The ground-water system as a whole is actually a three-dimensional flow field; therefore, it is very important understand how the vertical components of ground-water movement affect the interaction of ground water and surface water. A vertical section of a flow field implies the way potential energy is distributed beneath the water table in the ground-water process and how the energy submission can be used to determine vertical components of flow near a surface-water body. The term hydraulic head, that could be the sum of elevation and water pressure classified by the weight density of water, is used to describe potential energy in ground-water flow systems.
GROUND-WATER DISCHARGE: The quantity of ground-water discharge (flux) to and from surface-water bodies can be determined for a known cross section of aquifer by multiplying the hydraulic gradient, which is determined from the hydraulic-head measurements in wells and piezometers, by the permeability of the aquifer materials. Permeability is a quantitative measure of the ease of water movement through aquifer materials. For example, sand is more permeable than clay because the pore spaces between sand grains are larger than pore spaces between clay particles.