Distributed water erosion modelling at fine spatial resolution across Denmark

Water erosion on agricultural land and sediment delivery to streams are a major threat to soil productivity andsurface water quality. Climate change and different national and international societal drivers now requireDenmark to take action to protect soil and waterresources. Inthisstudy,weadapted the spatially distributedsed-iment transport model WaTEM using the best data available at national scale. To calibrate and validate the model,sediment yield data from 31 catchments and 189 slope units in Denmark were compared with the model output,which was produced at afine spatial resolution of 10 × 10 m.Residual analysis and cross-validation were used to identify potential catchment outliers and assess model ro-bustness. We obtained a median Nash & Sutcliffe model efficiency range of 0.06–0.6 for the Danish environment.Based on the equifinality concept, an ensemble of 100, NSE-weighted model realisations for acceptable transportcapacity coefficients was used to assess model uncertainty. The comparison between rill survey data and pre-dicted values indicates that although the model captures well the spatial variability in erosion, it may underesti-mate the long-term average of soil erosion. Based on the modelling, 71% of the agricultural land in Denmark ismapped as stable in terms of the amount of erosion and deposited material. Overall, 6.1% of the farmland is esti-mated to have unsustainable erosion and 0.9% of the farmland exceeds erosion rates of 7.5 t ha−1a−1.Sedimentexport into surface water in Denmark equals 92,000 t a−1, corresponding to an average sediment yield of2.7 t km−2.The performance of WaTEM is considered satisfactory in this study. Importantly, modelled water erosion exceedsthe perceived erosion risk in Denmark. Strengthened by distributed uncertainty assessment at national scale, ourstudy provides an important national knowledge base for engaging land users and regulators in the process oftargeted erosion mitigation planning that is required to comply with national and EU regulation. Future investi-gations concerning the deviation between predicted and observed data and specific catchment parameters forthe non-behavioural catchments are required as well as studies that include the establishment of catchment sed-iment budgets