Wednesday, June 13, 2012
U.S. Food Supply Vulnerable to Ground Water Depletion
University of Texas researchers didn’t mince words after studying how irrigated agriculture is depleting groundwater in some of the country’s semi-arid regions. “Basically, irrigated agriculture in much of the southern High Plains is unsustainable,” said Bridget Scanlon, senior research scientist at University of Texas at Austin’s Bureau of Economic Geology.
After taking a close look at irrigation practices and groundwater levels in California’s Central Valley and in the High Plains region, the scientists concluded that the nation’s food supply could be vulnerable to groundwater depletion. They hope the data will enable more sustainable use of water.
“We’re already seeing changes in both areas,” Scanlon said. “We’re seeing decreases in rural populations in the High Plains. Increasing urbanization is replacing farms in the Central Valley. And during droughts some farmers are forced to fallow their land. These trends will only accelerate as water scarcity issues become more severe.”
The study appears in the journal Proceedings of the National Academy of Sciences. It paints a high resolution picture of how groundwater depletion varies across space and time in California’s Central Valley and the High Plains.
Three results are particularly striking: First, during the most recent drought in California’s Central Valley, from 2006 to 2009, farmers in the south depleted enough groundwater to fill the nation’s largest man-made reservoir, Lake Mead near Las Vegas — a level of groundwater depletion that is unsustainable at current recharge rates. Second, a third of the groundwater depletion in the High Plains occurs in just four percent of the land area. And third, the researchers project that if current trends continue some parts of the southern High Plains that currently support irrigated agriculture, mostly in the Texas Panhandle and western Kansas, will be unable to do so within a few decades.
Combined, the two regions produced agricultural products worth $56 billion in 2007, accounting for much of the nation’s food production. They also account for half of all groundwater depletion in the U.S., mainly as a result of irrigating crops.
In the early 20th century, farmers in California’s Central Valley began pumping groundwater to irrigate their fields. Over time, groundwater levels dropped as much as 400 feet in some places. From the 1930s to 1970s, state and federal agencies built a system of dams, reservoirs, and canals to transfer water from the relatively water-rich north to the very dry south.
Since then, groundwater levels in some areas have risen as much as 300 feet. In the High Plains, farmers first began large-scale pumping of groundwater for crop irrigation in the 1930s and ’40s, but irrigation greatly expanded in response to the 1950s drought. Since then, groundwater levels there have steadily declined, in some places more than 150 feet.
Scanlon and her colleagues at the U.S. Geological Survey and the Université de Rennes in France used water level records from thousands of wells and data from NASA’s GRACE satellites, which monitor changes in Earth’s gravity field that are controlled primarily by variations in water storage.
Scanlon and her colleagues say replacing flood irrigation systems (used on about half of crops) with more efficient sprinkle and drip systems could make agriculture more sustainable. The team also recommended expanding the practice of groundwater banking, which means storing excess surface water in times of plenty in the same natural aquifers that supply groundwater for irrigation.
Groundwater banks currently store two to three cubic kilometers of water in California, similar to or greater than storage capacities of many of the large surface water reservoirs in the state. Groundwater banks provide a valuable approach for evening out water supplies during climate extremes ranging from droughts to floods.
Scanlon and other experts don’t think these or other engineering approaches will solve the problem in the High Plains. When groundwater levels drop too low to support irrigated farming in some areas, farmers there will be forced to switch from irrigated crops such as corn to non-irrigated crops such as sorghum, or to rangeland. The transition could be economically challenging because non-irrigated crops generate about half the yield of irrigated crops and are far more vulnerable to droughts.