b. The steps in the hydrologic cycle include precipitation, evaporation,
transpiration, infiltration-percolation, and runoff. Water is placed in the air by
evaporation from water and land surfaces and by transpiration from plants. It is then
condensed to produce cloud formations and returned to earth as rain, snow, sleet, or
hail. A portion of the precipitation evaporates, some flow over the earth as run off into
lakes and streams, and the remainder goes into the soil and thence into underlying rock
formations by percolation (also called infiltration or seepage). Eventually, some of the
water that has seeped through the earth will find its way to the surface through springs,
or wiII flow through porous media until intercepted by streams, lake, or the ocean.
c. The cycle does not always progress through a regular sequence. Steps may
be omitted or repeated at any point. For example, precipitation in hot climates may be
almost wholly evaporated and returned to the atmosphere. In such an instance, the
steps of infiltration, transpiration, and runoff are omitted. A large portion of the earth's
fresh water supply is trapped below the surface and never gets into the hydrologic
As shown in figure 1-1 part a, the surface of the earth is divided into three zones,
or layers: the zone of aeration; the zone of saturation; and the impervious zone.
a. Zone of Aeration. The zone of aeration is the uppermost soil layer. It is
within this zone that the roots of plants are found. Water that does not run off into
streams and lakes percolates through this earth layer.
b. Zone of Saturation. The zone of saturation is a pervious layer of sand,
gravel, or rock that is capable of becoming saturated and storing water. Such soil
formations are not always present. When they are present and contain usable
quantities of water, they are spoken of as aquifers.
c. Impervious Zone. The impervious zone is a layer of rock that is so dense
that no water wiII percolate through it.
d. Water-Table. The upper level of the ground water, or zone of saturation, is
known as the water table. The depth of the water table depends upon the particular soil
formation. When a depression in the surface falls below the water table, the result is a
spring, swamp, or lake. Ground water, following the law of gravity and hydraulic
principles, is constantly seeking a lower level. Sometimes a layer or several layers of
impervious earth or rock extend to below the general level of the water table and
confines a zone of saturation, or aquifer, below it (see figure 1-1 part b). This water
percolates into the aquifer in a lateral direction through the water bearing strata rather
than vertically. Since it is at a lower elevation than the water table, it is under pressure.
This pressure is referred to as artesian pressure. When a well is dug or drilled into an