Upscaling a simple erosion model from small areas to a large region

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Upscaling from a catchment scale to a regional scale is generally rendered difficult by the lack of relevant and precise data at the larger scale. In this case, a winter rill erosion hazard map was produced for the Nord-Pas-de-Calais region using a linear regression erosion model originally designed for the catchment scale. Upscaling entailed adapting and applying the model at the county scale for all of the counties within the region. In upscaling the model, the difficulties associated with the nature of the data were dealt with in three stages: (1) the modification of the model for the county scale as a function of the nature of the data available, (2) an analysis of the influence of the spatial distribution of the data, (3) an analysis of the effect of the loss in precision of the data on the model output. Reference areas were used to verify the accuracy of the upscaling process before applying it to all of the counties in the region. In this case, the most significant limitation was the spatial coverage of the data: the basic administrative unit for which data is collected is the county, and it does not correspond to the erosion process scale which is the catchment. Defining erosion risk in terms of hazard categories rather than estimated erosion rates overcomes this difficulty to some extent. The use of reference areas provides several advantages in an upscaling procedure: these are mainly related to minimizing data collection and obtaining a reliable estimate of the accuracy of the predicted output.

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