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U.S. GEOLOGICAL SURVEY
Data Series 284

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Evapotranspiration Instrumentation

ET instrumentation consists of a CSI eddy covariance system which includes a CSAT3 three-dimensional sonic anemometer, a KH20 krypton hygrometer, and a FW05 fine-wire thermocouple (type E, 0.0005 inch diameter). These instruments measure two main energy budget components: the transfer of water vapor (latent-heat flux) and the transfer of heat (sensible-heat flux) through the atmosphere, using the eddy covariance technique. Additional sensors were added to this instrumentation to document the two other principal energy budget components of net radiation and soil-heat flux. Net radiation is measured by a REBS Q7.1 net radiometer. Soil-heat flux is calculated by measurements obtained from ground sensors consisting of two REBS HFT3.1 heat-flow transducer plates, a TCAV-L (CSI) averaging soil-temperature probe, and a CS616 water content reflectometer. The eddy covariance system uses a HMP45C Vaisala temperature and relative humidity probe to derive other necessary variables for the eddy covariance technique.

The anemometer, hygrometer, and fine-wire thermocouple were mounted on their own tripod with cables running to a second tripod used to mount the net radiometer, Vaisala temperature and relative humidity probe, cellular phone antenna, and enclosure box containing a CSI 23X data logger, storage module, cellular phone and other terminals and controls. The installation geometry of the soil sensors is the same as at the weather station. The net radiometer was mounted 3 m above the ground and 3 m out from the tripod; this sensor placement was selected to provide a representative measurement of the land surface consisting of sparse plant canopy and bare soil. The temperature and humidity probe along with the anemometer, hygrometer, and fine-wire thermocouple were mounted 2 m above the ground surface.

The ET station was operated using a field program (flx232_3.csi, version 2.3, 22 March 2001) obtained from Campbell Scientific, Inc. (Ed Swiatek, Campbell Scientific, Inc., written commun., 2002). The field program was modified to the specific sensors used at the ADRS. The eddy covariance fluxes are computed for a 15-minute covariance period based on a 0.1-second execution or sampling interval. Other sensors generally sample data for a 1-second sampling interval and output data every 15 minutes.

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