Geography Reference
In-Depth Information
•
The number of sampling sites is dictated by the (1)
length of river to be measured, (2) its accessibility, and
(3) the location of temperature extremes within the river.
Care should be taken to avoid locally cool or warm
locations, as validation is best made where temperatures
represent the bulk of the region being observed.
•
Validation data can be collected fromautomatic gauges,
or fromsurveys with radiant or kinetic thermometers with
or without a data logger. A combination of devices can
provide an expanded network of observations in con-
junction with an image acquisition as long as all devices
are calibrated and care is taken in selecting measurement
locations.
◦
Measurements using hand-held kinetic thermome-
ters to measure T
k
(i.e., analog, digital, or data logger)
will generally be near the water surface, and because
inserting the thermometer into the water will break any
skin layer, suchmeasurements will always be of the bulk
water, not the skin itself.
◦
Analog thermometers are usually less expensive, but
digital thermometers may be easier to read and are
potentially more robust. Both can be used to rapidly
expand a sampling network with trained volunteers for
a single day of observations.
◦
Self-contained data loggers are the most expensive
option, but they can be used to monitor water temper-
ature deeper in the water column, for extended periods
of time, and in locations where regular access may
be difficult.
◦
Radiant thermometers can be used to survey a region
for appropriate sampling locations. They are, however,
generally sensitive to the entire TIR spectral range, and
require that a single representative
•
Precision of thermometers should be at least as good as
that expected from the TIR measurements.
5.7 Example 1: Illustrating the necessity
of matching the spatial resolution of
the TIR imaging device to river width
using multi-scale observations of
water temperature in the Pacific
Northwest (USA)
In this section, we show examples of TIR remote sensing
of water across a range of stream widths and pixel sizes,
to illustrate the necessity of matching the specifications
of the TIR imaging sensor to the river characteristics
such as the channel width. The combination of the spa-
tial resolution of TIR images and river width both affect
the accuracy and uncertainty of recovered in-stream T
r
(Handcock et al., 2006). The accuracy is of great impor-
tance in that any detectable spatial pattern in temperature
could be evidence for identifying thermal features such
as springs, seeping cold water or subsurface flow. To
illustrate the consistency between T
r
and T
k
fully, longi-
tudinal profiles of radiant temperature from downstream
to headwater stream reaches can be plotted in order to
compare with the
in situ
gage observations. The mean
difference and the standard error (deviation) of tempera-
ture are important metrics for evaluating the accuracy of
temperature extraction.
As an example of these concepts, we have provided a
group of images for reaches of the Green River, Washing-
ton (USA), as remotely sensed from TIR imaging sensors
with pixel sizes of 5m (MASTER), 15m (MASTER),
60m (Landsat 7 ETM
+
) and 90m (ASTER) respectively
(Figure 5.10a). Although Green River was clearly visible
under MASTER and Landsat sensors, this river reach was
obscured in the 90m pixel size of the ASTER image. In
our analysis, data were extracted along centre-lines of
the river, in order to remove geo-referencing errors and
possible along-stream mixed pixels. The results showed
that in the 5m MASTER image, the Green River along-
stream radiant temperature had a standard deviation of
0
be specified. Such
thermometers are also generally sensitive to their own
temperature so care should be taken when using them
in the field.
•
Measured T
r
represents the 'skin' layer at the water
surface. Depending on the amount of mixing in the river,
it may not be representative of T
k
further down in the
water column.
•
The effects of thermal stratification on data collection
can be minimised by collecting measurements in shallow,
well-mixed parts of the river because sites that are exposed
to solar radiation, slow moving currents, or substantial
cold-water inflows from seeps or springs may experience
substantial thermal stratification.
•
Water temperature should bemeasured close enough to
the time of the TIR image collection that the temperature
has not changed significantly (see Section 5.6.1).
ε
7
◦
C and the mean difference between radiant tem-
perature and kinetic temperature was
.
9
◦
C. The fact
+
1
.
7
◦
C variability was close to the NE
that the 0
Tfor
the MASTER sensor (0.46-0
.
71
◦
C; Hook et al., 2001)
indicated no obvious influence from warmer bank tem-
perature. In contrast, the standard deviation was 1
.
6
◦
C
and the mean difference was
.
Δ
1
◦
C for the 15m pixels
+
2
.
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