Prediction of Sediment Inflow to Gefersa Reservoir (Using Swat Model) and Assessing Sediment Redution Methods
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Date
2012-03
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Addis Ababa University
Abstract
Sediment transport is a worldwide environmental problem that degrades soil productivity, water
quality, causes sedimentation to the reservoirs and increases the probability of floods. Gefersa
reservoir, one of the surface water supply sources of Addis Ababa city for the last 70 years, face
this problem. The reservoir supplies an average of 30,000m3 of treated water per day to the city.
Based on the 1979 and 1998 bathymetric surveys, with the assumption of linear yearly siltation rate
there is 22,252 m3/year of sediment inflow to the reservoir. In terms of soil loss from the catchment
area, this constitutes a loss of 575 tons/km2/year contributed by the catchment area. In this study,
Soil and Water Assessment Tool (SWAT) was used to calibrate and validate a hydrologic
component and sediment yield of Gefersa watershed. Back-calculation process was carried out to
estimate the natural inflow to the Gefersa reservoir. Sensitivity analysis, model calibration and
validation were also performed to assess the model performance. Nine highly sensitive parameters
were identified of which curve number (CN2) was the most sensitive one. The coefficient of
determination (R
2
), Nash-Sutcliffe (E
NS
) and the percent difference for a quantity (D) was used to
evaluate model performance during calibration and validation. Results found were satisfactory and
plausible for ungauged station i.e. R
2
= 0.78. E
NS
= 0.77 and D=-15.3 for calibration and R
2
= 0.72,
E
NS
= 0.70 and D=-20.7 for validation period. Sediment were calibrated and validated on annual
basis using D i.e.-1.37 for calibration and -6.9 for validation. Four scenarios are developed to
observe the impact of land use changes. Based on this, change of 53% and 16% forest to agriculture
resulted in 74.5% and 52.89% increase in sediment load. And change of 35% and 18% rangeland to
agriculture land increase 40.47% and 29.51% increase in sediment load .Based on results of
modeling, sediment reduction methods in the catchment as well as recovering storage capacity are
proposed. Of the available reservoir sediment management approach, watershed management is the
best method to reduce the yield of sediment and its entry into the reservoir. Periodic sluicing of
sediments through operation of bottom outlet gates would also help to ease the problem.
Key words: - Gefersa, Sediment, SWAT Watershed, Bathymetric survey
Description
Keywords
Gefersa; Sediment; SWAT Watershed; Bathymetric survey