Conceptual & Visual Understanding of Hydraulic Head & Groundwater Flow — Andrew J.B. Cohen and John A. Cherry. The Groundwater Project. Free download from gw-project.org. ©The Authors.

5 Saturated Steady Flow at the Field Scale

On a field scale, hydraulic head can be measured using various types of wells (either installed separately, in a spatially clustered arrangement, or at the same horizontal location but at varying depths). Groundwater wells are essentially large-scale versions of the laboratory-scale piezometers used to illustrate Darcy’s law in preceding sections. The following presents the relationship of hydraulic head and flow in three basic formation types (confined aquifer, aquitard, unconfined aquifer) in the saturated zone, and Figure 19 provides a schematic overview.

A confined aquifer is bounded above and below by an aquitard, a semi-pervious formation that restricts flow between overlying and/or underlying aquifers. Interpolation of water levels measured in wells screened in a confined aquifer defines the potentiometric surface.

An unconfined aquifer is bounded only at its base by an aquitard. The top of the unconfined aquifer is the water table, which equals the observed water level in a screened well just deep enough to encounter standing water.

5.1 Hydraulics of Flow in Confined Aquifers

The experimental setups used to illustrate head and flow resemble conditions in a confined aquifer, because low-K confining layers bound it. When those layers have much lower hydraulic conductivity than the aquifer, they behave nearly as no-flow boundaries (as in the earlier experiments), although under most natural conditions some cross-flow through aquitards can still occur where gradients exist (Darcy’s law).

Central to confined systems is the potentiometric surface, an imaginary surface (Figure 20a) represented with equipotential contours in plan view (Figure 20b) and as a line in cross section (Figure 20c). Section 5.3 discusses aquitard effects in more detail.