WHEN DOES SURFACE DEFORMATION CORRESPOND WITH GROUNDWATER VOLUME CHANGE?ANSWERS FROM CALIFORNIA’S CENTRAL VALLEY (Invited Presentation)

Changes of groundwater level within an aquifer can cause vertical land motion (VLM) of the surface, which is due to poroelastic deformation of the porous medium. However, this response is sometimes inelastic. For instance, an aquifer system with thick aquitard lenses may undergo residual compaction due to lagged equilibration as a result of past overdrafts. Here, we focus on California’s Central Valley, a major US agricultural hub. We investigate the land subsidence during the recent drought periods of 2007-2010 and 2012-2015. We show that during droughts, the time series of VLM has a declining trend modulated with seasonal variations. However, the subsidence continues beyond the drought periods, although the groundwater levels have already stopped to decline. The seasonal groundwater oscillations correspond with that of VLM, disregarding the state of drought. Examining both data sets, we discuss the conditions required for VLM time series to correspond with the groundwater volume change and highlight the critical role of geodesy in understanding the aquifer-system dynamics, particularly when supplemented with hydro-geological data, such as groundwater levels [Shirzaei et al., 2019].

We further characterize the impact of drought on groundwater volume and aquifer storage loss. We use ~300 continuous GPS stations, 620 SAR images acquired by ALOS L-band and Sentinel-1A/B C-band sensors, and observation of groundwater level at ~1600 wells. JPL provided estimates of total water storage change calculated from gravity field observations by the GRACE satellites, which we converted into groundwater volume loss using hydrological datasets. We estimate maximum subsidence rates in the southern San Joaquin Valley of up to ~25 cm/yr and ~35 cm/yr for the 1st and 2nd droughts, respectively. Using a 1-D poroelastic calculation based on VLM data, we find a groundwater loss of 21.3±7.2 km³ for the entire Central Valley during 2007-2010 and of 29.3±8.7 km³ for the San Joaquin Valley during 2012-2015. The VLM-based estimates of groundwater loss are consistent with that of GRACE, considering uncertainty ranges. We further infer that due to overdraft during both droughts, the aquifer system storage capacity permanently reduced by up to 5% [Ojha et al., 2018; Ojha et al., 2019].

References

• Ojha, C., M. Shirzaei, S. Werth, D. F. Argus, and T. G. Farr (2018), WRR, 54(7), 4449-4460.
• Ojha, C., S. Werth, and M. Shirzaei (2019), JGR, 124(3), 3127-3143.
• Shirzaei, M., C. Ojha, S. Werth, G. Carlson, and E. R. Vivoni (2019), Sci. Adv., 5(eaav8038).