Groundwater depletion has been most severe in the purple areas
indicated on these maps of (A) the High Plains and (B) California's
Central Valley. These heavily affected areas are concentrated in parts
of the Texas Panhandle, western Kansas, and the Tulare Basin in
California's Central Valley. Changes in groundwater levels in (A) are
adapted from a 2009 report by the US Geological Survey and in (B) from a
1989 report by the USGS. (Credit: US Geological Survey)
The nation's food supply may be vulnerable to rapid groundwater
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 journal Proceedings of the National Academy of Sciences,
paints the highest resolution picture yet of how groundwater depletion
varies across space and time in California's Central Valley and the High
Plains of the central U.S. Researchers hope this information will
enable more sustainable use of water in these areas, although they think
irrigated agriculture may be unsustainable in some parts.
"We're already seeing changes in both areas," said 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 groundwater to fill the
nation's largest human-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 4% 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 is sometimes called the nation's "fruit
and vegetable basket." The High Plains, which run from northwest Texas
to southern Wyoming and South Dakota, are sometimes 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 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 crops. Over time,
groundwater levels dropped as much as 400 feet in some places. From the
1930s to '70s, 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, data from NASA's GRACE satellites, and computer models to
study groundwater depletion in the two regions.
GRACE satellites monitor changes in Earth's gravity field which 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
10th anniversary.
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 groundwater banking
-- storing excess surface water in times of plenty in the same natural
aquifers that supply groundwater for irrigation. Groundwater banks
currently store 2 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. Groundwater 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 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.
"Basically irrigated agriculture in much of the southern High Plains is unsustainable," said Scanlon.
No comments:
Post a Comment