In 2013, the International Association of Hydrological Sciences (lAHS) launched the hydrological decade 2013-2022 with the theme "Panta Rhei; Change in Hydrology and Society". The decade recognizes the urgency of hydrological research to understand and predict the interactions of society and water, to support sustainable water resource use under changing climatic and environmental conditions. This paper reports on the first Panta Rhei biennium 2013-2015, providing a comprehensive resource that describes the scope and direction of Panta Rhei. We bring together the knowledge of all the Panta Rhei working groups, to summarize the most pressing research questions and how the hydrological community is progressing towards those goals. We draw out interconnections between different strands of research, and reflect on the need to take a global view on hydrology in the current era of human impacts and environmental change. Finally, we look back to the six driving science questions identified at the outset of Panta Rhei, to quantify progress towards those aims. @en

One of the main obstacles in mapping flood hazard in data scarce areas is the difficulty in estimating the design flood, i.e. river discharge corresponding to a given return period. This exercise can be carried out using regionalization techniques, which are based on flood data of regions with similar hydro-climatic conditions, or employing physically based model cascades. In this context, we compared the flood extents maps derived for a river reach of the Blue Nile following two alternative methods: i) regional envelope curve (REC), whereby design floods (e.g. 1-in-20 and 1-in-100 year flood peaks) are derived from African envelope curves and ii) physical model cascade (PMC), whereby design floods are calculated from the physical model chain of the European Centre for Medium-Range Weather Forecasts (ECMWF,). The two design flood estimates are then used as input of a 2D hydraulic model LISFLOOD-FP and the simulated flood extents are quantitatively evaluated by comparing to a reference flood extent model, which uses design floods estimated from in situ data. The results show the complexity in assessing flood hazard in data scarce area as PMC largely overestimates the flood extent, while REC underestimates it. @en