Groundwater flow

Groundwater flow is the movement of water through saturated soil and permeable rock, driven mainly by differences in hydraulic head.

Darcy's law: An empirical law relating fluid flow through porous media to hydraulic gradient and material permeability. It quantifies how hydraulic gradients drive flow through saturated ground.
Aquifer: A permeable geological formation that stores and transmits usable quantities of groundwater. Aquifers are the principal formations through which groundwater flows.
Groundwater recharge: The process by which water enters an aquifer from the land surface or adjacent formations. Recharge supplies water that subsequently moves through aquifers.
Unsaturated flow: Water movement through soil whose pores contain both air and water. Unlike saturated flow, its pathways and conductivity change with moisture content.
Hydraulic head: The elevation-related energy per unit weight of groundwater, combining elevation and pressure effects. Differences in head provide the principal force moving groundwater.
Aquiclude: A geological unit that stores water but transmits it very slowly. Its low permeability restricts flow between adjoining aquifers.
Groundwater discharge: The movement of groundwater out of an aquifer into springs, surface waters, or other outlets. Discharge marks where subsurface flow returns to the surface or leaves an aquifer.
Darcy flow: Laminar fluid movement through porous media that follows Darcy's law. It describes the regime assumed in most groundwater-flow calculations.
Hydraulic conductivity: A measure of how readily a porous material transmits water under a hydraulic gradient. It controls how rapidly water moves through soil and rock.
Water table: The surface below which ground is saturated with water at atmospheric pressure. Its shape indicates the upper boundary and gradient of unconfined groundwater.

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