As discussed, one factor that contributes to balancing of groundwater pumping is the removal of water from storage in the pore spaces of the subsurface saturated zone. Coincident with storage depletion are water-level declines, either from a declining water table in an unconfined aquifer or from lowering of the potentiometric head in a confined aquifer. Groundwater storage depletion and accompanying water level declines have a number of consequences.
The volume of water that an aquifer releases from storage per unit surface area per unit decline in head in response to pumping is characterized by the dimensionless storage coefficient (S) (Lohman et al., 1972). In a confined aquifer, the water released from storage is derived from the expansion of water and compression of the aquifer as the head declines. In an unconfined (or “water table”) aquifer, the primary source of water from storage comes from gravity drainage of the pore spaces as the water table declines, and S is essentially equivalent to the dimensionless specific yield (Sy) (Lohman et al., 1972). In confined aquifers, values of S typically range from 5×10-5 to 5×10-3, whereas for unconfined aquifers the values of Sy typically range from 0.01 to 0.30 (Freeze and Cherry, 1979). Consequently, a given (or unit) withdrawal rate in a confined aquifer will result in greater drawdown near the well and a cone of depression that spreads faster and further than would result from an identical withdrawal rate in an unconfined aquifer.