of the measurements, as the metal rotor tends to
Air and water temperature
catch floating slush, or frazil, ice (Crantz et al.
Air and water temperature are relatively easy
1982a). However, since 1988, the USGS has used
to collect remotely, but some difficulties are still
the modified yoke Price-type winter meter as the
encountered. Air temperature is almost always col-
standard meter for discharge measurement
lected at project sites using a mercury thermom-
through an ice cover, allowing the use of solid plas-
eter or some type of digital thermometer, or tem-
tic rotors during slush ice conditions to reduce
peratures are collected from the nearest National
rotor plugging (Wagner 1994).
Weather Service site. As with any other type of
The current USGS standard method of dis-
instrument, a thermometer must be placed cor-
charge measurement of ice-covered streams (Rantz
rectly in order to obtain a good reading. Accuracy
et al. 1982a) requires the drilling of holes in the ice
to the nearest degree is often all that is desired (and
through which the current meter is immersed (un-
needed) for air temperature.
less open water exists relatively near the gaging
Such is not always the case with water tempera-
station). The use of the ice surface as a working
ture measurements. Frazil ice forms when water
platform can lead to concerns for personnel safety
supercools below the freezing point by only a few
and cause some unique problems. Discharge mea-
hundredths of a degree (Ashton 1986). However,
surements collected under ice cover conditions are
if the temperature measurement device is only
generally considered to have greater uncertainty
accurate to the nearest degree, water temperatures
of nearly 0.5C (warm enough to melt ice) and
than discharge measurements made under open-
0.01C (supercooled) will both register as 0C.
water conditions at the same location (Cobb and
Latkovich 1986). Ice does not even have to be
When estimates of frazil ice production are needed
present to affect the stage-discharge relationship;
(e.g., estimating when heavy frazil ice production
it has been demonstrated that decreases in water
may begin to impact navigation traffic, or when
temperature have an apparent impact on bed
river intake structures might be affected), an in-
strument capable of reading to the nearest 0.01C
roughness (Colby and Scott 1965, USAED, Omaha
1969). Discharges during an ice jam can create
may be needed. Typically, a glass-bead thermistor
stages vastly different than from a stage-discharge
is used in such situations. Generally, these ther-
curve; some type of on-site observation is almost
mistors are used in conjunction with a digital
always needed to verify if stages at a gaging loca-
multimeter for determining temperatures. They
tion are being affected by ice during a freezeup or
can be permanently installed and connected to a
breakup event.
data logger or DCP for recording temperature
Most reported discharges are determined from
data. When connected to a DCP, a voltage divider
a stage-discharge curve. Some USGS offices main-
circuit that converts resistance to voltage is needed.
tain separate rating curves for open-water and ice-
A good reference for permanent thermistor instal-
covered flow, but typically the USGS has not cor-
lation guidelines can be found in EM 1110-8-1(FR)
rected the daily discharges for ice effects until af-
(USACE 1990). Each thermistor is hand-made and
ter ice out, using the hydrographic and climatic
must be individually calibrated. The resistance of
comparison (Walker 1991). Walker (1991) con-
the calibrated thermistor is used in the Steinhart-
cluded that analytical methods could be better
Hart equation, usually with three terms, to deter-
than the subjective hydrographic and climatic
mine temperature.
comparison, but recommended further refinement
Thermistors are theoretically capable of a tem-
perature accuracy within 0.010.02C, but exami-
and investigation. Further work by Walker (1994)
suggests nationwide implementation of a method
nation of water temperature records at a specific
he calls the "first-visit complete-profile" for use
DCP site can reveal variations of several degrees
in improving the accuracy of discharge measure-
one day, consistent readings the next day, and
ments under ice covered conditions. Wagner (1994)
missing records the day after. It may seem when
notes that during the work of Melcher and Walker
going back over temperature records that such
(1992) in Iowa in the 1987-88 season a computer
problems occur most frequently during the most
program was developed that allowed for daily
crucial freezing periods, but this may seem so only
discharge adjustments via computer monitor,
because they are the periods of most interest (and
based on other nearby weather data and discharge
most scrutinized). Possibly this problem is not with
hydrographs. Wagner further states that this ca-
the thermistor itself, but with other components
pability is being incorporated into USGS nation-
in the DCP system being adversely affected by cold
wide streamflow computer programs.
air temperatures.
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