Environmental Engineering Reference
In-Depth Information
Figure 5.4
Hypothetical vertical
profiles of total head (
H
) within
the unsaturated zone indicating
(A) downward flux (no zero-flux
plane, ZFP); (B) upward flux due
to evapotranspiration (no ZFP);
(C) a single ZFP with upward flux
above and downward flux below;
and (D) two ZFPs. Total head is
equal to pressure head,
h
, minus
depth,
z
, and the top of each frame
represents land surface.
A
B
z
C
D
ZFP
ZFP
ZFP
H
evapotranspiration. There are no guidelines for
that apportionment, so it is left to the discre-
tion of the user.
Central to application of the ZFP method
is the assumption that water moves vertically
through the unsaturated zone as periodic, dis-
tinct “pulses” that can be monitored over time.
Steady flow through the unsaturated zone
(i.e. flow that occurs with no change in water
content) cannot be measured with this method;
likewise, the method cannot be applied when
water is moving downward throughout the
entire extent of the unsaturated zone. The ZFP
a p p r of a c h q u a n t i i e s t h e a m of u n t of f w a t e r m of v i n g
in each pulse. In many applications (particularly
those in areas of deep water tables), water-con-
tent measurements are made only to some finite
depth above the water table. Any depth interval
for measurements is sufficient as long as that
interval is below the ZFP and is of sufficient
thickness to include the leading and trailing
edge of each pulse of moving water. If meas-
urement depths extend to the water table, the
method produces estimates of actual recharge.
Otherwise, the estimates should be referred to
as drainage. Frequent water-content measure-
ments may be required to adequately capture
the movement of a water pulse through a depth
interval. Rapidly moving wetting fronts could
go undetected with insufficient measurement
f requenc y.
The ZFP moves vertically with time in
response to infiltration, evapotranspir-
ation, and drainage. Four vertical profiles of
hydraulic head are depicted in
Figure 5.4
to
illustrate hypothetical locations of the ZFP.
In Profile A, water is moving downward at
all depths; there is no ZFP in the subsurface.
Profile A would be expected during or imme-
diately following a large precipitation event.
The ZFP method cannot be applied under these
conditions; an alternative method, such as a
Darcy or other water-budget method, must be
used (e.g. Hodnett and Bell,
1990
; Roman
et al
.,
1996
). Profile B indicates that water is moving
upward at all depths, most likely in response
to evapotranspiration. There is no ZFP and no
drainage so the method cannot be used. Profile
C contains a single ZFP, and the ZFP method
can be applied to estimate drainage and evapo-
transpiration. Finally, profile D contains a
ZFP at two depths. Such an occurrence is not
uncommon and could result from precipita-
tion falling when ambient soil conditions were
similar to those in profile C. Usually, the two
ZFPs would converge within a short time. The
ZFP method can be applied under conditions
shown in Profile D, but only the change in stor-
age below the lowermost ZFP should be attrib-
uted to drainage.
The configuration of hydraulic heads within
the unsaturated zone changes over time. Any
one of the profiles depicted in
Figure 5.4
could
exist in the same column of the unsaturated
zone at different times of the year. The condi-
tions in profile A must exist from time to time
as infiltration occurs; otherwise, no drainage
through the column would occur. Frequent
