Geoscience Reference
In-Depth Information
Movement and Storage
of Groundwater
for plant absorption (Figure 15.2). Recall that the
wilting point
is the threshold at which, for any given soil, water is no longer
available for plant uptake. With increasing precipitation, the
pore spaces begin to fill with
capillary water
, which is water
held to soil particles by molecular attraction. Some of this water
is lost to the atmosphere through evaporation, but plants absorb
a great deal of it before the water is returned to the air through
the process of transpiration (see Figure 15.1). The combined
processes of evaporation and transpiration are often referred to
as
evapotranspiration
.
If precipitation continues to the point where soil pore
spaces become completely filled with water, the soil is consid-
ered to be at its
ield capacity
. Any excess water that does not
run off the surface is then free to move more deeply down into
the sediment or rock, under the force of gravity, as gravitational
water. As this water moves downward, it first passes through
an area called the
unsaturated zone
(see Figure 15.1 again),
which is the portion of the rock or sediment mass where pore
spaces may be partially filled with water, but also contain air
(i.e., oxygen and other gases).
As water continues to percolate downward, it will eventu-
ally reach a depth where pore spaces are entirely filled with
water and will stop moving. This part of the groundwater sys-
The logical place to begin a discussion of groundwater is with
a brief review of the hydrologic cycle presented in Chapter 7.
If you recall, the hydrologic cycle refers to the way water is
stored in various reservoirs (such as rivers) and how it moves
to other places (like the oceans). Earth's groundwater system
is also part of the hydrologic cycle because it is a place where
water can move to, be stored for a period of time, and flow to
some other reservoir.
In this context, it is time to examine the various parts of the
groundwater system and establish some fundamental terminol-
ogy. Start by considering what happens to water after it falls
to Earth as some form of precipitation. In most circumstances,
this water is absorbed by the soil, where it is stored in the
soil
-
water belt
(Figure 15.1). Remember from Chapter 11 that water
is absorbed into the soil by infiltration, which occurs through
pore spaces between sediment grains (Figures 11.3 and 11.4),
along pathways associated with the soil structure, or down plant
roots. When the soil is very dry, water is held tightly to sedi-
ment grains as
hygroscopic water
that is largely unavailable
Precipitation
Transpiration
(from plants)
Evaporation
(from land, water)
Overland runoff
to streams
Soil-water belt
Movement of
gravitational
water
Unsaturated zone
Water table
Groundwater zone
Saturated zone
Infiltration
Percolation
Figure 15.1 The groundwater model.
Precipitation is initially stored in the soil-water belt until it be-
comes saturated. After that occurs, water can either run off the surface in streams or percolate through
the unsaturated zone to the saturated zone.
Hygroscopic water
Soil water held so tightly by sediment
grains that it is unavailable for plant use.
Unsaturated zone
The area between the soil-water belt and
the water table where pore spaces are not saturated with water.
Field capacity
The maximum amount of water the soil can
hold after gravitational water has moved away.
