aquifer"; Section 4.3.3 discusses methods for determining ground water flow direction and
hydraulic gradient; Section 4.3.4 discusses methods for determining hydraulic conductivity;
and Section 4.3.5 discusses determining ground water flow rate. The special case of ground 
water flow in aquifers dominated by conduit flow is discussed in Sections 4.3.1 and 5.2.
Most of the discussions provided in Sections 4.3.3, 4.3.4, and 4.3.5 do not apply to this
special case.
Conventional ground water hydrology considers aquifers to be porous granular media
(either unconsolidated granular deposits or rock) having a well defined water table or
potentiometric surface. The flow of ground water in these types of aquifers is described by
Darcy's law:
Q =  KiA
Q =
quantity of flow per unit of time, in (volume/time)
K =
hydraulic conductivity, in (length/time)
i =
hydraulic gradient, in (length/length)
A =
cross sectional area through which the flow occurs, in (length
Darcy's law assumes laminar flow of individual particles of water moving parallel to
the direction of flow, with no mixing or transverse component in their motion. The right 
hand side of the Darcy equation is preceded by a negative sign because ground water flows
from high head to low head.
There are two types of ground water systems where the relationship expressed by
Darcy's law does not apply. These are systems where ground water flows through materials
with low hydraulic conductivities under extremely low gradients, and systems in which a
large amount of flow passes through materials with very high hydraulic conductivities
(turbulent flow). These two situations can be considered, respectively, as the lower and upper
limits of the validity of Darcy's law (Freeze and Cherry, 1979).
The Reynolds number (the ratio of inertial to viscous fluid forces) is a dimensionless
number used to define the limits of the validity of Darcy's law. The Reynolds number (R ) is
defined as:
v d
R =
November 1992
4 36






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