Impacts of irrigation scheduling on urban green space cooling

The increasing heat stress in cities due to climate change and urbanisation can prevent people from using urban green spaces. Irrigating vegetation is a promising strategy to cool urban green spaces in summer. Irrigation scheduling, such as daytime vs night-time irrigation and the frequency of irrigation in a day, may influence the cooling benefit of irrigation. This study aimed to investigate whether irrigation scheduling can be optimised to increase the cooling benefit and determine how the cooling benefit changes with weather conditions. A field experiment with twelve identical turfgrass plots (three replicates × four irrigation treatments) was set up to measure the afternoon cooling benefits of irrigation. The four treatments included: no irrigation, single night-time irrigation (4 mm d^–1), single daytime irrigation (4 mm d^–1) and multiple daytime irrigation (4 x 1 mm d^–1). The cooling benefit was defined as the air temperature difference measured at 1.1 m above the turfgrass between the irrigated and unirrigated treatments (air temperature sensor accuracy ± 0.2 °C). The afternoon (12:00–15:59) mean cooling benefit of multiple daytime irrigation (–0.9 °C) which was significantly stronger than that of single night-time irrigation (–0.6 °C) and single daytime irrigation (–0.5 °C). Regardless of irrigation scheduling, the afternoon mean cooling benefits of irrigation were greater for days when background air temperature, vapour pressure deficit and incoming shortwave radiation were greater. The findings suggested that irrigation scheduling can be optimised to increase the cooling benefit of urban green space irrigation without increasing overall water use.