Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/1999
Publication Date: N/A
Citation: N/A Interpretive Summary: Grass hedges are a new low-cost erosion control technology. Hedges are narrow parallel strips of tall stiff grass that are planted across cropped fields. They slow down runoff and trap eroded sediment at numerous points. It is usually recognized that tillage creates conditions that increase risks of water erosion, but direct soil movement by tillage is often not seen as a form of erosion in its own right. By dividing a hillslope into relatively narrow tilled strips, hedges make the impact of soil movement by tillage more important. The amount of soil moved by tillage is a function of the slope of the land; more soil moves down slope on steeper lands. The amount of soil moved locally downslope by each tillage operation does not change when hedges are established. However, the length of the field is greatly shortened as each tilled strip is effectively a separate field. If hedges divide a field into 10 strips, the effective tillage erosion rate per unit area (soil movement downslope per meter along a contour line divided by the up-and-down slope field length) is increased 10 fold. In this study, soil benches at tall as 0.4 m developed across 1.5 m wide grass hedges in only three years. Results indicate that direct tillage translocation could account for 30% to 60% of this soil movement. This study shows that tillage translocation can be very important when grass hedges are used on cropland and that long-term conservation planning should consider changes in landscape shape.
Technical Abstract: Grass hedges are narrow (1 to 2 m wide) parallel strips of stiff, erect, grass planted near to or on the contour of fields but crossing swale areas at angles convenient for farming. They serve as guides for contour cultivation, retard and disperse surface runoff, cause deposition of eroded sediment, and reduce ephemeral gully development. After three years of tilled fallow between mixed-species hedges, the average grade of 18-m wide tilled strips between 1.5 m wide hedges was reduced from 0.068 to 0.052 as a result of surface lowering below hedges and on the shoulders of swale areas combined with increases in elevation above hedges. Annual surveys show progressive lowering of high spots and filling of low spots as contours lines more closely aligned with hedges. Survey data indicated annual erosion rates of nearly 250 t ha-1 y-1. Both RUSLE and WEPP over-predicted erosion rates, partly because backwater and slope modification affects were not considered. A tillage translocation model predicted enough soil movement to account for 30 to 60% of the observed changes. A combination of tillage translocation and water erosion/deposition provides the best explanation for the observed aggradation/degradation patterns.