Groundwater Loss and Aquifer System Compaction in San Joaquin Valley During 2012–2015 Drought

Abstract

California's millennium drought of 2012–2015 severely impacted the Central Valley aquifer system and caused permanent loss of groundwater and aquifer storage capacity. To quantify these impacts within the southern San Joaquin Valley, we analyze various complementary measurements, including gravity changes from Gravity Recovery and Climate Experiment (GRACE) satellites; vertical land motion from Global Positioning System, interferometric synthetic aperture radar, and extensometer; and groundwater level records. The interferometric data set acquired by the Sentinel‐1 satellite only spans the period January 2015 and October 2017, while the other data sets span the entire drought period. Using GRACE observations, we find an average groundwater loss of 6.1 ± 2.3 km 3 /year as a lower bound estimate for the San Joaquin Valley, amounting to a total volume of 24.2 ± 9.3 km 3 lost during the period October 2011 to September 2015. This is consistent with the total volume of 29.25 ± 8.7 km 3 , estimated using only Global Positioning System deformation data. Our results highlight the advantage of using vertical land motion data to evaluate groundwater loss and thus fill the gaps between GRACE and GRACE‐Follow‐On missions and complement their estimates. We further determine that 0.4–3.25% of the aquifer system storage capacity is permanently lost during this drought period. Comparing groundwater level and vertical land motion data following September 2015, we determine an equilibration time of 0.5–1.5 years for groundwater levels within aquitard and aquifer units, during which residual compaction of aquitard and land subsidence continues beyond the drought period. We suggest that such studies can advance the knowledge of evolving groundwater resources, enabling managers and decision makers to better assess water demand and supply during and in‐between drought periods.

Seasonal and Long‐Term Groundwater Unloading in the Central Valley Modifies Crustal Stress

“…Excess irrigation water can also flow down through perforated groundwater well casings, which increase the vertical hydraulic conductivity between the shallow unconfined and deeper confined and semi‐confined aquifers and aid in aquifer recharge during this time (Faunt, ). It is also important to note that the largest seasonal amplitudes occur where we see peak recharge in the spring versus in the winter, showing that the largest seasonal recharge and discharge rates are dominantly controlled by agricultural activity (Ojha et al, ; Shirzaei et al, ).…”

“…The primary disadvantage with these load change measurements is their coarse spatial resolution (~200–300 km). Spaceborne InSAR has also been proven useful in measuring compaction due to the removal of fluid (Galloway et al, ; Miller et al, ; Ojha et al, ) and, thus, can be used to estimate groundwater volume loss (Ojha et al, ). Here we use published groundwater volume loss estimates from VLM (Ojha et al, ) found using a combination of InSAR line of sight (LOS) displacements and horizontal displacements from Plate Boundary Observatory network GPS stations to evaluate the effect groundwater loss has on vertical elastic deformation outside of the Valley and changes to the magnitude of stress along faults in California.…”

“…In this study, we focus on the Central Valley aquifer‐system in California, which has recently undergone two severe, prolonged periods of drought (2007–2010 and 2012–2015). Both droughts have resulted in dramatic groundwater losses caused by intense pumping activities and reduced natural recharge (Famiglietti et al, ; Faunt et al, ; Ojha et al, , ). Pumping in the Central Valley is not well documented; therefore, groundwater losses are difficult to quantify.…”

Spatially coherent regional changes in seasonal extreme streamflow events in the United States and Canada since 1950

“…In the Northeast United States, Collins (20) found minimal evidence for shifts in the timing of annual peak flows, but Hodgkins and Dudley (32) showed that the centroid of winterspring streamflow has shifted at 32 to 64% of snowmelt-affected sites in the eastern North America. In the western United States, drought has and is projected to limit snowpack generation and groundwater recharge (33)(34)(35), a pattern reflected in increased frequency of extreme low flows in the Pacific Coast and Pacific Northwest hydro-regions during summer and fall ( fig. S3).…”

Subsidence‐Derived Volumetric Strain Models for Mapping Extensional Fissures and Constraining Rock Mechanical Properties in the San Joaquin Valley, California

“…As long as pre‐consolidation stress is not exceeded, though, the deformation is recoverable (Galloway & Burbey, 2011). It has been shown that pumping in the San Joaquin Valley during recent droughts has caused inelastic deformation, permanently reducing aquifer storage (Chaussard & Farr, 2019; Ojha et al, 2019; 2020).…”