Under conservation tillage systems, the effectiveness of crop residues in reducing water erosion decreases if the mulch is carried away by runoff and/or if the runoff furrows the soil. This phenomenon is called mulch failure, characterizing the critical slope length, i.e., the distance on the ground after which the runoff shear stress is sufficient to overcome the resistance of the mulch to be carried away and/or the resistance of the soil to furrowing. The objective of this study was to define critical slope lengths in two sowing directions under different types and doses of crop residues, under no-tillage. The field work was conducted from May 2009 to May 2011, on a Nitosol (Alfisol) in São José do Cerrito, Santa Catarina, Brazil, in experimental units (3.5 x 11 m) with a mean slope of 0.144 m m-1. Transverse (contour) and parallel sowing directions (up-and-down) to the slope were tested using maize (Zea mays), wheat (Triticum aestivum) and soybean (Glycine max) mulches at doses of 50 and 100% of the total residue produced. During the research, rainfall was simulated using a rotating-boom rainfall simulator, with a rain intensity of 65 mm h-1 in 90 min, which was the time required until the runoff rate became constant. At the end of 90 min of rain, without disconnecting the rainfall simulator, increasing levels of extra flow of clean water were applied at the upper part of the plots, simulating additional runoff from longer slopes. The mulch failure was evaluated based on the theory of erosion, using the relationship between erosion rate (Dp) x runoff rate (q m) and sediment concentration in runoff (C) x runoff rate (q m). In all treatments the critical slope lengths were determined, and, in general, contour sowing and higher amounts of crop residues increased the critical slope length in relation to the up-and-down sowing and the lowest residue dose, respectively. These critical lengths ranged from 35 to 155 m. The point of failure of the theory of erosion was best identified by a combined analysis of the relationships Dp x q m and C x q m.
Simulated rainfall; mulch failure; conservation soil tillage; contour farming
