GPS Survey of the Sacramento Valley Subsidence Network

In 2008, DWR developed a series of survey monuments in the Sacramento Valley to create a subsidence monitoring network. The network encompasses all or part of 11 counties, from Shasta County at the north end of the valley to Solano and Sacramento counties in the south. It includes over 300 benchmarks with an average spacing of 4.3 miles (7 kilometers). During the spring/summer of 2008, DWR along with 25 local, State, and federal partners performed an initial GPS survey of the network to establish a baseline measurement to compare against future surveys. The initial survey was done using National Geodetic Survey standards. DWR resurveyed the monument network in 2017 with assistance from 19 State, county, and local agencies, and a private entity. The methodology and equipment used was similar to the 2008 survey. Analysis of the results was done to depict the change in height at each monument from 2008 to 2017. The Arbuckle area (Colusa County) showed the most subsidence with a maximum change of -2.14 feet (ft.). Surrounding stations and InSAR data confirm this result with changes ranging from -0.49 to -1.00 ft. In eastern Yolo County (Zamora to Davis), the largest spatial extent of station declines was observed with several benchmarks showing changes between -0.3 and -1.1 ft. In Glenn County (Artois and Orland area), three stations, ARTO, K852, and AGUI showed changes of -0.59 ft., -0.46 ft., and -0.44 ft., respectively. An area on the south side of the Sutter Buttes showed changes ranging from -0.19 to -0.36 ft. The remainder of the valley shows little change overall. During the time of the 2017 survey, groundwater levels in the Sacramento Valley were recovering from the severe drought of 2012-16. During the drought, groundwater levels hit historic lows in most wells in the Sacramento Valley with maximum decreases in Glenn and Colusa Counties of 58 ft. and 43 ft., respectively, compared to 2011 pre-drought conditions. During the survey field work in 2017, groundwater levels had recovered about 7 ft. on average since 2015. The period between the two surveys was nine years, and it was not possible to determine when during that time frame the changes shown took place. However, it is likely that the subsidence occurred during the drought in 2012-16 when groundwater levels in many wells reached historic lows due to increased groundwater pumping. To better bracket when changes occur, surveys at a more frequent interval (3-5 years), are recommended. Integrating groundbased subsidence monitoring such as continuous GPS sites and extensometers and remote sensing data, such as InSAR, into the subsidence monitoring network is also recommended.