Dry Heat, Not Moist Heat, Drives Irrigation’s Labor Capacity Benefits in Arizona’s Cities and Croplands

The impact of irrigation on outdoor labor capacity under heat stress has long been debated due to the competing environmental effects of cooling (i.e., benefits) and moistening (i.e., drawbacks). We quantitatively address this debate through proposing an interdisciplinary framework that couples a regional climate model with a human heat balance model. We apply this framework to assess the impact of irrigation on labor capacity in the arid/semi-arid environments of Arizona (USA) during an extremely hot-dry summer. Results reveal that irrigation-induced environmental changes primarily modify labor capacity through dry heat (sensible heat exchange between human and environment) rather than moist heat (evaporative heat loss from human to environment) exchange. Through reduction of daytime dry heat gain from environment to human, irrigation decreases the proportion of discouraged outdoor work hours by ~ 30% in the hottest urban (i.e., Phoenix-metro) and cropland areas. Nocturnally, despite reduced dry heat loss from human to environment, labor capacity changes occur only 2% of the summertime. Our results demonstrate complex interplays between humans and their ambient environment, underscoring the necessity of coupled meteorological, physiological, and human biophysical principles to properly assess outdoor labor capacity.