Recent shifts in the behaviour of natural watersheds suggest acute challenges for water planning under climate change. Shifts towards less annual streamflow for a given annual precipitation have now been reported on multiple continents, usually in response to a multi-year drought. Future drying under climate change may induce similar unexpected hydrological behaviour, and 15 this commentary discusses the implications for water planning and management. Commonly-used hydrological models poorly represent the shifting behaviour and cannot be relied upon to anticipate future shifts. Thus, their use may result in underestimation of hydroclimatic risk and exposure to “surprise” reductions in water supply, relative to projections. The onus is now on hydrologists to determine the underlying causes of shifting behaviour and incorporate more dynamic realism into 20 operational models. Main points 1. Drought-induced hydrological shifts towards less streamflow for a given precipitation have been reported across multiple continents. 2. Future drying under climate change may induce similar unexpected behaviour. 25 3. Such behaviour creates additional uncertainty in runoff projections, and may lead to ‘surprise’ reductions in future streamflow. Main text In a recent article, Peterson et al. (2021) reported shifts in hydrological behaviour induced by the “Millennium” drought (1997-2010) in Australia and persisting years after the drought ended. 30 Reductions in water resources during and after this drought were far more extreme than expected, even given low rainfall (Saft et al., 2015), because many watersheds shifted into a seemingly different state of streamflow behaviour. Concerningly, some watersheds remain in this state despite a return to near-average climate conditions, so that a year of average rainfall now produces less streamflow than it did before the drought (Peterson et al., 2021). With similar hydrological 35 shifts reported elsewhere in the world, including the USA (Avanzi et al., 2020), China (Tian et al.,