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Darcy's law is valid in case of laminar flow. When the flow is turbulent, Reynold's
number, NR, or dimensionless number is used to show the turbulence. It is
observed that Darcy's law is valid when NR < 1, and in certain cases do not
deviate much even if when NR = 10
Permeability & Transmissivity :
Permeability, expressed as a co-efficient, was defined by Meinzer (1923, in Todd,
1955) as the rate of flow of water in gallons per day through a cross sectional
area of one square foot under a hydraulic gradient of one foot per foot at a
temperature of 600 F. When permeability is determined in the field, adjustment
to the standard temperature is not made. Permeability is then understood to be
a field co-efficient at the prevailing temperature of water. In recent years, the
term hydraulic conductivity is being widely used in preference to co-efficient of
permeability. Permeability has got the units of velocity i.e. m/day.
Transmissivity
The overall capacity of an aquifer to transmit groundwater is dependent on the
thickness and hydraulic conductivities of the component parts of the aquifer.
(Theis (1935) (as in Todd) introduced the term co-efficient of transmissibility,
which was expressed as the rate of flow of water at the prevailing water
temperature, in gallons per day, though a vertical strip of the aquifer under the
hydraulic gradient of 100 per cent. Currently, in wide usage is the term
transmissivity which is the rate at which water of the prevailing kinematics
viscosity is transmitted through a unit width of the aquifer under a unit hydraulic
gradient (Lohman et al. 1972, as in Todd, 1955). From transmissivity (T),
average hydraulic conductivity (K) is obtained :
K = T/b [when b = thickness of the aquifer]
From these parameters, groundwater flow under a uniform hydraulic gradient
through a confined aquifer of constant thickness may be given as :-
Q = TIL
Where Q = Groundwater flow in m3 / day
T = Transmissitivity (Kb) in m2 / day
L = Length of the aquifer along the equipotential linc, in metres,
through which the discharge occurs.
I
= Hydraulic gradient
