Special issue  

Hydrological research and practice have traditionally been concerned either with predictions of water related hazards such as foods and droughts, or with water resources management. This has motivated us to focus on prediction of integral systems responses – mostly stream flow – using hydrological model structures that represent the process patterns and redistribution of water and mass inside a hydrologcal system, based on parsimonious (and therefore simplified) concepts. This story of ongoing success seems to require a) a certain minimum catchment size so that errors arising from simplified process conceptualisations have the opportunity to aveage out, and b) stationarity of both the climate conditions and of the hydrological system itself. Even when ignoring the challenge of coping with hydrological system change (an increasingly optimistic endeavour these days), we still struggle to provide predictions for systems that exhibit organised complexity at the intermediate scale of 5–200 km². Following Dooge (1986), these can be characterized as heterogeneous systems that display "some degree" of organisation, which is both too small to be treated with simple second order statistics and too large for the application of a reductionist, deterministic treatment based on the Darcy – Richards paradigm.

This special issue solicits contributions that address the following how, what, and why questions at catchment scales ranging from hillslope to lower mesoscale, thereby helping to link the bottom‐up and top‐down approaches and, in this way, help achieve movement towards unified theories at the catchment scale. Questions of interest include, but are not limited to:

• How to detect and quantify catchment structures, especially in the subsurface?
• How does structure control integral hydrological response at higher scales?
• How to infer model structures in a top‐down manner, building on representations of key landscape (or other) units in a more realistic manner?
• What model structures better allow reproduction of the current bio‐geo‐morphic system architectures (e.g., better reproduction of volumes of surface and subsurface stores and topologies of surface and subsurface flow paths)?
• How do ecological, pedological and geomorphological behaviours control processes and functioning of hydrological systems?
• How to account for the context dependence of process organisation within catchments?
• Why did a system evolve the way it did, in adaptive response to past hydro‐geo‐morphologic and biotic processes, and what can we learn from this to address future prediction challenges?
• What roles do feedbacks between biota and abiotic processes play in controlling structure formation and in stabilizing catchmen functions?