10 Transient Analytical Models for Pumping an Unconfined Aquifer
Unlike a confined aquifer, when an unconfined system is pumped, a portion of the unit is physically dewatered as water levels decline. If aquifer materials are isotropic and homogeneous, coarse‑grained, and the release of water from storage is more or less instantaneous, the response of the aquifer to pumping will mirror the Theis type curve. In this setting, the Theis equation with a storativity of Sy is used to represent the unconfined response to pumping (Figure 55a).
Figure 55 ‑ Schematics of the arithmetic time‑drawdown response of a pumping well and observation well finished in an unconfined aquifer that is pumped at a constant rate. A) Aquifer behaves as an isotropic homogeneous system with water instantaneously released from storage (Sy). b) Aquifer drawdown is affected by a delay in the release of water from storage. The period of delayed yield for the pumping well and the observation well is indicated by double black arrows and vertical dashed red and blue lines, respectively.
However, more often drawdown exhibits a time delay. This is generally attributed to the vertical flow of gravity drainage from pores above the water table that occurs as the water table is lowered. The resulting drawdown data often show an intermediate time reduction of slope and then a continued steeper decline (Figure 55b). The temporary reduction in drawdown is caused by what is referred to as delayed yield. Analysis of both these responses to pumping is discussed in this section.