The use of water irrigation to mitigate ambient overheating in the built environment: Recent progress

Since low evapotranspiration (ET) rates in the urban environment are partially responsible for higher temperatures in cities, irrigation has been regarded as a potential solution for mitigating urban temperature extremes and drought. Irrigation changes the surface water budget and modifies surface latent heat flux. As a result of irrigation, the evapotranspiration rate is increased which leads to air temperature decline and rising relative humidity. For decades, irrigation-induced climate effects have been thoroughly investigated in the field of agriculture. In contrast, urban irrigation cooling research is still at an early stage and existing studies in this field are rare. In addition, only a limited range of topics were studied including the quantification of irrigation-induced temperature decrease and humidity increase, the evaluation of the relationship between irrigation cooling and ET, as well as the integration of irrigation into mesoscale climate models. In this review, recent progress achieved on the above topics is presented and analysed. As a result, inconsistences were discovered in previous studies in the magnitude of temperature decrease. At the same time, vegetation related factors such as the urban vegetation fraction and the vegetation species are recognized as key factors in determining the irrigation cooling effect. Meanwhile, since irrigation cooling has a non-linear relationship with many factors like water consumption and background temperature, urban irrigation cooling requires an optimum plan to save water and energy.