Does irrigation cooling effect intensify during heatwaves? A case study in the Melbourne botanic gardens

Abstract

Reducing night-time air temperature by irrigating urban greenery can ameliorate heat stress in cities. Past studies often used modelling approaches to investigate the irrigation cooling effect during heatwaves. However, few observational studies have compared the differences in micro-scale irrigation cooling effect between heatwave and non-heatwave conditions. This study examines whether nocturnal irrigation cooling effects intensify during heatwaves, as there have been conflicting results in the literature regarding this issue. During the Australian summer of December 2013 to March 2014, this study was conducted in the Royal Botanic Gardens Victoria’s Melbourne Gardens (mixture of Australian and exotic vegetation) and in the Australian Gardens of its Cranbourne Gardens (specialises in Australian vegetation). We used the excess heat factor (EHF) to identify one severe heatwave (EHF = 34.9) and one extreme heatwave (EHF = 155.6) in our study period. Using meteorological observations, irrigation data and one-way ANOVA, we present the differences in cooling associated with irrigation between different types of landscape in the two gardens. During non-heatwave conditions, there was 0.5 °C – 1 °C of cooling associated with overnight irrigation of 20–25 mm. During heatwave conditions, several lawn areas recorded greater cooling (2 °C – 4 °C) associated with irrigation of similar amounts, but this phenomenon was not observed in areas dominated by hard surfaces. The magnitude and timing of the irrigation cooling effects also differed in various garden sites, with residual cooling effects observed lasting for several hours in certain sites following nocturnal irrigation events. Our results suggest that some significant cooling at the micro-scale is apparent from irrigated landscape in Melbourne, which has a temperate oceanic climate with occasional heatwaves. Moreover, the irrigation cooling effect in the Melbourne Gardens intensifies during heatwaves. Our findings could inform landscape succession planning to prepare for a hotter future climate.