Environmental Engineering Reference
In-Depth Information
CMB approach underestimated drainage when
steady-state conditions were not quickly rees-
tablished following land-use change; for such
cases, they suggested drainage be estimated
with Equation (
7.4
). In contrast, Scanlon
et al
.
(
2007
) found the standard CMB method to be
applicable at sites in the southern High Plains of
Texas that were converted from native vegeta-
tion to crops. In this region, the chloride bulge
that accumulated beneath native vegetation
was displaced downward in the unsaturated
zone in response to increased drainage under
cropland, indicating piston-like flow.
The peak-displacement method also can be
used to estimate the change in drainage rate
resulting from a land-use change. Soil-core
samples were obtained and analyzed from
fields under native eucalyptus and from nearby
agricultural fields in Victoria, Australia (Allison
and Hughes,
1983
). Equation (
7. 3
) was used with
Δ
z
T
set equal to the difference in depth of the
chloride front under native vegetation and that
under cropland (1 to 1.5 m); Δ
t
was set equal to
the time between land-use change and profile
sampling, 65 years. Multiplying the calculated
velocities by an average water content of 0.2
produced drainage estimates of 3 to 5 mm/yr
beneath the agricultural field.
Unsaturated zone chloride concentration-
depth profiles are useful qualitative tools for
identifying areas where drainage is occurring
in arid and semiarid regions. In the Southern
High Plains in Texas, Scanlon and Goldsmith
(
1997
) found much lower chloride concentra-
tions beneath playas than beneath interplaya
settings, indicating that the playas are an area
with much higher infiltration rates. The pro-
files can also serve as archives of past climates.
Bulge-shaped chloride profiles (
F ig u r e 7. 5b
) in
semiarid settings in the southwestern United
States are attributed to high water fluxes dur-
ing the Pleistocene when the climate was
cooler and wetter, followed by greatly reduced
fluxes during the Holocene period (Scanlon,
1991
; Phillips,
1994
; Tyler
et al
.,
1996
). Results
of numerical modeling of water and chloride
movement within thick unsaturated zones over
a period of 15 000 years support this hypothesis
(Walvoord
et al
.,
2002a
,
b
; Scanlon
et al
.,
2003
).
The time required to accumulate chloride
or the age of chloride and, by implication, that
of the water at a particular depth,
z
1
, may be
useful information in some studies. The chlo-
ride mass-balance age,
t
1
, can be calculated by
dividing the integrated mass of chloride in the
unsaturated zone from the surface to
z
1
by the
annual chloride input flux:
z
z
1
1
∫
∫
C
θ
dz
C
ρ
dz
'
(7.11)
uz
uz
b
0
0
t
=
=
()
z
11
PC
*
PC
*
P
P
where
C
uz
is chloride concentration in terms of
mass of chloride per unit mass of dry soil, and ρ
b
is soil bulk density. The chloride mass-balance
age is independent of soil-water content, and its
calculation, therefore, is not affected if samples
dry out prior to analysis.
In analyzing the CMB method, several points
should be considered. Uncertainties in chloride
deposition rates result in corresponding uncer-
tainties in estimated water fluxes because of
the linearity of Equation (
7.10
). Chloride pro-
files are generally insensitive to preferential
flow if chloride deposition is constant over
time. Unsaturated zone concentration-depth
profiles of bomb-pulse tracers such as
36
Cl and
3
H are generally more affected by preferential
flow (Scanlon,
2000
). Drainage estimates can be
obtained with a one-time collection of soil sam-
ples (although many depths need to be sampled),
and chloride analysis is relatively inexpensive.
There is no minimum drainage rate that can
be estimated, so the method is widely used in
arid regions. Because estimated drainage rates
are inversely related to chloride concentration,
the maximum rates that can be estimated are
limited by the analytical detection limits for
chloride. For this reason and because chloride
sources other than atmospheric deposition may
need to be considered, the unsaturated zone
CMB method is not often applied in humid
settings.
Example: Impacts of land use change on
groundwater recharge in the southern
High Plains and the Murray Basin
Environmental chloride has been used to
estimate impacts of changing from natural
ecosystems to agricultural ecosystems on
