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• Differential travel rates cause particle size distributions to vary with time. • Sediment discharges peak at drop size dependent flow depths. • Sediment concentrations increase before declining as flow depth tends to zero. In laboratory experiments undertaken using sand on a sloping surface in the 1970s, it was observed that while fine material moved as suspended load, fine to medium sand was induced to saltate as the result of raindrop impacts whereas coarse sand and gravel were induced to roll. The particle size distribution of the sediment discharge was initially much finer than the original bed and coarsened with time to approach that of the bed as time passed. At the time the exact nature of the raindrop induced transport mechanisms was not well known. Later, experiments using an apparatus that enabled the depth and velocity of rain-impacted flow to be controlled over an eroding surface, showed that raindrops induced some particles to travel limited distances that depended on a number of factors including flow velocity and particle size and density. The resultant differential travel rates of particles of different size and density provided an explanation as to why particle size distributions of sediments discharged by rain-impacted flows vary temporally, and why a non-cohesive layer develops on top of the surface of the cohesive matrix soil. The formation of the non-cohesive layer is indicative of erosion being transport limited when raindrop induced saltation occurs. The experiments also showed that sediment discharges produced by rain-impacted flows peaked at a drop size dependent flow depth so that a function that accounts for the interaction between raindrop size and flow depth could be developed for drops impacting at close to their terminal velocities. One of the implications of the observed responses to variations in rainfall and flow conditions is that the assumption that rain has certain erosive power that is defined simply by the product of flow discharge and rainfall factors such as rainfall intensity or the kinetic energy flux is a false assumption. [ABSTRACT FROM AUTHOR]

• Susceptibility of surface to detachment always limits raindrop driven erosion. • Sediment transport associated with drop impact is always a limiting factor. • Raindrop driven erosion is both detachment and transport limited at the same time. Soil erosion involves two liked processes, detachment of particles held within the soil surface by cohesive forces and inter-particle friction, and the transport of the detached material away from the site of detachment. It is commonly thought that erosion is either detachment limited or transport limited but this is not true for surfaces where raindrop driven sediment transport occurs. When detachment occurs by raindrop impact, transport away from the site of detachment can occur in the air and in rain-impacted flow. Splash transports both fine and coarse particles aerially. In rain impacted flows, fine material travels in completed suspension and at velocities that are at, or near the velocity of the flow. Coarse material may travel by raindrop induced saltation and rolling, or if flow conditions are appropriate, by flow driven saltation and rolling. Transport by splash and by raindrop induced saltation and rolling ceases when raindrop impact ceases even if previously detached material has not been discharged. Consequently, sediment transport limits erosion whenever raindrop impact is involved in transporting detached soil material over the soil surface. However, the amount of material available for transport is controlled by the ability of the impacting raindrops to cause detachment and the ability of the cohesive soil surface to resist detachment. Consequently, the discharged of coarse material by rain-impacted flows is both detachment limited and transport limited at the same time. Raindrop driven erosion is a complex process and the spatial and temporal variations in the transport mechanisms need to be considered when experiments involving raindrop driven erosion experiments are designed and the results analysed. [ABSTRACT FROM AUTHOR]

• Spatio-temporal variations of effective sediment sizes were investigated on field plots. • Sediment size was more susceptible to erosional degradation under low intensity rainfall. • Erosional degradation impact exhibited a unimodal variation with soil structure stability. • Rainfall intensity, pH and bulk density contributed jointly to sediment size (p < 0.001). • This study facilitated an understanding of sediment transport mechanisms on eroded lands. Land degradation by erosion alters soil properties to a varying extent and definitely influences the subsequent soil erosion. However, little is known about the impact of erosion-induced land degradation on the erosion process from the aspect of sediment size characteristics, which may not only indicate aggregate breakdown and sediment transport mechanism, but also influence sediment deposition. Here, the effective sediment size distribution (undispersed) and its dynamics during sheet erosion on different eroded lands (non, moderately and very-severely eroded) were studied under field rainfall simulation on five types of soils (Calcic Luvisols, Ferric Luvisols, Plinthic Alisols, Plinthic Acrisols and Acric Ferralsols) formed separately on loess deposits, quaternary red clays and basalt from temperate to tropical climate. The rainfalls at the intensities of 45 and 90 mm h−1 were simulated on pre-wetted bare fallow plots (3 × 0.8 m2, 10° slope). The effect of erosion degree on the effective sediment size showed a unimodal trend from temperate Luvisols to tropic Ferralsols, coupled with the influence of rainfall intensity to a varying extent. Under the low-intensity rainfall, except for Ferralsol, the moderate erosion degree increased the sediment size by −49% to 72% relative to non-eroded lands, in contrast to a decrease of 6–64% for the very-severe erosion degree. The high-intensity rainfall was shown to lessen the erosion degree effect for Luvisols, magnify it for Alisol, and change it to positive for Acrisol. Sediment transport was dominated by suspension-saltation of <0.10 mm size fraction for Luvisols regardless of erosion degree due to the weak soil structure, and shifted from both rolling (0.5–2 mm) and suspension-saltation on the non/moderately eroded lands to only suspension-saltation on the very-severely eroded lands for the other soil types. Rainfall intensity, pH and bulk density contributed jointly to sediment size (Adj-R2 = 0.76, p < 0.001), and exchangeable sodium showed a positive effect on its temporal variation (Adj-R2 = 0.36, p < 0.001). The varying effects of land degradation by erosion on sediment size and transport mechanism at the soil pedon and regional scales merit consideration in land management and rehabilitation as well as erosion modeling. [ABSTRACT FROM AUTHOR]

• Airborne material moves downstream during fall through flow. • Contribution airborne material generally ignored in sheet and interril experiments. • Contribution airborne material high in very shallow flows. • Dominant transport mechanism in deeper flows no longer operate in shallow flows. Soil material lifted by raindrop impacting flows a few millimetres deep are transported downstream as they fall back to the soil surface. That occurs not only with material that is lifted and remains in the flow, but also material lifted into the air that falls onto the surface of the flow. The amount lifted into the air varies with flow depth and the effect of the input from airborne material on sediment discharged by rain-impacted flows is not normally taken into account in experiments on, or models of, sheet and interrill erosion where flows are more than 1 mm deep. However, in very shallow flows, the airborne input to rain-impacted flows is high and the dominant transport mechanisms the operate in deeper flows no longer operate effectively. Further research into the mechanisms that operate in transporting sediment by rain-impacted flows on slopes with high gradient is warranted. [ABSTRACT FROM AUTHOR]