EVALUATION OF AGRICULTURAL CONSERVATION PRACTICES ON ECOSYSTEM SERVICES IN SASUMUA WATERSHED, KENYA USING SWAT MODEL

Abstract

Degradation of agricultural watershed reduces the capacity of agro-ecosystems to produce Ecosystem Services such as improving water quality and flood mitigation. Conservation of degraded watersheds can abate water pollution and  regulate stream flows by reducing flash floods and increasing base flow as a result of enhanced infiltration. The objective of this study was to evaluate the effect of agricultural conservation practices on hydrology and water quality in Sasumua watershed, Kenya using SWAT model. Filter strips, contour farming, parallel terraces and grassed waterways were represented by adjusting the relevant parameters in the model and the resulting effect on sediment yield and water yield assessed. It was found that the reduction in sediment yield increased with increase in width of the filter strip but the increase was not linear. Contour farming reduced sediment yield by 49%, decreased the surface runoff by 16% and increased base flow by about 7.5%. Simulation of parallel terraces reduced sediment load by 85%, decreased surface runoff by 22% and increased base flow by 10%. Both the contour farming and terraces had only a slight change in total water yield. Grassed waterway simulated for some drainage ditches in the watershed reduced sediment load by about 41% at the outlet downstream of the drainage channels and by 23.5% for the entire Sasumua sub watershed. Terraces were found to be the most effective practices but due to their cost filter strips and contour farming were  recommended for agricultural lands and grassed waterways on the seasonal stream channels. Filter strips would eventually evolve into bench terraces.