As long as effective stress stays below the historic maximum, the sediment skeleton behaves like a stiff spring: it compresses when heads fall and rebounds when they recover. Seasonal pumping rides this elastic range — small, reversible, and where every aquifer should be managed to stay.
When effective stress increases but stays below the preconsolidation stress σ′pc (the historic maximum the sediment has "felt" before), grains shift only slightly within their existing packing. Remove the load and they spring back. This recoverable deformation is governed by the elastic skeletal specific storage:
The compaction of a bed equals its storativity times the head change. In the elastic range the same coefficient applies to loading and unloading, so a head cycle that returns to its starting point leaves no net compaction.
Aquifer storativity has two parts: water expansion/compression and skeletal compression. The skeletal part, in its elastic range, is Sske. So elastic compaction is just the visible, vertical expression of releasing water from elastic storage.
Typical values: elastic skeletal specific storage is small — on the order of 1×10⁻⁶ to 1×10⁻⁵ ft⁻¹ for aquifer materials. Real but modest.
| Peak seasonal compaction (per cycle) | — in |
| Net compaction after full cycles | ≈ 0 (recoverable) |
Compaction shown in inches for a single representative interbed; the elastic loop closes because the same Ske applies on the way up and down.
If seasonal drawdown never pushes effective stress past σ′pc — i.e., heads never fall below the historic low — deformation is recoverable and net subsidence is negligible over the long run.
σ′pc records the deepest the skeleton has ever been loaded. Reach it and the response changes character: the stiff elastic spring gives way to soft, permanent virgin compression (page 04).
Beyond σ′pc, compressibility jumps by 1–2 orders of magnitude and the deformation no longer rebounds. The same head decline now produces far more — and permanent — subsidence.
In aquifers managed within the elastic range, water levels and the land surface oscillate seasonally and recover. Extensometer records from such sites show clean, closed annual loops (Riley 1969). This is the target condition for sustainable management.
The catch: even small inelastic "leakage" each year accumulates, and thick aquitards equilibrate so slowly (page 06) that an apparently elastic seasonal record can hide slow permanent compaction at depth.
The whole of subsidence theory turns on comparing two coefficients:
Elastic storage is what you get back. Inelastic storage is what you spend permanently. Telling them apart in field records is the subject of page 07.