INTERRILL ERODIBILITIES BASED ON THE RAINFALL INTENSITY FLOW DISCHARGE EROSIVITY FACTOR

Interrill erodibilities are often determined using a model which considers the interrill erosion rate to vary directly with rainfall intensity (1) squared. However, it has been shown that the sediment discharge from an interrill area varies directly with I rather than I2 when factors such as flow discharge are also considered. Analysis of data from the experiments used to determine interrill erodibilities in the Water Erosion Prediction Project (WEPP) in terms of a model that uses both rainfall rate and flow discharge indicates that the susceptibilities of some soils to interrill erosion may differ considerably from the erodibilities determined from the WEPP model. Apart from the need to use models which better account for the variations in erosive stress, consideration has to be given to the fact that erodibility is a dynamic factor that can vary rapidly between two extremes. One of these extremes is associated with detachment-limiting conditions that may often be controlled by cohesion. The other extreme is associated with the complete protection of the soil matrix by a layer of pre-detached particles. When this extreme results, interrill erodibilities are controlled by the particle size and density characteristics of the pre-detached particles. Difficulties arise in extrapolating information from one scale to another, and from one soil where erodibility experiments have been performed to others where they have not because of the dynamic nature of the rain-flow-soil surface interaction. Also, the effect of slope gradient in the interrill erosion environment varies between soils and experimental conditions because of this interaction and the differing susceptibility of soils to rilling.