Hydrological extremes (droughts and floods), have major impacts on society and ecosystems and are expected to increase in frequency and severity with climate change. An extreme case of systems experiencing both extremes are intermittent rivers, which range from near-perennial rivers with infrequent, short periods of zero flow to rivers experiencing flow only episodically following large rainfall events. Although floods and droughts are at the extreme end of the hydrological spectrum, they are governed by different processes, which means that they operate on different spatial and temporal scales and that different analysis methods and indices are needed to characterise them. But there are also many similarities and links between the two extremes that are increasingly being studied.

This general session on hydrological extremes aims to bring together the two communities in order to learn from the similarities and differences between flood and drought research. We aim to increase the understanding of the governing processes of both hydrological extremes, find robust ways of modelling and analysing floods and droughts, assess the influence of global change (including climate change, land use change, and other anthropogenic influences) on floods and droughts, and study the socio-economic and environmental impacts of hydrological extremes. There is a clear need to study the hydrology of natural intermittent and ephemeral streams to characterize their flow regimes, to understand the main origins of intermittency and how it shapes their biodiversity and ecosystem processes and to assess the consequences of altered flow intermittency (both increased and decreased) in river systems.

This session is jointly organised by the Panta Rhei Working Groups “Understanding Flood Changes”, “Changes in Flood Risk”, and “Drought in the Anthropocene” and will further stimulate scientific discussion on change detection and attribution of hydrological extremes and the feedbacks between hydrological extremes and society. The session is linked to the European Low Flow and Drought Group of UNESCO´s IHP-VIII FRIEND-Water Program, which aims to promote international drought research, and the SMIRES (Science and Management of Intermittent Rivers & Ephemeral streams) COST Action that focuses on better understand intermittent and ephemeral streams.