The effects of selected corn-soybean tillage systems on rill cross-sectional geometry and soil erosion for two Illinois soils

The objectives of this study were to quantify rill cross sectional geometry under different tillage, crop and soil conditions and to relate variations in rill geometric properties to measured sediment discharge and to soil erosion and sediment transport predicted by various models. In the experimental phase of this study, simulated rainfall and additional run-on water were applied to plots that had been subjected to alternative corn-soybean tillage treatments in two soils in central Illinois. Before and after rainfall simulations, a pin type rill meter was used to record soil surface elevations. For each soil surface profile recorded, a series of water surface elevations were assumed, and a top width, wetted perimeter, hydraulic radius, and cross sectional area were calculated for each depression in the soil surface. Non-linear regression analyses were conducted to estimate mean rill top width, wetted perimeter and hydraulic radius as functions of rill cross sectional area for each tillage treatment and rainfall simulation run. Rill geometric properties as functions of cross sectional area appeared to vary across tillage treatments, soils and rainfall simulation runs. Rill geometric properties were also related to estimates of rill flow. A rill width equation was identified that accounted for 86% of the variation in rill width calculated from the rill meter measurements, but for tillage treatments with residue cover ranging from 15% to 54%, this equation tended to underpredict rill widths by 20% to 40%. Multiple linear regression analyses indicated that most of the variation in measured sediment discharge could be accounted for with a model that included runoff rate, but rill geometric properties also added statistically significant information to such a model. Calculations with the multiple linear regression models suggested that a 25% increase in rill width may be associated with a 25% to 50% decrease in sediment discharge while runoff rate remains constant. In the modeling phase of this study, various erosion models were calibrated to the field conditions, and different estimates of rill geometry were used to estimate sediment discharge. For one tillage treatment, sediment discharge estimated by the excess shear model varied by 36% on average as rill width varied by an average of 54%.