Last week, there was much interest in the news that satellite data indicated [thanks to colleague John Lambie for this link] that groundwater pumping in California's Central Valley had extracted the equivalent of one Lake Mead (about 24 MAF at full pool) during the past six years. The data confirmed an earlier report by the U.S. Geological Survey.
Yes, Virginia, that's a lot of water, almost double the average annual flow of the Colorado River.
Recent reductions in surface water for irrigation have no doubt contributed to the high pumpage figures (see quote below).
But this realization is nothing new, although the amount of pumpage may be. Groundwater has been pumped unsustainably (mined or overdrafted) in the Central Valley for decades. When I took Aquifer Mechanics from the late Dr. Gene Simpson in 1971, the Central Valley was already one of the 'poster children' for groundwater mining (we didn't refer to it as 'unsustainable pumping') and land subsidence. Pumpage had long since exceeded recharge, and when coupled with the necessary geologic conditions, land subsidence resulted.
Here is a classic photo of USGS hydrologist Dr. Joe Poland standing by a utility pole near Mendota, CA, that graphically illustrates over 50 years of subsidence, 1925-1977. At this location the subsidence was about 29 feet, at the time the maximum amount of subsidence in the San Joaquin Valley.
Subsidence in the Valley had been noted as early as 1935, as mentioned in this 1970 paper by USGS hydrologist Ben Lofgren that we used in Gene's class:
So although the amount of groundwater pumping in the Central Valley may have increased in the past six years, it's been a much longer time since it was 'sustainable' (although that depends upon your definition of 'sustainable' - I'm using it in the sense of groundwater 'overdrafting' or 'mining').
My blogosphere colleague David Zetland has a somewhat different perspective on the Central Valley pumping. Give him a read.
So what do you think? Does California need to regulate groundwater on a statewide basis?