217
Swamp of Virginia and North Carolina (Oaks and
Whitehead 1979). Some scientists also consider
them as factors contributing to altered global
carbon biogeochemistry (Naiman 1988; Johnston
and Naiman 1990; Johnston 1994). Further,
muskrats burrow through wetlands. They are
believed to change flow patterns. Sometimes,
they play a role in changing the water level
directly. They are said to harvest a large amount
of emergent vegetation for their food and to build
winter lodges; thereby, they open up large areas
of marshes. In addition to the above, the geese,
especially the Canada geese (Branta canadensis)
and several varieties of snow geese (Chen spp.),
cause ‘eat-outs’, or major wetland vegetation
removal by herbivory, in many parts of the world
(Middleton 1999). However, the newly planted
wetlands are particularly susceptible to Canada
geese eat-outs in North America. These herbivores are said to re-set the successional status of
the wetlands by removing the vegetation cover.
In this way, they have a major impact on wetland
hydrology.
Notwithstanding the above, the American alligator (Alligator mississippiensis) is quite known
for its role in the Florida Everglades in constructing ‘gator holes’ which serve as oases for fish,
turtles, snails, etc., during the dry season.
14.3 Studies on Wetland
Hydrology
Many earlier investigations on wetlands had
dealt mainly with the relationships between
hydrologic variables (usually water depth) and
wetland productivity and species composition.
There had been many publications on the hydrology of wetlands (Bay 1967, 1969; Siegel 1983,
1988; Glaser 1997; Ford and Bedford 1987;
Doss 1993; Lide et al. 1995; and so on) But there
had not been much studies describing the hydrologic characteristics within specific wetland
types. However, a wealth of literature exists on
the works done in many countries which include
the former USSR, the British Isles and the North
America. Further, some of the significant hydrological studies in other types of wetlands in the
USA include the works done in salt marshes in
New England (Hemond and Fifield 1982),
Cypress swamps in Florida (Heimburg 1984)
and large-scale wetland complexes at the
Okefenokee Swamp in Georgia (Rykiel 1984).
Nevertheless, a major contribution to the understanding of wetland hydrology had resulted
from the papers published from the outcome of
an International Symposium on Wetland
hydrogeology held at the 28th International
Geological Congress in Washington D.C. in
1989 (Winter and Llamas 1993).
14.4 Wetland Hydro-period
The seasonal pattern of the water level of a wetland is called its ‘hydro-period’. It is like a hydrologic signature of each wetland type. It defines
the rise and fall of a wetland’s surface and subsurface water. It characterises each type of wetland, and the constancy of its pattern from year to
year ensures a reasonable stability for that wetland. The hydro-period is an integration of all
inflows and outflows of water, as influenced by
physical features of the terrain and by proximity
to other bodies of water. In the case of temporarily flooded wetlands, the amount of time that a
wetland is in standing water is called the ‘flood
duration’. The average number of times a wetland is flooded in a given period of time is called
the ‘flood frequency’. Both these terms are used
to describe periodically flooded wetlands, such as
the riparian wetlands.
Some of the most seasonally variable wetlands are the vernal pools of central California,
where surface water essentially disappears in the
Mediterranean type of climate for all but 4 or
5 months. The Cypress domes in central Florida
have standing water during the wet summer season and dry periods in the late autumn and early
spring. Low-order riverine wetlands, such as the
alluvial swamps in the southeast USA are said to
respond to local rainfall events rather than to general seasonal patterns. Further, the hydro-periods
of many bottom land hardwood forests and
swamps in colder climates have distinct periods
of surface flooding in the winter and early spring
14.4 Wetland Hydro-period
Swamp of Virginia and North Carolina (Oaks and
Whitehead 1979). Some scientists also consider
them as factors contributing to altered global
carbon biogeochemistry (Naiman 1988; Johnston
and Naiman 1990; Johnston 1994). Further,
muskrats burrow through wetlands. They are
believed to change flow patterns. Sometimes,
they play a role in changing the water level
directly. They are said to harvest a large amount
of emergent vegetation for their food and to build
winter lodges; thereby, they open up large areas
of marshes. In addition to the above, the geese,
especially the Canada geese (Branta canadensis)
and several varieties of snow geese (Chen spp.),
cause ‘eat-outs’, or major wetland vegetation
removal by herbivory, in many parts of the world
(Middleton 1999). However, the newly planted
wetlands are particularly susceptible to Canada
geese eat-outs in North America. These herbivores are said to re-set the successional status of
the wetlands by removing the vegetation cover.
In this way, they have a major impact on wetland
hydrology.
Notwithstanding the above, the American alligator (Alligator mississippiensis) is quite known
for its role in the Florida Everglades in constructing ‘gator holes’ which serve as oases for fish,
turtles, snails, etc., during the dry season.
14.3 Studies on Wetland
Hydrology
Many earlier investigations on wetlands had
dealt mainly with the relationships between
hydrologic variables (usually water depth) and
wetland productivity and species composition.
There had been many publications on the hydrology of wetlands (Bay 1967, 1969; Siegel 1983,
1988; Glaser 1997; Ford and Bedford 1987;
Doss 1993; Lide et al. 1995; and so on) But there
had not been much studies describing the hydrologic characteristics within specific wetland
types. However, a wealth of literature exists on
the works done in many countries which include
the former USSR, the British Isles and the North
America. Further, some of the significant hydrological studies in other types of wetlands in the
USA include the works done in salt marshes in
New England (Hemond and Fifield 1982),
Cypress swamps in Florida (Heimburg 1984)
and large-scale wetland complexes at the
Okefenokee Swamp in Georgia (Rykiel 1984).
Nevertheless, a major contribution to the understanding of wetland hydrology had resulted
from the papers published from the outcome of
an International Symposium on Wetland
hydrogeology held at the 28th International
Geological Congress in Washington D.C. in
1989 (Winter and Llamas 1993).
14.4 Wetland Hydro-period
The seasonal pattern of the water level of a wetland is called its ‘hydro-period’. It is like a hydrologic signature of each wetland type. It defines
the rise and fall of a wetland’s surface and subsurface water. It characterises each type of wetland, and the constancy of its pattern from year to
year ensures a reasonable stability for that wetland. The hydro-period is an integration of all
inflows and outflows of water, as influenced by
physical features of the terrain and by proximity
to other bodies of water. In the case of temporarily flooded wetlands, the amount of time that a
wetland is in standing water is called the ‘flood
duration’. The average number of times a wetland is flooded in a given period of time is called
the ‘flood frequency’. Both these terms are used
to describe periodically flooded wetlands, such as
the riparian wetlands.
Some of the most seasonally variable wetlands are the vernal pools of central California,
where surface water essentially disappears in the
Mediterranean type of climate for all but 4 or
5 months. The Cypress domes in central Florida
have standing water during the wet summer season and dry periods in the late autumn and early
spring. Low-order riverine wetlands, such as the
alluvial swamps in the southeast USA are said to
respond to local rainfall events rather than to general seasonal patterns. Further, the hydro-periods
of many bottom land hardwood forests and
swamps in colder climates have distinct periods
of surface flooding in the winter and early spring
14.4 Wetland Hydro-period