Environmental Engineering Reference
In-Depth Information
1999
). Anderson (
2005
) provided a more exten-
sive list and a summary of selected models.
Whether analytical or numerical models
are used, the general approach for determin-
ing water flux is similar and follows the gen-
eral model calibration guidelines described in
Section 3.2
and those presented by Niswonger
a nd P r u d ic (
2003
). Thermal properties are usually
held constant, and flux (or hydraulic conductiv-
ity and/or other hydraulic properties in the case
of a numerical model that solves a flow equation
in addition to Equation (
8.1
)) is adjusted between
model applications until simulated temperatures
agree with measured temperatures. Model cali-
bration can be accomplished manually (Lapham,
1989
; Ronan
et al
.,
1998
) or with a parameter
estimation program such as PEST (Bartolino and
Niswonger,
1999
; Prudic
et al
.,
2003
).
easily measured, logging can provide detailed
temperature-depth profiles at specific times.
Logging with a single sensor offers economy and
allows for recalibration of the sensor, but the pro-
cedure is time consuming. The detail provided
on the time history is dictated by the frequency
with which the logging is performed. Although
there are many exceptions, heat transport stud-
ies within the surficial zone usually are based
on fixed-depth temperature measurements,
whereas investigations within the geothermal
zone generally rely on temperature logging.
Recently developed distributed temperature
sensors (DTS) hold great promise for future appli-
cation in hydrologic studies (Selker
et al
.,
2006a
,
b
; Lowry
et al
.,
2007
). The sensors determine
temperature by measuring the scattering of
light along a fiber-optic cable. By laying out the
cable in a stream channel or hanging it down an
observation well, temperatures can be measured
over length increments of less than 1 m and at
frequencies greater than once per minute. DTS
provide the spatial detail of temperature logging
and temporal detail of fixed-depth sensors. Lowry
et al
. (
2007
) used the sensors to obtain streambed
temperatures at 1 m and 1 min intervals along
a 650 m stream reach in northern Wisconsin;
temperatures were averaged over 15 min peri-
ods, and a measurement accuracy of ±0.03°C was
determined. DTS systems are expensive, and the
cable is fragile. It is anticipated that the systems
will come into widespread use as these problems
are resolved.
Remote sensing of stream temperatures with
forward-looking infrared technology from heli-
copter platforms can provide detailed informa-
tion on spatial patterns of stream temperatures
(Mertes,
2002
; Loheide and Gorelick,
2006
).
However, helicopter flights are expensive, and
multiple flights are required to obtain temporal
patterns in temperatures.
8.2.1 Temperature measurements
A variety of sensors are available for measur-
ing temperature within the subsurface. These
include thermistor, thermocouple, resistance
type, and integrated circuit devices. Stonestrom
and Blasch (
2003
) reviewed the advantages and
limitations of the different types of sensors.
Sensor accuracy of 0.1 to 0.2°C is sufficient
for many studies within the surficial zone; an
accuracy of about 0.01°C is desirable for measur-
ing temperature profiles within the geothermal
zone. External data loggers allow temperatures
to be electronically recorded at fixed time inter-
vals; some newer temperature sensors have
internal recording capabilities.
Temperature sensors can be emplaced more
or less permanently at specific depths within the
subsurface with wire leads extending to land sur-
face.
Fixed-depth
sensors are usually set up to rec-
ord temperature at fixed time intervals and thus
are capable of providing detailed time histories
of temperature for specific locations. Expense,
however, may limit the number of locations that
can be monitored, and the sensors cannot be
recalibrated after emplacement. Temperatures
can also be logged within observation wells or
piezometers at discrete points in time. Logging,
usually a manual operation, consists of taking
temperature measurements at specific depths
with a single sensor. Because many depths are
8.3 Diffuse recharge
8.3.1 Diffuse drainage in the geothermal
zone
Temperatures within the geothermal zone
increase linearly with depth if there is no water
