Irrigation in and around Arizona cities: Assessing impacts on urban climate and human thermal comfort

Irrigation is an important component of the hydrologic cycle, especially in arid and semiarid regions. Phoenix is a semiarid metropolitan area with urban and peri-urban cropland irrigation. The impact of urban and cropland irrigation interactions on hydrologic cycle and associated thermal environment consequences remain understudies. However, limited research has focused on the separate and combined impacts of urban and peri-urban cropland irrigation on the climate and associated human thermal comfort in such areas. To fill the research gaps, we first added an irrigation scheme into the Weather Research and Forecasting model (WRF) to represent changes in soil moisture owing to irrigation. Three sensitive experiments (urban irrigation only, cropland irrigation only, and urban and cropland irrigation) and one control experiment (without irrigation) were conducted to evaluate summer climate responses to irrigation in Arizona cities (including Phoenix metropolitan area and Tucson) for 2023. Second, we deployed a leverages method of partitional calorimetry to model heat stress and further assessed human livability to heat for different irrigation scenarios. In this physiological approach, the WRF model provided input for ambient thermal environmental variables (air temperature, relative humidity, wind speed, and radiation). Results show that the WRF model well reproduced the diurnal cycle of observed near-surface temperature, wind speed, dew point, and relative humidity in urban areas and croplands over Arizona. In general, irrigation on both urban and croplands showed the best agreement with observation. Irrigation in and around Arizona cities decreased daytime temperature but increased nighttime temperature in summer. In Phoenix metropolitan area, urban irrigation, cropland irrigation, and irrigation on both led to a daily maximum decrease in air temperature of 0.35, 0.21, 0.47℃, respectively. Conversely, a daily maximum increase of 0.39, 0.58, 0.88℃, respectively, was simulated in the nighttime. Compared with the Phoenix metropolitan area, Tucson had stronger daytime cooling but weaker nighttime warming. Meanwhile, urban irrigation, cropland irrigation, and irrigation on both led to a daily average increase in water vapor in the Phoenix metropolitan area by 0.18, 0.11, and 0.15 g/kg, respectively, while in Tucson, an increase of 0.13, 0.11, and 0.18 g/kg was found. In the Phoenix metropolitan area, the three irrigation scenarios decreased daily average wind speed by 0.21, 0.22, and 0.24 m/s, respectively, while in Tucson, the decrease was 0.22, 0.23, and 0.33 m/s. Cropland irrigation only and urban and cropland irrigation alleviated daytime heat stress in Arizona urban areas, while urban irrigation only increased daytime heat stress. Different irrigation scenarios, however, led to trivial increases in heat stress at night. Our study underlined that the increase of moisture, both within and around cities, exerted a strong influence on summer climate and heat stress in Arizona urban areas, and highlighted that mitigation measures should be taken to alleviate the deleterious impacts of high temperature and raising humidity on human health.