Surface water contamination can often be reduced by passing runoff water through perennial grass filters. Research was conducted in 2006 to 2008 to evaluate the size of cool season grass filters consisting primarily of tall fescue (Festuca arundinacea Schreb) with some orchard grass (Dactylis glomerata L.) relative to drainage area size in reducing runoff sediment and phosphorus (P). The soil was Pohocco silt loam Typic Eutrochrepts with a median slope of 5.5 %. The grass filters occupying 1.1 and 4.3 % of the plot area were compared with no filter with four replications. The filters were planted in the Vshaped plot outlets which were 3.7 × 11.0 m in size. The filter effect on sediment and P concentration was determined from four natural runoff events when nearly all plots had runoff. Filter effect on runoff volume and contaminant load was determined using total runoff and composites of samples collected from 12 runoff events. Sediment concentration was reduced by 25 % with filters compared with no filter (from 1.10 to 1.47 g L −1 ), but P concentration was not affected. The 1.1 and 4.3 % filters, respectively, compared with having no grass filter, reduced: runoff volume by 54 and 79 %; sediment load by 67 and 84 % (357 to 58 kg ha −1 ); total P load by 68 and 76 % (0.58 to 0.14 kg ha −1 ); particulate P (PP) load by 66 and 82 % (0.39 to 0.07 kg ha −1 ); and dissolved reactive P (DRP) load by 73 and 66 % (0.2 to 0.07 kg ha −1 ), respectfully. A snowmelt runoff event had 56 % greater DRP concentration compared with rainfall-induced runoff events. Grass filters reduced sediment and P load largely by reducing runoff volume rather than reducing concentration. Well-designed and well-placed grass filters that occupy 1.0 to 1.5 % of the drainage area and intercept a uniform flow of runoff from a drainage area can reduce sediment and nutrient loss in runoff by greater than 50 %.
Climate change scenarios predict an increase in the frequency of heavy rainfall events in some areas. This will increase runoff and soil erosion, and reduce agricultural productivity, particularly on vulnerable mountainous agricultural lands that is already exhibiting high rates of soil erosion. Haphazard implementation of soil and water conservation (SWC) interventions on scattered fields is inefficient in reducing soil erosion. The objective of this study was to identify areas at high risk of erosion to aid the design and implementation of sustainable SWC using GIS analysis and farmers' participation approach. A 25 m digital elevation model (DEM) was used to derive layers of flow accumulation, slope steepness and land curvature, which were used to derive an erosion-risk (priority) map for the whole watershed. Boundaries of farmers' fields were mapped and verified by the community and each field was classified into high, moderate or low erosion risk. Fields with low flow accumulation (top of hill) and/or steep slope and/or convex slope were assigned high erosion risk and therefore high implementation priority. The study showed that more than 64% of the fields were classified into high erosion risk areas. Accordingly, a community-watershed plan was established, revised and approved by the community.Incentive loans to implement SWC measures were distributed to 100 farmers based on the priorities of their fields. Judged by local farmers and using 16 randomly selected fields, 90% of the targeted areas were correctly identified using the erosion risk map. After two years, the conservation measures had led to marked improvement of soil conservation. The approach is simple and easy to comprehend by the community and provides scientific basis to prioritize the implementation of SWC and to target the most degraded areas, which amplify the impact of these in reducing the vulnerability to land degradation.
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