In an increasingly urbanized world, water systems must be designed and operated according to innovative standards in terms of climate adaptation, resource efficiency, sustainability and resilience. This grand challenge triggers unprecedented questions for hydro-environment research and engineering. Shifts in paradigms are urgently needed in the way we view (circular) water systems, water as a renewable energy (production and storage), risk management of floods, storms, sea level rise and droughts, as well as their consequences on water quality, morphodynamics (e.g., reservoir sedimentation, scour, sustainability of deltas) and the environment. Addressing these issues requires a deep understanding of basic processes in fluid mechanics, heat and mass transfer, surface and groundwater flow, among others.