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One-pager 09 · Modeling & Management

Modeling, Consequences & SGMA Management

The theory from pages 02–07 is encoded in standard simulation tools and reduces, for managers, to one operating rule: keep heads above the critical head. This page covers the models, the damage subsidence does, and how critical head becomes a defensible SGMA minimum threshold.

SGMA Subsidence Series
Page 09 of 09
The simulation lineage

Helm 1-D aquitard drainage (1975–76)

The foundational model: couples Terzaghi consolidation (page 06) with stress-dependent elastic/inelastic storage (pages 04–05). It reproduced the Pixley extensometer record and remains the conceptual core that every later code — from MODFLOW to the modern Lees 1-D model — still inherits.

MODFLOW subsidence packages

The current standard is CSUB for MODFLOW 6 (Hughes et al. 2022): elastic and inelastic skeletal storage with optional delay (Terzaghi) interbeds, in the actively developed modeling framework. The earlier SUB package (Hoffmann et al. 2003), also with delay interbeds, is still widely used in MODFLOW-2005/NWT models. MODFLOW-OWHM (Hanson et al. 2014; v2 Boyce et al. 2020) improves on the 2005 SUB by merging SUB and SUB-WT into one modern, faster package with four interbed types (instantaneous/delayed × confined/stress-dependent) and by coupling compaction to its Farm Process — valuable where agricultural pumping is the driver. The original IBS (Leake & Prudic 1991) is now superseded, and stand-alone SUB-WT saw only limited adoption.

1-D models & InSAR calibration

Fast Helm-based one-dimensional codes are the workhorse for site and subbasin analysis. The Lees, Knight & Smith (2022) 1-D compaction model reconstructed 65 years of San Joaquin Valley subsidence and projects committed subsidence, calibrated against InSAR. InSAR fields are also inverted for interbed storage (Hoffmann, Galloway & Zebker 2003), and subsidence packages embed in regional models (e.g., C2VSim, Central Valley Hydrologic Model) for SGMA-scale projection.

Timeline of subsidence simulation tools
1975 1985 1995 2005 2015 2024 Helm 1-D theory 1975–76 aquitard-drainage foundation SUB 2003 delay interbeds OWHM SUB 2014 · v2 2020 SUB+SUB-WT fusion; +FMP CSUB · MODFLOW 6 2022 current standard IBS 1991 first interbed storage (superseded) SUB-WT 2007 water-table; limited use Lees 1-D 2022 Helm-based, InSAR-calibrated theory MODFLOW package current standard independent 1-D model
Figure 2. Development of the main tools for simulating aquifer-system compaction. Helm's 1-D aquitard-drainage theory (1975–76) underlies everything that follows. The MODFLOW family advanced from IBS (no drainage delay) to SUB (delay interbeds) to the water-table SUB-WT; MODFLOW-OWHM then merged SUB and SUB-WT into a single package (four interbed types) coupled to its Farm Process, and CSUB brought subsidence into the modern MODFLOW 6 framework — the current standard. In parallel, fast Helm-based 1-D codes (e.g., Lees et al. 2022) remain the workhorse for site and subbasin studies.
Interactive: project subsidence under a management choice

Management scenario

Ultimate permanent subsidence— ft
Subsidence at 10 / 30 / 50 yr— ft
Committed but not yet realized (yr 0)— ft

"Hold" causes essentially no new subsidence; "decline to threshold then hold" locks in a fixed permanent amount that arrives gradually; "continued overdraft" never stabilizes.

Figure 1. Projected land subsidence over 50 years for three strategies, given an inelastic storage coefficient and a residual-drainage time constant τ (page 06). Note the lag: even a one-time decline that "then holds" keeps producing subsidence for years as thick clays finish draining (the committed subsidence). Only holding heads at or above the current critical head flattens the curve. Illustrative — real projections use calibrated SUB/SUB-WT models.
Consequences that drive the management

Permanent & system-wide

  • Lost storage capacity — inelastic compaction permanently shrinks the aquifer (pages 04–05).
  • Differential subsidence — uneven sinking reverses gradients on the Friant-Kern Canal, California Aqueduct, and flood-control channels, cutting conveyance.
  • Earth fissures — tensional ground cracks where compaction is differential or near bedrock highs (Holzer 1984).
  • Well damage — protruding/collapsed casings as ground sinks around fixed casing.
  • Flooding — lowered land surface increases inundation risk in valleys and coastal zones.

Two endings, one rule

Santa Clara Valley recovered: imported water and recharge raised heads above σ′pc, and subsidence stopped. San Joaquin Valley keeps subsiding wherever drought pumping sets new record lows.

The difference is entirely whether heads were held above the critical head. That single rule — derived from effective stress, the e–log σ′ curve, and preconsolidation — is the whole of subsidence management.

Subsidence under SGMA

Setting thresholds

SGMA lists land subsidence as one of six sustainability indicators and requires GSAs to avoid "significant and unreasonable" subsidence. A defensible framework:

  • Map current and historic subsidence (InSAR + benchmarks, page 08).
  • Estimate σ′pc / critical head from extensometer–piezometer pairs (page 07).
  • Set a groundwater-level minimum threshold at or above the critical head in vulnerable areas, linking the level indicator to the subsidence indicator.
  • Track committed (residual) subsidence so thresholds account for the lag (page 06).

DWR's Land Subsidence Best Management Practice (2026) lays out this workflow for GSAs — identifying vulnerable areas, setting sustainable management criteria, and monitoring to avoid and minimize pumping-induced subsidence.

Why "manage the head" works

Subsidence is hard to measure quickly, permanent once it happens, and delayed in its response — a difficult thing to regulate directly. Head is easy to measure, controllable through pumping management, and mechanistically upstream of subsidence. Managing to a critical-head target converts an intractable surface problem into a tractable groundwater-level one.

This is the same logic that ties the series together: protect the surface by protecting effective stress, and protect effective stress by protecting head.

Key references

  1. Helm, D.C. (1975, 1976). One-dimensional simulation of aquifer system compaction near Pixley, California (Parts 1 & 2). Water Resources Research 11(3): 465–478; 12(3): 375–391.
  2. Hughes, J.D., Leake, S.A., Galloway, D.L. & White, J.T. (2022). Documentation for the Skeletal Storage, Compaction, and Subsidence (CSUB) Package of MODFLOW 6. USGS Techniques and Methods 6-A62. (The current-generation subsidence package.)
  3. Lees, M., Knight, R. & Smith, R. (2022). Development and application of a 1D compaction model to understand 65 years of subsidence in the San Joaquin Valley. Water Resources Research 58(6): e2021WR031390. (Modern Helm-based 1-D model, InSAR-calibrated.)
  4. Leake, S.A. & Prudic, D.E. (1991). Documentation of a computer program to simulate aquifer-system compaction using the modular finite-difference ground-water flow model (IBS). USGS TWRI 6-A2. (Original MODFLOW interbed-storage package; now superseded by SUB and CSUB.)
  5. Hoffmann, J., Leake, S.A., Galloway, D.L. & Wilson, A.M. (2003). MODFLOW-2000 ground-water model — user guide to the Subsidence and Aquifer-System Compaction (SUB) Package. USGS Open-File Report 03-233.
  6. Leake, S.A. & Galloway, D.L. (2007). MODFLOW ground-water model — user guide to the Subsidence and Aquifer-System Compaction Package (SUB-WT) for water-table aquifers. USGS Techniques and Methods 6-A23.
  7. Hanson, R.T., Boyce, S.E., Schmid, W., Hughes, J.D., Mehl, S.M., Leake, S.A., Maddock, T. & Niswonger, R.G. (2014). One-Water Hydrologic Flow Model (MODFLOW-OWHM). USGS Techniques and Methods 6-A51.
  8. Boyce, S.E., Hanson, R.T., Ferguson, I., Schmid, W., Henson, W., Reimann, T., Mehl, S.M. & Earll, M.M. (2020). One-Water Hydrologic Flow Model: a MODFLOW-based conjunctive-use simulation software (MODFLOW-OWHM v2). USGS Techniques and Methods 6-A60. (Enhanced SUB package: four interbed types, Farm Process coupling.)
  9. Hoffmann, J., Galloway, D.L. & Zebker, H.A. (2003). Inverse modeling of interbed storage parameters using land subsidence observations, Antelope Valley, California. Water Resources Research 39(2): 1031.
  10. Ellis, J.H., Knight, J.E., White, J.T., Sneed, M., Hughes, J.D., Ramage, J.K., Braun, C.L., Teeple, A., Foster, L., Rendon, S.H. & Brandt, J. (2023). Hydrogeology, land-surface subsidence, and documentation of the Gulf Coast Land Subsidence and Groundwater-Flow (GULF) model, southeast Texas, 1897–2018. USGS Professional Paper 1877. (Regional coupled groundwater-flow and subsidence model, Harris-Galveston Subsidence District.)
  11. Holzer, T.L. (1984). Ground failure induced by ground-water withdrawal from unconsolidated sediment. In Man-Induced Land Subsidence, GSA Reviews in Engineering Geology VI, p. 67–106.
  12. Galloway, D.L., Jones, D.R. & Ingebritsen, S.E. (1999). Land subsidence in the United States. USGS Circular 1182.
  13. Poland, J.F. & Ireland, R.L. (1988). Land subsidence in the Santa Clara Valley, California, as of 1982. USGS Professional Paper 497-F.
  14. California Department of Water Resources (2026). Land Subsidence — Best Management Practice for the Sustainable Management of Groundwater. SGMA BMP series (lead author J.H. Ellis). (Guidance for GSAs on avoiding and minimizing pumping-induced subsidence; finalized January 2026.)
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