The notion of hydrologic similarity underlies many if not most areas of hydrology. We depend on similarity to transfer information from one catchment to another. For example predictions in ungauged basins using either catchment-scale models or regionalization of statistical, physical and (or) climatological catchment properties form the basis for generalization of results beyond individual catchments and for the development of classification frameworks. Also, the similarity with previously studied catchments supports how we approach designing measurement networks in new locations. Due to the importance of this research topic, a variety of approaches – ranging from statistically- to physically-based methods – for quantifying hydrologic similarity or dissimilarity between catchments have been introduced and tested in the literature. However, a synthesis of these methods with their respective advantages and limitations remains an outstanding challenge. It is also not clear what these strategies can achieve given that some stress the uniqueness of catchments and hence the difficulty in formalizing any kind of similarity assessment.

This session welcomes a discussion on the diversity of ideas and approaches, in the context of hydrologic modeling, observation and theory, to:
• define and apply hydrologic similarity,
• quantify dissimilarity among catchments, or
• guide the selection of hydrologically similar reference catchments.
We also encourage submissions on experimental studies on inter-site comparison demonstrating their importance for identifying similarities and dissimilarities of catchments.