Losing storage above and below ground

Last week, researchers published evidence that intensive groundwater pumping in California during the 2020-22 drought led to permanent and irreversible damage in parts of the Sacramento Valley.1 This damage took the form of sinking land, one of the many undesirable costs of excessive groundwater use.

Land subsidence occurs when large amounts of water are withdrawn from an aquifer, leading to the compaction of soils. Over time, subsidence can reveal itself in damage on the surface — to roads, infrastructure (see the California Aqueduct), buildings and even entire communities (see Windsor Park in Las Vegas).2

But subsidence can take different forms, as the paper published last week explains. In some cases, an aquifer behaves dynamically. Land recovers once water is returned to the aquifer. But in other cases, the impacts can prove far more permanent and long-lasting, resulting in an inability of the aquifer to recover.

That latter “inelastic regime” is what the research team documented in parts of the Sacramento Valley. They write “rapid and extensive land subsidence indicates severe inelastic compaction and loss of storage capacity of the underlying aquifer system, which pose a serious threat to California’s water resources and infrastructure.”

It’s an indication of how quickly intensive pumping in a drought can affect a system. But the findings also speak to physical limits on the capacity to adapt when you are operating from a deficit. The loss of storage, a consequence of overuse, can narrow the options for recovering from overuse.

Aquifers, when sustainably managed, can act like bank accounts. That is, in wet years, precipitation (deposits) can replenish water taken out in dry years (withdrawals). But losing the physical space to store water underground can put a cap on the amount of water can be placed in the account. Christian Elliott over at the Science Reporter’s Cut has a further look at the study and offers some more meditations on subsidence.

One of the things I wanted to do here with this post was connect what happens underground with what how water is allocated on the surface.

Abridged (a wonderful nonprofit outlet in the Sacramento-area) reports that a potential reason for the rapid change in the aquifer dynamics during the 2020-22 drought was a decrease in surface water availability. “Surface water availability plays a significant factor in how much groundwater is extracted,” an assistant general manager for the Yolo County Flood Control and Water Conservation District said. “Landowners had to make a choice on what would be irrigated with groundwater versus not irrigated at all.”3

Historically, groundwater and surface water have been managed separately. But the ways in which they are connected — whether through hydrology, policy, or individual choices — are multifaceted. I’ve been thinking about this a lot over the past week.

Last week, the U.S. Department of the Interior released its long-awaited framework for managing the Colorado River. The basics: The department is adopting a 10-year framework that calls for new operating plans every two years. The first plan has yet to be released but initial cuts are expected to fall on the Lower Basin states of Arizona, California, and Nevada — with the states upstream spared from any mandatory cuts. It’s a band-aid deal, and a potential lawsuit still looms.4

I’ll write more about the Colorado River deal after the first operating agreement is released. But what is unfolding on the Colorado River is similarly a story about the challenges of adaptation amid a loss of storage, and the loss of flexibility that comes with it. While the Colorado River’s reservoirs still have the physical capacity to fill again, one day, the conversation now is about managing their depletion.

A primary goal is to keep Lake Mead and Powell above key thresholds below which physical access to hydropower and water are threatened. The cuts help to achieve this by rebalancing demand with supply, but it will be important to see how the operating plans deal with the question of where the saved water goes and how it’s accounted for. Do the savings benefit the system as a whole?

The Colorado River story is also a groundwater story. The two are connected. For years, Arizona (and Nevada) have stored excess Colorado River water in underground aquifers as a buffer against future shortages. As the Colorado River cuts loom, cities in Arizona have started to turn to their groundwater bank. Last month, the Arizona Water Banking Authority announced that it would use groundwater to make central Arizona cities whole, for at least a year, offsetting Colorado River cuts.5

• “California’s push to limit groundwater pumping could leave small farmers in the dust,” L.A Times

• “Court rules Sonoma County must study impact of water wells on fish, Russian River,” San Francisco Chronicle

• “Studies look at “less bad” options for farmers facing groundwater cutbacks,” SJV Water

• “Hacks on U.S. water supply follow years of warnings and neglect,” New York Times