The Dangers of Land Subsidence From California’s Groundwater Overdraft

Land subsidence from overpumping groundwater in the San Joaquin Valley has been called the largest human alteration of the Earth’s surface. 

When the last comprehensive surveys were made in 1970, subsidence in excess of one foot had occurred over more than 5,200 square miles (13,000 sq km) of irrigable land – half the entire valley. Southwest of Mendota, a town that prides itself on being the cantaloupe center of the world, maximum subsidence was estimated at 28 feet (8.5m). By this time, however, massive infusions of surface water were being delivered to the valley, and subsidence was slowing or had been “arrested.”

Then came a series of droughts and cutbacks in imported water that resulted in renewed overpumping and subsidence. During the severe drought of 1976-1977, surface water imports were sharply reduced. The six-year drought beginning in 1987 was the state’s first extended dry period since the 1920s into the 1930s. A severe drought from 2007-2009 marked the first time a statewide “proclamation of emergency” was issued. And then, in 2012, the worst drought on record began to grip the state. Paleoclimate investigations suggest that this was the most severe drought in 1,200 years, predating the Viking conquests in Europe.

This drought wasn’t just about lack of rainfall. What made it so extraordinary was the extreme heat that came with it. It was called the “Hot Drought.” What this meant is that all those vegetables and orchards needed more water than ever. Some areas of the San Joaquin Valley were sinking by almost a foot (0.3m) a year.

When groundwater is pumped from an aquifer system, hydraulic pressure decreases. This reduced pressure shifts the support for the weight of the overlying landmass from the water in the pores to the granular skeleton of the aquifer system. If the geologic materials are sediments, rather than hard rock, the increased load on the sediments causes them to compact, with associated land subsidence.

Basic geology explains why subsidence affects the San Joaquin Valley and not the High Plains. The High Plains is basically a huge erosional sand pile from the Rocky Mountains. Anyone who has walked along the saturated tide-line of a beach knows that your footsteps quickly rebound and disappear. In the San Joaquin Valley, however, deposits of silt and clay (known as “aquitards”) are sandwiched between, and within, aquifers. These clays are not only compressible, but if groundwater levels fall below critical thresholds, the compaction is mostly nonrecoverable – even if groundwater levels later recover. In other words, much of the land subsidence is permanent.

But that’s only the beginning of the San Joaquin Valley’s problems. Even if pumping returns to more normal rates, the subsidence will continue (albeit at a slower rate) long after water levels recover because of the slow drainage and response time of the aquitards. It will take decades for most of the pressure equilibration to occur, and for the ultimate compaction to be realized in some of the thicker aquifers in the valley. Meanwhile, things are never going to go back to the good old days. In the same way that a crushed soda can holds less water, nonrecoverable compaction leads to a permanent loss of aquifer storage. During the 1976-1977 drought, after only a third of the peak annual pumping volumes of the 1960s had been produced, groundwater levels rapidly declined more than 150 feet (45m) over a large area and subsidence resumed. That a relatively small amount of pumping caused such a rapid decline in water levels reflects the reduced groundwater storage capacity caused by compaction. And that was four major droughts ago.

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