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in the range of 0.00005 to 0.005, indicating that large pressure changes over
extensive areas are required to produce substantial water yields.
Storage co-efficient for an unconfined aquifer corresponds to its specific yield.
Movement of Groundwater and Darcy's law :
Groundwater in the upper layers is dynamic and moves from high potential (or
higher head) to lower potential (low head) through the porous medium of sand.
Darcy has shown that the flow of water through sand follows the property of
laminar flow and the rate of flow through the porous medium of sand is
proportional to the head loss and inversely proportional to the length of the
flow path. This is known as Darcy's law and is an established tool in the
study of hydraulics. Commonly Darcy's law can be expressed as :
[Source : Karanch, 2004]
Q = K(h
1
- h
2
) A
l
Where Q = Discharge in m
3
/day
K = Hydraulic conductivity in m
3
/day/m
2
or (m / day)
h1 - h2 = Head loss in meters over a distance of l, also in meter, in the
flow direction.
A = Cross sectional area of the porous medium through flow
occurs, in square metres, measured at right angles to the
direction of flow.
Sometimes negative (-) sign is used on the right hand side to show the flow of
water from high head to low head.
The expression Q/A represents velocity, but the velocity of groundwater through
the porous medium is very sluggish. Moreover, the water through the sand/
aquifers flow depending upon the pore size, grain distribution of sand etc. and
the expression Q/A for the aquifer is generally known as Darcy's velocity i.e.
V = Q = -K dh
A
dl
