Evaluation of sedimentation in run-off-river for hydroelectric power development in the western region of Uganda

Abstract

Soil erosion is the major cause of land degradation and reservoir sedimentation. Therefore, modeling of run-off-river and sediment yield at a watershed level is very important. A conceptual, distributed and continuous time, SWAT 2012 (Soil and Water Assessment Tool) model was selected for the simulation of the run-off and sediment yield from River Mpanga watershed. The main objective of this study was to quantify the river flows, sedimentation yield and its impact to hydropower development in Mpanga catchment in western Uganda. A 30 m DEM (Digital Elevation Map), 1 km spatial resolution of land use map and 10 km spatial resolution of soil map, daily precipitation and daily minimum and maximum air temperatures from 1979 to 2014 were used to set up the model for simulation. The stream flow data available from 1999 to 2012 was used to calibrate and validate the SWAT model for Mpanga catchment. The instant daily sediment record for a few of the months within a particular year from 2001 to 2007 was available. The model was calibrated by using both automated calibration and manual calibration for daily stream flow by using the flow data from 2007 to 2008 and validated for 2009 and 2004. The calibration for sediment was not conducted for the whole period for which the sediment data was available. The model was not validated for sediment yield as there was not enough length of data. The performance of the model was evaluated by using a time series plots of observed and simulated value and the statistical measures of coefficient of determination (R ) and the Nash Sutcliffe efficiency (NS). The statistical analysis of calibration results for Mpanga watershed showed unsatisfactory agreement between observed and simulated daily values, with NS of 0.44 in discharge simulation; and NS value during the validation period was 0.59. In general, the model was capable of simulating runoff and sediment from Mpanga watershed for mean annual and monthly flows; however, the result from sediment simulation was not as good as the runoff. This is because of the inability of the SWAT model to realistically simulate the sediment from gully erosion, landslide, mass wasting and insufficient dataset about the current management operations and practices within the catchment. The high sediment loading, especially suspended sediment is likely to have adverse effects on the established hydropower plants.