Evaluating the Potential of Irrigation for Mitigating Urban Heat:
Trade-off between Water Use and Heat Mitigation Capacity
Abstract
Our world has been continuously urbanized and is currently accommodating
more than half of the human population in cities. Despite that cities
cover only less than 3% of the Earth’s land surface area, they emerged
as focal points of human activities, and confront numerous environmental
challenges as a result of changes in landscapes, hydroclimate,
ecosystems, and biodiversity. In particular, the built environment
usually experiences exacerbated heat stress induced by global climate
and landscape changes, commonly known as the urban heat island effect.
Urban irrigation, as a climate adaptation and mitigation strategy, is
effective in cooling the built environment, but exhibits large
uncertainties in the trade-off between water use and heat mitigation
capacity. Here we show the efficiency of cooling effect induced by
irrigation of urban vegetation, represented by a novel metric, viz.
urban water capacity, analogous to the heat capacity, across the
contiguous United States (CONUS) during summertime via numerical
simulations. The urban water capacity is calculated as the average
irrigation depth per degree of urban temperature reduction; the values
are 4.52 ± 0.77 mm day–1 °C–1 and 7.27 ± 1.27 mm day–1 °C–1 (mean ±
standard deviation) for surface and near-surface air cooling,
respectively, over the CONUS. The robustness of urban water capacity is
further exemplified in an extreme heat wave event, during which the
warming anomaly is partially offset by the additional cooling from urban
irrigation. Estimates of water capacity provide a quantitative metric
for evaluating the efficacy of irrigation in urban planning under
current heat stress and future warming.