Environmental Engineering Reference
In-Depth Information
1991
). Drainage at borehole 6 is greater that at
borehole 4, and evapotranspiration is less. This
is attributed to denser vegetation near bore-
hole 4. Average annual drainage for the period
of study was 90 mm (13% of precipitation) at the
10 m depth and 70 mm (10% of precipitation) at
the 18 m depth.
The approach used to estimate the depth
of the ZFP in this study was viable because of
the uniform texture of the sediments, the rela-
tively deep water table, and the measurement
of the soil-water retention curves. Roark and
Healy (
1998
) and Schwartz
et al
. (
2008
) also used
water-content measurements to identify the
depth of the ZFP.
0
5
10
Jan 17
Apr 12
Aug 22
Dec 13
15
20
0
0.02
0.04
0.06
0.08
0.1
Water content (m
3
/m
3
)
Figure 5.8
Profiles of water content and locations of
zero-flux plane (arrows) for 4 days in 1985 at a site in
Western Australia (Sharma
et al
.,
1991
; reprinted with
permission of John Wiley & Sons, Inc.).
5.4 Darcy methods
2500
625
The Darcy equation was first introduced in
Section
2.3.5
to describe groundwater flow. A
variant of the equation may also be used to esti-
mate drainage through the unsaturated zone.
For vertical flow in the unsaturated zone, the
Darcy equation takes the form:
P
2000
500
D4
1500
375
D3
1000
250
D1
500
125
q
=−
KK h H
sr
()
∂ ∂
/
z
(5.13)
D2
0
0
where
q
is drainage rate;
K
s
K
r
(
h
) is the vertical
hydraulic conductivity at the ambient pres-
sure head,
h
;
H
is total head; and
z
is depth.
The unsaturated-zone Darcy method requires
measurements or estimates of the vertical total
head gradient (
Section 5.2.2
) and hydraulic con-
ductivity as a function of water content or pres-
sure head (
Section 5.2.3
). The method has been
applied in many studies under various climatic
conditions. Enfield
et al
. (
1973
), Stephens and
Knowlton (
1986
), and Sammis
et al
. (
1982
) used
the method in arid or semiarid environments.
It has also been applied in humid regions (e.g.
Ahuja and El-Swaify,
1979
; Steenhuis
et al
.,
1985
; Kengni
et al
.,
1994
; Normand
et al
.,
1997
;
and Coes
et al
.,
2007
).
Unlike the zero-flux plane method, applica-
tion of the Darcy method is not limited to certain
times of the year. Equation (
5.13
) can be expanded
to account for water movement in two or three
dimensions. If head measurements are made in
multiple dimensions, fluxes can be estimated for
1985
1986
1987
1988
Years
Figure 5.9
Precipitation (P) and drainage for 1985-1987
at a site in Western Australia. Drainage calculated with ZFP
method for borehole 4 for 10 m (D1) and 18 m (D2) depths
and for borehole 6 for 10 m (D3) and 18 m (D4) depths
(Sharma
et al
.,
1991
; reprinted with permission of John
Wiley & Sons, Inc.).
and 6 showed a similar seasonal trend (
Figure
5.9
); highest rates occurred during periods
when precipitation exceeded evapotranspir-
ation (which was also estimated by the ZFP
method). A seasonal trend was less apparent in
the 18 m drainage estimates. The two estimates
for 3 years of cumulative drainage at borehole
6 are in good agreement. The agreement is not
so good for the borehole 4 estimates, perhaps
because data collection was terminated at a
time when drainage rates at the deeper depth
would have been increasing (Sharma
et al
.,
