The influence of time and other factors on soil loss produced by rain-impacted flow under artificial rainfall

Erosion is caused by detachment of soil materials from the surface of the soil matrix where they are held by cohesion and inter-particle friction, followed by transport of the detached particles away from the point of detachment. It is well known that raindrop impact and surface water flow can be involved individually or together in causing erosion. When raindrop impact causes erosion, crusting may occur so that erosion rate varies with time. Flow depth influences detachment by raindrops in rain-impacted flows so that detachment varies in space and time. In addition, splash transport associated with splash erosion and the transport of coarse material by raindrop induced saltation in rain-impacted flows produces a layer of non-cohesive material that also influences detachment in space and time. Various detachment and transport mechanism operate on surfaces eroding under artificial rainfall. The spatial distribution of the mechanisms varies with flow conditions which are influenced by rainfall intensity, slope length and slope gradient together with the infiltration characteristics of the soil surface. Detached particles travel across the soil surface at rates that depend on the size and density and the transport mechanisms involved and the steady-state is not achieved until the slowest moving particle detached at the point farthest from the discharge point is discharged. Most experiments with artificial rainfall apply rain at a fixed intensity for a fixed time. Before the steady-state occurs, sediment discharge and composition produced when rainfall and runoff rates are both steady will vary with time. Total amounts or average rates of soil discharged in experiments of fixed duration will produce results that are dependent on the duration of the experiment. This needs to be taken into account when designing and analysing experiments using artificial rainfall to erode soils.