After five years, the rains and snow have returned to California. Powerful storms brought rain and flooding. Less than 10% of California remains in drought and the reservoirs are full, but some of the most expensive damage from the drought is just becoming visible. California experiences the most extreme variability in yearly precipitation in the nation. The potential for wide swings in precipitation from one year to the next requires that California must be prepared for either floods or drought in any year and has extensive water infrastructure-aqueducts, bridges, dams and more.
California uses about 37 million acre feet of water a year, 26 million acre feet for agriculture and 9 acre feet for all other users. An acre foot is about 326,000 gallons. In non-drought years 30-40% of the water is supplied by groundwater. However in a drought California draws more than 60% of its water from groundwater. The groundwater of the southern Central Valley of California has both an upper unconfined and deeper confined aquifer system. An unconfined, or water -table aquifer is an aquifer whose upper surface is the water table, and is at atmospheric pressure. The water table rises and falls with moisture content that is contained in the soil, and water can be extracted or recharged easily with only seasonal compaction and rebound of the land in wet years.
However, water-table aquifers are usually shallower than confined aquifers and because they are shallow, they are impacted by drought conditions much sooner than confined aquifers. Thus, most water wells draw from the deeper confined aquifers. The water is drawn from the fine-grained confining layers called aquitards. Water enters these aquitards very slowly and the danger is that the compaction of the layers will become permanent. If the water levels are drawn too low, then an irreversible compaction of the fined-grained confining layer occurs and there is permanent subsidence, permanently reducing the storage capacity of underground aquifers, threatening future water supplies; and also lowering the level of the land surface.
Subsidence caused by groundwater pumping in the Central Valley has been a problem in California for decades. Subsidence is also a serious problem for California's water managers, and their infrastructure. The subsidence puts the state and federal aqueducts, levees, bridges and roads at risk of damage. In the past few years subsidence has damaged thousands of public and private groundwater wells throughout the San Joaquin Valley. While there is no comprehensive estimate of damage costs associated with subsidence, California and the federal water agencies have spent an estimated $100 million on subsidence-related repairs since the 1960s and are now projecting $250 million in further repairs.
New NASA radar satellite maps prepared for the California Department of Water Resources show that the land surface continues to sink rapidly in certain areas of the San Joaquin Valley, putting state and federal aqueducts and flood control structures at risk of damage. New NASA Jet Propulsion Laboratory radar satellite maps prepared for the California Department of Water Resources in a report released last month, Subsidence in California, March 2015-September 2016, show that the land continues to sink in certain areas of the San Joaquin Valley.
The new NASA report shows that two main areas of subsidence covering hundreds of square miles in California’s Central Valley grew wider and deeper between spring 2015 and fall 2016. The subsidence also intensified at a third area, near Tranquility in Fresno County, where the land surface has settled up to 20 inches during the past 18 months in an area that extends seven miles.
The report also found that subsidence caused by groundwater pumping near Avenal in Kings County (not far from Visalia) has caused the Aqueduct to drop more than two feet. To avoid overtopping the concrete banks of the Aqueduct in these sections, water project operators must reduce flows. The Aqueduct in this area can now carry only 6,650 cubic feet per second (cfs) – 20% less than before.
“The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable,” said California Department of Water Resource 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.”
In response to the new findings, and as part of an ongoing effort to respond forecast longer droughts in the future the Department of Water Resources is investigating measures to reduce long-term subsidence risk. In addition, the Department of Water Resources will work with local water managers to incorporate reduction of subsidence risk into the implementation of the Sustainable Groundwater Management Act (SGMA) signed into law in 2014. Steps being considered are groundwater pumping curtailment, creation of groundwater management zones near critical infrastructure, and county ordinance requirements.