"There is enough water physically available to mitigate long-term groundwater overdraft," said Helen Dahlke of the University of California, Davis. "We just have to manage it more efficiently."

In years of high streamflow, the team’s analysis shows, over three cubic kilometres of excess surface water is exported from California’s Central Valley to the Sacramento–San Joaquin Delta, often when the required flows of the delta and its major rivers are exceeded. That’s potentially enough to boost or even replenish groundwater levels, over time.

Over the last 100 years, the volume of groundwater in California’s Central Valley has dropped by an estimated 185 cubic km, and continues to drop by 0.6 to 3.5 cubic km each year. Although this overdraft is small compared to the total level of groundwater, which is close to 4000 cubic km, it is significant compared with the groundwater that is of useable quality – typically the top 100–300 m. What’s more, extracting deeper groundwater at times of drought is not an option available to all landowners, with deep, high-capacity wells costing hundreds of thousands of dollars.

Together with Tiffany Kocis at UC Davis, Dahlke statistically analysed the flow of rivers in the Central Valley to see whether surface flow could alleviate the problem. Within the valley, the Sacramento River, the San Joaquin River and their tributaries transport huge volumes of water to the Sacramento–San Joaquin Delta. Much of this is vital to sustain the needs of fish and other aquatic species, to ensure minimum water-quality standards and to meet other requirements, but there is often a surplus – especially in winter, California’s rainy season.

The researchers analysed the flow records of 93 stream gauges in the Central Valley, and identified the volume of water transported in the top 10% of flows – storm flows – for each. By looking at how often these storm flows occurred, they were able to estimate the total amount of surplus surface water.

Suitable storm flows occurred in 7 out of 10 years in the Sacramento River basin, and in 4.7 out of 10 years in the San Joaquin–Tulare Basin, mostly from storms scattered over a few days between November and April. In those storm years, some 3.2 cubic km of surplus surface water headed towards the delta – water that could potentially be used to replenish groundwater supplies, via a process known as groundwater banking. This is done already, Dahlke says, "but not at the scale that we would like it to happen, since wet years might become more rare. Thus, we need strategies in place to make the most out of the surface water when it is available in abundance."

There are several methods to bank groundwater. One is to substitute surface water for groundwater to reduce groundwater use; another is to supply surface water to those who normally use groundwater; yet another is to actively replenish groundwater by letting water infiltrate the surface, or by injecting it in wells.

But perhaps the most promising method, according to Dahlke, is to flood farmland with surface water in the winter, so that the water is present as shallow groundwater for use later when surface water supplies are running low. "[This would allow] California to adapt to climate change and to make use of big storm flows when they become available in extreme wet years, as we just had," she said.

Dahlke and Kocis published their findings in Environmental Research Letters (ERL).

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