PUB00012

• Year: 1999
• Title: Sustainability of Ground-Water Resources: U.S. Geological Survey Circular 1186
• CWCB Section: Interstate & Federal
• Author: William M. Alley, Thomas E. Reilly, O. Lehn Franke
• Description: Illustration of the hydrologic, geologic, and ecological concepts to assure wise and sustainable use of ground-water resources
• Publications - Doc Type: Historical

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Velocities of ground-water flow generally are
low and are orders of magnitude less than
velocities of streamflow. The movement
of ground water normally occurs as slow
seepage through the pore spaces between
particles of unconsolidated earth materials
or through networks of fractures and solu-
tion openings in consolidated rocks. A
velocity of 1 foot per day or greater is a high
rate of movement for ground water, and
ground-water velocities can be as low as
1 foot per year or 1 foot per decade. In
contrast, velocities of streamflow generally
are measured in feet per second. A velocity
of 1 foot per second equals about 16 miles
per day. The low velocities of ground-water

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figure.5. A local scale ground-water-flow system.

In this loCal scale ground-water-flow system,
inflow of water from areal recharge occurs at the water
table. Outflow of water occurs as (1) discharge to the
atmosphere as ground-water evapotranspiration (tran-
spiration by vegetafion rooted at or near the water table
Or direct evaporation from the water table when it is at
or cIoseto the land surface) and (2) discharge of ground
water directly through the streambed, Short, shallow
flow paths originate at the water table near the stream.
As distance from the stream increases, flow paths to the
stream are longer and deeper. For lcmg-tenn average
conditions, inflow to this natural ground-water system
must equal outflow.

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flow can have important implications,

particularly in relation to the movement

of contaminants.

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Under natural conditions, gro~d water
moves along flow paths from areas of
recharge to areas of discharge at springs
or along streams, lakes, and wetlands.
Discharge also occurs as seepage to bays
or the ocean in coastal areas, and as transpi-
ration by plants whose roots extend to near
the water table. The three-dimensional body
of earth material saturated with moving
ground water that extends from areas of
recharge to areas of discharge is referred to
as a ground-water-flow system (Figure 5).

Transpiration
by vegetation
In

Water table

Unconfined aquifer

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EXPLANATION
o High hydraunc-conductivlty aquifer

~"",:~-~i Low hydraullc-conductivlty confining unit

Q Very low hydraulic--conductlvl1y ~rock

-+- Direction of ground-water flow

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