This workshop is the second workshop organized in the framework of Improving the Theoretical Underpinnings of Hydrologic Models initiative, following the first successful workshop held in Bertinoro (Italy) in April 2016 to launch the community initiative. This follow-up workshop in Sopron (Hungary) is organised to consolidate progress since the last workshop, showcase new and innovative research and bring hydrologists together to accelerate progress on the development of a long-term strategy for a more systematic development and testing of hydrological models. More details on the workshop can be found in the first circular in the attachment.

The initiative is driven by five main research questions:

1. What are major theories that should guide hydrologic model development?

2. How can we represent small-scale processes that combine to produce large-scale fluxes (emergent behavior, spacetime scaling) in a consistent way?

3. How can we make use of optimality principles (or ecological and landscape evolution principles) to constrain model behavior? Are such principles useful when dealing with anthropogenic interactions?

4. How can we reflect the structure of the landscape in the structure of models?

5. How can we develop a rigorous approach to evaluate and select among competing theories and algorithms in presence of highly uncertain observation data?