Are seemingly physically similar catchments truly hydrologically similar?

This paper discusses the notion of similarity often used in the regionalisation studies of hydrological models. We compare two different visions of similarity: the apparent similarity defined on the basis of observable catchment properties; and behavioural similarity judged through the use of hydrological models. These two visions are generally assumed to be merged in regionalisation studies: catchments having apparently similar physical characteristics are assumed to have a similar hydrological behaviour. In this paper, we wished to test the validity of this assumption. To this aim, we defined behavioural (hydrological) similarity on the basis of model parameter transferability. Then pools of hydrologically similar catchments are compared to pools of apparently physically similar catchments, as identified on the basis of physiographic catchment descriptors. The overlap between the two pools of similar catchments is analysed, making it possible to judge the efficiency of the physical similarity measure to identify hydrologically similar catchments in an ungauged context. The results show that the overlap between the two pools is significant for only 60% of the catchments. For the other catchments, two major reasons were identified as contributing to the lack of overlap: (1) these catchments often have a quite specific hydrological behaviour; (2) the role of the underground properties of the catchment on its hydrological behaviour was not found to be accurately described by the available physical descriptors, meaning that more relevant catchment descriptors should be sought to better describe the geological and lithological context in hydrological terms.