Ethiopia can realize its food security and sustainable development not by depending on rainfed agriculture but by utilizing its plentiful water resources and conserving soil resources. Proper utilization of these resources necessitates assessment and management of the quantity and quality of the water resources both spatially and temporally. To quantify such variability of hydrologic fluxes and flow of matter a distributed watershed model with a high resolution of space and time is necessary.

The objectives of this study were
1. to calibrate and validate SWAT model for flow and sediment yield,
2. to assess impacts of land management practices on water and sediment yield
3. to assess sensitivity of the hydrologic system of the watershed to the climate change
4. to compare sediment yield from SWAT model and 137Cs method

SWAT was successfully calibrated and validated for three measurement stations for flow and at one measurement station for sediment yield. The model performance evaluation statistics clearly show that the model can produce reliable estimates of daily, weekly and monthly discharge and monthly sediment yield. Results of this analysis demonstrated that SWAT is a capable modeling tool for analyzing hydrologic processes and water resources planning and management and sedimentation in the Upper Awash River Basin using the set of optimized parameters along with the other parameters.

Following calibration and validation of SWAT model different land management scenarios were tested with respect to sediment yield. Results of simulations were used to illustrate the potential reduction in sediment yield that could be expected by various land management practices. Results of the simulation show that some land management practices can result in reduction of sediment yield at Hombole station by about 10-72%. The simulation using parallel terraces with the reduction of slope length by 75% gave the highest reduction in sediment yield which is from 21.53t/ha/y (baseline scenario with current management practice) to 6.09t/ha/y.

The sensitivity of the hydrologic system and sediment yield of Upper Awash River basin to future climate change were analyzed using SWAT model by formulating climate change scenarios. A regression analysis of the flow responses for the scenarios with a precipitation decrease and increase resulted in a slope of 2.01, indicating that a unit percent increase in precipitation produced a 2.01 percent increase in flow for the Upper Awash River Basin. But sediment response to climate change behave differently it is expressed by exponential regression equation and as the rainfall increases the sediment yield increases exponentially. This is because of the limited capabilities of absorbing the water, which is falling as a rainfall as a result the surface runoff will increases, and hence sediment yield will increase.

The soil erosion and deposition rates using137Cs technique were calculated by proportional model (PM) and simplified mass balance model (MBM1) calibration models. Comparison was made between results from 137Cs technique and SWAT model. Based on this comparison the erosion rate simulated by SWAT model (21.53t/ha/y) and the annual soil erosion rate modeled by PM (21t/ha/y for Tede and 34t/ha/y for Ejere site) are almost in the same range. The erosion rates from the PM were extrapolated and maps soil erosion-deposition were created and this can be used to formulate appropriate soil conservation practices.

E-mail: dilnesaw@yahoo.com