According to the spatial scale, almost all of the soil erosion models can be classified as field-scale models, catchment-scale
models and region-scale models. Each type of
erosion model has its instinctive characteristics. As to the field scale, the analysis of the erosion process on field vertical zones and the interaction between them is very important. In addition to the analysis of soil erosion on hill slopes, effects of runoff and sediment from hill slope on erosion and sediment yield of the followed gully zone and the erosion caused by gravitation are complex and pivotal in the catchments scale. In this scale, the sediment delivery has a significant importance to the erosion modeling. Establishing region scale erosion model, the relationship and the interaction of the subordinate drainages together with the style of the runoff yield and runoff concentration should be taken into account. On different scale, the influence of the main factors on runoff and sediment yield and the composition of the main factors is not the same. The factors affecting the processes of soil erosion and sediment yield have more asymmetry and variability, enhancing sediment yield modeling complexity in different scales. So, scale transfer is the key to soil erosion and sediment yield modeling on different scales. Five kinds of methods of scale transfer in other fields listed in the end of this paper, among which maybe contend some useful ways to transfer scale in modeling soil erosion. Based on the analysis of the soil erosion process and the main factors and according to some valuable works and precious experience done by some authority researches in
loess hilly-gully region, this paper has put forward some existing problems in building soil erosion models. All these will provide some useful suggestions to soil erosion modeling in the future.