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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.

Air quality policies based on scientific information have proved to be effective for controlling air pollution and protecting public health. Intensive field studies provide knowledge that combined to data from emission inventories and air quality monitoring allows to understand the causes that trigger air pollution and catalyze the design of effective control measures. We review the case of Mexico City, where past international collaborative studies were fundamental to improve air quality, but a null progress and a possible reversal to high air pollution levels in recent years suggest that a new dedicated field measurement campaign is urgently needed.