Physics for Geologists, Second edition

Fluids and fluid flow 129
the liquid at the point where the pressure p was measured. h = z + p/pg (cf.
Potential).
Static head: the pressure head in a borehole that is not producing (cf. Static
level).
Hydraulic conductivity See Permeability.
Hydraulic radius A measure of the size and shape of the space through which
liquid flows, it is the volume of liquid divided by its wetted surface area.
Dimension [L].
Hydraulic gradient The difference of total head divided by the macroscopic
length of porous material between the points where the total head is mea-
sured. Ah11 (dimensionless). Properly it is the steepest gradient at a point in
the fluid. Note that the gradient of a potentiometric surface is not strictly the
hydraulic gradient unless the aquifer is horizontal (but it may be a sufficiently
close approximation). Hydraulic gradient is not the same thing as pressure
gradient.
Permeability The quality of rocks that allows the passage of a fluid; a measure
of the ease of passage of fluids. There are two measures of permeability:
Coefficient of Permeability or hydraulic conductivity: The coefficient K in
Darcy's law when written q = KAhIl. It takes into account both the
fluid properties and the properties of the permeable medium. Dimensions of
a velocity, LT-'.
Intrinsic permeability: A measure of the material properties allowing the flow
of fluids, usual symbol, k. Dimensions of an area, L2. The coefficient of
permeability is related to intrinsic permeability by K = kpglq.
Potential The energy per unit of mass of the liquid at a point where the
pressure p is measured. = gh = gz + p/p. Dimensions, L2T-2.
Potential gradient: The loss of energy per unit of length along the macro-
scopic flow path, gAh/l. Dimensions of an acceleration, LT-2 (cf. Hydraulic
gradient).
Potentiometric surface The surface obtained by mapping the total head of
an aquifer. The potentiometric surface can be contoured (equipotential lines)
and fluid flow is in the direction normal to these contours, down the slope
of the surface in the direction of decreasing total head or potential. The
synonym piezometric surface is not to be preferred because it is a matter of
potential, not pressure.
Static level The level of the water in a well that is not producing, and has
stabilized. This level may be given relative to the ground surface or the bottom
of the well; but if given relative to some horizontal datum surface becomes
synonymous with total head.
Water table In practice, the free water surface of an unconfined aquifer in
a well. Strictly, it is the level in the zone of saturation at which the pressure
is atmospheric, that is, water raised in rocks by capillary action is ignored.
Copyright 2002 by Richard E. Chapman