Study Finds Sacramento Valley Aquifer Has Permanently Lost Water Storage Capacity

Parched soil fissures are a stark but surface-level indication of drought.

To grasp the full consequences of persistent droughts intensified by climate change, scientists must also examine what is taking place far below ground, within the planet's aquifers.

These freshwater stores, which act rather like sponges, provide water to communities, farming and ecosystems at the surface. Yet when rainfall is insufficient to refill them, or when too much water is pumped out, aquifers may be unable to recover.

Sacramento Valley aquifer under pressure

The Sacramento Valley aquifer system has been strained by recent droughts. It supplies part of California's Central Valley, which, together with the San Joaquin Valley farther south, produces roughly one-quarter of the US food supply.

New data indicate that rapid, repeated freshwater extraction in recent years has permanently diminished this aquifer's storage capacity.

A study published in PNAS reports that land subsidence above the aquifer - the sinking of layers supported by the underground reservoir - has occurred quickly and broadly enough to point to an irreversible shift.

"All available measurements suggest that the accelerated subsidence starting in 2021 primarily results from inelastic compaction due to groundwater extraction," the researchers write in their paper.

"On average, the ground surface does not rebound to its pre-2021 state even as groundwater levels recover with seasonal recharge at the end of 2021, indicative of irreversible deformation."

Pressure inside aquifers has an effect on the land overhead. When water is removed, the upper layers may compact in a reversible, or elastic, way as the aquifer naturally refills, or in an irreversible, inelastic, way.

In the latter case, water loss and the resulting subsidence are too extensive for the geological layers to regain their former arrangement.

This permanently cuts the aquifer's capacity, limiting the quantity of water it can provide in future.

Satellite data reveals irreversible compaction

After assessing the scale, timing and geographical spread of deformation across the Sacramento Valley, the researchers conclude that inelastic compaction has become the leading process affecting the region.

"Such 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," the team writes.

To arrive at this finding, the researchers examined information from satellites carrying navigation systems and radar instruments. This supplied a broader, more precise record of how the ground changed over time, accurate to the millimetre.

Importantly, the satellite observations identified aquifer compaction that groundwater measurements alone would not have revealed.

Between 2016 and 2020, the data showed that ground levels generally returned to their previous position.

From 2021 onwards, however, some locations sank by as much as 50 centimetres each year - many times greater than the roughly 2 centimetres per year of reversible movement recorded in the preceding years.

"By imaging this sharp transition at the regional scale, our analysis demonstrates the potential of space-based monitoring for early detection of groundwater overdraft and the resulting loss of aquifer storage capacity worldwide," write the researchers.

Water storage losses and monitoring

The team estimates that annual loss of water-storage space rose to as much as 0.2 cubic kilometres from 2021, approximately five times the level seen in earlier years.

In capacity terms, that represents about 30 per cent of Los Angeles's yearly water use: the reduced aquifer will be unable to hold as much water from now on.

Improved data cannot itself undo global warming or make droughts less common, but it can support water management.

Here, satellites detected a major transition that was missed by a network of more than 2,500 groundwater wells - although the discovery came too late for action to be taken.

"Real-time monitoring of surface displacements through satellite geodesy, which provide an integrated measure of internal deformation over the entire sedimentary column, could have enabled early detection of this sharp transition and potentially reduced storage loss through timely remedial actions," write the researchers.

"Developing such capability is especially critical in regions where in situ monitoring is unviable but groundwater resources increasingly vital."

The research has been published in PNAS.

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