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California Must Prepare for Flood and Drought

February 21, 2017

California is experiencing record wet conditions following five consecutive years of drought.

  • In 2015, we had record low statewide mountain snowpack of only 5 percent of average.
  • The 4 driest consecutive years of statewide precipitation in the historical record were in 2012-14.
  • Water year 2017 (October 1, 2016-September 30, 2017) is now surpassing the wettest year of record (1982-83) in the Sacramento River and San Joaquin River watersheds and close to becoming the wettest year in the Tulare Basin (set in 1968-69).
  • Mountain snowpack is already well above the April 1 seasonal averages throughout the Sierra Nevada, with the southern Sierra being more than 200 percent of average for the year to date.

California experiences the most extreme variability in yearly precipitation in the nation. The summary on California Precipitation by the Center for Western Weather and Water Extremes at the Scripps Institution explains how large storms (often atmospheric river storms) contribute to those extreme changes. Water year 2017 has been an active year for atmospheric river storms.

The potential for wide swings in precipitation from one year to the next shows why we must be prepared for either flood or drought in any year. Although this year may be wet, dry conditions could return again next year. 2017 may be only a wet outlier in an otherwise dry extended period. Unfortunately, the scientific ability to determine if next year will be wet or dry (known as sub-seasonal to seasonal forecasting, or long-range weather forecasting) isn’t yet capable of delivering reliable predictions.

NASA Report: San Joaquin Valley Land Continues to Sink

West Washington Road where it crosses the Eastside Bypass, a constructed floodway for the San Joaquin River

Aqueduct before and after subsidence

Estimated subsidence in the San Joaquin Valley between 1949-2005

February 8, 2017

New NASA radar satellite maps prepared for the California Department of Water Resources (DWR) in the report, Subsidence in California, March 2015-September 2016, show that land continues to sink at in certain areas of the San Joaquin Valley, putting state and federal aqueducts and flood control structures at risk of damage.

“The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable,” said DWR Director William Croyle. “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.”

A prior August 2015 NASA report prepared for DWR documented record rates of subsidence in the San Joaquin Valley, particularly near Chowchilla and Corcoran, as farmers pumped groundwater in the midst of historic drought.  The report released today shows that two main subsidence bowls covering hundreds of square miles grew wider and deeper between spring 2015 and fall 2016.  Subsidence also intensified at a third area, near Tranquillity in Fresno County, where the land surface has settled up to 20 inches in an area that extends seven miles.

Additional aircraft-based NASA radar mapping was focused on the California Aqueduct, the main artery of the State Water Project, which supplies 25 million Californians and nearly 1 million acres of farmland.  The report shows that subsidence caused by groundwater pumping near Avenal in Kings County has caused the Aqueduct to drop more than two feet.  As a result of the sinking, the Aqueduct at this stretch can carry a flow of only 6,650 cubic feet per second (cfs) – 20 percent less than its design capacity of 8,350 cfs.  To avoid overtopping the concrete banks of the Aqueduct in those sections that have sunk due to subsidence, water project operators must reduce flows.

The California Department of Water Resources (DWR), which operates the State Water Project, is analyzing whether the subsidence-created dip in the Aqueduct will affect deliveries to Kern County and Southern California water districts.  If the State Water Project allocation is 85 percent or greater, delivery may be impaired this year due to the cumulative impacts of subsidence in the Avenal-Kettleman City area.

Read NASA’s latest report, Subsidence in California, March 2015-September 2016.

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