The fact that persistent spatial organization in catchments exists inspired many scientists to speculate whether this is a manifestation of self-reinforcing co-development due to an underlying organizing principle. Spatial organization of catchments manifests through different fingerprints and affects different processes:

  • Hillslope scale spatial organization of soil types manifests in a typical arrangement of soil types along the topographic gradient. This is of key importance for overland flow, sediment yields, water availability for evaporation and temporal stability of soil moisture patterns.
  • Spatially organized variability at the pedon scale (within a soil type) is reflected in a spatial correlation of soil hydraulic properties, which translates into spatially correlated storage and recharge.
  • Soils and unconsolidated rock are veined with connected networks of preferential flow paths either created by biota (worms, rodents, roots) or by abiotic processes (shrinkage cracks, pipes, rills). Activated preferential flow networks allow for high mass flows even at small driving gradients and thus dominate export and redistribution of water and matter across many scales.

Jim Dooge was to our knowledge the first hydrologist who realized that spatial organization alongside with stochastic heterogeneity leads to complex hydrological behavior at intermediate scales (5-250 km2). Dooge argued that these are systems of organized complexity; being already too large and heterogeneous to be treated in a reductionist deterministic manner, but yet too small for characterizing their behavior using first and second order statistics. The latter is possible at larger scales of organized simplicity. Despite great progress made since Dooge’s pioneering work in hillslope hydrology and  the conceptualization of organized simplicity, we feel still pretty “naked” at the intermediate scale of organized complexity. There is little agreement neither on how fieldwork-based understanding should be reflected in catchment models to improve hydrological predictions, nor on how an appropriate model structure should look like, nor on what the controls are that determine the emergent behavior.

The rationale of this Chapman conference is thus to foster the development of a holistic framework to explore and understand how spatial organization controls complex behavior of intermediate scale catchments with the emphasis set on short and long term catchment flows of water and energy. Here, “holistic” means to harmonize the triad of observation, theory/explanation and prediction. This objective will be achieved through three closely related topical sessions:

Topic 1) Organizing principles, catchment structure and catchment functioning – is there a connection? The target is here to link the how to the why question by synthesizing testable hypotheses that could explain why spatial organization has evolved as a fingerprint of past catchment flows of water and energy. Ecology, fluvial geomorphology and thermodynamics offer a large set of candidate organizing principles. We will discuss to which extent we can test their value for improving hydrological predictions

Topic 2) New experimental concepts to search for functional landscape entities and to explore their controls on catchment functioning: Here, we will discuss experimental strategies suited for exploring a hierarchy of functional entities in catchments, and learning in an exemplary manner whether/how their structural features control their functional characteristics and the different catchment functions. This essentially implies to discuss suitable metrics to discriminate functional and structural similarity from our data.

Topic 3) Reducing physical and structural bias in catchment models: necessary complexity versus parsimony: We will discuss model structures that rely on observable quantities (possibly functional units), joint explicit but parsimonious treatment of mass, energy and momentum balance. We will set a special emphasis on the role of symmetric and anisotropic landscape characteristics controlling gradients and flow resistances in the landscapes, their role for process symmetries and the way they may reduce degrees of freedom in model structures. Additionally, we will focus on thermodynamic consistency of models, which is necessary to test organizing principles, as well as on metrics to assess model prediction consistence.

After the conference a special issue is foreseen (preferably in the AGU Water Resources Research journal), possibly in combination with a review on the outcomes of the discussions, debates and workshops of the conference.