A transport‐distance approach to scaling erosion rates: 2. sensitivity and evaluation of Mahleran

In the first paper in this series, we demonstrated that most process‐based erosion models have a series of in‐built assumptions that led us to question their true process basis. An alternative soil‐erosion model (M ahleran – Model for Assessing Hillslope‐Landscape Erosion, Runoff And Nutrients) based upon particle‐travel distance has been presented in the first paper in this series and this paper presents the first of two evaluations of the model. Here, a sensitivity analysis shows that the numerical model is consistent with the analytical model of Parsons et al. (2004) and demonstrates that downslope patterns of sediment flux on hillslopes are a complex interaction of rainfall intensity, duration and pattern; hillslope gradient; surface roughness and sediment size. This result indicates that the spatial scaling of sediment transfers on hillslopes is a non‐trivial problem and will vary from point to point and from event to event and thus from year to year. The model is evaluated against field data from a rainfall‐simulation experiment on an 18 m × 35 m plot for which there are sub‐plot‐scale data on runoff hydraulics and sediment flux. The results show that the model is capable of reproducing the sedigraph with an overall normalized root‐mean‐square error of 18·4% and Nash–Sutcliffe efficiency of 0·90. Spatial and temporal patterns of particle‐size distributions of the eroded sediment are also reproduced very well, once erosion parameters have been optimized for the specific soil conditions.