More than 250 different crops, worth $30 billion per year are grown in the greater Central Valley but according to University of California researchers from the USDA-funded Secure Water Future project, crop-water demand has doubled since 1980, with much of that increase attributable to climate change.
They found that an increase in the demand explains a great deal of “half of the cumulative deficits of the agricultural water balance” over the past four decades. This means there is higher demand while groundwater supplies decrease. The researchers called this phenomenon of climate-induced increased crop-water demand an “invisible water surcharge.”
“With a high reliance on surface-water imports and groundwater in our region, the combination of groundwater regulation and increased evaporative demands, as shown in this study, will further challenge our water management options,” said Josue Medellin-Azuara, part of the research team. “Increased evaporative losses also apply to open water bodies like reservoirs, making managed aquifer storage more attractive during wet years.”
Researchers found that in the past decade, this invisible water surcharge on agriculture has increased the crop-water demand in the Valley by roughly 650 thousand acre-feet per year, a volume larger than the capacity of Millerton Reservoir on the San Joaquin River near Fresno. This represents a 4.4 percent increase with respect to the 1980-2011 baseline used in the study. Through an analysis of cumulative anomalies, researchers showed that the chronic increases in crop-water demand over the last four decades explain half of the cumulative deficits in the agricultural water budget, and this trend is expected to worsen in the coming years.
To compensate for the lack of precipitation during the growing season in the Central Valley’s arid Mediterranean climate, irrigation is provided through a combination of surface and groundwater sources. There is increased reliance on groundwater pumping where surface-water supplies are limited — such as during droughts — and when crop-water demands are high. Both conditions are often present in the Valley, where the region has been subject to accelerating the depletion of groundwater reserves over the past several decades.
Researcher Kelly Moyers, a post-doctoral scholar at the time, was the lead author of the study completed about increasing crop-water demand as the climate has warmed, recently published in PLoS Water. The SWF team included UC Merced professors John Abatzoglou and Medellin-Azuara, and UCLA Professor Alvar Escriva-Bou, as contributing co-authors. SWF director and UC Merced professor Joshua Viers was the senior author.
Researchers used data to estimate crop-water demand in the San Joaquin Valley over the past 40 years. They focused on the climate effects on crop-water demand, avoiding the confounding factors of changing land use and management practices.
Agricultural droughts are not only caused by lack of precipitation, but now more often by insufficient surface or groundwater reserves to fulfill increasing crop demands.
“The long-term decline in spring snowpack is a hallmark symptom of a warming climate in the state,” said Abatzoglou. “Our study shows this hidden tax of increased crop-water demands with warming is pouring salt in the wounds from the vantage of agricultural water resources.”
Despite potential agronomic adaptation and crop response to climate warming, increased crop water demand adds a stressor to the sustainability of the global fruit and nut supply and calls for changes in management and policies to consider the shifting hydroclimate.
