Environmental Engineering Reference
In-Depth Information
Table 6.3 Values of specific yield after Nwankwor
et al.
(
1984
).
careful planning; useful guidelines are pro-
vided by Walton (
1970
), Stallman (
1971
), and
Batu (
1998
).
The volume-balance method combines a
traditional aquifer test with a water budget
of the cone of depression created by the with-
drawal of water from the pumping well. Specific
yield is defined as:
Method
S
y
Elapsed time (min)
Neuman (1972)
0.07
Boulton (1963)
0.08
Volume balance
0.02
15
0.05
40
0.12
600
0.20
1560
S VV
=
/
(6.10)
y
wc
0.23
2690
0.25
3870
where
V
w
is the volume of water pumped out of
the system at some point in time and
V
c
is the
volume of the cone of depression (the region
between the initial and the final water table)
at that same time (Clark,
1917
; Wenzel,
1942
;
Remson and Lang,
1955
). Nwankwor
e t a l
. (
1984
)
used this method to analyze results from an
aquifer test at the Borden field site in Ontario,
Canada. Their results (
Table 6.3
) show a trend
of increasing
S
y
with increasing time: the
longer the pump test, the larger the calculated
value of
S
y
became. These results were attrib-
uted to delayed drainage from the unsatur-
ated zone. Wenzel (
1942
) observed a similar
trend in a sand-gravel aquifer. Difficulties in
accurately measuring the volume of the cone
of depression limit the applicability of this
method, but advances have been made with
ground-penetrating radar for these measure-
ments (Endres
et al
.,
2000
; Bevan
et al
.,
2003
;
Ferré
et al
.,
2003
).
Laboratory (
θ
s
−
θ
r
)
0.30
surface reservoirs, Δ
S
sw
(including water bod-
ies as well as ice and snowpacks), the unsatur-
ated zone, Δ
S
uz
, and the saturated zone, Δ
S
gw
.
Substituting into Equation (
6.11
) and recalling
that Equation (
6.2
) can be used to determine
Δ
S
gw
, we can write:
∆
S
gw
=+ − − − −
=
P
Q
ET
∆∆
S
sw
S
uz
Q
on
off
(6.12)
SHt
∆∆
/
y
Rearranging this equation produces an esti-
mate for
S
y
:
S
=+ − −− −
(
P
Q
Q
ET
∆ ∆ ∆∆
S
S
)/
H
/
t
sw
uz
(6.13)
y
on
off
Gerhart (
1986
) and Hall and Risser (
1993
) applied
this method during winter periods (usually of
week-long duration) with the assumption that
ET
, Δ
S
uz
, and net subsurface flow were zero.
Crosbie
et al
. (
2005
) and Saha and Agrawal
(
2006
) used similar approaches for estimat-
ing
S
y
. Best results should be expected in areas
where surface and subsurface flows are easily
determined.
w a t e r
-
b u d g e t
m e t h o d s
Walton (
1970
) proposed using a water budget in
conjunction with Equation (
6.2
) to estimate
S
y
during winter periods when evapotranspiration
is low and soil is near saturation (so change in
storage in the unsaturated zone is low). A sim-
ple water budget for a basin can be written as:
g e o p h y s i c a l
m e t h o d s
The temporal gravity technique of Pool and
Eychaner (
1995
) for estimating change in sub-
surface water storage (
Section 2.3.3
) can also
be used to estimate specific yield. Pool and
Eychaner (
1995
) made microgravity measure-
ments over transects that were kilometers
in length. Changes in gravity over time were
attributed to changes in subsurface water
P
= =++
Q
ET
∆
S
Q
off
(6.11)
on
where
P
is precipitation plus irrigation,
Q
on
and
Q
off
are surface and subsurface water flow
into and out of the basin,
ET
is evapotranspir-
ation, and Δ
S
is change in water storage. Δ
S
can
be written as the sum of change in storage of
