Do deterministic sediment detachment and transport equations adequately represent the process-interactions in eroding rills? An experimental field study

This paper tackles two main questions by linking observations, determination of hydraulic parameters and measurement of sediment transport with the formulae used in soil erosion models. First, do constant shear stress values in different rills with constant soil parameters result in the same soil detachment values? Secondly, deterministic soil erosion models make the assumption that there is a relationship (often further assumed to be linear) between shear stress and soil detachment; is this suitable for representing real erosion process combinations in natural rills? Following most process based deterministic soil erosion models, derived hydraulic and erosion parameters should be similar. However, the results from the different experiments showed clear differences in sediment concentration, transport rates and other measured as well as calculated values. In contrast to our experimental results, a model simulation would produce erosion parameters with low variations, represented by the relative measurement error and the empirical variation coefficient. This reveals the general problems of using process based deterministic models for erosion in shallow rills. While soil erosion models simulate the processes resulting from the shear forces of flowing water on the soil surface, other processes like side wall failure, headcut retreat and plunge pool dynamics are not taken into account. Our results suggested that these other processes may contribute substantially to rill erosion processes. The results of this study strongly suggested that the model concept of most physical based soil erosion models is inadequate for modeling rill erosion processes. Measured total rill erosion rates were the sum of erosion rates caused by a combination of different soil erosion processes with different spatial and temporal distribution. This combination cannot be described by a single equation.

► We accomplished experiments under field conditions in naturally developed rills.
► We tested some basic equations used in physically based soil erosion models.
► Used equations are not suitable to describe process interactions in natural rills.
► Processes are deepening, headcut retreat, transport of loose material, bank failure.