Environmental Engineering Reference
In-Depth Information
Lacking such detailed data, it is possible to determine change in volume with
reasonable accuracy by making some basic assumptions related to wetland geome-
try. Assuming a symmetrical wetland basin with the deepest part located at the
center of the basin, wetland area can be determined by making an assumption about
the change in slope of the wetland basin with distance from the center. Using this
approach, Hayashi and van der Kamp (
2000
) developed the following relation:
2
p
H
H
0
A
¼
s
(3.4)
where
A
is wetland surface area,
H
is wetland depth,
H
0
is unit depth (e.g., 1 m),
s
(m
2
) is a scaling factor that is equal to the wetland surface area at
H
0
, and
p
is a
dimensionless scaling factor that is related to the shape of the wetland basin. For
example, if the profile of the wetland bed extending from the center to the perimeter
is a straight line, then
p
is equal to 1. If the wetland is bowl shaped, then
p
is close to
2. If the wetland has a broad, flat basin that steepens near the wetland edge, then
p
is
somewhere between about 5 and 100, with
p
increasing to infinity for a rectangular
cross section. Wetland volume also can be approximated using the same fitting
factors and the equation
H
1
þ
2
=
H
0
2
p
s
V
¼
(3.5)
1
þ
2
p
=
=
(Hayashi and van der Kamp
2000
).
An example of comparing fitted and measured values for area and wetland area
and volume is shown in Fig.
3.4
. Even with an irregularly shaped wetland basin,
Eqs.
3.4
and
3.5
approximate values for
A
and
V
reasonably well based on measured
bathymetry.
3.3.3 Sources of Error
Measurement of wetland stage is conceptually very simple and any given observa-
tion has a high likelihood of being very accurate. However, several sources of error
can increase as study duration extends to multiple months or years and greatly
diminish the accuracy of wetland stage that is very important to a water-budget
analysis. The significance of these errors depends on the accuracy requirements.
The U.S. Geological Survey, for example, requires an accuracy of
0.01 ft (3 mm)
for water-level measurements over the range typically encountered in most wetland
settings (Sauer and Turnipseed
2010
).
If daily water budgets are a goal, then measuring stage to a level of precision and
accuracy similar to hydrologic fluxes summed over a day would be appropriate.
