Three results of the new study are particularly striking: First,during the most recent drought in California's Central Valley, from2006 to 2009, farmers in the south depleted enough groundwater tofill the nation's largest man-made reservoir, Lake Mead near LasVegas a level of groundwater depletion that is unsustainable atcurrent recharge rates. Second, a third of the groundwater depletion in the High Plainsoccurs in just 4% of the land area. And third, the researchersproject that if current trends continue some parts of the southernHigh Plains that currently support irrigated agriculture, mostly inthe Texas Panhandle and western Kansas, will be unable to do sowithin a few decades. California's Central Valley is sometimes called the nation's "fruitand vegetable basket." The High Plains, which run from northwestTexas to southern Wyoming and South Dakota, are sometimes calledthe country's "grain basket." Combined, these two regions producedagricultural products worth $56 billion in 2007, accounting formuch of the nation's food production. They also account for half ofall groundwater depletion in the U.S., mainly as a result ofirrigating crops. |
In the early 20th century, farmers in California's Central Valleybegan pumping groundwater to irrigate their crops. Over time,groundwater levels dropped as much as 400 feet in some places. Fromthe 1930s to 70s, state and federal agencies built a system ofdams, reservoirs and canals to transfer water from the relativelywater-rich north to the very dry south. Since then, groundwaterlevels in some areas have risen as much as 300 feet.
In the HighPlains, farmers first began large-scale pumping of groundwater forcrop irrigation in the 1930s and 40s; but irrigation greatlyexpanded in response to the 1950s drought. Since then, groundwaterlevels there have steadily declined, in some places more than 150feet. Scanlon and her colleagues at the U.S. Geological Survey and theUniversit de Rennes in France used water level records fromthousands of wells, data from NASA's GRACE satellites, and computermodels to study groundwater depletion in the two regions. GRACE satellites monitor changes in Earth s gravity field whichare controlled primarily by variations in water storage.
ByronTapley, director of the university's Center for Space Research, ledthe development of the GRACE satellites, which recently celebratedtheir 10th anniversary. Scanlon and her colleagues suggested several ways to make irrigatedagriculture in the Central Valley more sustainable: Replace floodirrigation systems (used on about half of crops) with moreefficient sprinkle and drip systems and expand the practice ofgroundwater banking storing excess surface water in times ofplenty in the same natural aquifers that supply groundwater forirrigation. Groundwater banks currently store 2 to 3 cubickilometers of water in California, similar to or greater thanstorage capacities of many of the large surface water reservoirs inthe state. Groundwater banks provide a valuable approach forevening out water supplies during climate extremes ranging fromdroughts to floods. For various reasons, Scanlon and other experts don't think these orother engineering approaches will solve the problem in the HighPlains.
When groundwater levels drop too low to support irrigatedfarming in some areas, farmers there will be forced to switch fromirrigated crops such as corn to non-irrigated crops such assorghum, or to rangeland. The transition could be economicallychallenging because non-irrigated crops generate about half theyield of irrigated crops and are far more vulnerable to droughts. "Basically irrigated agriculture in much of the southern HighPlains is unsustainable," said Scanlon.
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