Environmental Engineering Reference
In-Depth Information
15.0
14.5
14.0
13.5
13.0
90
60
30
0
1950
1951
1952
Figure 6.9
Hydrograph of average groundwater level and
bar graph of weekly average precipitation for Beaverdam
Creek Basin, Maryland (after Rasmussen and Andreasen,
1959
). Dashed lines represent the expected level to which
the water table would have receded in the absence of
precipitation.
Figure 6.9
shows the average water level in
the observation wells and precipitation on
a weekly basis. Water levels were highest in
late winter and early spring. Precipitation was
fairly evenly distributed throughout the year.
Recharge was calculated with Equation (
6.2
)
on a monthly basis. A water-budget approach
(Equation (
6.13
)) was used to develop a
basin-wide estimate of 0.11 for
S
y
. The authors
noted that this value is less than the average
value of 0.21 determined in the laboratory
on core samples obtained from the basin and
attribute this difference to inadequate time
for the sediments to fully drain between suc-
cessive water-level rises. Δ
H
was taken as the
cumulative rise in water level for the month
(i.e. the sum of all rises that occurred). Water
levels prior to rises were extrapolated to their
expected positions had there been no pre-
cipitation (dashed lines in
Figure 6.9
). The
rise was then determined as the difference
between the peak level and the extrapolated
antecedent level at the time of the peak.
Table
6.4
shows monthly estimates of recharge
for April 1950 through March 1952. Average
annual precipitation for the 2-year period
The 51 km
2
drainage basin ranges in ele-
vation from 4 to 26 m above sea level and
receives on average 1.09 m of precipitation
annually. Beneath the basin, which is located
on the Atlantic Coastal Plain, the unconsoli-
dated sedimentary rocks consist mostly of
sand, silt, clay, greensand, and shell marl.
The water table is generally within a few
meters of land surface. Quaternary-age surfi-
cial sands and silts of up to 22 m in thickness
overlie Tertiar y-age sand aquifers. Twenty-five
observation wells, ranging in depth from 3 to
8 m, were distributed fairly uniformly (sub-
ject to constraints of accessibility) across the
watershed. Groundwater levels were manu-
ally measured on a weekly basis. Stream dis-
charge was monitored at the outlet of the
basin. Precipitation was measured weekly at
12 sites within the basin.
