Saturday, September 19, 2015

#262 / That Sinking Feeling

The picture above comes from an online article published in January 2014 by the Rural Community Assistance Corporation, based in West Sacramento. It shows damage to the Delta Mendota Canal caused by subsidence. The ground subsidence that caused this damage was itself caused by the over-pumping of groundwater aquifers in the Central Valley.

Michelle Sneed, a USGS hydrologist, was quoted in this January 2014 article as saying that the rate of subsidence was "nearly a foot a year—some of the fastest rates recorded since the 1950s.”

On Thursday, August 20, 2015, an article in the San Jose Mercury News updated this 2014 report. According to the Mercury article, which is accompanied by a picture of Michelle Sneed standing by this same crack, "parts of the great San Joaquin Valley are sinking almost 2 inches every month."

Two inches per month translates to two feet a year, which means that subsidence is now proceeding twice as fast as Sneed reported in January 2014!

Not only does this kind of ground subsidence damage vital water infrastructure (witness the picture), it also compresses the water-bearing strata that make it possible for groundwater to be recharged into the soil. 


If we don't live within the limits of the Natural World, in the short-term, the result will be the long-term inability of the Natural World to rebound and respond to our needs.

Image Credit:

1 comment:

  1. The first article [1] refers to a study [2, 3] which actually shows that the majority of the valley experienced land subsidence less than 0.12/2 = 0.06 meters of land subsidence per year from 2008 to 2010. Only one small region south of El Nido experienced land subsidence at a rate of 0.48/2 = 0.24 meters per year (See Figure 9-C on page 19). This is not significantly different from the historical rate of 8.5/44 = 0.19 per year from 1926 to 1970 (See Figure 9-A on page 17). But it is measurably higher because "diminished deliveries of imported water prompted pumping of groundwater to meet irrigation demands. This increased groundwater pumping resulted in water-level declines and periods of renewed compaction." (Page 48)

    The second article [4] refers to a more recent study [5, 6] which again shows severe land subsidence in concentrated areas. The same region south of El Nido saw a rate higher than 25/3.5 = 7.14 inches per year = 0.181 meters per year between June 2007 and December 2010 (See Figure 1 on page 6). This agrees with the prior study. It also shows a rate of 12/(264/365) = 16.59 inches per year = 0.421 meters per year May 3rd, 2014 to January 22nd, 2015 i.e. (See Figure 2 on page 7). That's only 1.38 feet per year, not 2 feet as you've incorrectly stated.

    Your math was wrong but land subsidence has doubled due to groundwater taxation. It's a problem for the valley, but the costs and benefits of water sources and uses have nothing to do with living within the "limits of the Natural World". They have to do with feeding people, supporting farmers, and protecting the environment.



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