Environmental Engineering Reference
In-Depth Information
any climatic setting can have reaches that gain
and reaches that lose; some reaches can gain flow
at certain times of the year and lose flow at other
times. Hydrologic processes are variable in space
and time; the very water that becomes recharge
at one point in a stream may be discharged to
the same stream at a point 100 m down chan-
nel. Hence, the location and timing at which a
method is applied can affect estimated recharge
rates, a point that underscores the importance of
developing a sound conceptual model of recharge
processes in any study.
If the water table intersects the stream
channel, then stream loss represents actual
recharge (
Fig ure 4.1b
). However, if an unsatu-
rated zone exists between the channel and the
water table (
Fig ure 4.1c
), stream loss represents
potential recharge because all of the water
leaving the stream may not travel to the water
table; some may be diverted to evapotranspira-
tion, for example. Streams and rivers tend to be
more dynamic than lakes and aquifers in the
sense that residence times for water in streams
and rivers are typically much less than those
for lakes and the subsurface (Healy
et al
.,
2007
).
Recharge can be slow and steady over time or
highly episodic. A stream can quickly change
from gaining to losing as a result of a sudden
rise in stream stage associated with a large
precipitation event. As the stage rises, water
flows into stream banks. If the water table is
connected to the stream, this inflow from
the stream is called
bank storage
. This focused
recharge is an important source of recharge for
some aquifer systems, but in many other sys-
tems the stored water resides in the subsurface
for only short periods. As stream stage recedes
to levels less than that of groundwater, water
flows back from the stream banks to the stream.
Release of bank storage can sustain streamflow
for extended periods following a runoff event
depending on the duration of high stage and the
hydraulic properties of the stream banks. If the
water table is not connected to the stream, little
of the water lost from the stream may return
to the stream. This lost water is regarded as
potential recharge - it may percolate down to
the water table, or it may be evapotranspired to
the atmosphere.
Flow direction
Unsaturated
zone
Water table
Saturated zone
GAINING STREAM
(a)
Unsaturated
zone
Water table
Saturated zone
LOSING STREAM
(b)
Unsaturated
zone
Water table
Saturated zone
(c)
DISCONNECTED STREAM
Figure 4.1
Schematics showing (a) gaining stream, stream
stage is below water table; (b) losing stream connected to
aquifer, stream stage is above water table; and (c) losing
stream disconnected from aquifer. (Winter
et al
.,
1998
).
the stream is a
losing stream
and surface water
flows into the subsurface (
Fig ures 4.1b
and
c
).
Paradoxically, both stream losses and stream
gains are associated with recharge. A losing
stream is a source of focused recharge to the
aquifer. Groundwater discharge (base flow) to
a gaining stream is water that recharged the
aquifer, usually as diffuse recharge, at some
point upgradient from the discharge point.
In arid and semiarid regions, focused recharge
from ephemeral streams is often the dominant
recharge mechanism. These streams usually have
water only after rainfalls or snowmelts. Such
streams generally have steep slopes and lose water
over most reaches. Streams in humid regions are
generally gaining streams. However, streams in
