Ringside: How Dredging the Delta Enables Groundwater Recharge

ProPublica, a nonprofit news organization and winner of multiple Pulitzer prizes, recently published a report “The Drying Planet.” They report that “Moisture lost to evaporation and drought, plus runoff from pumped groundwater, now outpaces the melting of glaciers and the ice sheets of either Antarctica or Greenland as the largest contributor of water to the oceans.”

That’s a big claim, but the authors base it on a study, also published last month, “Unprecedented continental drying, shrinking freshwater availability, and increasing land contributions to sea level rise,” written by a team led by Hrishikesh A. Chandanpurkar, a researcher at Arizona State University. 

Unprecedented.” “The Drying Planet.” The message is clear: we face a climate emergency.

What does this have to do with dredging? We will get to that.

According to the study, about one millimeter per year of sea level rise is attributable to the reduction in land-based “total water storage” and 68 percent of that — or not quite 7/10ths of a millimeter per year comes from groundwater depletion. The study finds this amount to be roughly equal to the estimated contribution from Greenland’s icecap melting, and to exceed the estimate for Antarctica, which is around 4/10ths of a millimeter per year.

While the cumulative effect of all this may be of concern, how reliable are these numbers? For example, according to NASA, Greenland has shed about 5,400 gigatons of ice, cumulatively, over the last 20 years. But Greenland’s ice cap is estimated to be 2.9 million gigatons. Do we really possess sensors able to assess, on an island that’s 836,000 square miles in size, a cumulative net loss of total ice mass over 20 years of only 0.19 percent?

As for the measurement of groundwater depletion, there are a lot of variables involved, each of which is difficult to estimate, much less know how much weight to assign to it in the overall calculation. According to the Food and Agricultural Organization of the United Nations, total global water withdrawals per year are about 4 trillion cubic meters, which is equal to about 950 cubic miles per year. Of that, according to UNESCO, 70 percent is for agriculture, and 25 percent of agricultural water is sourced from aquifers. Meanwhile, of the remaining water withdrawals for urban and industrial use, 50 percent comes from aquifers. So far, then, we may assume that about 309 cubic miles of groundwater is extracted from the earth every year. Dumped into the world’s oceans, that would equal 3.6 mm per year, far in excess of what the study estimates, 0.7 mm per year. There are many ways to explain that.

The study’s authors evidently concluded that around 90 percent of groundwater withdrawals do not make it to the ocean because of either percolation (which can recharge aquifers), evapotranspiration, or reuse. And yet they found that the amount they arrived at, 0.7 mm/year, to match Greenland’s contribution and actually exceed Antarctica’s 0.4 mm/year contribution. At the same time, they concluded that the entire net impact of other forms of “global drying” (glacier melt, reduced water in lakes and snowpacks, and net loss of water bearing flora) only added 0.3 mm/year to sea level rise. The total, from all causes: 2.1 mm/year. If it continues at that rate, sea level will rise by two meters over the next century. Well, maybe. An annual rise estimated at 2.1 millimeters over 140 million square miles of ocean (321 million cubic miles) is, even in this age of satellites and sensors, a difficult measurement.

In any case, from the perspective of eons, 2.1 mm per year is nothing. Natural fluctuations in sea level as we have cycled through ice ages and experienced massive tectonic shifts have caused sea level rises of up to 700 feet higher than today. Sea level fluctuations happen with or without our help. We will adapt.

The point here is not merely to suggest we apply our own numeric reasoning skills to agenda-driven studies that are routinely reported with awful certainty. We also may want to recognize that putting climate change window dressing on a problem as serious as groundwater depletion can become a distraction from other causes. Groundwater depletion may play a small role in sea level rise, but its far more serious consequence is land subsidence.

California has a serious problem with groundwater depletion, most notably in the San Joaquin Valley. The problem, which had been creeping along for decades, only became serious in the last 20 years, when farmers converted from flood irrigation to drip irrigation. That, in turn, was forced upon them by new regulations that favor reserving water for increased flow in the delta and rivers instead of allocating it to farmers. And that change in policy, as usual, was well wrapped in climate change rhetoric.

But increasing flow through the delta has not helped. Maybe the health of the Sacramento-San Joaquin Delta, and the species therein, would be better served if the channels could be dredged back to the historic 10-12 foot depth from the now silted up 2-4 foot average. That would provide cool, deep water channels, and accommodate a much higher volume of water, both of which might finally help revive our threatened fish populations.

The hazard inherent in wrapping every environmental challenge in the language of a climate crisis is that it can limit what might otherwise be effective solutions. Here in California, we could cool the migratory pathways for our anadromous fish populations by dredging our delta channels, like we used to do regularly until the 1970s. If that were done, we could save the fish at the same time as we release additional millions of acre feet of water back to the San Joaquin Valley farmers, so they could grow crops while also recharging their depleted aquifers.

Or we can continue to frame every problem in terms of the climate emergency, while embracing the futile imperatives of scarcity politics as our only option. We may then reinforce this mentality by promulgating studies distilled into press releases, which we accept without question.