INTERA Incorporated (INTERA) supported the California Department of Water Resources (DWR) in developing Appendix I, Update on Land Subsidence in California, for the 2025 update of California's Bulletin 118. The work assesses land-surface subsidence across California’s Central Valley, documenting nearly a century of aquifer-system compaction and land-surface change from 1926 through 2026. 

Land subsidence has affected thousands of square miles of the Central Valley and has damaged canals, roads, bridges, flood-control facilities, and other critical infrastructure. While many of California’s largest areas experiencing subsidence were documented during the mid-20th century, groundwater-level declines associated with drought and increased reliance on groundwater have led to substantial renewed and ongoing subsidence in several regions over the past two decades. The Bulletin 118 assessment integrates historical and modern datasets to provide a consistent long-term perspective on how subsidence patterns have evolved across the Valley. 

In support of the assessment, INTERA compiled and analyzed more than a century of groundwater-level, land-surface elevation, and subsidence observations and developed records of groundwater conditions, aquifer-system compaction, and cumulative subsidence at 50 representative locations throughout the Central Valley. These records provide a foundation for evaluating historical trends, understanding present-day subsidence conditions, and supporting groundwater sustainability planning under California’s Sustainable Groundwater Management Act (SGMA). 

“Understanding subsidence requires understanding the long-term stress history of the aquifer system,” said John Ellis, PG, INTERA’s National Subsidence Lead and technical lead for the subsidence work with DWR. “That perspective is essential for interpreting present-day compaction behavior and evaluating future subsidence risk.” 

A major technical challenge involved reconstructing subsidence histories from monitoring data collected over many decades using different methods and reference systems. While historical leveling surveys, satellite-based measurements, GPS observations, extensometer records, and groundwater data provided valuable information, each covered different time periods and spatial scales. INTERA developed methods to integrate these datasets into continuous cumulative subsidence histories, providing one of the longest and most comprehensive reconstructions of subsidence available for the Central Valley. 

The project also required development of new approaches for interpreting aquifer-system stress history and evaluating present-day critical head conditions from groundwater flow and compaction models. Working alongside INTERA modeling and subsidence code developers, Jeremy White, PhD and Joe Hughes, PhD, the team created a scripted workflow capable of processing model results across multiple hydrostratigraphic units and translating detailed subsurface behavior into representative critical head estimates suitable for groundwater sustainability planning. 

“This project went beyond estimating subsidence and developing models,” Ellis said. “It was about building a technically defensible and transparent framework that stakeholders can continue to use and build upon as subsidence management evolves under SGMA.” 

The workflows and model archive were published in a publicly accessible GitHub repository, allowing technical users to reproduce analyses, evaluate assumptions, and adapt the tools for site-specific applications throughout the Central Valley. 

The update can be found here: California’s Groundwater: Bulletin 118—Update 2025. Appendix I: Update on Land Subsidence