Geography Reference
In-Depth Information
effects (where a dam or other obstruction results in
changed temperature upstream), and groundwater seeps,
which can change the bulk T
k
of the main river channel
locally. Fine-scale examples can be seen in Figures 5.1-5.3
in which seeps, springs, and tributaries affect water tem-
perature at spatial scales of a few to 100s of m. The mixing
of two very large tributaries, such as the Mississippi and
Ohio rivers (USA) (Figure 5.7) can affect water tempera-
ture for many kilometers. In measurements made on the
Columbia River (USA) using a hand-held thermometer,
thermal eddies were revealed at the edge of the wide river,
suggesting that more frequent measurements (
oceans, so beyond the potential for wind mixing, river
flow around and over rocks, woody debris and other
obstructions induces additional mixing and will break
the skin layer. Because of this additional mixing, tem-
perature derived from TIR images for turbulent rivers
should be more representative of T
k
than data for more
static circumstances.
5.6.4 Measuringrepresentative temperature
The use of hand-held, ground-based TIR thermometers,
or small imaging radiometers, provides the most direct
validation of remotely determined temperature. How-
ever, obtaining accurate temperature from such devices
can be difficult. Because TIR radiation is emitted by all
objects at all temperatures, the potential for contami-
nation of readings from a radiant thermometer is high.
Contact sensors, which include analog and digital sen-
sors, aremore robust and accurate, but theymeasure T
k
at
depth, thereby reducing their utility for comparison with
T
r
.T
k
is the environmentally and ecologically important
temperature but is not a direct comparison with what
the remote TIR sensor detects. For example, in oceano-
graphic applications where SST is of interest, a regression
is usually applied between
in situ
T
k
measured frombuoys
and ships and T
r
fromTIR sensors (Robinson et al., 1984)
to convert the skin temperature measurement of T
k
into
T
k
from deeper in the water column.
This regression approach is especially important for
SST measurements since bulk T
k
measurements have tra-
ditionally been collected at depths of up to 1m. Such
measurements can lead to values of T
k
that are 0.1 to
1.5 K warmer than near-surface measurements of T
k
.
Capturing representative river temperature is potentially
more difficult than in the ocean as water depths can
change, there can be exchanges of water between the
water and the streambed, and there can be spatial vari-
ability of temperature along and across the channel. For
that reason, the collection of representative temperature
of streams and rivers must be handled with care, both
for ground-truth evaluation of remote sensing prod-
ucts and for capturing the ecologically sensitive values
of T
k
.
Precision of thermometers should be at least as good as
that expected from the TIR measurements. For example,
if the TIR sensor provides temperature with an NE
Δ
T
of 0
.
5
◦
C, a bulb thermometer should have lines at least
every 1
1min
−
1
)
might be necessary when the river is not well-mixed or in
the presence of obstructions (Handcock et al., 2006).
∼
5.6.3 Thermal stratificationandmixing
As suggested byWunderlich (1969), the effects of thermal
stratification on data collection can be minimised by col-
lecting 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 strat-
ification (Nielson et al., 1994; Matthews and Berg, 1997;
Torgersen et al., 2001). Thermal stratification is possible
during sunny conditions or slow river flows when solar
heating of the surface layer is not compensated by vertical
mixing. For example, thermal stratification was observed
by Torgersen et al. (2001) in some areas of the river that
were not well mixed. Nielsen et al. (1994) found signifi-
cant vertical stratification for flows of less than 1ms
−
2
in
the Middle Fork Eel River in northern California. They
found that at depth, water could be as much as 7
◦
C cooler
than at the surface. These examples highlight that TIR
images only measure the water's surface, and that there
are complex mixing processes deeper in the water col-
umn. For a good discussion of this issue and the physics
of mixing, see Torgersen et al. (2001).
Wind and the breaking of waves can also disturb the
skin layer and leave the surface well mixed, as has been
seen in studies of ocean waves (Jessup et al., 1997), so
that the measured T
r
approaches the T
k
of the water at
the depth of mixing. Hook et al. (2003) investigated the
difference between T
k
in the surface layer and the T
r
in the skin layer of lakes using four monitoring stations
permanently moored on Lake Tahoe, California-Nevada
(USA). They found a difference between T
k
and T
r
that
varied over the diurnal cycle, which they attributed to
solar heating and lower wind speeds in the morning.
Water in rivers is more turbulent than in lakes and
0
◦
C so that temperature can be measured and
recorded with a precision of 0
.
5
◦
C.
.
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