Parts of California are Sinking a Foot a Year

Since the 1920s, excessive pumping of groundwater at thousands of wells in California’s San Joaquin Valley has caused land in sections of the valley to subside, or sink, by almost 30 feet. This subsidence is exacerbated during droughts, when farmers rely heavily on groundwater to sustain one of the most productive agricultural regions in the nation. Once the land subsides it's capacity to hold groundwater cannot be restored.

from Vasco et al

Subsidence induced by groundwater depletion is a grave problem in California’s Tulare Basin, and while certainly not surprise, scientists at NASA have been alarmed by how much faster the areas of the San Joaquin Valley are sinking during this extended drought period. NASA found that sections of the Tulare Basin are sinking at a rate of about a foot a year.

Subsidence can also cause structural damage to the infrastructure we build on top of the earth- like roads, bridges, and pipes.  Often, permanently losing storage space for water as the earth compacts. A group of scientists supported by their various organizations combined the data from two orbiting satellite-based systems to monitor the variations within the Tulare basin at various timescales.

Using the Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) observations, which provide estimates of the displacements of the Earth’s surface, and terrestrial water storage changes measured from NASA’s Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-on (FO) missions the scientists were able to model the groundwater changes.

 However,  changes in the gravity field sensed by GRACE and GRACE-FO can be traced to a variety of sources such as ground movement, soil moisture, water table variations, and snow cover. Thus, it was  diffcult, if not impossible, for the scientists to distinguish between water mass changes in the shallow unconfined aquifer and in the underlying confined aquifer using gravitational observations alone.

Corcoran clay separates the shallower unconfined aquifer from the confined aquifer below. Recharge occurs in the unconfined aquifer from snow, runoff, and precipitation. Groundwater storage in both the unconfined and confined aquifers is detected by the GRACE satellites. Compaction, believed to occur predominantly in the confined aquifer, causes the displacement of the Earth’s surface measured by Sentinel-1 Synthetic Aperture Radar (SAR) satellites.

Though much more work needs to be done, results suggest that available Sentinel and GRACE satellite data can indeed monitor hydrological variations over time. With future improvements in observations, and additional satellites planned for 2023, there should be even better monitoring of the changes in the Tulare basin in the future.

Read the article here if you are interested.

Vasco, Donald W., Kim, Kyra H., Farr, Tom G., Reager, J. T., Bekaert, David, Sangha, Simran S., Rutqvist, Jonny Beaudoing, Hiroko K., 2022/03/09: “Using Sentinel-1 and GRACE satellite data to monitor the hydrological variations within the Tulare Basin, California,” Scientific Reports, 3867 vol 12 issue 1.