Towards a hydrological classification of European soils: preliminary test of its predictive power for the base flow index using river discharge data

Papers published in Hydrology and Earth System Sciences Discussions are under open-access review for the journal Hydrology and Earth System Sciences Abstract Predicting discharge in ungauged catchments requires knowledge on the distribution and spatial heterogeneity of hydrological soil properties. Because hydrological soil information is not available on a European scale, we reclassified the Soil Geographical Database of Europe (SGDBE) in a hydrological man-5 ner by adopting the Hydrology Of Soil Types (HOST) system developed in the UK. The HOST classification describes dominant pathways of water movement through soil and was related to the base flow index (BFI) of a catchment (the long-term proportion of base flow on total stream flow). In the original UK study, a linear regression of the coverage of HOST classes in a catchment explained 79% of BFI variability. 10 We found that a hydrological soil classification can be built based on the information present in the SGDBE. The reclassified SGDBE and the regression coefficients from the original UK study were used to predict BFIs for 103 catchments spread throughout Europe. The predicted BFI explained around 65% of the variability in measured BFI in catchments in Northern Europe, but the explained variance decreased from North 15 to South. We therefore estimated new regression coefficients from the European discharge data and found that these were qualitatively similar to the original estimates from the UK. This suggests little variation across Europe in the hydrological effect of particular HOST classes, but decreasing influence of soil on BFI towards Southern Europe. 20 Our preliminary study showed that pedological information is useful for characteris-ing soil hydrology within Europe and the long-term discharge regime of catchments in Northern Europe. Based on the results, we draft a roadmap for a refined hydrological classification of European soils.