Scale effects on runoff and erosion losses from arable land under conservation and conventional tillage: The role of residue cover

A literature survey as well as our own observations on runoff and soil losses measured under conventional and conservation tillage (CT) show that differences in runoff and erosion between both tillage techniques are scale-dependent: the difference in runoff and erosion response between conservation and conventional tillage increases with the length of the plot/field considered. The relative scale effect is more important for erosion than for runoff. The scale effect implies that plot measurements may lead to an underestimation of the effectiveness of conservation tillage in reducing runoff and erosion at the field or catchment scale. We tested experimentally the hypothesis that this scale-dependency can (partly) be explained by the occurrence of runoff transmission losses along the hillslope. In a 2.3m long soil tray, filled with a silty loam soil, a seedbed was simulated and covered with different amounts of straw and maize residues. Our data showed that transmission losses can indeed be important and that they depend on cover percentage, discharge, time of discharge application and residue type. A simple model exercise shows that, under realistic assumptions, the effect of transmission losses on runoff and erosion on arable land may be highly significant. At present, most erosion models explicitly or implicitly assume a linear increase of runoff with slope length: taking into account transmission losses may contribute to a better estimation of runoff and soil losses. Since no interaction effect between cover percentage and inflow rate was found, the relative difference in runoff and soil loss between bare and residue-covered surfaces did not change with scale. Thus, other factors than those observed are responsible for the observed increasing differences between conservation and conventional tillage with increasing scale.