|MCGREGOR, KEITH - Retired ARS Employee|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2011
Publication Date: 1/1/2012
Citation: Dabney, S.M., Wilson, G.V., Mcgregor, K.C., Vieira, D.A. 2012. Runoff through and upslope of contour switchgrass hedges. Soil Science Society of America Journal. 76(1):210-219.
Interpretive Summary: A grass hedge is a special class of vegetative buffer that is effective in trapping sediment. Less is known about the effects of grass hedges in reducing and/or redirect runoff. We studied narrow switchgrass hedges on ¼ acre plots at Holly Springs, MS over eight years. During the first four years, special care was taken not to till directly next to the hedges and to smooth any tillage marks created. In the last four years, tillage was done right up to the edge of the hedges, more like on a working farm, and this resulted in the formation of a soil berm at the upslope margin of the hedges. The tillage berm greatly affected runoff flow patters. Without a berm most runoff passed through the hedges, but with the berm most runoff from small storms flowed upslope and along the hedges rather than through them. Even for the largest storms, more than half of the runoff flowed upslope of the hedges. Hedges reduced the fraction of rainfall that left the plots as surface runoff and hedges with soil berms reduced this runoff even more. The grass hedges decreased sediment delivery by 75% and this was not altered by the presence of the berm. These studies demonstrate the runoff and erosion control benefits that can be achieved by grass hedges and results support the current design criteria in the USDA-NRCS National Practice Standard, “Vegetative Barriers, code 601.”
Technical Abstract: Grass hedges are specialized vegetative buffers effective in trapping sediment but less is known about their ability to reduce and/or redirect runoff. Runoff and sediment yield from natural rainfall were measured during eight years from 0.1-ha contour-planted plots with and without 1-m wide switchgrass hedges (Panicum virgatum, L.) at their lower end. Plots had slope lengths of 22 m with a steepness of 5%, were located on silt loam (Typic Fragiudalfs) soils near Holly Springs, Mississippi, USA, and were cropped to conventional-tillage corn (Zea mays, L.) each year. During the first 4 years of the study, care was taken to conduct tillage is such a way that no soil was thrown into the grass hedge, while during the last 4 years of the study, primary disk tillage was performed immediately adjacent to the grass hedges, resulting in the progressive formation of a soil berm that caused the hedges to behave as low terraces. Rainfall-runoff relationships were interpreted in terms of the curve number (CN) and results showed that (1) hedges reduced the fraction of rainfall that ran off the plots and (2) hedges with soil berms reduced runoff even more. When berms did not exist, 95% of runoff passed through the hedges. In contrast, when berms existed, most runoff (>70% from events <10 mm/d; >55% for events <80 mm/d) passed above and parallel to the hedges. Results suggest that for the short slope lengths investigated, the hydraulic resistance of the grass vegetation had little effect on runoff flow paths, but that berms created by typical tillage operations upslope of hedges created oriented roughness that enhanced the conservation benefit of stiff-grass hedges. Without or with a berm, grass hedges decrease sediment yield by a factor of 0.25 to 0.28.