A method to compute the effect of vertical mulching on the reduction of surface runoff

Six plots (three as controls) were established in southern Brazil to assess the reduction in runoff by vertical mulching (vm). In the other three, ditches (0.08-m wide, 0.4-m deep) were cut perpendicular to the slope, 2-m and 12-m upslope from the bottom of the plot where a flume was installed to measure runoff. Three simulated rainfalls (80 mm h^-1 for 120 min, 80 mm h^-1 for 65 min, 97 mm h^-1 for 65 min) were applied with a sprinkler system to generate runoff. Compared to the controls, vm delayed its beginning by 10 to 20 min and reduced its volume by 34 to 39%, because initially all and later some runoff was caught in the ditches from where it infiltrated into the surrounding soil. Based on the experimental results, we derived a method to compute the runoff reduction by vm as influenced by ditch spacing (ds), initial soil moisture content (θ_i), and rainfall rate and duration. Computations for a combination of four ds (5 to 20 m), five θ_i (16 to 32%-vol), and three rainfall regimes (124 mm h^-1 for 20 min, 66 mm h^-1 for 60 min, and 42 mm h^-1 for 120 min) showed an increasing runoff reduction as ds or θ_i decreased. In some cases, runoff was even completely prevented. The reductions were larger in the 60-min than in the 20-min rainfall. For the 120-min rainfall, there was no runoff with any combination of ds and θ_i.