What's at stake?
Rapid groundwater pumping due to expanding farmland and drought has caused some parts of the Central Valley to sink. It doesn't have to be permanent, researchers found.
Not many stories about water these days have a happy ending, but a June study from Stanford University researchers proves that when we act to solve subsidence – one of our most challenging drought problems – things actually can get better.
The San Joaquin Valley is no stranger to subsidence – the word used to describe the phenomenon of land sinking, due to over-pumping of groundwater. Across the valley, markers on telephone lines and street signs have marked the slow but steady decline of land levels since the late 1920s, after the first electricity-powered groundwater pump enabled the emptying of the aquifers beneath the ground, creating one of the world’s most productive agricultural regions.
Subsidence is a costly problem. When land sinks, even a few inches, it can create devastation on many scales. Canals – like the mighty Friant-Kern, delivering water to farms along the valley’s eastside, and the California Aqueduct to the westside farms and onto southern California – can shift, stopping water from following the designs of gravity downhill and preventing its efficient delivery.
Nearly 10 feet of subsidence has occurred along the Friant-Kern Canal in Tulare and Kern County, forcing hundreds of millions of dollars in repairs to allow water to move efficiently all the way from the headwaters of the San Joaquin River to Kern County.
New research from Matthew Lees, a doctoral student of geophysics at Stanford University, found that when groundwater levels rise – subsidence can actually reverse. The study was published on June 2 in the journal Water Resources Research.
“If you don’t get these water levels to come back up, then the land is going to sink, potentially tens of centimeters per year, for decades. But if they go up, you can get rewarded very quickly. You almost immediately improve the situation,” said Lees, in an interview.
His research findings were based on a model of groundwater conditions around Hanford, but can be applied to anywhere in the Central Valley, he said, as underground conditions are relatively similar.
Stabilizing water levels not enough to stop sinking
This is a new understanding of subsidence: previously, water managers and researchers had thought that subsidence could stop simply by halting further decline of groundwater levels. This assumption is baked into SGMA, the state’s groundwater management law, which requires local groundwater agencies to stabilize aquifers and prevent further decline.
It could have important ramifications for current efforts by local groundwater agencies, many of whom are working on revisions to their groundwater sustainability plans, after the state rejected earlier drafts in December and January. The agencies now have until July to submit updated plans to the state’s Department of Water Resources.
“What we’re saying is, unfortunately, you can still see subsidence, potentially decades after you stabilize the groundwater levels. So groundwater management practices should be adapting to keep up with findings like this,” added Lees.
The good news from the research, according to Lees? Groundwater levels don’t have to recover significantly to reduce or eliminate subsidence completely. An improvement of groundwater levels by 20-25% will still yield a significant improvement, says Lees.
Even so, as groundwater managers are learning, it is one thing to know how to improve water levels, and a completely different skill to actually be able to pull it off, in concert with the thousands of growers subject to new pumping restrictions and fees designed to make sure more water is going into the ground rather than pumped out.
Up and down the valley, newly-formed groundwater agencies are grappling with how to meet the requirements of the state law – which only requires aquifers to stabilize (or in some places, rise to protect drinking water wells from dewatering) – in order to prevent further subsidence.
There are a few choices on the table for water managers: take farmland out of production, switch to water-wise crops, recharge excess water in wet years back into the ground, or buy up someone else’s water rights. Most are planning to implement some combination of all of these options.
That work has been difficult, with the Kings River East Groundwater Agency recently saying in a letter to DWR that it’s ‘unreasonable’ to be responsible for protecting drinking water wells. In Kern County’s Semitropic Irrigation District, growers are facing a steep groundwater overdraft fee – meant to push some land into permanent retirement. In Madera County, growers have bitterly debated who should be subject to new pumping fees.
While many growers and groundwater managers have stated that simply adding more recharge basins would be enough to stem groundwater decline, researchers at the Public Policy Institute of California have found, there is simply not enough water – or canals – to take water in wet years and bring it to the driest parts of the state.
How water managers act during the increasingly shorter – but critical – wet times matters greatly.
“I think the key challenge facing us, moving forwards – if and when there is another wet year – if we can improve the recovery of the aquifer,” said Lees. “In 2017, we saw recovery in some places of 10 meters. Let’s see if next time we can get it to 15 or 20.”