May 30, 2012
Pumping the Well Dry?
DM Crumbliss for RedOrbit.com
Groundwater is being used faster than it can be replenished in the High Plains states and in California´s Central Valley, according to a study published in the Proceedings of the National Academy of Sciences. Most of the water brought up in wells is used for irrigation in these vital agricultural regions. A new study based at the University of Texas finds that rapid depletion of groundwater is a threat to the country´s food supply.
Scanlon and her colleagues at the U.S. Geological Survey and the UniversitÃ© 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.
"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."
The study found that some areas will be made unfit for agriculture, and this could be devastating. California's Central Valley is sometimes called the nation's "fruit and vegetable basket." The High Plains states, from northwest Texas to Kansas and 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.
Groundwater levels have declined steadily for years in both places as irrigation has increased.
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.
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.
The authors of the study encourage farmers in the Central Valley to replace flood irrigation systems 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.
Image 2 (below): 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