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U.S. GEOLOGICAL SURVEY
Data Series 284

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Micrometeorological Instrumentation

Micrometeorological sensors consisting of a tipping‑bucket precipitation gage, an air-temperature and relative‑humidity probe, a barometric-pressure sensor, a pyranometer, and an anemometer with wind vane were installed in December 1997. A net radiometer was installed at the site in July 1998. Soil-temperature probes and two soil-heat-flux plates were installed in September 1999. A soil-moisture probe was installed in January 2002. Data from two accumulating precipitation gages periodically were read after storm events to verify the data values obtained by the tipping-bucket precipitation gage.

Data from all the sensors were recorded using a Campbell Scientific, Inc. (CSI), 23X data logger using a 10-second sampling interval. The logger was programmed to output data in three formats: daily mean values (except for precipitation where only daily accumulated totals are output); hourly mean values (except for precipitation where only hourly accumulated totals are output); and 5‑minute totals for precipitation (to define storm event timing rather than to collect data values and are not included in this data report). The data logger is inter­faced to a telephone modem permitting automated communication and data retrieval using an off-site computer. The data logger reference is Pacific Standard Time (PST) throughout the year.

The air-temperature relative-humidity sensor, pyranometer, and anemometer with wind vane are mounted on a CM10 tripod at heights of 1.5 m, 3 m, and 3 m above the ground surface, respectively. The barometric-pressure sensor is mounted 1 m above the ground and housed with the data logger in a shed approximately 30 m north of the tripod (fig. 2). The net radiometer is mounted 1.5 m above the ground and approximately 10 m from the tripod. The net radiometer monitors principally bare soil radiation within the undisturbed vegetated area. The precipitation tipping-bucket precipitation gage was installed on its own mount approximately 5 m from the tripod at a height of 1 m. Adjacent to the tipping-bucket precipitation gage is an accumulating plastic precipitation gage made by Tru-Chek mounted at a height of 1 m along the south fence of the instrument shaft area (fig. 2), and a second Tru-Chek precipitation gage was at the south end of the waste-trench area (fig. 3). Two soil-heat-flux plates are buried in the near-surface soil at a depth of about 0.08 m approximately 2 m from the weather-station tripod. Between the flux plates and the soil surface, the aver­aging soil-temperature probes are buried at depths of 0.02 and 0.06 m. The soil-moisture probe is buried to measure average soil moisture in the depth interval between the flux plate depth of 0.08 m and the soil surface. The probe has two 0.3 m rods spaced about 0.03 m apart. The rods are inserted into the ground at a slight angle to integrate the water content of soil over the depth interval.

The accuracy of the data is dependent on the sensors being used. Instrumentation manuals from the manufacturers (vendors) contained the following sensor specifications. The tipping-bucket precipitation gage is a WeatherMeasure model P-501 with a resolution of 0.25 mm representing one tip of the bucket, and an accuracy of 0.5 percent at 12.7 mm/h. The air-temperature relative-humidity sensor is a Vaisala HMP35C from CSI with a temperature accuracy of ±0.4°C over a range of -24 to 48°C, and measures a relative humidity accuracy of ±2 percent within the range from 0 to 90 percent and ±3 percent within the range from 90 to 100 percent. The Vaisala temperature and humidity sensor is mounted inside a 12‑plate gill radiation shield. Solar radiation is mea­sured with a LI-COR LI200X silicon pyranometer cal­ibrated against an Eppley Precision Spectral Pyranometer, and has a maximum error of ±5 per­cent. The net radiometer is a Radiation and Energy Balance Systems (REBS) Q7.1 net radiometer, which has a spectral response from 0.25 to 60 µm, with a nom­inal resistance of 4 ohms. Wind speed and direction are measured by a Met One 034A-L Windset with a wind speed accuracy of ±0.12 m/s, a threshold of 0.28 m/s, and wind direction accuracy of ±4 degrees. The barometric-pressure sensor is a CSI SBP270 with a pressure range from 800 to 1,100 mbar and an accuracy of ±0.2 mbar. Soil temperature is measured with a TCAV-L averaging soil-temperature probe manufactured by CSI with two junctions at two depths and constructed using a Type-E thermocouple (chromel-constantan) wire. The four thermocouples and associated reference temperature define an aver­aged soil temperature with a typical uncertainty of 0.5°C, but the uncertainty can be as high as 1.6°C. The soil-heat flux is measured with two REBS HFT3.1 heat-flow transducer plates with a nominal resistance of 2 ohms and a thermal conductivity of 1.00 Watt per meter per degrees Kelvin (W/m/K). The soil-heat-flux plates have an error of about ±5 percent. The near-surface soil moisture is measured using a CS615 water content reflectometer designed to measure volumetric water content derived from the probe sensitivity to the unique dielectric content of the soil which changes with changing moisture conditions. The soil-moisture probe has an accuracy of ±2 percent when calibrated for a specific soil. The soil moisture values have been calibrated to the specific soil at the site by periodic field sampling of soil between the 8-cm flux-plate depth and the soil surface. Samples were analyzed to determine gravimetric water, bulk density, and volumetric water content.

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