219
the upstream reaches of a watershed. These wetlands are often important water flow regulators
for downstream rivers. Further, some wetlands
have surface outflows which may develop only
when their water storages exceed a critical level.
Further, the wetlands are subjected to surface
inflows of several types. The non-channelised
sheet flow, called ‘overland flow’, usually occurs
during and immediately following rainfall of a
spring thaw. A wetland which is influenced by a
drainage basin may receive channelised stream
flow during most or all of the year. Wetlands are
often an integrated part of a stream or a river, e.g.
as in-stream FW marshes or riparian bottomland
forests. In addition to the above, wetlands are
greatly influenced by the seasonal stream flow
patterns of the river which are formed in wide
shallow expanses of river channels or floodplains.
Further, wetlands may also receive surface inflow
from seasonal episodic pulses of flood flow from
adjacent streams and rivers which may otherwise
not be connected hydrologically with the
wetland.
14.5.3 Water Flow Measurement
Surface outflow from a wetland could be determined with the general equations for surface
flow. When we want a continuous record, a rating
curve related to stream stage could be developed.
When a weir or other control structure is used,
the outflow of the wetland could also be estimated to be a function of the water level in the
wetland itself according to the equation given
below:
S
xL
y
o =
where:
S o = Surface outflow
L = Wetland water level above a control structure
crest (level at which flow just begins)
x, y = Calibration coefficients
14.5.4 Evapotranspiration
The water which vaporises from water or soil in a
wetland is called ‘evaporation’. And the moisture
which passes through vascular plants to the atmosphere is called ‘transpiration’. ‘Evapotranspiration’
is a combination of the two.
The meteorological factors which affect evaporation and transpiration are almost similar as
long as there is adequate moisture, a condition
which almost always exists in most wetlands.
The rate of evapotranspiration is proportional to
the difference between the vapour pressure at the
water surface (or at the leaf surface) and the
vapour pressure in the overlying air. This is
explained in a version of Dalton’s law:
E
u e e
=
-
(
)
c f
w
a
( )
where:
E = Rate of evaporation
c = Mass transfer coefficient
f (u) = Function of wind speed, u
e w = Vapour pressure at the surface or saturation
vapour pressure at wet surface
e a = Vapour pressure in surrounding air
It may be noted here that evaporation and transpiration are enhanced by the same meteorological conditions, such as solar radiation or surface
temperature, which increase the value of the
vapour pressure at the evaporating surface. Also,
decreased humidity or increased wind speed
decrease the vapour pressure of the surrounding
air. However, this equation assumes an adequate
supply of water for capillary movement in the
soil or access by rooted plants. Further, transpiration may also be physiologically limited in plants
through the closing of stomata in leaf despite
adequate moisture during periods of stress, such
as anoxia.
14.5.5 Effects of Vegetation
on Wetland Evapotranspiration
There is a pertinent question about evapotranspiration from wetlands which does not elicit a uniform answer in the literature. The question is:
Does the presence of vegetation in a wetland
increase or decrease the loss of water compared
to that which would occur from an open body
of water without vegetation? In this regard, data
from individual studies are conflicting. Obviously,
14.5 The Wetland Water Budget
the upstream reaches of a watershed. These wetlands are often important water flow regulators
for downstream rivers. Further, some wetlands
have surface outflows which may develop only
when their water storages exceed a critical level.
Further, the wetlands are subjected to surface
inflows of several types. The non-channelised
sheet flow, called ‘overland flow’, usually occurs
during and immediately following rainfall of a
spring thaw. A wetland which is influenced by a
drainage basin may receive channelised stream
flow during most or all of the year. Wetlands are
often an integrated part of a stream or a river, e.g.
as in-stream FW marshes or riparian bottomland
forests. In addition to the above, wetlands are
greatly influenced by the seasonal stream flow
patterns of the river which are formed in wide
shallow expanses of river channels or floodplains.
Further, wetlands may also receive surface inflow
from seasonal episodic pulses of flood flow from
adjacent streams and rivers which may otherwise
not be connected hydrologically with the
wetland.
14.5.3 Water Flow Measurement
Surface outflow from a wetland could be determined with the general equations for surface
flow. When we want a continuous record, a rating
curve related to stream stage could be developed.
When a weir or other control structure is used,
the outflow of the wetland could also be estimated to be a function of the water level in the
wetland itself according to the equation given
below:
S
xL
y
o =
where:
S o = Surface outflow
L = Wetland water level above a control structure
crest (level at which flow just begins)
x, y = Calibration coefficients
14.5.4 Evapotranspiration
The water which vaporises from water or soil in a
wetland is called ‘evaporation’. And the moisture
which passes through vascular plants to the atmosphere is called ‘transpiration’. ‘Evapotranspiration’
is a combination of the two.
The meteorological factors which affect evaporation and transpiration are almost similar as
long as there is adequate moisture, a condition
which almost always exists in most wetlands.
The rate of evapotranspiration is proportional to
the difference between the vapour pressure at the
water surface (or at the leaf surface) and the
vapour pressure in the overlying air. This is
explained in a version of Dalton’s law:
E
u e e
=
-
(
)
c f
w
a
( )
where:
E = Rate of evaporation
c = Mass transfer coefficient
f (u) = Function of wind speed, u
e w = Vapour pressure at the surface or saturation
vapour pressure at wet surface
e a = Vapour pressure in surrounding air
It may be noted here that evaporation and transpiration are enhanced by the same meteorological conditions, such as solar radiation or surface
temperature, which increase the value of the
vapour pressure at the evaporating surface. Also,
decreased humidity or increased wind speed
decrease the vapour pressure of the surrounding
air. However, this equation assumes an adequate
supply of water for capillary movement in the
soil or access by rooted plants. Further, transpiration may also be physiologically limited in plants
through the closing of stomata in leaf despite
adequate moisture during periods of stress, such
as anoxia.
14.5.5 Effects of Vegetation
on Wetland Evapotranspiration
There is a pertinent question about evapotranspiration from wetlands which does not elicit a uniform answer in the literature. The question is:
Does the presence of vegetation in a wetland
increase or decrease the loss of water compared
to that which would occur from an open body
of water without vegetation? In this regard, data
from individual studies are conflicting. Obviously,
14.5 The Wetland Water Budget