EFFECTS OF INITIAL WATER-CONTENT AND SOIL MECHANICAL STRENGTH ON THE RUNOFF EROSION RESISTANCE OF CLAY SOILS

Effects of antecedent water content and soil strength on the resistance to erosion by overland (rill) flow were tested for two clay soils of the eastern Darling Downs, Queensland. Both shear and unconfined compressive strength of wet soil (for soil wet to saturation immediately prior to application of rill flow) mere higher for soil with initially high water contents than for soil initially air-dry. Rates of runoff erosion did not show a simple relationship with soil strength across the two soils, though for each soil, higher strength was associated with much lower rates of erosion. The results show that variations in initial water content can be associated with large chang;es in soil erodibility. Particularly for the initially wet soils of higher strength, rates of runoff erosion were controlled by rates of detachment of sediment. From size distributions of wet aggregates and of sediment, and from measured water contents of wet soil, it can be suggested that the extent of incipient failure of aggregates on wet;ting was a major factor controlling ease of detachment by rill flow, as it can be inferred that detachment of sediment involved breakdown of aggregates. Consistent with this, rates of runoff erosion across the two soils showed a direct relationship with the amount of water uptake on wetting, which appears to be a useful measure of susceptibility to detachment by rill flow. Water uptake on wetting would be an indirect measure of incipient failure and, hence, of aggregate strength.