Environmental Engineering Reference
In-Depth Information
P
ET
uz
Land surface
R
off
Zero-flux plane
D
ET
gw
Water table
ET
sw
R
Q
bf
Stream
Q
gw
on
Groundwater
Q
gw
off
Bedrock
Q Q
= ++
gw
RQ
bf
(2.6)
Figure 2.2
Schematic diagram of a vertical cross section
through a watershed showing water-budget components
(large arrows) and directions of water movement (small
arrows). Recharge,
R
, occurs at the water table. Drainage,
D
, is water movement between the zero-flux plane and the
water table.
off
off
off
Incorporating Equations (
2.3
through
2.6
) into
Equation (
2.2
), the water-budget equation for
the watershed can be expressed as (
Figure 2.2
):
P
+ + = + + +
+ + + + ++
Q
sw
Q
gw
ET
sw
ET
gw
ET
uz
∆
S
sw
on
on
evapotranspirative demand of the atmosphere.
Water below that plane moves downward to
eventually recharge the groundwater system.
The zero-flux plane is sometimes equated
with the bottom of the root zone. Δ
S
gw
refers
to changes in storage in the saturated zone; as
written here, it actually includes all storage
changes that occur at depths greater than that
of the zero-flux plane.
Flow out of the watershed,
Q
off
, can be
divided into surface-water flow,
Q
sw
off
, and
groundwater flow,
Q
gw
off
(which could include
extraction of groundwater by pumping wells
or groundwater flow to adjacent watersheds).
In addition,
Q
sw
off
can be split into direct run-
off,
R
off
, which now includes water that may
have infiltrated and traveled through a part of
the unsaturated zone but was diverted to land
surface before reaching the saturated zone,
and base flow,
Q
bf
, which is water that has been
discharged from the saturated zone to springs
and streams and is sometimes referred to as
groundwater runoff:
∆ ∆∆
S
snow
S
uz
S
gw
Q
gw
RQ
bf
off
off
(2.7)
Equation (
2.7
) can be refined further, if need be.
On the other hand, for some watersheds, many
of the terms in the equation will be negligible
in magnitude and can be ignored.
Several of the parameters described in this
section are also components of the water budget
of an aquifer. A water-budget equation for an
aquifer equates change in groundwater storage
with the difference between flow into and out
of the aquifer. Inflow is in the form of recharge
and groundwater flow; outflow is by ground-
water flow, base flow, and evapotranspiration.
The equation can be written as:
R
+ = ++ +
Q
gw
∆
S
gw
Q
bf
ET
gw
Q
gw
(2.8)
on
off
Equation (
2.8
) is a form of the groundwater-
flow equation that is solved by numerical
groundwater-flow models.
