Irrigation in and around Arizona cities: Assessing impacts on urban climate and heat stress through meteorological and physiological pathways

The addition of irrigated water to regions with naturally limited surface water is bound to have thermal environmental impacts. Recent studies have begun exploring irrigation-induced impacts on heat stress. However, no studies have integrated regional climate and physiological modeling to examine these impacts through meteorological and physiological pathways. To fill this gap, we implemented an irrigation scheme into the Weather Research and Forecasting (WRF) model to simulate irrigation-induced meteorological processes across Arizona cities during the summer of 2023. Then, thermal environmental factors derived from the WRF simulations were input into a human heat balance model to evaluate changes in livability (maximum safe and sustained activity). Our findings showed that, during the day, irrigation led to a redistribution of surface energy budget (a greater conversion of sensible and ground heat into latent heat). Consequently, reduction in air temperature and mean radiant temperature (MRT) offset the negative effects of increased moisture in Phoenix and Tucson, two large semi-arid metropolises, resulting in decreased human dry heat gain and increased livability. Conversely, in Flagstaff and Prescott, two mountainous cities, irrigation increased daytime livability by reducing humidity. Although irrigation slightly decreased nighttime livability, its daytime benefits outweighed the minor drawbacks at night in the four main Arizona cities. This study underlined the value of coupling regional climate and human physiological modeling to quantify summer heat stress variations driven by meteorological and physiological processes.