Sagar Parajuli, Trent Biggs, Fernando De Sales, Miguel Zavala, Cenlin He, Charles Jones, CAllum Thompson, Nicolas Lopez Galvez, Haley Ciborowski, Tiago Quintino, Claudia Di Napoli, Aliasghar Montazar, Tayebeh Hosseini Yazdi, Monica Soucier

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 exacerbate heat stress, quantification of which is crucial in intensely irrigated agricultural lands such as the Imperial Valley (IV) in southern California. We present high-resolution maps of wet bulb globe temperature (WBGT), a key indicator of heat exposure in humans, over the IV and quantify the impact of irrigation during day and night in agricultural and urban settings. We derive WBGT from a high-resolution regional climate model (WRF), which shows robust performance against station-derived WBGT metrics yielding R-square up to 0.95 and RMSE as low as 0.71 ℃ in agricultural sites. We find that irrigation reduces WBGT by 0.3-1.3 ℃ during the wet season in the daytime due to strong evaporative cooling. However, during dry season, irrigation increases WBGT by 0.4-1.3 ℃ at night, when the large increase in humidity sufficiently raises the wet-bulb temperature (WBT) with added increase in dry-bulb temperature (DBT) and black globe temperature (BGT), surpassing the weaker evaporative cooling. We also find that the urban and fallow areas adjacent to the crop fields experience increased heat stress due to moisture advection. Modeled WBGT frequently exceeds the regulatory threshold of 24.4 ℃ in the crop fields during key harvest seasons with exceedances greater than 50, 150, and 300 hours in April, June, and August 2020, respectively. The heat stress modeling framework presented serves as a prototype to develop climate change adaptation strategies for the agricultural regions of the Imperial Valley as well as the broader Central Valley and inform labor and environmental policies in California and elsewhere.