ARS | Publication request: Topographic placement of management practices to reduce water quality impacts from pastures

Submitted to: Landscape Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 11, 2012
Publication Date: October 22, 2012
Citation: Piechnik, D.A., Goslee, S.C., Veith, T.L., Bishop, J.A., Brooks, R.P. 2012. Topographic placement of management practices to reduce water quality impacts from pastures. Landscape Ecology. 27:1307-1319.

Interpretive Summary: Best Management Practices (BMPs) protect streams by excluding cattle from streambanks and by filtering the run-off flowing from animal heavy use areas such as feeding and watering stations. Conservation standards recommend placing buffers and filter strips downslope from heavy use areas, but do not explain how to do so. We compared two methods of siting BMPs: placing them at the closest point on the stream, or using elevation maps to trace topographic effects on water movement from the concentration area to the stream. Fine (1m resolution), medium (10m) and coarse (30m) elevation maps all showed that water from heavy use areas can enter the stream a long distance from the closest point. Flow paths and stream entry points delineated using elevation data differed from the straight-line results, demonstrating that the most direct solution is not always the most effective one for conservation planning. Streamside areas differed considerably in the drainage area that flowed through them; this information could be used to site riparian buffers so as to filter as much overland flow as possible. Placement of actual BMPs and animal heavy use areas in the Spring Creek watershed of central Pennsylvania were used as a case study of topographic methods. The medium-resolution elevation maps (10m) were most useful for planning the locations of new BMPs.

Technical Abstract: Best management practices (BMPs) such as streambank fencing and vegetative buffers lessen environmental impacts on streams from pasture-based agriculture by limiting livestock time in streams and intercepting sediment and nutrients in overland flow. Placing streamside BMPs at the closest point to the heavy use area may be adequate in high relief areas, but in more homogeneous landscapes the path of water movement and the location where it enters the stream may not be obvious during on-farm examination. Analysis of digital elevation models (DEMs) offers a more accurate approach to siting BMPs, and conversely to placing new heavy use areas to take advantage of existing BMPs. Advances in geospatial data collection and processing capabilities have made feasible the use of these data for conservation planning. Aerial photography of three headwater catchments of the Spring Creek watershed in central Pennsylvania was used to digitize agricultural areas, livestock heavy use areas, and existing BMPs. Flow path lengths, stream entry points, and area drained by each streambank cell were evaluated at three levels: the entire study region, only agricultural areas, and livestock heavy use areas; and using elevation data at 1m, 10m, and 30m resolutions. Flow paths and stream entry points delineated using elevation data differed from the straight-line results, demonstrating that the most direct solution is not always the most effective one for conservation planning. Streamside areas differed considerably in the drainage area that flowed through them; this information could be used to site riparian buffers so as to filter as much overland flow as possible. The 30m DEM was too coarse to effectively model hydrology at the scale of agricultural land use in this watershed, while small variations in the 1m data led to inaccurate flow paths and drainage areas. Of the currently available options, we recommend the 10m elevation data for agricultural planning. Existing BMPs placed following conventional procedures did not buffer the majority of the mapped heavy use areas. Using GIS tools and topographic mapping can improve the efficiency of BMP placement by targeting the most important streambank areas and ensuring that animal heavy use areas are properly mitigated.