Environmental Engineering Reference
In-Depth Information
8.4 Focused recharge
range than groundwater temperatures; hence, a
deeper, wider temperature envelope is expected
beneath a losing stream relative to that beneath
a gaining stream. The temperature envelope
beneath a gaining stream is condensed in terms
of depth and width because the discharging
groundwater has a relatively constant tempera-
ture. Temperature envelopes for diurnal cycles
display patterns similar to those in
Figure 8.5
,
only with less temperature difference at any
depth. The depths to which temperature
envelopes extend beneath streams vary with
exchange rates, climate, and sediment prop-
erties. For diurnal patterns, depths can range
from a few tenths of a meter to several meters.
To monitor diurnal temperature patterns, sen-
sors are usually installed at fixed depths, and
measurements are typically recorded every 15 to
60 min. The envelope for annual temperatures
can extend past a depth of 15 m (Bartolino and
Niswonger,
1999
); annual temperature trends
are usually measured by logging observation
wells on a weekly or monthly schedule.
Numerical or analytical solutions to the
Equation (
8.1
) can be used to simulate trends
in subsurface temperatures. Whereas methods
applied for estimating diffuse drainage attempt
to match spatial patterns in temperature that
are steady in time, methods applied for focused
drainage attempt to match temperature fluc-
tuations over time at specific locations. Diurnal
temperature cycles are usually analyzed (Ronan
et al
.,
1998
), but the approach can also be used to
analyze annual or seasonal temperature trends
(Lapham,
1989
; Bartolino and Niswonger,
1999
)
and to study infiltration events that may occur
over periods of hours to days on ephemeral
streams (Blasch
e t a l
.,
2006
; Hoffmann
e t a l
.,
2007
).
Surface-water temperature and temperature at
one or more depths within the diurnal or annual
temperature envelope must be measured.
Numerical simulations usually consider one-
dimensional vertical flow, but two-dimensional
simulations also have been used (Prudic
et al
.,
2003
,
2007
; Essaid
et al
.,
2008
), and three-di-
mensional simulations are possible. The upper
boundary condition consists of the streambed
and is usually treated as a specified pressure
head and temperature boundary. Boundary
Heat-tracer techniques, when applied in the
surficial zone, are most often used to esti-
mate rates of exchange between surface and
groundwaters. The discussion here will be on
streams, with the understanding that identical
principles hold for other surface-water bodies.
Temperatures in natural streams fluctuate on
daily and annual patterns in response to fluc-
tuations in air temperature and net radiation.
Stream temperatures can also be affected by
human activities, such as dam releases and cool-
ing operations at industrial facilities. Variability
in heat exchange between a stream and the sub-
surface is displayed by fluctuations in time and
space in subsurface temperatures.
Temperature envelopes in sediments under-
lying the center of a stream for an annual cycle
for losing and gaining streams are shown in
Fig u re 8.5
. (The temperature envelope is defined
by annual minimum and maximum tempera-
tures at each depth.) Water percolating down-
ward beneath a losing stream carries heat with
it, thus contributing an advective component to
the heat flow. On an annual basis, surface-water
temperatures fluctuate over a much wider
Stream
Winter
Summer
z
Downward flux
Upward flux
Temperature
Figure 8.5
Hypothetical annual temperature envelopes
beneath gaining and losing streams (after Stonestrom and
Constantz,
2003
). The envelope is defined by the annual
minimum and maximum temperatures at all depths (
z
). The
envelope is wider and deeper for a losing stream because
surface-water temperatures fluctuate more over a year
than do groundwater temperatures.
