Using Coastal Fog to Support Sustainable Water Use in a California Agricultural System

Abstract:

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. The future of California’s multibillion-dollar agricultural industry depends on increasing water use efficiency on farms. In coastal California, the growing season of economically important crops overlaps with the occurrence of coastal fog, which buffers the summer dry season through shading effects and direct water inputs. While the impacts of coastal fog on plant biology have been extensively studied in natural ecosystems, very few studies have evaluated its direct effects on the water and energy budgets of agricultural systems. The objective of this study was to develop a mechanistic understanding of the relationships between coastal fog and the water and energy budgets of croplands in order to improve estimates of crop-scale evapotranspiration rates, which has potential to curtail groundwater use based on local cloud meteorology. We established three sites on strawberry farms along a coastal-inland gradient in the Salinas Valley, California. At each site, we installed a passive fog collector and a micrometeorological station to monitor variation in microclimate conditions. Flow meters were installed in drip lines to quantify irrigation amount and timing. To assess plant response to foggy and non-foggy conditions, we collected measurements of photosynthesis and transpiration rates at the leaf and canopy-scale between June-September 2015. We found that canopy-level transpiration rates on foggy days were reduced by half compared to sunny, clear days (1.5 and 3 mmol H₂O m^-2 s^-1, respectively). Whereas the amount of direct fog water inputs to the soil did not differ significantly between foggy and clear days, average photosynthetically active radiation between 0900-1100 hr. was reduced from 1500 to 500 μmol photons m^-2 s^-1 between these sampling periods. Our results provide convincing evidence that shading effects by fog is a primary influence on crop transpiration rates in coastal agricultural fields during summer. The outcome of our research provides a first step towards making more accurate estimates of how much irrigation water may be reduced during foggy periods without sacrificing crop yields on coastal agricultural lands.