Land subsidence, or sinking, is a severe and growing problem in California, and it is most evident in the San Joaquin Valley, one of the nation’s most productive agricultural regions. The U.S. Geological Survey (USGS) has been monitoring land subsidence in the San Joaquin Valley since the 1960s.
Since the 1920s, the pumping of ground water at literally thousands of wells in the valley for agricultural irrigation has caused sections of the valley to sink or subside by as much as 28 feet (8.5 meters). And of course, during droughts, when farmers are relying heavily on ground water, the subsidence problem is exacerbated even more.
Besides the obvious effects seen in cracks in walls and foundations of buildings and damage to roads and canals, there is also the groundwater-level declines and associated aquifer-system compaction that result in permanent aquifer system storage loss. This, in turn, has resulted in reduced flow capacity of the Delta-Mendota Canal—as well as the California Aqueduct and other canals that transport floodwater and deliver irrigation water, says the USGS.
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Drought and drought-like conditions in 1976-77, 1986-92, 2007-09, and 2012-2015 caused increased pumping of groundwater in the San Joaquin Valley and a decline in aquifer levels to near or beyond historic lows. In 2012, at the beginning of the latest drought, the California Department of Water Resources (DWR) commissioned NASA’s Jet Propulsion Laboratory in Pasadena, California to set about collecting and analyzing airborne and satellite data with regard to the land subsidence monitoring being done.
In 2015, an initial report was released by JPL covering the period between 2006 and early 2015. JPL used data obtained from japan’s PALSAR (2006 to 2010), Canada’s Radarsat-2 (May 2014 to Jan. 2015) and UAVSAR (July 2013 to March 2015).
But with the drought continuing, the DWR commissioned a new report covering 2015 and 2016 with data that could be used to update the first report. The new images are from ESA’s Sentinel-1A satellite from March 2015 to Sept. 2016, and from NASA’s airborne Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) from March 2015 to June 2016.
“The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable,” said DWR Director William Croyle on seeing the report. “Subsidence has long plagued certain regions of California. But the current rates jeopardize infrastructure serving millions of people. Groundwater pumping now puts at risk the very system that brings water to the San Joaquin Valley. The situation is untenable.”
It is hoped that an upcoming NASA and ISRO (Indian Space Research Organisation) radar mission, NISAR, that will systematically collect data over California and the world and will be ideal for measuring and tracking changes to the land subsidence associated with groundwater pumping, as well as uplift associated with natural and assisted groundwater recharge, will be completed. But that mission will depend on the scope of the budget cuts to NASA.