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.
There had been no appreciable subsidence monitoring program in California since the last comprehensive survey in 1970. The problem simply dropped off the radar. By the turn of the millennium, land subsidence in the San Joaquin Valley had disappeared as a major issue. The consequences of this oversight began to be realized during the 2007-2009 drought. The California Department of Water Resources conducted GPS surveys, two years apart, in an area near the critical Delta-Mendota Canal. The remarkable difference in elevations sounded the alarm.
Michelle Sneed, a USGS hydrologist who is an expert on land subsidence in California, was tasked to confirm the results. “Not only did we confirm the results,” Michelle says, “but we found this very large subsidence area that was covering 1,200 square miles,” an area about the size of Rhode Island. Much of this area was away from historical centers of subsidence.
To help offset the monitoring gap, an innovative remote-sensing technology known as Interferometric Synthetic Aperture Radar (In-SAR) allows scientists to measure changes on the Earth’s surface as small as a few millimeters. A sensor on an Earth-orbiting satellite bounces radar signals off the ground surface. During repeat passes of the satellite over the same targeted ground surface, it is possible to precisely estimate changes in distances between the satellite sensor and the ground surface as it uplifts or subsides. The problem is that you have to know where to look. Surprises, like the new area near the Delta-Mendota Canal, had not been considered.
During 2012 to 2015, instead of rain, the dominoes were falling. A severely reduced snowpack in the Sierras lessened the streamflow, which led to inadequate reservoir supplies, which in turn reduced water allocations for much of the state. The situation was exacerbated by court-mandated reductions in surface-water deliveries in order to maintain adequate freshwater for fish and other environmental needs in the Sacramento-San Joaquin Delta.
Incrementally, the State Water Project and the federally run Central Valley Project were providing less and less water. In early 2014, the drought had become so severe that both projects announced that they would cut off water deliveries to farmers in the San Joaquin Valley.
To save their crops, farmers began pumping at record-breaking rates from an already depleted aquifer system. Drillers had more business than they could handle deepening or drilling new wells. In some areas, groundwater levels dropped to more than a hundred feet (30 meters) below previous historical lows. “I’ve been studying subsidence throughout the west for 20 years, and I’ve never measured rates like this before,” noted Michelle Sneed.
Infrastructure can’t handle this unprecedented rate of drop. If the entire San Joaquin Valley were subsiding at the same rate and in the same way, it would still be bad – just not as bad. The problem is that different amounts of subsidence in different places are wreaking havoc with canals, pipelines, dams, levees, roads, railways, bridges, building foundations, sewer lines and laser-leveled fields. Wells have also been damaged, because compacting clay causes well casings to buckle and eventually collapse.
Canals are particularly sensitive because subsidence affects the gradient that moves water by gravity in much of the system. When one part of a canal subsides, it reduces (or destroys) the conveyance capacity for all downstream parts of that canal. This makes downstream farmers even more dependent on groundwater. In addition, as the land sinks, bridges sink with it.
One of the most extreme cases is the Russell Avenue Bridge north of Mendota. Before subsidence, canal inspections were conducted from a boat that passed easily under the bridge. Today, canal water laps against the bridge, thereby reducing flow capacity by about 45 percent.
Bordered on both sides by mountain ranges, the San Joaquin Valley has a spectacular history of flooding. During a killer drought, this version of water abundance can almost sound good – until it happens. Both the Central Valley Project and the State Water Project were constructed with the dual purpose of supplying surface water and mitigating the frequent and devastating spring floods that can overwhelm the valley.
The Eastside Bypass, a key flood control channel, is so severely impacted by subsidence that a large part of the valley is now in danger of massive flooding. At the Chowchilla Bypass, subsidence is so severe that its capacity to carry water away during a flood is expected to decrease by more than 50 percent. If the bypass is breached, an area a few miles wide and about 25 miles (40km) long could be inundated by as much as 30 feet (9m) of floodwater.
Farmers in the San Joaquin Valley have developed an agricultural system dependent on plentiful imported surface water. This water wealth has allowed farmers to convert their fields from row crops to higher value nut trees (primarily almonds) and other permanent crops that require year-round watering. These vast fields can no longer be fallowed. When a drought hits and imported water is reduced, farmers start overpumping and subsidence continues. Michael Campana, an internationally recognized groundwater expert, sums up the problem: “Here we are 40-some years later, and they’re still doing the same thing. It’s like the classic definition of insanity.”
Excerpted from “High and Dry: Meeting the Challenges of the World’s Growing Dependence on Groundwater” by William M. Alley and Rosemarie Alley. Reprinted by permission of Yale University Press.
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