Finding the right direction: An approach to determine cropland management direction and its implications for soil erosion risk assessments on large spatial scales.

Land degradation (LD) threatens soil health and ecosystems globally, which is why the European Commission formulated the objective of transitioning towards healthy soils by reducing LD by 2030. Water erosion, one of the most severe forms of LD, is influenced by management direction. This is because it affects the roughness created by tillage, seeding ridges, and tramlines, which, in turn, can either exacerbate or mitigate erosion. Therefore, management direction is an important variable in the RUSLE-framework (Revised Universal Soil Loss Equation) used within  European Union Soil Observatory (EUSO) to assess and monitor soil health (Panagos et al. 2024). However, spatial data on management direction, especially at policy-relevant scales, is scarce and often not adequately integrated into model-based assessments. 

This study develops a straightforward and fast approach to derive management direction and assesses its implications for soil erosion risk at the national level of Germany. Using field geometries from Tetteh et al. (2024) for 2021, we assumed that management direction follows the longest field side of the “minimum rotated rectangle” around each field. The derived management direction was validated using 1155 mapped arable fields randomly selected across Germany. In combination with a 10x10 m DEM, we then derived the contouring index (CI, 0-100%) as a measure to represent the relationship between management direction and the contour for each cell. The CI was then used to calculate the support management practice factor (P) based on the RUSLE to estimate the implications for soil erosion risk. 

Results showed that the management direction was correctly modelled in 79% of observations assuming a 10° tolerance. The CI averaged at 37±26%, indicating high variability both between and within fields. Management direction of 2021 was found to reduce soil erosion risk by 7.6%. The currently low CI for Germany shows high potential for further reducing soil erosion risk by implementing contour parallel management in cropland and enhance model predictions by providing P-factor maps with high spatial resolution. Despite these promising results, challenges persist in accurately predicting management direction in fields with highly irregular shapes or unique environmental conditions. Our approach provides a feasible method to predict and monitor management direction with sufficient accuracy for large scales. The approach can be incorporated into other models requiring management direction of cropland (e.g. tillage erosion) and applied for other data sources (e.g. LPIS geometries). This can improve modelling and monitoring LD on field level for large scales, supporting landowners and policy makers towards improving soil health.

References:

Panagos, P., Borrelli, P., Jones, A., & Robinson, D. A. (2024). A 1-billion-euro mission: A Soil Deal for Europe. European Journal of Soil Science, 75(1), e13466. 

Tetteh, G. O., Schwieder, M., Blickensdörfer, L., Gocht, A., & Erasmi, S. (2024). Agricultural land use (vector): National-scale crop type maps for Germany from combined time series of Sentinel-1, Sentinel-2 and Landsat data (2017 to 2021).https://doi.org/10.5281/zenodo.10619783  

Acknowledgement:

P.S., F.M.,  K.K. and P.B, were funded by the European Union Horizon Europe Project AI4SoilHealth (Grant No. 101086179).