This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. ARTICLE HISTORY

[1] Floodplains are critical landscape features that are highly distinguished from neighboring uplands in terms of their hydrologic, geomorphologic, biogeochemical, and ecological processes. Thus an essential step in floodplain studies is the determination of the lateral floodplain extent and its variability within a basin. Recent advances in topographic mapping and GIS-based analysis provide a means for developing automated techniques to determine floodplain extent. In this study, we propose a hydrogeomorphic floodplain delineation which links a simplified inundation algorithm with the river basin properties. We utilize the method to investigate the physical controls on floodplain formation, as captured by scaling relations between floodplain width and contributing area. Our hypothesis is that an efficient delineation grounded in hydrogeomorphic theory can improve our understanding of floodplain geometry across a range of conditions. We test the method for two distinct watersheds, a humid catchment in Italy and an ephemeral basin in New Mexico, and note the existence of power law scaling of the floodplain width after averaging over stream orders. An analysis of the algorithm parameters is conducted to identify possible limitations. Further systematic tests are needed to establish the generality of the floodplain scaling and its control on floodplain processes.

Identifying floodplain boundaries is of paramount importance for earth, environmental and socioeconomic studies addressing riverine risk and resource management. However, to date, a global floodplain delineation using a homogeneous procedure has not been constructed. In this paper, we present the first, comprehensive, high-resolution, gridded dataset of Earth’s floodplains at 250-m resolution (GFPLAIN250m). We use the Shuttle Radar Topography Mission (SRTM) digital terrain model and set of terrain analysis procedures for geomorphic floodplain delineations. The elevation data are processed by a fast geospatial tool for floodplain mapping available for download at https://github.com/fnardi/GFPLAIN. The GFPLAIN250m dataset can support many applications, including flood hazard mapping, habitat restoration, development studies, and the analysis of human-flood interactions. To test the GFPLAIN250m dataset, we perform a consistency analysis with floodplain delineations derived by flood hazard modelling studies in Europe.

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