Environmental Engineering Reference
In-Depth Information
focused on themost important hydrologic components of a particular wetland setting or
type. The importance of quantifying individual hydrological components also depends
on the issues and questions being asked. For example, at an extensively studied wetland
in the prairie-pothole region of North Dakota, groundwater discharge was a small
component (3.5 %) of the water budget, small enough that it might be ignored.
However, groundwater discharge delivered a large percentage of chemicals to the
wetland and was an important contributor to wetland chemistry (LaBaugh et al.
2000
).
A wetland water budget can be written as
Δ
V
Δ
t
þ
R
¼
P
þ
O
f
þ
S
i
þ
G
i
ET
S
o
G
o
(3.1)
where
t
is the change in volume of surface water in the wetland per time,
P
is
precipitation,
O
f
is overland flow,
S
is surface water,
G
is groundwater,
ET
is evapo-
transpiration, and
R
is the residual, or unaccounted water, in the water budget. Subscripts
i
and
o
refer to water flowing into or out of the wetland. This basic equation should be
modified to suit specific wetland settings. For example, some wetlands will have dewfall
or stem flow that is substantial and quantifiable whereas other wetlands will not have any
surface-water inputs or losses. Many wetlands in northern latitudes also have an input
term associated with drifting snow (e.g., Hayashi and van der Kamp
2007
). Some
wetlands will rarely contain surface water, in which case
Δ
V
/
Δ
t
can be based on changes
in volume of surface water, groundwater, and soil-moisture storage over time. If surface
water is not present, hydrologic fluxes are distributed over an area based on criteria other
than areal extent of surface water, perhaps the areal extent of wetland vegetation. In this
chapter we will restrict discussion primarily to settings where surface water is present.
Equation
3.1
can be rearranged to solve for any of the components provided the
others are known. An example is presented later for determining
G
i
and
G
o
as the
unknown entities of the water-budget equation.
ET
also can be a difficult value to
obtain and is occasionally solved as the unknown of a water-budget equation.
However, the uncertainty associated with
ET
commonly is much smaller than the
uncertainty associated with quantifying
G
i
or
G
o
. In many wetland settings, errors
associated with quantifying groundwater exchange are so large that solving for
ET
as the residual would be meaningless.
Δ
V
/
Δ
3.2.1 Determining the Accounting Unit
As mentioned earlier, the change in wetland storage,
V
, integrates all of the input
and loss terms of a hydrologic budget. This term can also be approximated as
Δ
h
A
!
þ
Δ
A
2
Δ
V
Δ
(3.2)
where
A
is wetland surface area and
h
is wetland stage. Details for determination
of
V
in the typical case where
A
changes with depth are provided in Sect.
3.3.2
.
