Cooling an arid city using irrigated vegetation: Assessing the energy-water trade-off with the latest WRF-urban canopy model

Faithful representation of hydrological processes in urban areas has been a challenge to urban hydroclimate modelers and practitioners. Recently, we have developed and implemented an improved urban canopy model (UCM) in the newly released WRF-ARW v3.7. The new WRF-UCM framework incorporates enhanced modeling of hydrological processes in a built environment, including (1) anthropogenic latent heat, (2) urban irrigation, (3) evaporation over water-holding engineered pavements, (4) urban oasis effect, and (5) green roof systems. It thus enables us, in particular, to assess green urban infrastructure as sustainability solution for, e.g. mitigating urban heat island (UHI) effect and improving building energy efficiency. In this study, we apply the new WRF-UCM to investigate the impact of urban vegetation, viz. green roofs and lawns with irrigation, on local and regional hydroclimate. Offline case studies show that green roof is capable of reducing urban surface temperature and sensible heat flux effectively, and modifying local and regional hydroclimate throughout the year. Online simulations in the WRF platform with nested schemes are evaluated against field observations in Phoenix and Houston metropolitan areas. Results indicate that model performance, especially with respect to the moisture dynamics, is significantly improved. In addition, adopting the new WRF-UCM, we devise a measure of energy-water trade-off to evaluate the environmental sustainability of different irrigation schemes of urban lawns, and their impact on human thermal comfort as well as building energy efficiency. It is found that that a well-designed soil-temperature-controlled irrigation scheme has a great potential in saving building energy consumption in an annual scale, with a combined saving up to $2.5 per square meter wall area per month. In addition, smart irrigation scheme can substantially enhance outdoor thermal comfort of pedestrians in hot seasons, leading to potentially attractive sustainability solutions for UHI mitigation.