Coastal low cloudiness and fog enhance crop water use efficiency in a California agricultural system

Impacts of climate change threaten California farmers in a number of ways, most importantly through a decline in freshwater availability, concurrent with a rise in water demand. In coastal California, the growing season of economically important crops, such as strawberries, overlap with the occurrence of summertime coastal fog, which buffers the summer dry season through shading effects and direct water inputs. The impacts of coastal fog on plant physiology have been extensively studied in natural ecosystems. Yet, very few studies have evaluated its direct effects on crop water use and demand, which has potential to curtail groundwater use. We established two sites on large, conventional strawberry farms along a coastal-inland gradient in the Salinas Valley, California, where we monitored variation in microclimate conditions and measured strawberry plant physiological responses to foggy and non-foggy conditions between June September 2015. Spatial analysis of coastal low clouds and fog from satellite imagery was preformed to quantify and characterize fog events at seasonal and diel time scales. We found strong agreement between field and satellite-derived observations of coastal fog events. Canopy-level conductance and whole-plant carbon uptake were reduced by 60% and 30%, respectively, on foggy compared to clear-sky days. Leaf-level photosynthesis and stomatal conductance were 30% lower on foggy compared to clear-sky days, which was driven by reduced photosynthetically active radiation and cooler temperatures during fog events. Taken together, we found that whole-plant water use efficiency increased significantly during foggy periods, and these patterns were driven by changes in the radiation balance and atmospheric water stress. Our results provide evidence that the shading effect by fog is a primary influence on crop water use efficiency in coastal agricultural fields during summer. The outcome of our research can inform estimates of how much irrigation water may be reduced during foggy periods without sacrificing crop yields on coastal agricultural lands.