By David Boyer
The California Department of Water Resources (DWR) has officially finalized its updated guidance on land subsidence, transitioning from the Draft Subsidence Best Management Practices (BMP) of July 2025 to the Final Land Subsidence BMP issued in January 2026. The final document introduces substantial operational and legal changes. The Final BMP reflects a strategic tightening of regulatory standards, an expansion of liability triggers, and a refined approach to how subsidence metrics like “Critical Head” are defined and applied.
The Final Land Subsidence BMP: Groundwater Sustainability Agencies
Refining “Critical Head” and Zonal Management
The most significant technical and legal shifts in the Final BMP revolve around the definition, application, and certainty of “Critical Head,” which is the threshold below which permanent, inelastic subsidence begins. A major shift occurs in how this metric is applied geographically and geologically. While the Draft BMP suggested that critical head could be estimated broadly for entire “aquifer units,” the Final BMP refines this approach. Specifically, Section 4.3.2 allows critical head to be estimated for “aquifer units, or vertical intervals of aquifer units, at specific locations.” This change is significant for Groundwater Sustainability Agencies (GSAs) managing multi-zoned aquifers, as empowering them to set thresholds for specific “vertical intervals” encourages management by zones. Consequently, pumping in deep aquifers can be subjected to different critical head thresholds than shallow aquifers, limiting broad-brush liability and allowing for more targeted regulatory action.
Furthermore, the Final BMP softens the standard of certainty surrounding critical head, moving away from the Draft BMP’s presentation of the metric as a strict “quantitative target.” The finalized guidance, found in Section 5.3, explicitly requires that critical head “should be communicated as a relative indicator with associated uncertainty conveyed to stakeholders.” This shift officially recognizes the inherent uncertainty associated with using critical head as a metric for subsidence. To ground these estimates, the Final BMP anchors the critical head definition to a state-specified dataset, noting in Section 4.3.2 that representative critical head has been estimated for “a total of 50 sites in Bulletin 118 Appendix I.” This establishes a baseline for “best available science,” meaning GSAs located near these 50 sites will likely have their localized critical head estimates judged against DWR’s published figures.
The End of Proxy Monitoring in Subsiding Basins
Regarding monitoring practices, the Final BMP imposes a strict prohibition on the use of water level proxies in certain contexts. The Draft BMP previously contained permissive language suggesting groundwater levels could be used as a proxy for subsidence. However, Section 6.8 of the Final BMP explicitly bans this practice in basins actively experiencing land subsidence. The document states that the use of groundwater levels as a proxy is not appropriate because subsidence can be measured and monitored directly. As a result, GSAs in subsiding basins may no longer rely solely on groundwater level data to prove Sustainable Groundwater Management Act (SGMA) compliance regarding subsidence and are now mandated to invest in direct subsidence monitoring technologies, such as InSAR, extensometers, or continuous GPS.
Expanded Liability: Inclusion of Well Casings
The scope of infrastructure liability is notably expanded in the new document. While the draft guidance primarily focused on surface infrastructure such as canals, bridges, and levees, the Final BMP explicitly adds “well casings” to the list of infrastructure heavily impacted by subsidence. Adding well casings dramatically broadens the scope of potential damages. GSAs must now consider the collapse of private agricultural and domestic wells as a direct “undesirable result” of subsidence, opening the door for localized damage claims from individual well owners.
Hardening Cross-Basin Legal Standards
The updated guidance also hardens the legal framework for cross-boundary disputes by adding explicit statutory citations regarding a GSA’s duty to its neighboring subbasins. Section 7.2 of the Final BMP notes that DWR is required to evaluate whether a Plan “adversely affects the ability of an adjacent basin to implement their groundwater sustainability plan,” explicitly citing Water Code § 10733(c) and 23 CCR § 355.4(b)(7). If pumping in one basin causes a drop in critical head and subsequent subsidence in an adjacent basin, the offending GSA is on clear notice that DWR will evaluate this under specific statutory authority.
Conclusion and Implications
Ultimately, the January 2026 Final Land Subsidence BMP requires GSAs to be more precise in their depth-specific management while simultaneously forcing them to acknowledge the inherent uncertainty in their models by explicitly defining critical head as a “relative indicator” rather than a strict target. Furthermore, the explicit prohibition of water level proxies in subsiding basins and the inclusion of well casings as protected infrastructure significantly raises the compliance requirements for GSAs across the state.
For more information, see: https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Groundwater-Management/Sustainable-Groundwater-Management/Best-Management-Practices-and-Guidance-Documents/Files/Land_Subsidence_BMP.pdf
