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USGS hydrologists address groundwater resources assessment tool

The Association of California Water Agencies conference Dec. 1-4 in San Diego included a December 2 presentation by three U.S. Geological Survey hydrologists on a new system to assess groundwater resources in the Central Valley and likely elsewhere in North America.

Claudia Faunt, Randy Hanson, and Steve Phillips collaborated on the presentation “California’s Central Valley Groundwater Study: A New Tool to Assess Water Resources in the Central Valley”. The Central Valley Hydrologic Model helps predict water supply scenarios. “It also provides a tool for local, state, and federal agencies to address complex environmental and hydrologic issues,” Faunt said.

More than 250 different crops are grown in California’s Central Valley, which totals approximately 20,000 square miles. The approximate annual crop production value is $17 billion, and the Central Valley produces approximately 25 percent of table food consumed in the United States. The Central Valley comprises approximately 17 percent of the nation’s irrigated land, and approximately 20 percent of the country’s agricultural pumping is from the Central Valley Aquifer.

The Central Valley Hydrologic Model has three elements: a texture analysis which utilizes sediment characteristics to estimate hydrologic properties, a farm process component which estimates agricultural use, and a groundwater model. “It’s capable of being accurate, but a lot of the things can be improved,” Faunt said.

The texture analysis utilizes a three-dimensional model based on 8,500 drilling logs. The data is interpolated to one-mile grids and to 50-foot depth intervals. The farm process is a systemic approach to estimate water budget components and is based on the use of water by crops as well as nearby urban draws; the farm process component recognizes that weather varies both seasonally and annually. The Central Valley Hydrologic Model covers 21 urban, nature, and crop land use regions.

The data from 1962 through 2003 indicates a cumulative loss of 58 million acre-feet of groundwater, mostly in the southern part of the Central Valley. The natural recharge and discharge volume is 2 million acre-feet annually while the annual engineered volume during the study period was 12 million acre-feet. Surface water deliveries since 1960 have reduced the dependence on groundwater for the area, although that supply is currently at risk.

Hanson noted that surface water and groundwater are interdependent for conjunctive use irrigation. “It’s really one water nowadays,” he said. “You have to account for all of it.”

The second version of the farm process model was released in mid-November. It not only calculates farm demand and estimates groundwater pumping but can also simulate aquifer storage recovery systems, natural and riparian vegetation, and supply and demand analysis. Its applications include stacking on priority crops in the event of deficit irrigation. “It has a lot of options,” Hanson said.

The expected applications also include determining which response actions to a salinity, drainage, or other situation are likely to be most effective.

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