Looking at catchments as closely coupled hydro-geo-ecosystems reveal a highly organized architecture that is characterized by typical patterns of topography, soil and vegetation and self-similar flow networks at all scales. These patterns, biota and networks organise storage of water, nutrients and energy exchange as well as mass export (water, sediments) across large-scale gradients. While catchments exhibit complex internal organisational structures, understanding the similarity across catchments with respect to both underlying processes and responses can provide a pathway to a common framework for catchment classification. Understanding similarity is also fundamental for building more realistic dynamic catchment models that balance the necessary complexity with the greatest possible simplicity and to address the problem of transient hydro-geo-ecosystems.

The linkages between catchment organisation and similarity could provide further insight towards addressing a range of hydrologic questions including, but not limited to:

- the degree of model complexity needed to characterize catchment processes and response,

- the relative effects of hydroclimatology and physical properties on catchment response,

- the utility to trade space for time in understanding the effects of change on catchment response, and

- prediction in ungauged basins.

This session welcomes a combined discussion on ideas and studies that synthesize and (or) further our understanding of catchment organisation and similarity.