km−2 year−1 to 1817 t km−2 year−1 with an average of 1054 (± 446) t km−2 year−1. Since the variability of SSY is high between the studied reservoirs, care should be taken in the study area to adopt representative SSY values during reservoir and soil water conservation planning. The PSIAC SSY prediction is found to fit well with observed SSY without adjustment. While the FSM was found to have, after modifying the description of factors and incorporating new controlling factors, a good fit between the predicted and observed SSY. Studies of the relationship between the known sediment yield rates and the catchment conditions using semi-quantitative approaches such as PSIAC and FSM can be of substantial benefit in extrapolating data for areas where no detailed information is available in a cheap and quick way. However, calibration and modification of such models may be necessary if they are to be used beyond the region where they were developed.