Environmental Engineering Reference
In-Depth Information
Table 3.1 Errors indicated in % for water-budget components of selected studies conducted on
lakes, reservoirs, and wetlands (- indicates parameter was not determined; calc indicates value was
calculated as the residual)
P
E or ET
S
G
Of
Δ
V
Winter (
1981
)
5-10
10-15
5-10
13-36
-
-
LaBaugh (
1985
)
33
10
5-15
-
-
10
Belanger and Kirkner (
1994
)
10
10
50
50
-
10
LaBaugh et al. (
1995
,
1997
)
5
10
-
50
-
5
Lee and Swancar (
1997
)
10
16
-
102-106
-
5
Sacks et al. (
1998
)
5-9
10
30-100
calc
-
5
Choi and Harvey (
2000
)
8.5
20
10
10
-
15
Harvey et al. (
2000
)
15
10
10-15
10
-
15
Motz et al. (
2001
)
5
20
11-15
50
100
5
Rosenberry and Winter (
2009
)
5
15
5
25
-
10
Median
9
10
10
36
10
Maximum
33
20
100
106
15
Minimum
5
10
5
10
5
Δ
h
is small and
A
is much greater than
Δ
A
,
When
this relation often is
simplifiedbyassumingthat
A
is constant (i.e.,
Δ
A
¼
0). A minimum measurable
change in wetland stage is, therefore, a logical accounting unit in a wetland water
budget. Precipitation and evapotranspiration already are usually expressed in
terms of depth applied over the wetland surface per time (commonly mm/day).
Other water-budget components more commonly measured in terms of volume
per time, such as surface-water or groundwater inputs and losses, can be
expressed as
h
by dividing by
A
. This seemingly simple task can be a substantial
problem at many wetlands, as evidenced by the relatively large errors associated
with the
Δ
V
term listed in Table
3.1
; errors of 10-15 % are common. Since
measuring stage is quite simple and can be done very accurately, often with
accuracies of
Δ
3 mm or better, the estimation of surface area is the source of
most of this error.
The shoreline must be identified before wetland area can be determined. Unfor-
tunately, an indistinct shoreline as shown in Fig.
3.1
is common. In some cases, an
area of dense emergent vegetation forms an abrupt boundary, not at the shoreline
but at the edge of the open-water portion of the wetland, that confounds the
determination of the actual shoreline. If this border occurs at a water depth of 0.3
or 0.5 m, an example of which is shown in Fig.
3.1
, the actual shoreline, where
water depth decreases to zero, can be many meters away and obscured by additional
dense emergent vegetation.
For wetlands situated in low-gradient settings, the shoreline can move laterally a
large distance in response to a small stage change (e.g., Lee et al.
2009
). An accurate
bathymetry map, and associated stage-area and stage-volume plots, are particularly
important for minimizing error when determining
V
. Generating an accurate stage-
area plot is not nearly as onerous as it once was (see Chap.
2
on wetland bathymetry).
Δ
