draft and the maximum size of the reservoir needed to provide this draft. One other
useful bit of information can be derived from this figure. If a line is drawn parallel to
the chosen draft, but tangent at the lowest point of the drought and extended
backward in time until it again meets the runoff curve, this will indicate the time
at which the reservoir would have had to have been built in order for it to have filled
prior to the onset of the design drought. Thus it may be seen that this figure
represents a simplified balance between stream runoff as it varies, the amount of
withdrawal from the stream, and the storage that must be provided prior to the
withdrawal exceeding the stream flow. It must be pointed out that the demand curve
does not have to be a constant value; however, at no time can the total demand
exceed the sum of the inflow and the storage available.
9 Ground Water
Ground water represents a significant source of water supply. In the order of
30 billion gal/d (110 million m
3 /d) of ground water is used in the USA. It is the
prime source of water for drinking in rural areas. Many medium-sized cities and
numerous industries use ground water as their source of supply for drinking,
processing, and cooling. One principal advantage of ground water is its relatively
cool and constant temperature. Furthermore, the soil acts as a filter, removing
particulates including bacteria. Thus, under most circumstances, ground water is
not subject to contamination as is surface water. Potential sources of contamination
include industrial discharges, landfills, and chemical dump sites. These are discussed
in more detail in a later section.
Ground water results from the infiltration of precipitation into the soil. The water
will proceed downward vertically until it reaches the first confining layer. Above this
layer there is a saturated layer of soil containing the ground water. The upper edge of
this saturated area is called the ground water table. This will vary with the amount of
precipitation and the time since the last precipitation. There is flow in this saturated
area in a downhill gradient. At some point the water in the saturated zone may flow
under a confining zone. This now produces a confined aquifer. An aquifer is defined
as a geologic formation that contains water and transmits it from one point to another
in quantities sufficient to permit economic development. This usually defines a sand
or gravel soil. An aquiclude is a formation that contains water, but cannot transmit it
rapidly enough to furnish a significant supply. This is usually represented by clay,
which holds much moisture but does not transmit it. An aquifuge is a formation such
as rock that neither holds nor transmits water. There may be many intermittent levels
of aquifers, aquicludes, and aquifuges through the soil. Water may occur at any
depth and there are few locations where there is no water whatsoever beneath the soil
surface. However, in general, as the depth increases, the total dissolved salts
increase, with saline water being found at great depths. Thus, the useable quality
of the water decreases with depth.
266
D. B. Aulenbach et al.
useful bit of information can be derived from this figure. If a line is drawn parallel to
the chosen draft, but tangent at the lowest point of the drought and extended
backward in time until it again meets the runoff curve, this will indicate the time
at which the reservoir would have had to have been built in order for it to have filled
prior to the onset of the design drought. Thus it may be seen that this figure
represents a simplified balance between stream runoff as it varies, the amount of
withdrawal from the stream, and the storage that must be provided prior to the
withdrawal exceeding the stream flow. It must be pointed out that the demand curve
does not have to be a constant value; however, at no time can the total demand
exceed the sum of the inflow and the storage available.
9 Ground Water
Ground water represents a significant source of water supply. In the order of
30 billion gal/d (110 million m
3 /d) of ground water is used in the USA. It is the
prime source of water for drinking in rural areas. Many medium-sized cities and
numerous industries use ground water as their source of supply for drinking,
processing, and cooling. One principal advantage of ground water is its relatively
cool and constant temperature. Furthermore, the soil acts as a filter, removing
particulates including bacteria. Thus, under most circumstances, ground water is
not subject to contamination as is surface water. Potential sources of contamination
include industrial discharges, landfills, and chemical dump sites. These are discussed
in more detail in a later section.
Ground water results from the infiltration of precipitation into the soil. The water
will proceed downward vertically until it reaches the first confining layer. Above this
layer there is a saturated layer of soil containing the ground water. The upper edge of
this saturated area is called the ground water table. This will vary with the amount of
precipitation and the time since the last precipitation. There is flow in this saturated
area in a downhill gradient. At some point the water in the saturated zone may flow
under a confining zone. This now produces a confined aquifer. An aquifer is defined
as a geologic formation that contains water and transmits it from one point to another
in quantities sufficient to permit economic development. This usually defines a sand
or gravel soil. An aquiclude is a formation that contains water, but cannot transmit it
rapidly enough to furnish a significant supply. This is usually represented by clay,
which holds much moisture but does not transmit it. An aquifuge is a formation such
as rock that neither holds nor transmits water. There may be many intermittent levels
of aquifers, aquicludes, and aquifuges through the soil. Water may occur at any
depth and there are few locations where there is no water whatsoever beneath the soil
surface. However, in general, as the depth increases, the total dissolved salts
increase, with saline water being found at great depths. Thus, the useable quality
of the water decreases with depth.
266
D. B. Aulenbach et al.