Abstract
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.,