Modeling edge effects of tillage erosion

Tillage erosion has been recognized as an important factor in redistribution of soil over time and in the development of morphological changes within agricultural fields. Field borders, fences, and vegetated strips that interrupt soil fluxes lead to the creation of topographic discontinuities or lynchets. When tillage tools that preferentially throw soil to one side are used repeatedly to move soil in one direction, rather than alternating with each pass, they create berms at the receiving side of the tilled domain and a “dead furrow” or channel at the contributing side. However, even tillage implements that are symmetrical in throwing soil equally in both lateral directions on flat surfaces may throw some soil beyond the implement width and so contribute to soil berms formation just beyond the tilled zone that can affect water flow paths. We developed a two-dimensional Tillage Erosion and Landscape Evolution Model that allows complex internal boundaries to be defined within the simulation domain. In this paper we develop and demonstrate techniques and tools to allow prediction of the formation of edge-of-field berms by defining alternative boundary conditions. The derivation and assumptions of the model are presented and then it is applied and compared to survey results from two field studies: one an experimental field in Coffeeville, Mississippi, where grass hedges were planted close to field elevation contours to evaluate their effectiveness as an erosion control measure and were monitored over a 16-year period; and the other a set of 0.1 ha plots located near Holly Springs, MS where the effect of edge-of-field berm formation on runoff partitioning was evaluated during an 8-year study. Results demonstrate the ability of the model to correctly reproduce the location and magnitude of soil loss and accumulation. At Coffeeville, erosion averaging over 20 cm in the downslope side of each grass hedge and deposition taking place near the slope ends led to the formation of lynchets up to 0.8 m high, and the average slope steepness in the cropped areas between hedges decreased from an average of 7.2% in 1993 to about 3.7% in 2009. In Holly Springs, repeated tillage conducted next to grass hedges planted along the hillslope bottoms led to the formation of berms with average height of 13 cm, which may significantly alter field-scale hydrological, erosion, and sediment transport processes.