WETLANDS - Water Quality Engineering in Natural Systems - page 291

conditions. The balance between inflows and outflows
defines the water budget of the wetland, whereas the
surface contours and subsurface conditions define the
capacity of the wetland to store water. For wetlands that
are not subtidal or permanently flooded, the amount of
time that a wetland contains standing water is called the
flood duration , and the average number of times that a
wetland is flooded in a given period is known as the
flood frequency . Each depth of inundation has a range
of possible flood durations, and each duration has a cor-
responding flood frequency.
The hydrologic budget of a wetland is given by the
relation

the permeability of the soils, which is partially affected
by vegetation and soil type. The drawdown of the water
table caused by urbanization of nearby areas can have
a deleterious effect on wetland function. The effects of
water table drawdown on wetlands include reduced
hydroperiod, lower water levels, a shift to drier, nonwet-
land plant species, death of animal species, loss of fish
and amphipods, reduced use by birds and wildlife, and
increased fire damage.

8.2.3.3 Evapotranspiration. Evapotranspiration typ-
ically accounts for loss of water ranging from 20% to
80% of the annual water budget in most wetland systems
(Mitsch and Gosselink, 2007). ET plays a role in the
attenuation of floodwaters, as well as maintenance of
the soil-water redox conditions in a wetland. Changing
the water input, loss of plants, and changes in soil condi-
tions will affect ET and in turn change the functioning
of the wetland. Transpiration can exceed open-water
evaporation losses in prairie pothole systems (LaBaugh
et al., 1998).
As a general rule, natural wetlands are more preva-
lent in cool or wet climates than in hot or dry climates,
and steep terrain tends to have fewer wetlands than
gently sloping landscapes. Wetlands occur most exten-
sively in regions where precipitation is in excess of
losses, such as ET and surface runoff. The approximate
dividing line between precipitation excess in the eastern
United States and precipitation deficit in the western
United States is the Mississippi river.

∆
∆

V
t

n ET

(8.1)

= P S G + + −

n n

where Δ V is the volume of water added to the wetland
in time increment Δ t , P n is the incremental net precipita-
tion, S n is the incremental net surface water inflow,
including flooding streams, G n is the incremental net
groundwater inflow, and ET is the incremental evapo-
transpiration (ET). The incremental net precipitation is
equal to the incremental precipitation minus the amount
of this precipitation intercepted by the vegetation. The
components of the wetland water budget are discussed
below.

8.2.3.1 Net Surface Water Inlow. Wetlands receive
surface inflows in many forms. Overland flow is a non-
channelized sheet flow that usually occurs during and
immediately following rainfall or a spring thaw, or as
tides rise in coastal wetlands. Channelization of flow,
which is usually associated with urbanization, has a sig-
nificant impact on the functioning wetlands with signifi-
cant overland inflow. In addition, roads can block or
severely alter the outflow dynamics of the system, and
increased flow rates through culverts can be a major
impediment to wetland functioning, as well as fish
migration. A special case of surface flow occurs in ripar-
ian wetlands that are in floodplains adjacent to rivers or
streams and are occasionally flooded by those rivers or
streams. Examples of riparian wetlands are the delta
marshes of the Mississippi river and the hardwood
swamps of the southeastern United States. Some wet-
lands can be more isolated, receiving only intermittent
surface water input. These systems include vernal pools
in California, prairie pothole wetlands in the midwest,
and playa lakes in the southern high plains.

8.3 CONSTRUCTED TREATMENT WETLANDS

Constructed treatment wetlands are engineered systems
that utilize natural processes involving wetland vegeta-
tion, soils, and their associated microbial assemblages to
assist, at least partly, in treating a wastewater effluent or
other water source (U.S. Environmental Protection
Agency [USEPA], 2000a). The primary goal of con-
structed treatment wetlands is to improve water quality.
Constructed treatment wetlands are used to treat waste-
water from municipal and industrial sources, as well as
contaminated water from agricultural and stormwater
sources. The advantages of using constructed treatment
wetlands include low construction and operating costs,
and they are appropriate for small communities and as
a final-stage treatment in large municipal systems. Con-
structed wetlands for treating municipal wastewater are
an excellent option for small communities where inex-
pensive land is available and skilled operators are hard
to find. In arid regions and communities reaching their
limits of water availability, water reuse using these
systems is an attractive option that can help achieve

8.2.3.2 Net Groundwater Inlow. Wetlands can
recharge groundwater or can be located in areas where
groundwater is discharged to the wetland. Movement of
groundwater into or out of a wetland is a function of