RILL EROSION OF A VERTISOL WITH EXTENDED TIME SINCE TILLAGE

Reduced tillage and no-tillage cropping systems have periods of time l asting several months or years between tillage operations. If erosion is to be predicted for these systems, then there is a need to characte rize the effects of such extended time since tillage on rill erodibili ty. Most erosion studies have been conducted on freshly tilled or dist urbed soils which may respond differently to erosive forces than undis turbed soils. The objective of this study was to evaluate the effects of extended time since tillage on critical shear stress and rill erodi bility for a Heiden vertisol under corn production. Rainfall simulatio n studies were conducted in March and August, before corn planting and after corn harvest, over a period of two years with time since tillag e ranging from 7 to 624 days. Tests were replicated four times across a slope of 4 to 6%. The resulting data expands erosion information on one site included in the 1987 national tour under the Water Erosion Pr ediction Project (WEPP) studies. As time since tillage increased, the flow width in rills increased, and the hydraulic radius and hydraulic shear converged to more stable values. Three soil strength measurement s, cumulative rainfall, or cumulative rainfall energy were not consist ently correlated to rill erosion parameters and did not seem to be pot entially useful for future predictions. Different approaches may be ne eded to relate repetitive wetting and drying cycles to measured soil p roperties and erodibility. Critical shear stress ranged from 0.3 to 2. 47 Pa which indicated that rill erosion started at lower flow velociti es or shallower flow depths than characterized by the 1987 WEPP data. Rill erodibility was found to be highly variable (0.73 - 16.31 x 10(3) s/m) on freshly tilled treatments, but variability decreased signific antly for 89 days or more time since tillage, stabilizing in the range of 2 - 5 x 10(3) s/m. Erosion should be more accurately predictable o n sites with extended time since tillage, for the soil in this study.