Looking at catchments as closely coupled hydro-geo-ecosystems reveals an organized architecture that is characterized by typical patterns of topography, soil and vegetation and self-similar flow networks at many different scales. These patterns and networks are a result of processes that occur over a large range of time scales and with different degree of persistence, and they can organize storage of water, nutrients and energy exchange as well as mass export (water, sediments) across large-scale gradients. This empirical evidence supports the idea that fundamental organizing principles exists that shape the landscape, and these organizing principles may lead to similarity across catchments. 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 organization and similarity could provide further insight towards addressing a range of hydrologic research 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 uncertainties involved in identification of dominating processes and hydrologic response behavior,

- the underlying organizing principles that shape the catchments,

- 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 organization and similarity.