NASA satellites spot drought effect on groundwater 

Arbuckle’s imminent demise highly overrated 

The town of Arbuckle isn’t sinking into the ground as reported by Bay Area media, but a small survey point near the town is two feet lower than when the State Department of Water Resources last measured it in 2008. 

Colusa County Water Resources Manager Mary Fahey, along with Bill Ehorn, DWR supervising engineering specialist, presented the findings from the Northern Sacramento Valley GPS subsidence report to the Board of Supervisors on March 5.  

About 25 local, state, and federal agencies assisted with the 2017 NASA survey of 317 sites spanning 11 counties from Redding to Sacramento, Ehorn said. 

The results were collected as the state was recovering from the severe drought of 2012-16, when groundwater levels in much of the state reached historic lows.  

Fahey said that subsidence (defined as land surface elevation decreases greater than or equal to 0.17 feet, or 2 inches) in the Arbuckle area did not come as a surprise. 

“In 2015, we were at the height of a severe drought,” Fahey said. “We know there was declining groundwater levels in the Arbuckle area, and during that period there were several domestic well outages reported, and the county took action restricting well development.” 

In 2016, Fahey said a DWR study (separate from the 2017 NASA study) was conducted to determine if the decline was a result of crop changes or drought itself.

“That report determined that drought conditions were really at the root of the groundwater declines,” she said. “It was historic drought conditions during that period and a decade-long period of dry condition in the area and in California.” 

It was also during that period, the Tehama Colusa Canal Authority allotted no surface water allocations, which resulted in farmers having to draw heavily on groundwater to keep their trees and crops alive. 

“The media reported that Arbuckle is sinking,” Fahey said. “Arbuckle is not sinking. It’s just one survey point.” 

Fahey said while significant subsidence was reported at only one point in Colusa County, it is not something that can be ignored. 

“Subsidence is something we will have to address under the Sustainable Management Groundwater Act, so it will be part of the groundwater sustainability plans that we are putting together right now,” she said. 

Ehorn said the 2016 NASA satellite survey also shows several other statistically significant levels of subsidence. 

Those decreases were observed at three survey sites in Glenn County (between .44 and .59 feet of subsidence) and five survey sites in Sutter County (between .20 and .36 feet of subsidence). 

Yolo County experiences some subsidence with 31 survey sites measuring a land surface decline between .3 and 1.1 feet.

“All the other areas of the valley were below the threshold and did not show significant subsidence at all.”  

Since the drought, Ehorn said groundwater levels have recovered an average of seven feet, but more frequent and more comprehensive monitoring may be needed to more accurately detail the impacts of droughts and high-water years on groundwater levels and subsidence.

While the drought had depleted groundwater in parts of Colusa County (medium to deep wells) by as much as 52 feet, and in parts of Glenn County by as much as 58 feet, the survey, comparing Spring of 2015 to Spring of 2017, found that groundwater levels in the Sacramento Valley had recovered an average of seven feet since the drought’s end, Ehorn said. 

Ehorn said DWR plans to resurvey the same points in three to five years. ■