Abstract:
Soil and Water Assessment Tool (SWAT) was used to simulate streamflow in
the flood prone Nyando basin in Western Kenya. The objective of this study
was to evaluate the impact of changes in land cover, climate and reservoir
storage on flooding in the basin. Calibration of SWAT against streamflow in
the Nyando basin attained a coefficient of determination (R2) values ranging
from 0.45 to 0.72. From scenario simulation it was observed that 100% forest
cover reduced the simulated peak flow from 111 m3/s (base simulation) to
69 m ' Is at river gauging station 1 GD03. On the other hand, 0% forest
increased peak simulated flow from 111 m3/s to 121 m3/s at 1GD03. Rainfall
increase by 15% would increase peak flow from 111 m3/s to 159 m3/s. Change
in temperature was associated with reduced peak flow and lower dry season
flow; however, the effect of the expected change was less than the change
associated with rainfall. The proposed reservoirs would significantly alter the
streamflow characteristics. For instance, the proposed reservoirs would reduce
the peak flow from 60 m3/s (base simulation) to 34 m3/s at 1GD07. However,
in Ainabng'etuny tributary the only proposed reservoir would not be effective,
since it reduces the peak flow by 1 m3/s only at 1GB03. It was therefore
concluded that the current trend of land use change aggravates flooding in the
basin and should be controlled. Also, the proposed four reservoirs are
immediate solution to the flooding problem, while other alternatives such as
pan storage could be considered for the Ainabng'etuny tributary.
