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An aquifer is a permeable geological formation containing and transmitting groundwater. A confined aquifer lies beneath a confining layer; an unconfined aquifer instead has a free water table.

The Dupuit approximation treats flow in a shallow unconfined aquifer as predominantly horizontal, with hydrostatic pressure $p=\rho g(h-z)$ and horizontal pressure gradient independent of depth. Integrating Darcy's law over the saturated depth then gives a volume flux per unit width depending only on the groundwater height and its slope.

The water table is the upper surface of the saturated zone in an unconfined aquifer, where liquid pressure equals atmospheric pressure in the simplest drainage model. Under the Dupuit approximation, its height determines hydrostatic pressure throughout the saturated region and therefore the lateral Darcy flux.