Environmental Engineering Reference
In-Depth Information
Short Exercise 12: Estimation of Specific Yield
When inflow to and outflow from a wetland containing no surface water are
negligible over a short-duration storm, the change in subsurface storage (
S
sub
)is
approximately equal to the net vertical input or loss of water from the wetland
(
P
Δ
E
) (see Eq.
3.48
and the associated paragraph). Assuming that
E
is much
smaller than
P
during the storm, specific yield can be estimated as the proportio-
nality constant between
Δ
S
sub
(
P
) and increases in the water table (
Δ
h
) caused
by storms:
Δ
S
sub
¼
S
y
Δ
h
(3.63)
The figure embedded in Table
3.12
below shows the water-table elevation recorded
beneath Wetland 109 in the St. Denis National Wildlife Area in Saskatchewan,
Canada (see Hayashi et al.
1998
for the site condition), in July-August 1995 when
the water table was mostly below the sediment surface (551.68 m). During this
period, there were five storms that caused measurable increases in the water table
without bringing it to the surface (see Table
3.12
below).
Table 3.12 Total precipitation and water-table increases during storms recorded in July-August
1995 at Wetland 109. The graph shows the water-table elevation and cumulative precipitation
Date
P
(mm)
Δh
(mm)
July 30
7.8
193
552.0
120
Aug. 7
10.1
188
551.8
100
Aug. 8
7.6
149
sediment
surface
Aug. 26
3.8
94
551.6
80
Aug. 29
10.8
240
551.4
60
40
551.2
pond stage
cum. pcp.
551.0
20
550.8
0
7/29
8/3
8/8
8/13
8/18
8/23
8/28
12
10
8
6
4
2
0
0
50
100
150
200
250
Δ
h
(mm)
