A one-step similarity approach for the regionalization of hydrological model parameters based on Self-Organizing Maps

•Development of a novel one-step regionalization approach based on Self-Organizing Maps (SOM).•Whole model parameter sets were transferred to ungauged catchments based on catchment similarity.•Relationships between model parameters and catchment descriptors don't need to be predefined but result automatically.•The relationships are more transparent compared to a known transfer function based approach.

SummaryA novel one-step similarity approach for the regionalization of hydrological parameters has been developed. The method automatically associates model parameters (MP) with catchment descriptors (CD), using techniques based on Self-Organizing Maps (SOM-B). The combination of the one-step approach with the automatic association reduces the problem of equifinality and avoids subjective predefinition of relationships between CDs and MPs. Further positive properties of the SOM-B are the feasibility to use discrete variables as CDs and the ability to capture interdependencies between MPs.The SOM-B was validated against a known one-step regionalization method which is based on linear transfer functions (LT-B). Furthermore the influence of different factors like combination CDs, or size of the SOM, on the regionalization performance of the SOM-B was analyzed. The hydrological model used for the evaluations was a modified version of the HBV-IWS, which simulates continuous hourly discharge series. The investigations were carried out in 41 catchments of the Aller-Leine river basin in Northern Germany. For the validation a spatial split-sampling in 12 donor and 29 validation catchments, as well as a temporal split-sampling were applied.The results showed that: (I) MPs regionalized with the SOM-B approach led to similar or slightly better model performance than with the LT-B method, (II) some hydrologically plausible relationships between MPs and CDs were adjusted automatically without any predefinition by the SOM-B, (III) the relationships between CDs and MPs were more transparent using the SOM-B approach, (IV) the sensitivity of different factors of the SOM-B on the performance was smaller than expected.