Summable C factors for contemporary soil use

Abstract Crop cultivation, crop stages and the seasonal distribution of rainfall erosivity are continuously changing in response to changes in climate and socio-economic conditions. Therefore, the crop and cover factor (C factor) of the (Revised) Universal Soil Loss Equation should also be adjusted continuously. Within the framework of the (R)USLE, C factors can only be calculated for crop rotations. However, for large-scale and regional modeling of soil erosion on arable land and targeted subsidy schemes for the implementation of soil protection measures, C factors are required that quantify the effect of individual crops and management options on the risk of soil erosion. We therefore develop a method for deriving summable C factors that can easily be combined to derive C factors for crop rotations. These summable C factors also account for carry-over effects that influence the risk of soil erosion in subsequent crops. Using the latest data on the temporal distribution of rain erosivity and approximately 3.5 million observations of crop stages, summable C factors were derived for 57 crops and crop management options, including double cropping, which is currently becoming more prevalent in temperate areas. These C factors apply for Germany. However, the regional variation of summable C factors within Germany was small and comparison with Swiss data indicated that our summable C factors will also apply in neighboring countries in Central Europe. Changes in the seasonal distribution of rain erosivity and in crop development due to climate change caused some convergence of the summable C factors for different crops, i.e. the C factors for crops where the risk of soil erosion potential had previously been low increased, while for those crops where the risk of erosion had previously been high the C factors decreased. Of the arable crops, potatoes had by far the highest summable C factor, whereas sod-forming crops had negative summable C factors, leading to low C factors for crop rotations. The sod crops seem to be largely responsible for the low level of soil erosion found on many organic farms and in Switzerland, where sod crops account for a large share of arable land.