220
the presence of vegetation could retard evaporation from the water surface. But the question here
is whether the transpiration of water through the
plants equals or exceeds the difference (Kadlec
1989). Studies revealed evaporation from bogs
in Germany to be generally less than that from
open water, except during the summer months
(Eggelsmann 1963). Bay (1967) recorded
88–121 % of open-water evaporation in small
bogs in Minnesota. Further, Eisenlohr (1976) had
reported 10 % lower evapotranspiration from
vegetated Prairie potholes than from nonvegetated ones in North Dakota. It had been estimated by Hall et al. (1972) that a strand of
vegetation in a small New Hampshire wetland
had lost c. 80 % more water than did the open
water in the wetland.
14.6 Specific Effects of Hydrology
on Wetlands
(a) Hydrology may result in a unique vegetation
composition but may limit or enhance species richness.
(b) In wetlands, the primary productivity and
other ecosystem functions are often enhanced
by flowing conditions and a pulsing hydroperiod. They are often depressed by stagnant
conditions.
(c) Aggregation of organic materials in wetlands
is often controlled by hydrology. It happens
through its influence on primary productivity, decomposition and export of particulate
organic matter.
(d) Hydrologic conditions significantly influence
both nutrient cycling and nutrient availability.
14.7 Accumulation of Organic
Matter and their Export
Excess organic matters may be accumulated in
the wetlands. This could be as a result of
increased primary productivity or decreased
decomposition and export. Peat accumulates, to
some extent, in all wetlands due to these processes. In fact, wetlands are the carbon sinks in
the biosphere worldwide. More information in
this aspect could be obtained from Brinson
(1977), Odum and Heywood (1978), Deghi et al.
(1980), etc. Nevertheless, decomposition of
organic detritus generally requires electron
donors (usually oxygen; but alternative chemicals, e.g. sulphate and nitrate, may be effective
under anoxic conditions), moisture, inorganic
nutrients and microbes which are capable of
metabolising in the specific environment concerned. Further, the observed rate of organic
decomposition is also influenced by the ambient
temperature and by the activities of the microdetritivores. However, hydrology, possibly,
modifies many of these variables. Further, the
importance of hydrology in export of organic
carbon is obvious. Wetlands having continuous
inflow and outflow channels may be expected to
portray higher rate of export. Concomitantly, the
riparian wetlands often contribute a large amount
of organic detritus to the streams. Conversely,
the hydrologically isolated wetlands, e.g. the
northern peat lands, have much lower organic
export.
Summary
1. Hydrologic conditions are of paramount importance for the maintenance of a wetland’s structure and function. They have impact on many
abiotic factors, like soil anaerobiosis, nutrient
availability and salinity. These, in turn, play a
key role in developing the biota in a wetland.
Finally, the biotic components are active in altering the wetland hydrology and other physicochemical features and the cycle is completed.
2. The ‘hydro-period’ or hydrologic signature of
a wetland is the result of the balance between
inflows and outflows of water (called the
‘water budget’), the wetland basin geomorphology and sub-surface conditions.
3. The hydrology of a wetland creates the unique
physico-chemical conditions which make such an
ecosystem quite different from both well-defined
terrestrial and deep water aquatic systems.
Hydrologic pathways, such as precipitation, surface run-off, groundwater and flooding rivers,
transport energy and nutrients to and from wetlands. Hydrology also transports sediments,
nutrients, etc., into wetlands. It, thus, further
influences the physico- chemical environment.
14 Wetland Hydrology
the presence of vegetation could retard evaporation from the water surface. But the question here
is whether the transpiration of water through the
plants equals or exceeds the difference (Kadlec
1989). Studies revealed evaporation from bogs
in Germany to be generally less than that from
open water, except during the summer months
(Eggelsmann 1963). Bay (1967) recorded
88–121 % of open-water evaporation in small
bogs in Minnesota. Further, Eisenlohr (1976) had
reported 10 % lower evapotranspiration from
vegetated Prairie potholes than from nonvegetated ones in North Dakota. It had been estimated by Hall et al. (1972) that a strand of
vegetation in a small New Hampshire wetland
had lost c. 80 % more water than did the open
water in the wetland.
14.6 Specific Effects of Hydrology
on Wetlands
(a) Hydrology may result in a unique vegetation
composition but may limit or enhance species richness.
(b) In wetlands, the primary productivity and
other ecosystem functions are often enhanced
by flowing conditions and a pulsing hydroperiod. They are often depressed by stagnant
conditions.
(c) Aggregation of organic materials in wetlands
is often controlled by hydrology. It happens
through its influence on primary productivity, decomposition and export of particulate
organic matter.
(d) Hydrologic conditions significantly influence
both nutrient cycling and nutrient availability.
14.7 Accumulation of Organic
Matter and their Export
Excess organic matters may be accumulated in
the wetlands. This could be as a result of
increased primary productivity or decreased
decomposition and export. Peat accumulates, to
some extent, in all wetlands due to these processes. In fact, wetlands are the carbon sinks in
the biosphere worldwide. More information in
this aspect could be obtained from Brinson
(1977), Odum and Heywood (1978), Deghi et al.
(1980), etc. Nevertheless, decomposition of
organic detritus generally requires electron
donors (usually oxygen; but alternative chemicals, e.g. sulphate and nitrate, may be effective
under anoxic conditions), moisture, inorganic
nutrients and microbes which are capable of
metabolising in the specific environment concerned. Further, the observed rate of organic
decomposition is also influenced by the ambient
temperature and by the activities of the microdetritivores. However, hydrology, possibly,
modifies many of these variables. Further, the
importance of hydrology in export of organic
carbon is obvious. Wetlands having continuous
inflow and outflow channels may be expected to
portray higher rate of export. Concomitantly, the
riparian wetlands often contribute a large amount
of organic detritus to the streams. Conversely,
the hydrologically isolated wetlands, e.g. the
northern peat lands, have much lower organic
export.
Summary
1. Hydrologic conditions are of paramount importance for the maintenance of a wetland’s structure and function. They have impact on many
abiotic factors, like soil anaerobiosis, nutrient
availability and salinity. These, in turn, play a
key role in developing the biota in a wetland.
Finally, the biotic components are active in altering the wetland hydrology and other physicochemical features and the cycle is completed.
2. The ‘hydro-period’ or hydrologic signature of
a wetland is the result of the balance between
inflows and outflows of water (called the
‘water budget’), the wetland basin geomorphology and sub-surface conditions.
3. The hydrology of a wetland creates the unique
physico-chemical conditions which make such an
ecosystem quite different from both well-defined
terrestrial and deep water aquatic systems.
Hydrologic pathways, such as precipitation, surface run-off, groundwater and flooding rivers,
transport energy and nutrients to and from wetlands. Hydrology also transports sediments,
nutrients, etc., into wetlands. It, thus, further
influences the physico- chemical environment.
14 Wetland Hydrology