Runoff and sheet erosion from tillage trials under artificial rainfall at Harden, New South Wales

Abstract

Artificial rainfall was applied to g-m-long plots after the harvest of a canola crop in experiments on a grazed cropping system using a rotation that alternated wheat with lupins and canola. Three tillage treatments were selected for the experiments: one was a direct-drill treatment, while the other two involved the use of a reduced tillage practice during the preparation of the seed bed. Tillage treatment was found to have no substantial effect on runoff and sediment concentration resulting from sheet erosion during the post-harvest period. At sowing time, tillage again had no significant effect on sediment concentration but did have a significant effect on runoff. The roughness produced by the tillage provided a substantial capacity to store water and detached soil particles in surface depressions immediately after cultivation, and the decline in this surface roughness during rainfall was sufficiently small to cause little or no runoff when 1h of 70 mm/h rain was applied to a cultivated surface. The results support indications from SOILOSS, a model that provides local implementation of the Revised Universal Soil Loss Equation (RUSLE), that reduced-till may be as effective as direct-drill at conserving soil at this site. However, since the experiments were restricted to sheet erosion, and no experiments were performed between sowing and harvest or with other crops in the rotation, the results do not provide unconditional support for the SOILOSS result. Despite the result of the experiments reported here, soil erosion risk is likely to be substantially reduced on a field scale through the use of tillage practices which, like the direct-drill treatment, maintain anchored and unanchored crop residue during the fellow and post-sowing periods. Data were also collected in relation to an examination of the ability of the Soil Water Infiltration and Movement (SWIM) model to predict runoff during the post-harvest period. SWIM tended to underpredict runoff but the difference between the observed amount of runoff and that predicted by SWIM was statistically not significant.