Impact of irrigation on farmworker’s heat stress in California differs by season and during the day and night

Farmworkers, the frontline workers of our food system, are often exposed to heat stress that is likely to increase in frequency and severity due to climate change. Irrigation can either alleviate or exacerbate heat stress, quantification of which is crucial in intensely irrigated agricultural lands such as the Imperial Valley in southern California. We investigate the impact of irrigation on wet bulb globe temperature (WBGT), a key indicator of heat exposure in humans, using a validated high-resolution Weather Research and Forecasting (WRF) regional climate model, during day and night and in different seasons. We find that irrigation reduces WBGT by 0.3–1.3  °C during the daytime in summer due to strong evaporative cooling. However, during the summer nights, irrigation increases WBGT by 0.4–1.3 °C, when a large increase in humidity sufficiently raises the wet-bulb temperature. Urban and fallow areas adjacent to cropped fields also experience increased heat stress due to moisture advection from irrigated areas. Our results can inform heat-related policies in agricultural regions of California and elsewhere.