Environmental Engineering Reference
In-Depth Information
Low temperatures and rates of evapotranspiration characterize polar re-
gions. Thus, wetlands, lakes, and streams can form with moderate amounts
of precipitation.
Human activities can alter global patterns of precipitation and the hy-
drologic cycle in unpredictable ways. As the earth warms in response to
increases of greenhouse gasses (primarily CO
2
), evaporation and precipita-
tion will likely increase worldwide, but variability and distribution will also
change. How such changes will influence local weather patterns is uncertain.
Because freshwater habitats are influenced greatly by the balance between
precipitation and evapotranspiration, predicting the impacts of the green-
house effect and global change on specific habitats is difficult. The strongest
effects will likely occur in areas that are currently arid or where precipita-
tion is equal to or less than potential evapotranspiration (Schaake, 1990).
MOVEMENT THROUGH SOIL AND GROUNDWATER
Water can either flow across the surface of soil
(sheet flow)
or move
down into porous soils
(infiltration).
Sheet flow often ends up directly in
stream channels, whereas infiltration can percolate to groundwater. Several
regions below the surface of the soil that receive infiltration have been de-
scribed (Fig. 4.4). The dry or moist sediments below the surface soil layers
form the
unsaturated zone
(also called the
vadose zone
). The depth of the
unsaturated zone can vary from zero (where groundwater reaches surface
water) to more than 100 m (in some deserts). The
capillary fringe
is the
area where groundwater is drawn up into the pores or spaces in the sedi-
ment by capillary action. This zone is generally 1 m or less above the
wa-
ter table,
which is defined as the top of the region where virtually all of the
pore space is filled with groundwater. Below the water table is the ground-
water habitat. A continuous groundwater system is called an
aquifer;
I use
this definition in the topic, but some authors use the term aquifer only for
FIGURE 4.4
Various subsurface habitats.
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