Reassessment of the potential risk of soil erosion by water on agricultural land in Germany: Setting the stage for site-appropriate decision-making in soil and water resources management

Abstract Accelerated soil erosion by water on agricultural land is considered one of the most critical forms of soil degradation, as it threatens both agronomic productivity and environmental quality. Given that both the costs and the benefits of soil erosion control measures are subject to spatial variation, the criterion of cost-effectiveness calls for spatially heterogeneous soil conservation policies. Under the German Direct Payments Regulation, federal state authorities are obliged to assess the potential risk of soil erosion by water on arable land in accordance with ABAG – the German version of the well-established Universal Soil Loss Equation (USLE) – and to impose appropriate management restrictions on farmers. However, the responsible federal state authorities have considerable degrees of freedom with regard to the methodology of erosion modelling - and thus in the designation of soil erosion protection areas: First, classification is carried out by multiplication of soil erodibility (K) and slope steepness (S) factors; the inclusion of rainfall erosivity (R) and slope length (L) factors is only optional. Second, classification of RKS- and RKLS-factors corresponds to the assumption of cross-regional standard estimates for R and L, which do not adequately reflect the heterogeneity of local conditions. This inevitably leads to over- and underestimation of local soil erosion hazards. As a consequence, the official risk assessment of soil erosion by water in Germany consists of a patchwork of cadastral risk maps at federal state level, which are neither publicly available nor fully comparable with each other. Hence, under current legislation there is a great danger that management requirements will be imposed on German farmers, which are economically inefficient, distort competition and do not achieve their environmental objectives. The aim of this study is to identify methodological and conceptual potential for improvement and harmonization in the risk assessment of soil erosion by water in Germany. For this purpose, high-resolution nationwide maps of rainfall erosivity (R-factor), soil erodibility (K-factor), slope length (L-factor), and slope steepness (S-factor) are produced in accordance with the revised ABAG using High Performance Computing (HPC) resources. The study area covers arable land, permanent grassland, vineyards, and orchard plantations. The modelling shows that the potential risk of soil erosion by water on agricultural land in Germany is much higher than previously assumed.