Moving Beyond the catchment scale: value and opportunities in large-scale hydrology to understand our changing world

Hydrological research is often focused at the catchment scale; but there are significant benefits from taking a broader spatial perspective (i.e. comparative hydrology) to advance understanding of hydrological processes, especially in the context of global change. Indeed, many of the recently described ‘unsolved problems in hydrology’ (Blöschl et al., 2019) refer to either global-scale processes (e.g. climate change), the hydrology of major physiographic zones (e.g. semi-arid or snowmelt regions) or require extensive comparisons across catchments. Moving beyond the catchment-scale frequently provides more holistic insights into the varying spatio-temporal response of hydrological systems to climate variability and change, as well as to the myriad of other anthropogenic influences on water. This knowledge is key for both mitigation of, and adaption to, hazards under an increasingly changed water cycle (Abbot et al., 2019). Moreover, a large-scale viewpoint is essential to inform appropriate water management towards socio-economic development, water-food-energy security and ecosystem health (e.g. WWAP, 2019). Here we contest that taking a large-scale perspective can bring significant benefits to understanding hydrological processes under change. After making the case for a need for large-scale hydrology, we then explain the benefit of a large-scale hydrology approach for investigating global change, its causes, as well as water management in the present day and into the future. We conclude by identifying challenges and opportunities to advance research in large-scale hydrology and hydrological process understanding beyond the individual catchment.