Irrigated agriculture is rapidly depleting groundwater resources in parts of the High Plains and the Central Valley region of California, which are both critical regions for food production, according to a new study. According to the study, if groundwater depletion were to continue at current rates, 35 percent of the southern High Plains will no longer be able to support irrigation within the next 30 years.
With climate change projections showing that more severe droughts in both the Southwest and High Plains are likely as the climate continues to warm, groundwater resources are going to be even more highly stressed in the coming decades, the study says.
Satellite image of fields that have been irrigated by central pivot systems, which use less water than many other irrigation methods. Credit: Wikipedia Commons.
The groundwater resources that sustain agricultural production in California’s Central Valley and the High Plains enabled farmers to produce $56 billion in agricultural products in 2007 alone, the study reported, and these two areas comprise the country’s most productive agricultural lands. The High Plains is commonly known as America’s “bread basket,” while the moniker of the country’s “fruit and vegetable basket” is sometimes applied to California’s Central Valley.
The research, published in the journal Proceedings of the National Academy of Sciences, integrates water observations from different sources, including NASA satellites, about 11,300 wells, and computer models to produce one of the most comprehensive looks yet of how irrigated agriculture is drawing down vital groundwater supplies.
The picture that emerges from the study is more complex than was previously thought, with groundwater depletion varying in different areas and at different times. For example, due to the region’s coarse soil, the study found that there is little to no groundwater depletion occurring in Nebraska, but there is a large amount of depletion taking place in the southern regions that are fed by the same Ogallala Aquifer.
In fact, about a third of the groundwater depletion that has occurred in the High Plains has taken place beneath just 4 percent of the land area, the study found.
Lead author Bridget Scanlon of the University of Texas at Austin said the rapid depletion of what is “essentially fossil groundwater,”—dating back as far as 13,000 years—in the southern High Plains is especially troubling, because that groundwater cannot be easily recharged.
Recharge rates of the Ogallala Aquifer, which stretches from Nebraska south to Texas. The blues show positive recharge in Nebraska, contrasted with orange colors, showing net losses of groundwater, farther south. Credit: Scanlon et al.
In the U.S., 60 percent of agricultural irrigation relies on groundwater supplies, and two of the nation’s most heavily irrigated regions — California’s Central Valley and the High Plains, which stretches from Nebraska to Texas — are responsible for about half of all the groundwater depletion that has occurred since 1900.
At the global level, irrigation consumed about 90 percent of freshwater resources during the past century, and according to the researchers, global groundwater depletion has doubled between 1960 and 2000.
In the Central Valley of California, massive water projects have created a system that transports water from mountainous parts of the Golden State to drier southern parts of the state, resulting in booming agricultural production in an area that wouldn’t otherwise be suitable for it.
“Essentially they’re growing food in a desert,” Scanlon said.
In the Central Valley, groundwater tends to be depleted during periods of drought, but partially recovers during wetter times, as farmers tend to be equipped to use both surface water and groundwater. During a recent extended drought that lasted between 2006 and 2009, the amount of groundwater that was depleted was equivalent to the entire capacity of Lake Mead, the largest reservoir in the U.S., the study found.
Is “Groundwater Banking” a Possible Solution?
Climate studies show that groundwater depletion is likely to accelerate in coming years as temperatures increase and more severe and longer lasting droughts occur. In addition, the intensification of the water cycle is beginning to result in a feast-or-famine precipitation pattern, in which floods are soon followed by drought.
Two USDA researchers download data about the movement of a center-pivot irrigation system to reconstruct the amount of water and time it took to irrigate an area. Credit: USDA photo by Scott Bauer.
One possible way for farmers to use less groundwater would be by practicing more efficient irrigation methods, but such practices are already in use in Texas, for example. Instead, Scanlon said in some areas farmers might need to switch from irrigated crops to rain-fed crops. This would require that farmers make a transition away from growing thirsty crops like corn, to less water-intensive ones, such as cotton or sorghum. But that could cost farmers money, in the form of reduced crop yields.
Switching to rain-fed crops is not a viable option, though, in southern parts of the Central Valley, since so little rain falls there. For this region, Scanlon and her colleagues from the U.S. Geological Survey recommend using a process called “groundwater banking” or “managed aquifer recharge,” through which excess surface water would be stored underground during wet periods, which cuts down on any increase in evaporation associated with global warming. This water would then be tapped during times of drought, and it would reduce the need to use older, less rechargeable, groundwater supplies.
Scanlon said that a groundwater banking program helped San Antonio, Texas, weather the state’s worst one-year drought on record last year. “San Antonio survived last summer because it had aquifer storage and recovery,” she said. Other cities, such as Austin, saw their water supplies dwindle to perilously low levels.
Even with groundwater banking and other innovative solutions, this study and other research show that managing water supplies is going to be one of the toughest environmental challenges during the next century. Water demand will rise along with the population, with the U.S. expected to have about 440 million residents by 2050, up from 312 million in 2011. At the same time, water supplies will be on the decrease.