Field rainfall simulator studies on two clay soils of the Darling Downs, Queensland. I. The effect of plot length and tillage orientation on erosion processes and runoff and erosion rates

Abstract

Runoff and sediment concentrations were measured under simulated rainfall for a range of plot lengths, tilled either across or up and down the slope, on two soils, both of 4% slope. Sediment was moved by either rain-flow transport (the combined action of drop impact and shallow overland flow), or rilling, in which overland flow alone was the main agent of detachment and transport. Rain-flow transport occurred on all plots, while rilling developed only at plot discharges greater than 0.3-0.7 L s-1. For both processes, sediment was dominantly bed-load, and on both soils the concentration of bed-load carried by rills was approximately 4.8 times that carried by rain-flow. From this and other evidence it was concluded that sediment concentrations were controlled by transport capacity rather than the supply of detached sediment. For both rainflow and rill transport, sediment concentrations were constant over a range of plot discharges. It is suggested that rainflow concentrations were controlled largely by rainfall intensity, which was constant. Rills were observed to widen rather than deepen with increasing discharge, which would minimize changes in rill transport capacity. Tillage across slope delayed runoff and reduced total runoff and soil loss. Tillage orientation affected the plot discharges needed for rill initiation, but did not affect the sediment concentrations transported by either of the two main sediment transport processes that were observed.