U.S. Aquifers
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.
By B
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