Environmental Engineering Reference
In-Depth Information
basis, water draining beneath the zero-flux
plane that has not yet reached the water table
would be counted as recharge, instead of what
it actually should be called, which is drainage.
By our definition (Equations (
2.5
) and (
2.8
)), both
Δ
S
gw
and
R
include actual recharge (indicated
by change in water table height) and drainage
(indicated by a change in water storage within
the interval between the zero-flux plane and
the water table - that water does not become
actual recharge until it arrives at the water
table). Different boundaries for describing sub-
surface storage are used in different studies, so
it is important that those boundaries be clearly
defined.
Estimates of drainage produced by the
zero-flux plane method (
Section 5.3
) are based
on changes in storage within some interval
between the zero-flux plane and the water
table; changes in water-table height have no
effect on those estimates if the water table
does not intersect the measurement window.
On the other hand, the water-table fluctuation
method (
Section 6.2
) ignores any change in
storage between the zero-flux plane and the
water table; estimates of recharge are obtained
by looking only at the changes in groundwater
storage that occur with rises and falls of the
water table:
Land surface
z
=
0
S
uz
Zero-flux plane
z
=
z
zfp
S
subs
S
gw
Water table
Aquifer base
z
=
z
base
Figure 2.5
Schematic of soil column extending from land
surface to the base of the aquifer showing different storage
compartments.
S
subs
represents all water stored in column,
S
uz
is water stored between land surface and the zero-flux
plane, and
S
gw
is water stored between the zero-flux plane
and the base of the aquifer. z
ZFP
is depth to zero-flux
plane; z
base
is depth to base of aquifer.
where z
ZFP
is the depth to the zero-flux plane.
The relevant equation for calculating Δ
S
uz
, as
defined in Equation (
2.5
), is:
∆= −
S
(
S
S
) /(
tt
−
)
uz
uz
uz
(2.21)
i
i
i1
−
i
i1
−
where i is an index of time,
t
. Water-budget
methods for estimating evapotranspiration are
often based on Equations (
2.20
) and (
2.21
). For
determining the amount of storage underlying
the zero-flux plane,
S
gw
, the equation becomes:
z
=
∫
ZFP
base
∆ ∆∆
S
gw
=
y
SH t
/
(2.23)
θ
(2.22)
S
dz
gw
z
where
S
y
is specific yield and Δ
H
is change in
water-table height over time interval Δ
t
.
Measurement of soil-water content by gravi-
metric and geophysical techniques and calcu-
lation of Δ
S
uz
are described in some detail in
Chapter 5
. Soil-water content data are not as
widely available as precipitation and other cli-
matological data, but there are some sources
of data. The Global Soil Moisture Data Bank
(Robock
et al
.,
2000
;
Table 2.1
) contains soil-wa-
ter content data obtained at regular time inter-
vals, to depths of about 1 m, at over 600 fixed
locations worldwide. These data have been used
for assessing regional trends over decades (e.g.
Robock and Li,
2006
) and for evaluating simu-
lation results of general circulation and land
surface models (e.g. Fan
et al
.,
2006
). The NRCS
And an equation analogous to Equation (
2.21
) is
used to define Δ
S
gw
.
There are many different bookkeeping
schemes for subsurface water-storage account-
ing, so some clarification is in order. Our
approach divides the soil column into two com-
partments separated by the zero-flux plane. A
problem with this approach (and, indeed, a prob-
lem with many other water-budget approaches)
is that time lags may not be properly taken into
account. Water moves at different rates through
different parts of the hydrologic cycle; it can
take weeks or months for precipitation from
a day-long storm to move from land surface
to the water table. If a water-budget equation
such as Equation (
2.9
) is tabulated on a weekly
