The heavy rain and snow over the past six months in California could reverse the infamous decline of the state’s groundwater stores, but the relief may last only a season or two, according to a hydrologist with University of California, Davis, who says water agencies must find efficient ways to refill depleted aquifers.

Thomas Harter, a professor with the university’s Department of Land, Air and Water Resources, says the rainy winter will make a dent in the state’s groundwater deficit, especially for the Central Valley.

“Hopefully, even after the irrigation season, we’ll have a few million acre-feet more water in storage than we did last fall,” he tells Water Deeply in an interview.

However, in the long term, California’s groundwater supply remains slowly but steadily shrinking, Harter says, as most years we draw water from the ground faster than the water is being replaced by downward percolation. The challenge, then, will be for Californians to identify methods for diverting surplus surface water during wet winters into areas where it can sink into the ground and recharge overdrawn aquifers.

With such a system in mind, the past winter may look like a lost opportunity. According to Harter, as much as 350 million acre-feet (430 billion cubic meters) of water may have fallen from the sky over California in the past eight months. That’s perhaps twice the average annual precipitation for the state. While a great deal of that water flowed out to sea, much to the outrage of agricultural interests, a lot of 2016–2017’s precipitation remains in the mountains as snowpack and in the state’s reservoirs. These two water sources, Harter says, can hold 15 million acre-feet (18 billion cubic meters) and 40 million acre-feet (50 billion cubic meters), respectively. An acre-foot is about the amount of water that would submerge a football field under a foot of water, or a city block under about 4in (10cm).

California’s groundwater reserves, by contrast, are vast. In the porous soils below the ground there may be a billion acre-feet of water – roughly the amount it would take to fill a skyscraper 60,000 miles (97,000km) in height. It’s this underground reserve that has received a significant recharge this year and which water policy analysts want to replenish through innovative diversion and storage systems.

California Department of Water Resources senior engineering geologist Chris Bonds measures and records groundwater discharge from a production well to a nearby agricultural canal in Yolo County. (John Chacon/California Department of Water Resources)

According to a recent post by Harter on the California WaterBlog, published by the U.C. Davis Center for Watershed Sciences, a dry year in California results in a net loss of several million acre-feet of water from underground stores. A wet year, Harter writes, creates a net gain of about the same. “Ideally, over the longer term, the savings match the withdrawals – groundwater recharge matches groundwater pumping,” he writes.

But they don’t in California. Dominated by an arid Mediterranean climate, California has more dry years than wet years. This has led to an overall loss of groundwater. In the past century, Harter says, Californians have caused a net decline in groundwater volume of about 150 million acre-feet (185 billion cubic meters), mostly in the heavily cultivated but extremely arid San Joaquin Valley, which is also the heart of the state’s agricultural economy. This has caused the ground to subside – essentially caving in on storage space that can never be regained, since the sunken land cannot be elevated again. It has also damaged surface infrastructure, such as canals that carry water.

While the wet winter of 2016–2017 has relieved surface drought conditions, dry weather is certain to return – possibly for another long interval. This, says Harter, makes it all the more critical that local agencies establish sustainable groundwater use plans, as the 2014 California Sustainable Groundwater Management Act requires them to do by the end of June.

It’s probable that many agencies will come up with proposals to accelerate groundwater recharge into California’s enormous subterranean reservoir space. The best way to do this, Harter says, would be to divert surplus flows in wet years onto undeveloped land and allow it to sink. Floodplain areas, such as the vast Yolo Bypass – a sprawling region covering about 60,000 acres (24,000 hectares) west of Sacramento – provide some opportunity for this, though Harter writes in his California WaterBlog post that the soil in such areas may be of fine sediment and relatively impermeable, allowing virtually no significant recharge.

The greatest opportunities, then, lie on agricultural land, Harter says. Here, looser soils over millions of acres could mean doubling recharge rates in wet years. We might even offset overdraft in dry years and turn around California’s growing groundwater deficit.

But we probably won’t, he says.

“I don’t think we’ll refill our storage space,” he says. “I think the best we can do is draw a line in the sand and say we can’t go further down than we already have.”

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