The nation's food supply may be vulnerable to rapid ground water depletion from irrigated agriculture, according to a new study by researchers at The University of Texas at Austin and elsewhere.
The study, which appears in the journalProceedings
of the National Academy of Sciences, paints the highest resolution
picture yet of how ground water depletion varies across space and time in
California's Central Valley and the High Plains of the central United States.
Researchers hope this information will enable more sustainable use of water in
these areas, although they think irrigated agriculture may be unsustainable in
"We're already seeing changes in
both areas," says Bridget Scanlon, senior research scientist at The
University of Texas at Austin's Bureau of Economic Geology and lead author of
the study. "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."
Three results of the new study are
particularly striking: First, during the most recent drought in California's
Central Valley, from 2006 to 2009, farmers in the south depleted enough ground water
to fill the nation's largest man-made reservoir, Lake Mead near Las Vegas – a
level of ground water depletion that is unsustainable at current recharge rates.
Second, a third of the ground water depletion in the High Plains occurs in just
4 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.
California's Central Valley sometimes is
called the nation's "fruit and vegetable basket." The High Plains,
which run from northwest Texas to southern Wyoming and South Dakota, sometimes are
called the country's "grain basket." Combined, these 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 ground water
depletion in the United States, mainly as a result of irrigating crops.
In the early 20th century, farmers in
California's Central Valley began pumping ground water to irrigate their crops.
Over time, ground water 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, ground water levels in some areas have risen as
much as 300 feet. In the High Plains, farmers first began large-scale pumping
of ground water for crop irrigation in the 1930s and 1940s; but irrigation
greatly expanded in response to the 1950s drought. Since then, ground water
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, data from NASA's GRACE satellites, and
computer models to study ground water depletion in the two regions.
GRACE satellites monitor changes in
Earth's gravity field that are controlled primarily by variations in water
storage. Byron Tapley, director of the university's Center for Space Research,
led the development of the GRACE satellites, which recently celebrated their
Scanlon and her colleagues suggested
several ways to make irrigated agriculture in the Central Valley more
sustainable: Replace flood irrigation systems (used on about half of crops)
with more efficient sprinkle and drip systems, and expand the practice of
ground water banking – storing excess surface water in times of plenty in the
same natural aquifers that supply ground water for irrigation. Ground water
banks currently store 2 cubic kilometers to 3 cubic kilometers of water in
California, similar to or greater than storage capacities of many of the large
surface water reservoirs in the state. Ground water banks provide a valuable
approach for evening out water supplies during climate extremes ranging from
droughts to floods.
For various reasons, Scanlon and other
experts do not think these or other engineering approaches will solve the
problem in the High Plains. When ground water 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.
"Basically, irrigated agriculture
in much of the southern High Plains is unsustainable," says Scanlon.