Landscape benching from tillage erosion between grass hedges

Grass hedges are narrow (1-2 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) year(-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 30-60% of the observed changes. A combination of tillage translocation and water erosion/deposition provides the best explanation for the observed aggradation/degradation patterns. (C) 1999 Elsevier Science B.V.