How about this post's title!
WTF?
Just what you needed to see and read, now that we are just a few days beyond Gov. Jerry Brown's (D-CA) signing of California's landmark groundwater management legislation into law, bringing the Golden State into the 1980s:
Download Groundwater_Signing_Message
Back to subsidence. For background, here is a good discussion of subsidence from the USGS and a recent report on land subsidence in California from the California Water Foundation.
It is difficult to to believe that a groundwater geek such as WaterWired, who is fond of picking on California and its (formerly?) inept management of groundwater and groups that marginalize the importance of groundwater, would suggest that land subsidence due to excessive groundwater pumping could be considered good. How can the accompanying picture, depicting land subsidence in California's Central Valley (almost 30 feet maximum), be illustrative of anything good?
Land subsidence due to excessive groundwater pumping is the poster child for poor, unsustainable groundwater management. I do not need to list the deleterious effects of subsidence: change in topography and drainage patterns; destruction of infrastructure; damage to aquifers (reduction in storage and permeability); and ancillary effects that accompany declining groundwater levels (reduced environmental flows, greater energy costs for pumping, sea-water intrusion, etc.).
But land subsidence due to groundwater pumping - the kind prevalent in California's Central Valley - produces a certain amount of water on a one-time only basis. This 'slug' of water is expelled by the consolidation (compaction) of the porous medium - the subsidence. You cannot get the storage back because the consolidation is often nonlinear and inelastic. The porous materials remain in the consolidated state and cannot be 'inflated' back to their original volume by simply replacing the water pumped out.
The kind of subsidence to which I refer is the 'classical kind' where the low permeability rock/sediment layers - the clays and silts - are the formations doing most of the consolidating. Clays and silts are generally more compressible than sands and gravels and thus bear the brunt of consolidation. Imagine them as wet sponges - squeeze them, they surrender their water. But unlike sponges, they don't 'pop' back into their original shape; the water they gave up has been pumped out of the ground.
It should be noted that this slug of water is produced because the aquifer was pumped unsustainably. It's a 'booby prize' for behaving badly. But it does have consequences - just look at the picture.
So how much water is this one-time only amount? In the Central Valley, USGS hydrogeologist Lenny Konikow, in his excellent report on US groundwater depletion, estimated that the total amount of of groundwater depletion in the Central Valley (page 24) in the 20th century was 113 cubic kilometers (91.6 MAF). Of that amount, 20% (22.6 cubic kilometers or 18.3 MAF) was related to land subsidence. That's a lot of water!
Given the consequences, was 18.3 MAF of water worth it? What's the benefit-cost ratio? I'm guessing it was not worth it, but I don't live there.
Maybe it was not worth it as far as the Central Valley goes. But there are no doubt places where stakeholders will choose to pump their groundwater reservoirs unsustainably and risk subsidence (and other consequences) because they believe the 'extra water' is worth it. We will likely see more of this behavior as surface water supplies desiccate due to global warming.
Their decision? Or not?
"Someone told me that each equation I included in the book would halve the sales." - Stephen Hawking (thanks to John A. Conners)
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