California is a leading contributor to the agricultural activity in the United States and home to the greatest share of the nation’s population, 95% of which lives in highly irrigated urban areas where irrigation can account for more than 50% of municipal water consumption. In addition to the implications of climate change for state’s water resources, climate change is feared to increase water demands and therefore push California’s water supply-demand imbalance to levels beyond the range of past experiences.

Many past climate modeling studies support the notion that rising atmospheric temperatures have and will continue to lead to increases in moisture-holding capacity of the atmosphere, which in-turn lead to higher evapotranspiration (ET) rates and water demands. However, there are also reports of observed decreasing trends in ET across the globe during the first decade of 21st century. In this study we target this contradiction and quantify the impacts of rising atmospheric temperatures (under midcentury RCP4.5 and 8.5) on ET and irrigation water demand in agricultural and urban areas across California using a high-resolution regional climate model and realistic representation of irrigation processes.

We attribute mid-century ET changes to different forcing factors in the Penman–Monteith equation and show that higher temperatures and vapor pressure deficit trigger a vegetation response through increases stomatal resistance, that can significantly offset increasing atmospheric evaporative water demands. We further assess the implications of heat mitigation measures (i.e., cool roofs) for evaporative water demand in cities, contextualizing the local and global drivers of irrigation water demand in cities.