California’s drought causing the bottom to drop out of its aqueducts

Groundwater depletion in San Joaquin Valley is slowing its aqueducts.

While the Oroville Dam in northern California has been flirting with disaster due to excessive rainfall, the water story in the Golden State has been much more about lack than excess in recent years. In an unfortunate double consequence, the recent drought not only took a toll on California's water supplies—it damaged the water infrastructure as well. Ground water depletion gave the Earth a sinking feeling that accelerated damage to the California Aqueduct, which carries water to 25 million people and a million acres of farmland.

California’s San Joaquin Valley is (unhappily) famous for a history of groundwater depletion that has actually caused the land surface to sink in elevation. This subsidence is due to the fact that the water in the sediment beneath our feet actually bears some of the weight of everything above it. Removing that water from the tiny spaces between grains of material allows the sediment to compact down more tightly, causing subsidence up at the surface. The effect is not subtle in California—some places have sunk more than 30 feet (9 meters) over the years.

In recent years, incredibly precise satellite measurements have been used to map out the subsidence in detail, highlighting fast-dropping hotspots as well as changes along the length of the California Aqueduct.

A new report released by researchers at NASA’s Jet Propulsion Laboratory last week updates the map with measurements covering May 2015 to September 2016. In just 16 months, surface elevations in the San Joaquin Valley dropped by as much as 56 centimeters (22 inches), with a significant area sinking by at least 20 centimeters (eight inches).

Between 2013 and 2016, the California Aqueduct experienced as much as 64 centimeters (25 inches) of subsidence, with nearly five miles (8km) of the aqueduct sinking by more than 25 centimeters (10 inches).

That's critical because the flow of water through the aqueduct is based on a constant, gradual slope. According to the California Department of Water Resources, the location of greatest subsidence has created a sort of choke point for flow in the aqueduct, limiting the aqueduct to just 80 percent of its designed capacity.

A section of canal parallel to the aqueduct called the Eastside Bypass also runs through a trouble spot that has subsided considerably since it was first constructed. The Eastside Bypass is intended to carry off spring floodwater coming down the San Joaquin Valley from the Sierra Nevada. Extensive repairs were undertaken in 2000 to counteract subsidence, but the Department of Water Resources estimates that another $250 million would be required to just get it back to operating at its initial capacity.

Apart from highlighting locations where repairs may be necessary, this subsidence monitoring informs the state’s efforts to rein in the worst of the groundwater depletion that causes it. But when drought comes, farmers can be forced to rely even more heavily on groundwater—which further threatens the infrastructure that delivers whatever surface water is available. In the Department of Water Resources’ press release, Director William Croyle didn’t mince words: “Groundwater pumping now puts at risk the very system that brings water to the San Joaquin Valley. The situation is untenable.”