By Thomas H. Clarke, Jr.
For the last three years, California has been experiencing a major drought; only this season has the rainfall been "normal", probably due to El Nino. Yet, during the last six years, even before the onset of the drought, crop irrigation in California's Central Valley has removed groundwater at unsustainable rates.
The California Central Valley covers 52 thousand square miles and is one of the world's most productive agricultural regions. Farmers in the region produce about 250 different crops with a value around $17 billion (1/12th of U.S. agricultural production).
Satellite data indicates that more than 20 cubic kilometers of groundwater has been pumped from the Central Valley aquifers since October, 2003. Using data from the GRACE mission, researchers calculated that 31 cubic kilometers of water were lost from the San Joaquin and Sacramento River basins. Approximately 1/3 of the total was lost due to evaporation from soil or flowed out to sea after melting from the region's snowpack or being taken from surface reservoirs. The balance, 20.3 cubic kilometers, was pulled from groundwater; the researchers believe most of this pumping was for irrigated agriculture. On average, water tables across the region dropped 24 centimeters (~9½") per year during the 66-month period of the study. Since most of the loss occurred in the San Joaquin River basin, water tables in that area dropped on average 50 cm (~19¾") per year.
A draft of the study presented at the American Geophysical Union conference in S.F. in December, 2009 can be found at http://www.hydro.washington.edu/Lettenmaier/Publications/Gao_water-budget_IJRS_2009.pdf.
Anecdotal reports have noted that the drop in surface elevations has been sufficiently large to threaten the structural integrity of some of the canals carrying water from Northern to Southern California.
Thomas H. Clarke, Jr., J.D., M.S., is Chair of the Ropers Majeski Kohn & Bentley Environmental Practice Group and has over 30 years of environmental consulting and litigation experience.