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Title: Two-dimensional flow patterns near contour grass hedges

item Vieira, Dalmo
item Dabney, Seth

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2011
Publication Date: 11/18/2011
Citation: Vieira, D.A., Dabney, S.M. 2011. Two-dimensional flow patterns near contour grass hedges. Hydrological Processes. 26(15):2225-2234. DOI: 10.1002/hyp.8262.

Interpretive Summary: Grass hedges are narrow strips of stiff vegetation planted on elevation contours to slow runoff and trap eroded sediments. Grass hedges modify the distribution of runoff over agricultural fields by creating areas of ponding and by diverting water to run laterally, alongside the hedges. Soil deposition near the hedges caused by tillage operations may reduce the amount of runoff that crosses the hedges. This can be beneficial if water is transported without causing erosion, or detrimental if large flows accumulate creating localized erosion and the contributing to the formation of ephemeral gullies. A grid-based numerical model was developed that allows the computation of flow depths and velocities resulting from rainfall events. The model determines flow patterns over an entire field, allowing the identification of areas where flow concentration can lead to potential erosion. Field measurements of runoff crossing and being diverted by grass hedges planted on plots in North Mississippi were used in combination with model simulations to investigate how grass hedges redistributed runoff. The model was also used to determine how the presence of grass hedges modified flow patterns over a small watershed in western Iowa. The model simulations provided flow data that improved the understanding of how grass hedges can redistribute runoff and affect local erosion. When used with high-resolution terrain elevation data, simulations with the newly developed model can identify potential problems and improve the design of contour grass hedges as an erosion control measure in agricultural fields.

Technical Abstract: Grass hedges are narrow strips of stiff-stemmed vegetation used to control erosion and sediment delivery. When planted on the contour, the hydraulic resistance of the vegetation slows runoff, creates ponding, and promotes sediment deposition. In addition, when tillage is performed between grass hedges, soil may be thrown against the vegetation, where it settles to form a berm within the hedge. Tillage-induced berms divert part of runoff, causing it to flow alongside the hedge without crossing it. Such flow partitioning created by grass hedges was measured on experimental plots located on silt loam loess soil near Holly Springs, Mississippi, USA, where hedges planted at the bottom of 5%, 22 m-long slopes evolved berms averaging 0.13 m in height. They diverted about 80% of the runoff for events smaller than 5 mm, and about 50% for large events. A two-dimensional model was developed to determine overland flow patterns over complex terrains, accounting for oriented roughness created by tillage corrugations, crop rows, and larger features such as berms and vegetative barriers. The model was used to investigate how grass hedges redistributed runoff. It was used to reproduce the flow partition observed in the field experiments, and to determine how berm height and slope steepness and length affected runoff redistribution. Numerical simulations indicated that for most runoff events, and for berm heights and slope lengths similar to those of the experimental plots, ponded runoff depths were not high enough to overtop the berm, but rather crossed the hedges through cracks and gaps, represented in the model as small triangular weirs. A larger portion of runoff crossed hedges with lower berm heights or berms installed at the bottom of longer slopes. The model was also applied to a 6.0 ha watershed in western Iowa, USA, where nine grass hedges were planted across a 12~16% slope. Using high-resolution topographic data (0.5 m), the model computed dynamic flow properties to identify where high flow concentration occurred, and how the presence of hedges, with and without tillage-induced berms, affected runoff distribution over the watershed. Results showed that berms increased the amount of runoff flowing laterally above the hedges and that a large portion of the runoff crossed the vegetative strips at a few locations and with high flow depths, increasing the risk of development of ephemeral gullies.