Trade-off between water-saving and cooling objectives: restricting irrigation increases the number of hot days

Abstract Irrigating urban greenery can be an effective strategy to mitigate heat. However, as drought-heatwave combined events become more prevalent, cooling efforts may be compromised by water-saving strategies. This paper considers the heat implications of restricting irrigation and aims to further understanding of how water-saving and cooling objectives compete. Using ecohydrological modelling, validated for Melbourne, Australia, five irrigation scenarios were simulated over 10 years. To determine the level of heat stress under the five irrigation scenarios, the ecohydrological model, Urban Tethys-Chloris (UT&C), simulated temperatures and thermal stress in a Melbourne street. Results predict that heat stress is reduced with increasing irrigation amount. Every 1.5 mm of additional irrigation each week (up to 28 mm wk −1 ) is predicted to reduce the number of heat stress days (where maximum Universal Thermal Climate Index (UTCI) > 32 °C) at the study site by one per year, with irrigation most effective at cooling when air temperatures are the hottest. The number of days with a mean air temperature over 28 °C is predicted to increase by 15% and 20%, respectively, if irrigation is restricted or stopped entirely. Therefore, ensuring the maintenance of irrigation during hot periods may prevent deaths. Our results suggest that the amount of irrigation required to decrease the mean air temperature by 1 °C is approximately 7 mm d −1 for oceanic climates, consistent with predictions in previous studies. We also predict the effect of irrigation amount on the number of heat stress days (UTCI > 32 °C). This relationship is important in understanding and communicating the impacts of water restrictions on urban heat. Furthermore, innovative urban planning and design, including onsite alternative water sources for irrigation, may enable irrigation for cooling, without exascerbating water stress.