Size distributions of the solids in runoff water were measured for two clay soils subjected to simulated rain under a range of plot lengths and two tillage orientations. Selective transport did not appear to have affected the sediment size distributions. Therefore, these could be used as a measure of soil structure and aggregate breakdown by rainfall and runoff. There was little dispersed clay, most of the sediment remaining aggregated. For each soil, sediment size distributions were bimodal, peaks in sediment size being related to orders of aggregation in each soil. Concentrations of dispersed clay provide evidence that stresses on aggregates moved by rain impact on flowing water were greater than on those moved in rills by flowing water alone. Consistent with this, sediment size distributions showed much less breakdown to sizes <0.125 mm in rills. Suspended load (sediment < 20 �m) showed little temporal fluctuation, and little or no decrease with time, suggesting that for these soils, aggregate disruption by raindrops and overland flow provides a continuous source of suspendable material. Bed-load was more variable and saltating and contact load appeared to be complementary to some extent. Large differences between the two soils in measured sediment concentrations could not be explained by slight differences in sediment size. However, large differences between the soils in the water content and density of saturated aggregates were found. Transport equations for bed-load sediment suggest that the measured difference in aggregate density is sufficient to explain the difference between the soils in rates of sediment transport.
Relative Transportability for Non-uniform Bed Load Sediment Transport