NOT A WATERSHED MOMENT

For environmentalists, the results of the Tri-Valley Groundwater Modeling Project were supposed to be a stick-it-to-’em watershed moment that would finally force a historically reluctant board to accept that pumping restrictions are necessary.

That didn’t happen Wednesday night at the Tri-Valley Groundwater Management District board meeting in Benton.

“This is as far as the research has gone… and this is as far as it’s going to go?” one attendee asked, visibly stunned, after the consultant unveiled his findings.

Funded through a state grant secured by Mono County and administered by the Owens Valley Groundwater Authority (OVGA), the $200,000 model was designed to simulate groundwater conditions across the Tri-Valley basin under a range of management scenarios. For years, understanding of the basin has relied largely on limited monitoring and anecdotal evidence.

But the model wasn’t the silver bullet many had imagined. As the consultant, Gus Tolley with DBS&A Geo-Logic, made clear, building a truly reliable groundwater model takes years and a lot more money. Even then, it’s not a perfect science.

“All models are wrong, but some of them are useful,” he said mid-presentation.

Based on the available data and the model it presented, the consultant concluded that uniform pumping reductions over the next 30 years would improve groundwater levels in Benton and Hammil valleys, but would result in only modest improvements to spring discharge at Fish Slough. Land fallowing produced similar results.

“Modest results at Fish Slough” didn’t sit well with many in the room.

Fish Slough is a high desert wetland, making it an ecological anomaly home to endangered pupfish, unique microfauna, and culturally significant artifacts like petroglyphs and obsidian flakes. It’s also drying up, which is why so many people keep showing up to these meetings.

But Tolley stressed that the takeaway wasn’t that pumping has no effect, but that the system is not fully understood.

Fish Slough, he said, “appears to be controlled by more complex hydrogeologic processes.”

Nick Buckmaster, an off-duty CDFW employee, wasn’t convinced.

The model for the two monitoring stations at Fish Slough do reproduce much of the last 30 years with reasonable accuracy, but break down during wetter periods in the mid-2000s and overestimate the wet year of 2023, something the consultant described as “a real head scratcher.”

“If the model isn’t representing that system correctly,” Buckmaster said, “then conclusions about how pumping affects it should be treated carefully.” 

Part of the problem is data. Until recently, there were no deep monitoring wells in the Fish Slough area, making it difficult to understand what’s happening beneath the surface. New wells have since been installed, but incorporating that data would require additional modeling work, and additional funding.

That limitation is not unique to Fish Slough.

Across the Tri-Valley, large areas lack reliable monitoring data, leaving what are effectively blind spots in the model. In some cases, the consultants had to add “synthetic” wells to account for irrigation demand that couldn’t otherwise be explained.

“It tells us the water has to be coming from somewhere,” Tolley said.

More accurate modeling would require detailed information from local growers, specifically, which wells irrigate which fields, and how much water is being used. That data is largely not publicly available, and only one landowner responded to a request for input during the study. That one landowner ultimately didn’t submit any data.

There are also limits to what the district can control.

Notably, the model did not include reductions to pumping by the Los Angeles Department of Water and Power (LADWP), which operates wells in the basin but falls outside the district’s authority.

Even with these shortfalls, however, the model still offers a clear big-picture takeaway that groundwater levels in the Tri-Valley are declining. Over the 30-year simulation period, the basin shows a net loss of roughly 200,000 acre-feet of stored groundwater.

If nothing changes, groundwater levels are expected to continue declining across much of the basin. 

Water levels could drop roughly 9 to 10 feet in the Benton and Chalfant areas, and as much as 35 to 40 feet in Hammil Valley. Further south, the model projects more modest declines around 10 feet, where conditions are more strongly influenced by model boundary assumptions and external pumping. In Fish Slough, the model estimates declines of about 2.5 feet.

As a state-recognized groundwater sustainability agency, the Tri-Valley Groundwater Monitoring District has the authority to implement management measures, but the board members remained largely silent throughout the presentation and discussion.

Tolley is required submit a report to to Mono County by June 30, but the submission is not expected to carry any required action.