Preface : Linking landscape organisation and hydrological functioning: From hypotheses and observations to concepts, models and understanding

The link between landscape properties and hydrological functioning is the very foundation of hydrological sciences. The fundamental perception that landscape organisation and its hydrological and biogeochemical processes co-develop is often discussed. However, different landscape characteristics and hydrological processes interact in complex ways. Hence, the causal links between both are usually not directly deducible from our observations. So far no common concepts have been established to connect observations, properties and functions at and between different scales. This special issue hosts a broad set of original studies indicating the current state and progress in our understanding of different facets of dynamic hydrological systems across various scales. It is organised as a joint special issue in HESS and ESSD, with the purpose of providing the scientific insights in combination with the underlying data sets and study design. While the individual studies contribute to distinct aspects of the link between landscape characteristics and hydrological functioning, it remained difficult to compile their specific findings to more general conclusions. In this preface, we summarise the contributions. In the search for ways to synthesise these individual studies to the overall topic of linking landscape organisation and hydrological functioning, we suggest four major points how this process could be facilitated in the future: (i) formulating clear and testable research hypotheses, (ii) establishing appropriate sampling designs to test these hypotheses, (iii) fully providing the data and code, and (iv) clarifying and communicating scales of observations and concepts as well as scale transfers