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homogeneous porous medium). Another approach is to use a flow net (described earlier) to graphi-
cally solve Laplace's equation for relatively simple cases. More complex cases, however, have to be
solved mathematically, most commonly with computerized groundwater modeling programs. The
most popular of these programs is MODFLOW-2000, published by the USGS.
21.4.1 s teady F loW in a C onFined a quiFer
If steady movement of groundwater in a confined aquifer and the hydraulic heads do not change
over time, we can use another derivation of Darcy's law directly to determine how much water is
flowing through a unit width of aquifer, using the following equation:
dh
dl
q
′ =−
b
(21.7)
where
q ′ = Flow per unit width (L2/T). 2 /T).
K = Hydraulic conductivity (L/T).
b = Aquifer thickness (L).
dh / dl = Hydraulic gradient (dimensionless).
21.4.2 s teady F loW in an u nConFined a quiFer
Steady flow of water through an unconfined aquifer can be described by
2
2
1
2
hh
L
1
2
q
′ =−
K
(21.8)
where h 1 and h 2 are the water level at two points of interest, and L is the distance between these two
points. This equation states that the hydraulic gradient is equal to the slope of the water table; the
streamlines are horizontal and the equipotential lines are vertical. This equation is useful, particu-
larly in field evaluations of the hydraulic characteristics of aquifer materials.
REFERENCES AND RECOMMENDED READING
Baron, D. (2003). Water: California's Precious Resource . California State University, Bakersfield.
Davis, M.L. and Cornwell, D.A. (1985). Introduction to Environmental Engineering , 2nd ed. McGraw-Hill,
New York.
Fetter, C.W. (1994). Applied Hydrology , 3rd ed. Prentice-Hall, New York.
Gupta, R.S. (1997). Environmental Engineering and Science: An Introduction . Government Institutes,
Rockville, MD.
Odum, E.P. (1993). Ecology and Our Endangered Life Support Systems . Sinauer Associates, Sunderland, MA.
USEPA. (1994). Handbook: Ground Water and Wellhead Protection , EPA/625/R-94/001. U.S. Environmental
Protection Agency, Washington, DC.
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