Catchments are systems that often consist of an organized architecture of typical patterns of topography, soils, and vegetation and flow networks. These patterns are largely the geomorphic-, and biologic -response to temporally and spatially variable environmental conditions or human interference. This organization of catchment components will control the storage and release of water and nutrients. Consequently, understanding catchment organization is critical for:

- Creating catchment models that balance necessary complexity with possible simplicity,
- Understanding the degree of similarity between catchments, with the prospect of developing hydrological theories that are transferable in space and/or time,
- Increase our understanding of catchment processes and behavior across various spatial and temporal scales, and
- Predicting the future evolution of catchment properties and hydrologic response in a non-stationary environment.

This session welcomes studies that increase our understanding of catchment organization, similarity, and evolution in the context of the current hydrologic reality; we live in a world where catchments have varying degrees of human interference, hydrological data is uncertain and often limitedly available, and both climate and humans are changing the hydrology around us. Example topics can include (but are not limited to):

- the degree of model complexity needed to characterize catchment processes and response,
- systematic multi-catchment analysis of the degree of similarity in climate, landscape, and hydrology
- the relative effects of climate, landscape and human interventions on catchment response,
- the uncertainties involved in the identification of dominating processes and hydrologic response behavior
- the underlying organizing principles that shape the catchments
- the measure (i.e., quantification) of human impacts and the consequent change on catchment response behavior and similarity

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