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 resistanc e to erosion by overland (rill) flow were tested for two clay soils of the eastern Darling Downs, Queensland. Both shear and unconfined comp ressive strength of wet soil (for soil wet to saturation immediately p rior to application of rill flow) mere higher for soil with initially high water contents than for soil initially air-dry. Rates of runoff e rosion did not show a simple relationship with soil strength across th e two soils, though for each soil, higher strength was associated with much lower rates of erosion. The results show that variations in init ial water content can be associated with large chang;es in soil erodib ility. Particularly for the initially wet soils of higher strength, ra tes of runoff erosion were controlled by rates of detachment of sedime nt. From size distributions of wet aggregates and of sediment, and fro m measured water contents of wet soil, it can be suggested that the ex tent of incipient failure of aggregates on wet;ting was a major factor controlling ease of detachment by rill flow, as it can be inferred th at detachment of sediment involved breakdown of aggregates. Consistent with this, rates of runoff erosion across the two soils showed a dire ct relationship with the amount of water uptake on wetting, which appe ars to be a useful measure of susceptibility to detachment by rill flo w. Water uptake on wetting would be an indirect measure of incipient f ailure and, hence, of aggregate strength.