One-page technical summaries on the geomechanics of land subsidence caused by groundwater withdrawal — the science behind another of SGMA's six sustainability indicators. A companion to the Streamflow Depletion series, each page stands alone as a printable reference and reads together as a curriculum, from effective stress through critical head, inelastic compaction, and management.
Aquifer-system compaction vs. other causes; the classic California case histories (San Joaquin & Santa Clara valleys); permanent storage loss, fissures, and infrastructure damage.
Terzaghi's principle σ = σ′ + u, the constant geostatic load, and how lowering head transfers support from pore water to the granular skeleton — with an interactive stress-vs-depth explorer.
Reversible seasonal deformation in the recompression range, elastic skeletal specific storage Sske, and how it connects to the storativity practitioners already use.
Virgin compression past the historic maximum stress, the e–log σ′ consolidation curve, and why Sskv ≫ Sske makes most subsidence non-recoverable.
The threshold that separates recoverable from permanent: how the historic low water level sets the preconsolidation stress, and why each new record low ratchets it down.
Terzaghi 1-D consolidation in thick clays, Helm's aquitard-drainage model, the time constant τ, and why subsidence keeps accruing for years after heads stabilize.
Riley's stress–strain method: reading Sske, Sskv, and preconsolidation stress from paired extensometer and water-level records, checked against lab consolidation tests.
Borehole extensometers, InSAR, continuous GPS, leveling, and compaction recorders — what each method measures, and why they are strongest in combination.
MODFLOW IBS/SUB/SUB-WT, earth fissures and differential subsidence, and managing to a critical-head target under SGMA minimum thresholds — with a scenario projector.
Land subsidence is one of the six sustainability indicators under California's Sustainable Groundwater Management Act, and it is unique among them: aquifer-system compaction is largely permanent. When fine-grained interbeds compact inelastically, the lost pore volume — and the groundwater storage it represented — does not come back when water levels recover.
The consequences are concrete and expensive: damaged aqueducts and flood-control channels, earth fissures, collapsed well casings, and a shrinking aquifer. These summaries demystify the underlying geomechanics so that GSAs, stakeholders, and consultants can reason clearly about what drives subsidence, which water-level changes are safe, and how to set defensible thresholds.