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Part of California is sinking, here’s where and why

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    Dr. Mario Picazo
    Meteorologist, PhD

    Wednesday, September 5, 2018, 17:31 - Researchers from UCLA and the University of Houston have been tracking changes in the groundwater table that runs below California’s San Joaquin Central Valley. The study directed by UCLA’s Department of Geography Dennis Lettenmaier shows how between 2002 and 2016 there was a significant loss of groundwater from what is considered one of the largest agricultural hubs in the United States, as it provides more than half of the country’s fruit, vegetable and nut supply.

    The period used in this study published last year in Geophysical Research Letters, included two droughts, one from 2007 to 2009 and a more severe dry period from 2012 to 2016. During the two drought periods a total of 13.3 cubic miles of water were lost. Most of the loss came from a reduction in precipitation and snowmelt, a change in the type of crops and warmer temperatures. 

    According to Dr. Lettenmaier, “the amount of material associated with the 1980 eruption of Mount St. Helens was about one cubic kilometer, so, we’re talking about 40 times that amount in the recent drought.”

    RELATED: Study cites longer dry spells as fueling US wildfires

    USGS official measures groundwater in Santa Barbara County.


    With the use of satellite imagery, NASA scientists have also discovered that the same region depleted from a large portion of its groundwater is sinking faster than originally thought. The recent images show how some areas of the state are sinking 2 inches (5.q centimeters) per month. This subsidence effect of the ground is not new, and has been a problem for quite some time, especially since the occurrence of the two major drought periods mentioned earlier. The extreme drought in such a vivid agricultural area has fueled prolonged voracious groundwater pumping. 

    Mark Cowin, director of California's Department of Water Resources, said in a past statement that due to the increase in water pumping, groundwater levels have been at record lows -- up to 100 feet (30 meters) lower than in past records. As a consequence, land is sinking faster, putting nearby infrastructure at risk with all the costs that may mean.  

    NOAA graph 1900 to current on California Drought


    But the main concern for scientists is the long term effect's this frantic pumping of groundwater may have. If the land continues to shrink at the current rate, and for a considerable period, it can permanently lose its capacity to store water.

    Although this has been going on for years as a consequence of increasing water demand for agriculture, United States Geological Survey reports show how some areas of the California's Central Valley are now close to a dozen feet lower than they were back in 1925.

    Folsom Dam apparent amid California drought. Courtesy: NOAA

    Although precipitation both in the form and rain and snow improved the groundwater conditions in 2017, the drought has returned, and groundwater pumping has again increased leading to more subsidence episodes.

    Earth and Atmospheric Sciences Researchers Kyle Murray and Rowena Lohman from the University of Cornell, have collaborated the findings of others confirming that after a pause in the land subsidence and even an uplift in some areas thanks to the recent rains, the sinking is back. Like many other regions in the western U.S., ongoing groundwater extraction is happening faster than it can be replenished.

    About 80 percent of the groundwater use in California is agricultural, and since 2011, the persistent drought periods experienced by the state have led to a considerable parching of the Central California valley.

    Groundwater is incredibly important to communities in the region both for agricultural and municipal reasons. The sinking observed is a clear sign of how much groundwater is being pumped out of the ground.



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