Rainfall Detachment and Deposition: Sediment Transport in the Absence of Flow‐Driven Processes

Abstract Erosion of soils in nonchannelized soil surfaces areas is generally driven by the agents of raindrop impact and overland flow. This study considers soil erosion in portions of the landscape where the combination of land slope and water flux is relatively low so that overland flow driven processes are absent and raindrop impact is the only erosion agent. A model of soil erosion was developed to describe situations where the soil surface is covered by a water layer, and where the only processes acting are rainfall detachment and deposition. The model uses physical principles and recognizes that once soil has been detached by the impact of raindrops, soil aggregates or particles return to the bed by deposition to form a deposited layer from which they can be subsequently redetached. Since sediment in the deposited layer can have quite different characteristics from the original soil, it was necessary to distinguish between rainfall detachment of the original bed and rainfall redetachment of the deposited layer. The principle of mass conservation was then applied both to water and to soil, considering the settling velocity characteristic of sediment. This yields equations describing sediment transport in gentle overland flow on a plane land surface where the thresholds of entrainment and reentrainment by flow have not been exceeded. These equations were solved for the equilibrium situation only, and these solutions discussed and limitations outlined. It was proposed that three distinct regimes exist for shallow flow low slope erosion, each with a different dependency on slope steepness, when: (i) the depth is less than or equal to a breakpoint depth and flow‐driven processes are inactive, erosion is independent of slope; (ii) flow depth is greater than the breakpoint depth and flow‐driven processes are inactive, erosion is slightly dependent on slope; and (iii) the combination of slope and discharge is such that flow‐driven processes are active, then erosion is strongly dependent on slope.