Environmental Engineering Reference
In-Depth Information
area, such as millimeters per day. Equation (
2.1
)
states that the change in storage within the
column is balanced by the difference between
flow into the column, from precipitation, and
flow out of the column, by either evapotrans-
piration or drainage through the bottom of the
column.
Drainage,
D
, is equivalent to recharge,
R
,
only if the bottom of the column extends to the
water table. According to the definitions given
in
Chapter 1
, draining water is not referred to
as recharge until it actually arrives at the water
table. Some methods described in this and fol-
lowing chapters produce estimates of drainage;
others produce estimates of actual recharge.
The distinction between the two phenom-
ena is a matter of timing; if the soil column
extends beyond the bottom of the root zone,
water draining out the bottom will eventually
become recharge, but the time it takes for that
water to move to the water table is not known.
Approaches for describing this travel time are
discussed in
Chapter 3
. Distinguishing between
drainage and recharge may be important if tem-
poral trends of recharge are of interest. Land-
use and climate changes can result in current
drainage rates that are substantially different
from historical rates of recharge. But if recharge
is being estimated for input to a steady-state
groundwater flow model, the two terms can be
considered synonymous. It is not always clear
whether a particular method is providing an
estimate of drainage or recharge; many authors
do not even make this distinction. In this chap-
ter, and in the remainder of this topic, if it is
clear that a method estimates drainage and not
actual recharge, then the estimate is referred to
as drainage (
D
). If it is clear that a method esti-
mates actual recharge or if there is any ambigu-
ity as to what type of estimate is provided, the
estimate is referred to as recharge (
R
).
The water budget equation for a watershed
and the underlying unsaturated and saturated
zones can be written in a form similar to that
of Equation (
2.1
):
subsurface water flow out of the watershed.
Equation (
2.2
) can be easily expanded and
refined, as the following discussion illustrates.
Precipitation can occur in the form of rain,
snow, dew, or fog drip; irrigation could also be
explicitly included. Water flow into the water-
shed can be written as the sum of surface-water
flow,
Q
sw
on
(which could include diversion from
another watershed), and groundwater flow,
Q
gw
on
(which could include subsurface injection):
QQ
=
sw
+
Q
gw
on
(2.3)
on
on
Evapotranspiration can be divided on the
basis of the source of evaporated water:
uz
(2.4)
ET
=
ET
sw
+
ET
gw
+
ET
where
ET
sw
is evaporation or sublimation of
water stored on land surface,
ET
uz
is bare soil
evaporation and plant transpiration of water
stored in the unsaturated zone, and
ET
gw
is eva-
potranspiration of water stored in the saturated
zone. (Although distinguishing among
ET
sw
,
ET
gw
, and
ET
uz
can be accomplished in theory,
in practice, it may be problematic because
standard measurement techniques deter-
mine the flux of water vapor from land surface
to the atmosphere and are essentially blind to
the source of the evaporating water. Available
measurement techniques, their accuracy, and
their cost are important considerations when
fashioning a water-budget equation.)
Storage of water in a watershed occurs
in surface-water reservoirs, in ice and snow-
packs, in the unsaturated zone, and in the
saturated zone. Change in water storage can
be written as:
∆∆ ∆ ∆ ∆
(2.5)
S
=
S
sw
+
S
snow
+
S
uz
+
S
gw
where the superscripts refer to the aforemen-
tioned compartments. Δ
S
sw
can include storage
in surface depressions, in plants, and in the
plant canopy (due to interception of precipita-
tion). Δ
S
uz
refers to changes in water storage
in the unsaturated zone at depths equal to or
less than the zero-flux plane, ZFP, which is a
plane at some depth in the subsurface where
the magnitude of the vertical hydraulic gradi-
ent is 0 (
Figure 2.2
;
Section 5.3
). Water above
that plane moves upward in response to the
(2.2)
P
+ =++
Q
ET
∆
S
Q
on
off
where
Q
on
is surface and subsurface water flow
into the watershed; and
Q
off
is surface and
