Modelling the Effect of Irrigation on Urban Microclimate in a Mixed Development Suburb

In conventional urban areas, the majority of rainfall is exported out of the environment through the stormwater drainage system. This loss of stormwater is balanced by importing potable water, which is used for irrigation and gardening watering. Stormwater runoff can reduce urban moisture availability, leading to reduced evapotranspiration and increased sensible heat flux, which may cause higher urban air temperatures. This is particularly relevant in Australia, which has experienced extended dry periods and heat waves over the last two decades, especially in the major southern cities: Perth, Adelaide, and Melbourne. The ongoing drought has placed pressure on potable water resources and led to water restrictions and irrigation bans. Residents have become diligent at conserving water and many residents have adapted gardening approaches to cope with less potable water supplies by planting more drought-tolerant species. These compounding consequences of drought: water restrictions, xeric gardening practices, and reduced health of urban vegetation, further exacerbate urban warming and energy demands. Reintegrating stormwater into the urban environment, may help to modulate the effects of urban warming, while also improving stream ecology and conserving valuable potable water resources. However, the climatological implications of water scarcity, stormwater reintegration, and changing irrigation practices, for urban climate are often ignored in urban scenario and mitigation modelling research.

The objective of this research is to understand the effects of irrigation and stormwater reintegration on local-scale air temperature in a suburban environment. The analysis is conducted using the Town Energy Balance (TEB) model. We implement an irrigation scheme within the TEB model, and test a series of irrigation regimes for a mixed-residential suburb in Adelaide, Australia. Overall, irrigation and stormwater reintegration (processes which also have ecological benefits) show potential to cool daytime local-scale climate in suburban areas.

Drawing on a high resolution observational dataset (27 static weather stations), collected in Adelaide during summer 2011, we also consider the role of scale in urban climate. We highlight the large amount variability at the microscale compared with typical scales associated with urban climate modelling. The scale of urban climate that people are exposed to (microscale climate) is at a different resolution to that of urban climate models, such as TEB, which are typically designed to operate at the local to mesoscale. The implications and limitations of this scale issue are discussed in terms of urban heat mitigation and scenario modelling research.