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