The Answer to Groundwater Resources Comes from High in the Sky

Their findings, published recently in the Nature journal Scientific Reports, suggest that aquifer volume is influenced by variations in effective pressure in the ground surrounding documented wells. For the periods studied, the spatial patterns of changes in aquifer volume vary significantly among years representing drought (October 2015 to 2016), precipitation (2017), and near-average rainfall (2018), reflecting the deviations in groundwater pumping, which change depending upon surface water deliveries to farmers.

The researchers believe their methods could be relatively simple for local groundwater sustainability agencies to implement because the spatially explicit estimates of aquifer volume loss are relatively inexpensive to obtain, and can be used by local agencies to estimate localized areas of intensive pumping and groundwater overconsumption.

“This technique makes it possible to assess groundwater resources not just at the very local level, but at a larger scale, in order to be of benefit to multiple stakeholders,” Nico said.

Hyperlocal Approach Runs Risk of Double-Counting Available Water
For a long time, groundwater was regularly pumped in California to meet water needs when surface water coming from rivers, lakes, and streams was inadequate – so much so during the many years of persistent drought that massive over-pumping pushed the state to pass the SGMA legislation.

SGMA requires water users to halt overdraft and bring critically overdrawn groundwater basins into balanced levels of pumping and recharge by 2040. Local groundwater sustainability agencies charged with developing sustainable management policies for oversubscribed groundwater basins now face the difficult task of estimating groundwater use and aquifer storage capacity without sufficient systematic and quantitative monitoring, according to the Berkeley Lab researchers.

Unfortunately, there’s no cost-effective method that has proven useful for monitoring aquifer storage in sufficiently high resolution yet over needed spatial extents, even in highly developed agricultural areas such as California’s Central Valley, according to Don Vasco, Berkeley Lab senior scientist and lead author of the paper for Scientific Reports.

For example, an approach utilizing observed water levels in monitoring boreholes throughout the Central Valley to indicate aquifer status is largely ineffective, because boreholes are usually shallow and may not accurately represent the health of deeper aquifers. Nor would the typically sparsely distributed boreholes provide information over sufficient spatial extents to detect localized areas of high water use.

“Monitoring is critical in assessing the loss of storage capacity in a given aquifer – and in determining the rate of groundwater depletion – at a resolution that corresponds with the needs of agencies responsible for groundwater management, from a few kilometers to tens of kilometers,” said Vasco.

The Public Policy Institute of California recently evaluated 36 groundwater sustainability plans submitted to the state for 11 critically overdrawn basins in the San Joaquin Valley – California’s largest farming region. Their review identified the potential pitfalls of hyperlocal management, showing that while water supply estimates may appear reasonable when the plans are considered individually, reviewing the plans in aggregate exposes the potential that exists for double-counting water supplies by taking such a localized approach.

This perspective highlights the need for an integrated monitoring tool that can be applied at the small scale but also provide an integrated look across large jurisdictions encompassing multiple management areas to prevent any impacts from falling through the cracks.

“If adopted statewide, the computing approach that we have developed could allow the new local groundwater sustainability agencies (GSAs) established under SGMA to estimate the spatial variation of groundwater usage for their own area and the larger region at the same time,” Nico said.

The technique could also become a new approach that stakeholders in other U.S. states as well as other nations could consider as part of their water management strategies, the researchers said.

The effort is supported by Berkeley Lab’s Laboratory Directed Research and Development (LDRD) program, through which the Lab directs funding to specific areas of research.