Impact Evaluation Of Soil And Water Conservation Measures On Ecosystem Services In Thikachania Catchment, Kenya
Gathagu, John Ng'ang'a
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Across the world, ecosystem services such as water provision, sediment trapping and floods regulation are threatened by the increased degradation of catchments. This is as a result of increased pressure from population growth, improper farming methods and encroachment into riparian areas leading to soil erosion and water quality deterioration. These ecosystem services could be restored by implementing best management practices to enhance flow regulation and control sediment yield. In Thika-Chania catchment, the failing services are manifested in the deterioration of water quality, sedimentation of reservoirs and inadequate surface water in dry seasons. The aim of this study was to evaluate the impacts of implementing soil and water conservation measures on water and sediment yield. The Soil Water and Assessment Tool (SWAT) was used to evaluate contour farming, filter strips, terraces and grassed waterways on water and sediment yield. Calibration and validation of SWAT to simulate streamflow was conducted using SWATCUP-SUFI2 at RGS 4CB05. Results indicated respective monthly NSE, coefficient of determination and PBIAS of 0.66, 0.69 and 10.3 and 0.73, 0.75 and 7.2 during calibration and validation. Sediments calibration and validation was achieved using data obtained from a bathymetric survey conducted in the catchment. The average annual sediment yield was found to be 21.507 t/ha at the outlet of the catchment and average annual surface runoff of 202.28 mm. Model parameters were adjusted to simulate the impacts of contour farming, filter strips, terraces and grassed waterways. Terraces were found to be most effective in reducing sediment yield by 80.70% while grassed waterways, 3m-filter strips and contour farming reduced average annual sediment yield by 53.97%, 46.04% and 35.81%. A combination of various soil and water conservation methods were also evaluated. Surface runoff and total water yield was only influenced by contour farming and terraces. We concluded that soil and water conservation methods are effective in improving the provisioning and regulating services of the ecosystem. Combining more than one conservation method would have a greater impact on sediment yield reduction. A cost benefit analysis of the conservation methods need to be carried out before their implementation at the catchment or sub-catchment level.