MODELING OF SEDIMENT YIELD USING SWAT AND STOCHASTIC MODELS (CASE STUDY ON MEGECH DAM WATERSHED, BLUE NILE BASIN, ETHIOPIA)

Abstract

Megech Dam is sited at the out let of Megech watershed of area 408 km 2 where siltation is a threat for future service life of the reservoir. This is because of 59.13% of the watershed is cultivated, even steep slope is under cultivation. Accordingly, the two objectives of this study were to simulate the sediment yield inflowing to the reservoir site and to propose possible mitigation measures to reduce the quantity of sediment yield inflowing to the dam site. The SWAT2009 model was calibrated and validated against measured flow and sediment data. Both, calibration and validation results, showed a good match between measured and simulated flow and suspended sediment.This calibrated tool was used to predict sediment yield, identify spatial distribution of sediment, and to test the potential of watershed management interventions in reducing sediment load. In This research, two mitigation measures were used to reduce the sediment inflowing to the dam site. These were: (1) implementation of terrace in the agricultural HRUs of potential sub basins and (2) Area enclosure (afforestation) of any land use on steep slope (greater than 25%) in the whole Megech Dam watershed. Use of terrace in the agricultural HRUs and afforestation of steep slope areas reduced sediment yield (SYLD) inflowing to the reservoir by 23% and 42% respectively. The third objective of this study was to model sediment yield of Megech Dam watershed using stochastic models. Stochastic analysis modeling and simulation (SAMS) model and Thomass Fiering model were used for this objective. Annual and monthly sediment load characteristics in the temporal domain were modeled and 50 years of annual sediment yield (t/ha) data that have similar statistical characteristics with the historical SYLD data were generated using AR(1) and Thomas Fiering model.