GROUNDWATER SUPPLIES
Groundwater is both an important direct source of supply that is tapped by wells and
a significant indirect source, since surface streams are often supplied by subterranean
water.
Near the surface of the earth, in the zone of aeration, soil pore spaces contain both
air and water. This zone, which may have zero thickness in swamplands and be several
hundred feet thick in mountainous regions, contains three types of moisture. After a
storm, gravity water is in transit through the larger soil pore spaces. Capillary water is
drawn through small pore spaces by capillary action and is available for plant uptake.
Hygroscopic moisture is water held in place by molecular forces during all except the
driest climatic conditions. Moisture from the zone of aeration cannot be tapped as a
water supply source.
Water Supply  109
In the zone of saturation, located below the zone of aeration, the soil pores are
filled with water,  and this is what we call groundwater. A stratum that contains a
substantial amount of groundwater is called an aquifer. At the surface between the
two zones, called the water table or phreatic sugace, the hydrostatic pressure in the
groundwater is equal to the atmospheric pressure. An aquifer may extend to great
depths, but because the weight of overburden material generally closes pore spaces,
little water is found at depths greater than 600 m (2000 ft).
The amount of water that can be stored in the aquifer is the volume of the void
spaces between the soil grains. The fraction of voids volume to total volume of the soil
is termed porosity, so that
Porosity = (Volume of voids)/(Total volume).  (6.1)
However, not all of this water is available because it is so tightly tied to the soil particles.
The amount of water that can be extracted is known as speciftc yield, defined as the
percent of total volume of water in the aquifer that will drain freely from the aquifer.
The flow of water out of a soil is illustrated in Fig. 6-2 and analyzed using the
continuity equation, as