IMPACTS OF BEST MANAGEMENT PRACTICES (BMPS) ON WATER QUANTITY AND QUALITY OF MALEWA BASIN

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%.