Abstract:
The use of rainfall-runoff modeling in the decision making process of water resources
planning and management has become crucial. Extreme Runoff coupled with faulty
management systems may result in high rate of soil erosion and increased sediment
transport by changing the magnitude and pattern of runoff and sediment yield. The main
problem in the study area is erosion, sediment transport, and sedimentations also water
resource in the basin is scare due to limited rainfall, high evapotranspiration and
ongoing land degradation. The aim of this study was to model rainfall runoff process
and sediment yield in Geba River Watershed using Soil and Water Assessment Tool
(SWAT) model. Simulation was carried out using meteorological, hydrological and
spatial data that was collected from different sources. Model calibration period (2001-
2010) and validation period (2011-2015) were performed for monthly flow and sediment
data using Sequential Uncertainty Fitting (SUFI-2) within SWAT Calibration of
Uncertainty Program (SWAT-CUP). Model performance efficiency was checked by
coefficient of determination (R2
), Nash-Sutcliffe model efficiency (ENS), observation
Standard Deviation Ratio (RSR) and percent bias (PBIAS) indicating well performance
of model estimation. For flow, the values of R
2
, NSE, RSR and PBIAS were 0.71, 0.7, 0.55
and 6.7 during calibration period and 0.74, 0.7, 0.55 and 3.6 during validation period,
respectively and for sediment yield 0.72, 0.66, 0.61 and -8.7 during calibration period
and 0.79, 0.72, 0.53 and -11.3 during validation period, respectively. Average annual
sediment yield from Geba watershed after calibration and validation at Geba gauging
station was total sediment loading 18.440 ton/yr. Spatial variability of sediment yield was
performed using the simulated sediment yield results. Also based on the spatial result for
the critical sub-watersheds the design and development of best management practices
were proposed under different scenarios. Scenarios result showed that average annual
sediment yield reduction at entire watershed level after application of grassed waterway,
filter strips, terracing and contouring were 20.3%, 54.7%, 78.8% and 61.57%
respectively. Therefore, practicing terracing for Geba watershed should be developed
and encouraged for efficient sediment reductions.
