Measurements
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Program Overview
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Why A USCRN is Needed
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Who Can Benefit
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Site Selection Criteria
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What is Measured
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Station Instruments
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Site Photos
The primary purpose of the USCRN network is to monitor air temperature, precipitation, and soil
moisture/soil temperature. In addition to these parameters, each station measures ground surface
(IR) temperature, solar radiation, wind speed, relative humidity, wetness from precipitation, and
several values that monitor the operating condition of the equipment. Some of the secondary
parameters contribute to improving the confidence in the observational measurements, and provide
insight into the reliability and performance of the primary sensors.
Highly accurate measurements and reliable reporting are critical. Station
instruments
are calibrated annually and maintenance includes
routine replacement of aging sensors. The performance of each station's measurements is monitored on
a daily basis and problems are addressed as quickly as possible, typically within days. Each station
transmits data hourly to a geostationary satellite. Within minutes of transmission, raw data and
computed summary statistics are made available on the USCRN web site. This page describes the
details of the data stream.
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Air Temperature
USCRN stations are equipped with three independent thermometers which measure air
temperature in degrees Celsius. The station's datalogger computes independent 5-minute
averages using two-second readings from each thermometer. These multiple measurements are
then used to derive the station's official hourly temperature value.
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Precipitation
Each station has a weighing precipitation gauge which is equipped with three load cell
sensors to provide three independent measurements of depth change (in millimeters) at
5-minute intervals. The three series of 5-minute values are then used in an algorithm to
derive the station's official 5-minute and hourly precipitation value.
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Soil Moisture and Soil Temperature
USCRN stations are equipped with three soil probes measuring temperature and moisture at
5, 10, 20, 50, and 100 cm depths (when possible). Each sensor records measurements at
5-minute intervals but only the observations at the 5-cm depth are saved to the datalogger.
However, hourly observations (which are derived from 5-minute observations) are saved for all
depths. Soil temperature is measured in degrees Celsius, and dielectric measurements are
converted to fractional volumetric water content measured in cubic meter of water per cubic
meter of soil (m
3
/m
3
).
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Surface (Skin) Temperature
Each USCRN station measures surface temperature in degrees Celsius using an infrared
sensor pointed at the ground. Every five minutes, the datalogger averages two-second
thermocouple-measured temperature readings to obtain 5-minute values. Beginning in January
2013, a correction was applied to surface temperatures at or above 15 degrees C. Please
refer to documentation on the
instruments
page for
details.
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Solar Radiation
USCRN stations measure global solar radiation from the entire hemisphere (direct plus
diffuse) using a pyranometer. Every five minutes, the datalogger converts the sensor's
two-second voltage output values into watts per meter squared (W/m
2
) and averages
them into 5-minute values.
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Wind Speed
Each station uses an anemometer to measure wind speed at a height of approximately 1.5
meters above the surface. Every five minutes, the datalogger averages two-second pulse
counts to derive 5-minute average wind speed in meters per second (m/s).
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Relative Humidity
USCRN stations have a single relative humidity sensor placed in the first of the three
air temperature instrument shields alongside the primary thermometer. The relative humidity
is measured as a percent of atmospheric capacity, from slightly above 0 percent to 100
percent. These measurements are made by a thin-film capacitive humidity sensor and reported
as 5-minute averages.
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Wetness
The presence of precipitation is detected through the use of a wetness sensor, or
disdrometer. When a water droplet falls on the detector pad, a circuit is completed and
resistance in one channel of the instrument drops to very small numbers, indicating the
presence of precipitation. This information is used in interpreting the rain gauge depth
changes and assigning them to actual precipitation when warranted.
Details on Measured Parameters
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Surface Air Temperature
Each USCRN station has three thermometers which report independent
temperature measurements each hour. These three observed temperature
value are used to derive a single
official USCRN temperature
value for the hour. This single value is sometimes a median and
sometimes an average of various combinations of the three observed
values, depending on information about which instruments agree in a pairwise comparison within
0.3°C. Each station transmits the three independent observed
values; the computation of the official USCRN temperature value is
done after these values arrive at NCEI. The discussion below
describes the details of the three observed values.
Each station has three Thermometrics platinum
resistance thermometers, each of which is housed in its own Met One
076B 7308 aspirated solar shield. Each thermometer measures the
temperature (in degrees Celsius) every 2 seconds. Every 5 minutes the station datalogger
computes the average of these 2-second values, giving 12 5-minute
averages for each thermometer. Standard deviations are also calculated for each thermometer.
Finally, a moving 5-minute average displaced 10-seconds at a time is used to determine the
maximum and minimum 5-minute periods ending within the hour in question.
In addition to the thermometer values, the station also
measures the speed of the fan in each aspirated shield.
As the shield's fan rotates, a contact closes and generates a pulse
twice per rotation. The datalogger counts these pulses every two
seconds. Every hour these 2-second values are averaged to obtain an
average number of pulses per second for the hour. The hourly
data stream from the station thus include the average pulse rate per second for each of the
three sensors. The actual speed of the fan in revolutions per second is half the pulse rate.
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Precipitation
Each USCRN station measures precipitation with a Geonor T-200B
precipitation gauge. This gauge produces twelve independent observed depth
measurements during each hour for each of three load cells. These observed values
are used to derive a single
official USCRN precipitation
value for the hour. Each station transmits the observed
values; the computation of the official USCRN precipitation value is
done after these values arrive at NCEI. The discussion below
describes the details of the observed values.
The Geonor T-200B uses a collection bucket which is
suspended by three vibrating wire strain gauges. Each wire, when
excited with 12V DC, vibrates with a frequency relative to the weight
in the collection bucket. The gauge is surrounded by a small
wind/snow shield, and a controlled heater device is attached to the
gauge to prevent ice buildup.
The station datalogger measures the frequency of each vibrating wire
and converts it to a gauge depth (in mm) every five minutes.
A Hydrological Services Tipping Bucket Rain Gauge Model TB-3 is
installed at most sites for comparison purposes only. Its data are
not quality controlled and are not considered official USCRN
precipitation readings, but can assist in cases where the Geonor unit fails during warm
conditions.
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Soil Moisture and Soil Temperature
For every USCRN site with deep soils, a total of 15 Stevens Water Monitoring Systems, Inc.,
Hydra Probe II (SDI-12) units are placed in the ground in three plots at five depths
(5, 10, 20, 50, and 100 cm) to measure soil moisture and soil temperature. These probes use
reflected electromagnetic radio waves at 50 MHz to determine the dielectric permittivity of the
soil in which the probe is inserted, which can be converted to volumetric soil moisture units
(m3m-3) by use of a calibration equation. The probe also contains a thermistor to measure
ambient temperature in the face plate pressing against the soil. All 15 probes are interrogated
every two minutes and their measurements averaged over 5-minute periods for output purposes.
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IR Ground Surface Temperature
An Apogee Instruments infrared temperature sensor measures
the infrared ground surface temperature (in degrees Celsius) at each station. The datalogger
samples the sensor every two seconds. Every five minutes these
two-second samples are averaged to obtain 5-minute values.
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Solar Radiation
A Kipp & Zonen SP Lite Pyranometer measures
solar radiation (watts per meter squared, W/m
2
) at each station. The datalogger
samples the sensor every two seconds. Every five minutes these
two-second samples are averaged to obtain 5-minute values.
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Wind Speed
A Met One Model 014A anemometer measures
wind speed (in meters per second) at each station. The datalogger
samples the anemometer every two seconds. Every five minutes these
two-second samples are averaged to obtain 5-minute values.
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Relative Humidity
The Vaisala HMT 337 HUMICAP instrument is a capacitive thin-film device that measures
relative humidity by the flow of electricity across two electrodes separated by a polymer
film. The capacitance of the film is related to relative humidity by a calibration equation
that converts the flow of electricity to relative humidity. These values are averaged over
5-minute periods to produce final data for this variable.
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Wetness Sensor
In addition to the above elemental observations, the hourly data stream from each station
includes measurements from a disdrometer, or wetness sensor. A Vaisala DRD11A Rain Detector
produces two variables, one of which provides yes/no information with regards to the impact of
hydrometeors on a slanted sensor plate, and the other indicating the intensity of the
precipitation. The former is used to provide information at least two times every 5 minutes as
to whether precipitation is falling.
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Miscellaneous
In addition to the above elemental observations, the hourly data stream
from each station includes the following values:
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FGBV
= Battery voltage for fan and GOES transmitter battery
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FGBV
full
= Battery voltage for fan and GOES transmitter battery under full load
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DLBV
= Battery voltage for datalogger
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DLDO
= Number of minutes in this hour that the datalogger door
was open
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