California’s Sinking San Joaquin Valley

Published on December 14th, 2013

I have written often about California’s worsening water resource problems. At the same time our population and water demands are growing, with no end in sight, the long-term availability of freshwater is diminishing. This relationship can be depicted in a simplified schematic:

The schematic shows a growing human figure representing both increasing population and water demand in California over time. The dwindling droplet represents California’s decreasing per capita water supply. To a large extent the rising population/consumption causes the falling water supply; some of the shrinkage, however, is due to a warming climate, which is reducing the Sierra Nevada snowpack and subsequent snowmelt upon which so much of California depends.

A recent Associated Press article sheds light on another hidden facet of California’s overexploited water resources. The article reported on a U.S. Geological Survey (USGS) study whose findings indicate that land in the San Joaquin Valley (the southern portion of the Central Valley) is sinking more rapidly than usual because of increased water pumping from local aquifers. The USGS study found that land subsidence reached nearly one foot per year at one site.

This subsidence in turn is reducing the gradient and flow capacity of the Delta-Mendota Canal and the California Aqueduct, two major sources of water. Because canals are built with a small slope to help propel the water by gravity, differential land sinking can change that slope in random areas and thus impact flows.

Overpumping groundwater is a common cause of sinking land surfaces, called land subsidence, around the world. According to the USGS:

Land subsidence occurs when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rock falls in on itself.

In the U.S. alone, subsidence has directly affected more than 17,000 square miles in 45 states, an area approximately equal in size to New Hampshire and Vermont combined.

Land surfaces have been subsiding in the San Joaquin Valley for many years, but the rate had slowed in recent decades since construction of the California Aqueduct and Delta-Mendota Canal. These two canals help provide surface water to ever-thirsty San Joaquin Valley agriculture and to booming Southern California cities, which in turn reduced the pumping of groundwater.

However, subsidence in the two years studied (2008-2010) was aggravated by drought, the USGS report found.

In those dry years, not only was rainfall reduced, but water regulators also placed mandatory limits on surface water use to conserve water. This led farmers and others to increase their pumping of groundwater from aquifers; California does not restrict groundwater withdrawals.

The USGS study said that canals are not the only infrastructure at risk from subsidence. Railways, roads and pipelines – all linear structures that extend over a long distance – are also under threat. According to the California High Speed Rail Authority, the state’s bullet train project, which would cut through the Central Valley, is already taking into account the land subsidence problem among other engineering considerations.

In the years ahead, California’s incessant, unsustainable population growth will only continue to exacerbate complex, interconnected problems like this. Proactively reducing excessive immigration and fertility – before Nature forces us to – would enable the Golden State to halt population growth, stabilize our numbers and “help save some America for tomorrow.”

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