User:Sir Jacob III/Central Valley land subsidence

Monitoring[edit]
Land subsidence in the Central Valley is monitored by various government agencies including NASA, the California Department of Water Resources, the USGS, and various local agencies or businesses within the valley.

The NASA Jet Propulsion Laboratory uses InSAR to remotely measure topographic change over time. In a monitoring study from May 2015 to September 2016, NASA found areas that were subsiding as rapidly as two feet per year. The data also revealed that southern El Nido and Corcoran, California were experiencing the most rapid sinking in a bowl shaped manner.

In the Sacramento Valley, the DWR reports using GPS surveys to study elevation changes in a study known as the Sacramento Valley Height-Modernization Project. Their study includes 339 survey monuments throughout the valley. The DWR also has eleven borehole extensometers to collect detailed information at designated sites. The GPS survey gives a broad picture of change, and the borehole extensometers can focus on key areas.

The USGS collects and synthesizes data from other agencies to make reports on land subsidence in the valley. They use InSAR data from NASA's Jet Propulsion Laboratory in conjunction with interferograms to interpret the topographic change captured by satellite imagery. Continuous Global Positioning Survey station data from UNAVCO, Scripps Orbit and Permanent Array Center, Caltrans, and others are obtained for topography changes over a large region. Spirit Leveling data from the DWR, Delta-Mendota Water Authority, the Central California Irrigation District, and others are used to measure elevation change in a smaller area. The USGS uses the data from borehole extensometers provided by the DWR, and also utilizes information from piezometers to track changes in groundwater.

As of 2018, the USGS has been using large hexagonal-shaped sensor rigged to a helicopter to measure groundwater salinity and map underground aquifers in the San Joaquin Valley. These sensors work by sending in small electromagnetic waves into the subsurface level, and receiving information as these waves are returned.

Key components of studying land subsidence in the Central Valley include large and small scale monitoring, studying topography in relation to groundwater, and a multi-agency approach. Discoveries made by monitoring may allow for better mitigation practices and policies to prevent subsidence.

Specific to the Central Valley Aqueduct system, the USGS has implemented the Central Valley Hydrologic Model (CVHM) to closely analyze and predict the changes in groundwater levels and consequential land subsidence. The CVHM considers surface level water supply to estimate groundwater use, as well as monitoring recharge rates. Updates and improvements have been made to the CVHM since its creation. For example, the contribution of groundwater recharge from un-gaged watersheds to the Central Valley Aquifer were previously not contributed in the model for the 1961-2009 data analysis. Changes such as this are being made to improve the accuracy of the model.

Infrastructure damage[edit]
Infrastructure damage caused by accelerated land subsidence threatens the effectiveness of many dams and canals sourcing surface water to the central valley. Damage has been noted in the California Aqueduct system and is attributed to the rate at which the valley floor is settling. In a 2019 study, researchers found that stretches of the California Aqueduct had sunk in a way that have caused up to 20% loss in carrying capacity of water. Buildings, foundations, pipelines, and other infrastructure may be at risk of damage as well. Areas with substantial subsidence may be at higher risk of flooding. This is in large part due to the levee designed to prevent flood risk are typically in high-subsidence areas. The Delta-Mendota Canal and Chowchilla Basin are evidence of this in the Central Valley With California introducing the Sustainable Groundwater Management Act in 2014, new estimations of subsidence based on water usage plans have revealed that stretches of the California Aqueduct are still at a substantially high risk of subsidence. These projections range from 10-15 feet of subsidence in the most at risk areas.

The Kern-Friant Canal and the Delta-Mendota Canal are two of four projects that are being considered for repaired from subsidence damages. For example, Senate Bill 559 sought to provide over $400 million in funds to repair the Kern-Friant canal. However, Governor Newsome vetoed the bill in search of broader state-wide solutions to subsidence damage. With some facilities and water transport infrastructure being federally owned and operated and some state owned and operated, it often requires the collaboration of the State Water Board and DWR to organize and fund repair projects.

Groundwater Recharge
Groundwater recharge is the process of pumping excess surface water into the underground aquifers. Groundwater recharge is vital to mitigating subsidence and minimizes the consequential damage. The amount of recharge that occurs in the San Joaquin Valley is very dependent on the amount of rain received in winter and spring months and county policy. Roughly 55% of groundwater in the San Joaquin Valley is recharged via local precipitation, while 45% is recharged via river water recharge. With anthropologic climate change increasing temperatures and therefore higher evaporation rates, groundwater recharge via local precipitation may be in danger of decreasing over time. Furthermore, the snowpack volume has already shown signs of decreasing. This could have an effect on the summer water supply to the state, forcing surface water to be used at a higher capacity percentage and less available for recharge.