Modelling Switzerland’s actual erosion risk on arable land

Abstract

In Switzerland, soil erosion by water is a challenge due to the complex topography. It is stipulated by law that a soil removal rate of 2 t/ha/yr (for soil depth < 70 cm) or 4 t/ha/yr (for soil depth > 70 cm) on arable land must not be exceeded as a long-term average. Furthermore, an impairment of soil fertility and of water bodies and near-natural habitats due to erosion should be avoided by all means.In this study, we calculated the actual erosion risk on arable land of Switzerland for 2021 using newly available data sources on crops and tillage practices. The erosion risk was calculated with the Revised Universal Soil Loss Equation:A = R∗K∗L∗S∗C∗PWith R = rainfall and surface runoff factor, K = soil erodibility factor, L = slope length factor,  S = slope gradient factor,  C = soil cover and cultivation factor, and P = erosion control factor.The product of the factors R∗K∗L∗S represents the potential erosion risk. For the calculation, we used the available potential erosion risk map by Bircher et al. (2019).To derive realistic C factors adapted to Swiss climate conditions, the following procedure was applied: average crop-specific C  factors for two geographical regions (valley, mountain) and three distinct tillage types (plough mulch, direct seeding) were deduced from data of the Swiss agri-environmental monitoring programme (SAEDN; Gilgen et al. 2023), which provides detailed management data on the field level of approximately 300 farms since 2009. The categorised C  factors were allocated to individual parcels using geo-referenced crop area polygons of the Swiss cantons as well as direct payment data from the Federal Office for Agriculture on crop-specific tillage types.Since no data on the P factor was available, it was estimated using expert knowledge. Calculation was carried out on a 2x2m grid and summarised at municipality level.Our calculations represent the best yet for Switzerland's actual erosion risk. However, a comparison with the erosion that has actually occurred, as carried out by Bircher et al. (2022), is still pending. The described method can be used in the future for monitoring that focuses on the environmental impact of farmers' management changes. A prerequisite for this is that sufficiently detailed data on farm management continues to be available.