Incorporating sediment transport competence into existing soil erosion models

Competence in this thesis is defined as a limit to the maximum size of particle that can be detached and transported in rain-impacted interrill overland flow. Although there is evidence to show that there is some form of size selectivity occurring in rain-impacted interrill overland flow (i.e. competence), most modern soil erosion models do not simulate competence as a limit to erosion.;Existing competence equations were not developed in the shallow rain-impacted flow that occurs in interrill areas. A new competence equation was developed in the laboratory under rain-impacted flow.;The new competence equation was used to form the basis of an algorithm designed to incorporate competence in existing soil erosion models. SMODERP was chosen as a suitable model used to assess the effect of competence on rain-impacted interrill erosion.;The code of SMODERP was studied and the variables required by the competence algorithm located.;The competence algorithm required an input of erosion per model time step, SMODERP did not provided this and had to be modified to yield erosion per time step.;The new versions of SMODERP were tested on plot scale data. The effect of competence was found to be large, reducing erosion by a factor of between 3 and 65 times. Competence had the greatest effect on erosion on lower rainfall intensity events.;The competence algorithm assumed that there was no spatial or temporal change in surface texture. This assumption was investigated at the field, plot and laboratory scale. There was found to be some temporal and spatial variation in surface texture but only at the laboratory scale and to a lesser extent at the plot scale. This suggests that at smaller scales there is a spatial and temporal variation in surface texture but this variation does not occur at larger scales where other processes may dominate.;This thesis has identified a limit (competence) not simulated in most soil erosion models and provided an approach to including this limit into soil erosion models. The effects of competence was shown to be large but more work is need in this area to more fully assess the effect of incorporating sediment transport competence into existing soil erosion models.