Abstract

Review studies indicate that conservation tillage effectively reduces runoff generation measured on large plots, but less so on smaller plots. These tillage and scale effects have never been measured in a single study for temperate mechanized agriculture. With the objective to study how runoff production changes with scale as influenced by tillage we set up rainfall-runoff plots of 5 m2, 30 m2 and 180 m2 size, on a field split into a reduced tillage (RT) and a conventionally ploughed field (CT) during two crop seasons (maize and wheat). In addition, we conducted rainfall simulations on small plots (0.7 m2) and measured soil surface properties. Average scale effects, defined as reduction of runoff coefficients with increasing plot size, were 57% (RT) and 37% (CT) between 5 m2 and 30 m2, and − 24% (reversed) (RT) and 27% (CT) between 30 m2 and 180 m2. Tillage effects, defined as the reduction in runoff coefficients of RT compared to CT were 65% at 5 m2, 86% at 30 m2 and 31% at 180 m2. On average, scale effects were 15% stronger on RT compared to CT. The overall level of runoff coefficient was most clearly affected by crop type, with winter wheat having 1.7–30 times lower runoff coefficients than maize. Crop development and soil moisture negatively affected runoff coefficients on Wheat, while rainfall intensity affected runoff coefficients positively on Maize. No evidence for interactions of these variables with either scale or tillage were found. Runoff coefficients of CT and RT diverged with scale (i.e. runoff accumulation) between 5 m2 and 30 m2, which was expected and can be explained with higher residue cover and macropore density on reduced tillage than conventional tillage slowing down runoff and increasing infiltration opportunities. Unexpectedly, between 30 m2 and 180 m2, on Maize, runoff coefficients converged strongly, which was interpreted with erosion of sealing on conventional tillage. In summary, this study confirms previous findings on the effects of tillage on runoff coefficients, but finds indications that important process transitions take place at the field scale that are affected differently by tillage, and that are not yet sufficiently studied or accounted for in models.

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