Scale effects on the estimation of erosion thresholds through a distributed and physically-based hydrological model

Slope incision and subsequent development of rills, gullies and channels are responsible for significant soil losses and are often irreversible with very high restoration costs particularly in semiarid environments. The location of potential areas of erosion where these processes occur is vital to land management and conservation. The study of the relationship between the local slope (S) and the drainage area (A) through the geomorphological relationship S = αAb, combined with hydrologic simulation models, has proved to be appropriate for the identification and characterisation of potential areas of incision, especially when it takes into account the spatial distribution of soil properties and the evolution of hydrological processes. However, physical properties of soil, digital elevation models (DEMs) and flow algorithms used may affect the results. This study employs a distributed, physically-based hydrological model to evaluate the infiltration-runoff relationships and their influence on selecting critical area from three DEMs with different resolutions. The results show a significant scale effect on flow distribution and the location of threshold points on slopes. The results obtained from a 30-m DEM significantly differ from those obtained from 10 and 5-m DEMs because the former was unable to capture the spatial variability of geomorphic processes. The selected set of critical points shows high S-A correlations for different values of critical shear stress. The physical model confirmed the dominance of surface runoff in the study site and was validated from field identification of erosion risk areas, although for incision areas < 2 ha, an appreciable error remains in relation to the calculation algorithm used for the drainage network and DEM resolution.

► Erosion processes particularly severe in arid environments. ► Physically-based and distributed hydrological modelling to evaluate the risk areas. ► Scale effect that affects the location of threshold points. ► Errors associated with the drainage calculation method and DEM resolution.