Environmental Engineering Reference
In-Depth Information
low hydrologic throughput
(ombrotrophic).
Compared with lakes and
streams, water loss by plant transpiration is usually more important
to the hydrology of wetlands, which tend to be shallow and densely
vegetated. Tidal action can influence wetland hydrology where wetlands
are contiguous with the ocean, as in coastal estuaries and mangrove
swamps.
Climate interacts with hydrology to constrain hydrodynamics and the
plants found in wetlands. Given the hydrogeomorphic properties, wetlands
can then be classified by function (Table 4.5). Those wetlands that are fed
by constant flows of groundwater may be influenced little by seasonal fac-
tors that control surface water flows. At the other extreme, seasonal wet-
lands, such as playas or riparian wetlands,
can fill during wet seasons and remain dry
throughout the rest of the year.
Wetlands can be further categorized by
nutrient input (eutrophic or oligotrophic),
salinity, pH, and other water chemistry. Be-
cause of the variety of hydrogeomorpholgy,
climate, and other factors, vegetation can
range from dense forest to tundra or from
macrophytes to trees (Table 4.4). A variety
of different subhabitats also occur within
each wetland, depending on the degree and
duration of inundation and the water depth
(Fig. 4.13).
Wetlands may be strongly influenced by
global change because the water level in many
wetlands is highly sensitive to changes in rates
of precipitation and evapotranspiration. Such
changes may directly affect input and output
of water, or they may indirectly affect wet-
land water levels by altering the height of the
groundwater table. A warmer climate will
produce higher evapotranspiration rates,
which can lower water levels even if precipi-
tation rates remain constant. Given the com-
plex and variable hydraulic characteristics
across the different types of wetlands, the
magnitude and direction of the changes in
these habitats are not easy to predict.
In addition to evapotranspiration, biotic
factors may also influence wetland hydrol-
ogy. Human activities have strong influences
on wetland hydrology. A large-scale exam-
ple of this is the effort to manage the Ever-
glades in Florida (Sidebar 4.3). Other factors
that may be important include beavers, alli-
gators, and other large freshwater verte-
brates. For example, beavers have altered
the geomorphology of entire valleys (Naiman
1985 United States Food Security Act:
The
term "wetland," except when such term is part
of the term "converted wetland," means land
that (i) has a predominance of hydric soils; (ii)
is inundated or saturated by surface or
groundwater at a frequency and duration suf-
ficient to support a prevalence of hydrophytic
vegetation typically adapted for life in satu-
rated soil conditions; and (iii) under normal cir-
cumstances does support the prevalence of
such vegetation.
1995 Committee on Wetlands Characteriza-
tion, U.S. National Research Council:
A wet-
land is an ecosystem that depends on constant
or recurrent, shallow inundation or saturation
at or near the surface of the substrate. The
minimum essential characteristics of a wet-
land are recurrent, sustained inundation or sat-
uration at or near the surface and the pres-
ence of physical, chemical, and biological
features reflective of recurrent, sustained in-
undation or saturation. Common diagnostic
features of wetlands are hydric soils and hy-
drophytic vegetation. These features will be
present except where specific physicochemi-
cal, biotic, or anthropogenic factors have re-
moved them or prevented their development.
Some other words historically used to de-
lineate wetlands or specific types of wetlands:
The term wetland has only been in regular use
by scientists since the mid-1900s. Terms used
prior to this, or to indicate specific types of
wetlands, include bog, bottomland, fen, marsh,
mire, moor, muskeg, peatland, playa, pothole,
reedswamp, slough, swamp, vernal pool, wet
meadow, and wet prairie.
Search WWH ::
Custom Search