Special issue  

The hydrological cycle, from catchments to global scales, has for thousands of years been intimately linked with human activity, in forms including irrigation, water storage, domestic water use and flood defence. Today, people control or impact on hydrological systems in a multitude of ways. We modify land use and climate, change flow pathways and sources of nutrients and sediments, and disrupt natural feedbacks between water and ecosystems. Rural and urban areas use and transport water in different ways and at different scales, and our trade has an embedded water footprint. In many regions, there are severe challenges associated with societal interactions with water, including water scarcity, pollution and flooding. Societies must make decisions on water governance, ownership and management, despite large uncertainties in the coupled hydrological–societal system.

In response to the clear imperative to include human impact as integral to hydrological research, the International Association of Hydrological Sciences (IAHS) launched Panta Rhei, the scientific decade 2013-2022 with the central theme "Change in Hydrology and Society" (www.iahs.info/pantarhei). The research agenda aims to address understanding, prediction and management of hydrological systems under environmental change and human impact. This special issue reports on progress by the Panta Rhei working groups towards many of the questions described above, during the first Panta Rhei biennium 2013-15. It also contains opinion papers, as we explore and reflect on the most pressing issues in research into hydrology and change. In particular, many articles will aim to quantify progress against the six driving science questions set out at the beginning of the Panta Rhei initiative:

1. What are the key gaps in our understanding of hydrological change?

2.How do changes in hydrological systems interact with, and feedback to, natural and social systems driven by hydrological processes?

3. What are the boundaries of coupled hydrological and societal systems?

4. How can we use improved knowledge of coupled hydrological–social systems to improve model predictions, including estimation of predictive uncertainty and predictability?

5. How can we advance our monitoring and data analysis capabilities to predict and manage hydrological change?

6. How can we support societies to adapt to changing conditions by considering the uncertainties and feedbacks between natural and human-induced hydrological changes?