Abstract:
This paper evaluated the impacts of best management practices (BMPs) on water quality and quantity of
Malewa selected subbasins. Soil and Water Assessment Tool model was used in evaluating the effects of
implementing BMPs. The model was calibrated and validated before doing BMPs scenarios of changing filter
widths from 0 to 5m to 10m and altering the USLE-P factor from default value of (no conservation measure) to
0.65 and 0.1 respectively. The two scenarios were done independently. From the results, it was observed that
filter strips were having varying effectiveness at reducing overland flow, sedimentation, and removing nutrients.
The hydrologic benefit of riparian buffers increases with width. Considerable reductions in sediment
concentrations occur when 5m wide filter strips are simulated. However, increasing the filter strips by an
additional 5m (total 10 m) does not produce the same level of reductions as was observed for the 0 to 5m
condition. This suggests that benefits from implementing filter strips will taper off for further increases in filter
width. Reductions were slightly higher for sub-with moderate slope gradient compared to sub-basins with steep
slopes. Also headwater sub-basins recorded greater reductions in sediment exports (e.g., 17, 13) compared to
sub-basins located downstream (e.g., 23 and 19). Clearly greater improvements in water quality could be
achieved by targeting headwater sub-basins. The impact of simulating filter strips on the sediment load at the
main watershed outlet was also determined. The 5m filter scenarios produces a 17% reduction in sediment load,
whereas doubling the filter widths only decreases the load by an additional 5%.
