The volume of water released from storage per unit surface area of aquifer per unit decline in hydraulic head (dimensionless).

The specific pressure storativity is the volume of water released from saturated pore storage, per volume of porous medium, due to a unit drop in fluid pressure. SUTRA computes the specific pressure storativity from the fluid and solid compressibilites under the assumption that individual solid grains are incompressible and that the total stress (the sum of the fluid pore pressure and the intergranular stress) remains constant. See Section 2.1 of the SUTRA documentation for details.

the volume of water an aquifer released from an aquifer per unit surface area of the aquifer and per unit change in head.