Water storage plays an important role in mitigating heat and flooding in
urban areas. Assessment of the water storage capacity of cities remains
challenging due to the inherent heterogeneity of the urban surface.
Traditionally, effective storage has been estimated from runoff. Here,
we present a novel approach to estimate effective water storage capacity
from recession rates of observed evaporation during precipitation-free
periods. We test this approach for cities at neighborhood scale with
eddy-covariance based latent heat flux observations from fourteen
contrasting sites with different local climate zones, vegetation cover
and characteristics, and climates. Based on analysis of 583 drydowns, we
find storage capacities to vary between 1.3-28.4 mm, corresponding to
e-folding timescales of 1.8-20.1 days. This makes the storage capacity
at least one order of magnitude smaller than the observed values for
natural ecosystems, reflecting an evaporation regime characterised by
extreme water limitation.