Effectiveness and Cost of Improving Vegetated Filter Zones by Installing Level Spreaders to Disperse Agricultural Runoff
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Date
2000-12-13
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Abstract
Recent studies of agricultural watersheds have indicated that vegetated filter zones (VFZ) maynot function optimally because of channelized surface runoff through the zone. One proposedsolution has been to disperse channelized surface runoff in the receiving portion of the filter zonewith level spreaders. An initial feasibility study demonstrated that level spreaders substantiallyimproved filter strip performance and reduced non-point source pollution (NPSP) outputs tosurface water. However, widespread adoption of level spreaders as a best management practicerequires designs that are both functional and cost effective over a range of site characteristicsincluding filter zone (FZ) condition and with varying sources of NPSP and under differing ratesof loading.The main objective of this research was to evaluate different level spreader designs andconfigurations for dispersing channelized agricultural runoff and to evaluate them for enhancing FZ effectiveness on several sites with greatly differing watershed characteristics. Otherobjectives included estimating construction and maintenance costs and developing recommendations for level spreaders for specific watershed and FZ conditions. Level spreaders with associated instrumentation were constructed on eight watersheds from 1989to 1997 representing a wide variety of watershed and FZ conditions. Spreaders withoutassociated instrumentation were constructed on three watersheds. Source areas included cropsunder both conventional and conservation tillage, a pasture, a dry-lot for dairy cattle, and a pavedand partly-roofed cattle containment area. All spreaders tested were designed to be permanentinstallations. Designs tested included commercial galvanized gutters, treated wood, fabric-linedditches with gravel just above and below the ditch, and vegetated berm and trench. Reductions in NPSP through-puts were a function of filter zone size, input concentration, runoffvolume, and season. Filter zones where FZ area was greater than two percent of source-area sizegenerally removed a least a third of each analyte. Of the six sites with dispersed flow which didnot have large unaccounted contributions to the FZ, only one failed to reduce N by 30 percent.All spreader designs improved FZ performance. Level spreaders with larger cross-sectional areaswere more effective for high peak-flow events. However, spreaders with limited cross sectionalsuch as above-ground gutters have potential where excavation of ditches or shaping of spreaderswith large equipment is a problem such as in forests or on steep slopes. The most easilymaintained design is a vegetated berm and trench spreader shaped from soil. However, its use ispractical only where tree roots are minimum and where farm equipment can maneuver duringinstallation. This design also allows limited vehicle traffic over the spreader.
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PhD
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Forestry
