Campbell Scientific HFP01 Open Path Eddy Covariance (OPEC) - Page 10

OPEC Open-Path Eddy-Covariance System 1.3 Extended OPEC Energy balance sensors can be added to a basic OPEC system to also measure the net radiation, soil heat flux, soil temperature, and soil water content. The sensors required for these additional measurements are a NR-LITE, CNR2, NR01 or CNR4 net radiometer; two or four HFP01 or HFP01-SC soil heat flux plates, one or two TCAV averaging soil temperature probes, and one or two CS616 or CS650 soil moisture reflectometers. The number of soil sensors will vary with the site. A heterogeneous site will require more sensors to get adequate spatial representation of the site. The datalogger can accommodate the additional measurements with a multiplexer. Energy balance sensors no longer carried by Campbell Scientific are also supported by the extended OPEC datalogger program, including the Q7.1 and CNR1 net radiometers, and HFT3 soil heat flux plates. 1.4 Additional Fast Response Sensors If the application requires a direct measurement of sensible heat flux, a FW05 can be added to the system. In the absence of a FW05 or if the FW05 breaks, the sensible heat flux can be found using the sonic temperature corrected for humidity. A KH20 Krypton hygrometer, instead of the IRGASON gas analyzer, can be used to measure the latent heat flux. The KH20 can not be used to measure an absolute concentration of water vapor, because of scaling on the source tube windows caused by disassociation of atmospheric continuants by the ultra violet photons (Campbell and Tanner, 1985 and Buck, 1976). A slow response humidity sensor is used to measure the absolute humidity required to compute the air density. Air density is needed to scale the covariances into sensible heat flux and momentum flux. The rate of scaling on the KH20 windows is a function of the atmospheric humidity. In high humidity environments, scaling can occur within a few hours. That scaling attenuates the signal and can cause shifts in the calibration. However, the scaling over a typical flux averaging period is small. Thus, water vapor fluctuation measurements can still be made with the hygrometer. The effects of the scaling can be easily reversed by wiping the windows with a moist swab. 2. Installation and Mounting When making eddy-covariance measurements near the surface (less than 3 meters), mount the datalogger enclosure between the legs of the CM106 tripod, on a separate tripod, or user-supplied drive stake. Also, mount any sensor electronics boxes as far from the fast response sensors as possible and always use the tripod guy kit. This will minimize potential flow distortions and tower sway caused by wind blowing against the fiberglass enclosure. See the tripod manuals for detailed installation instructions. FIGURE 2-1 depicts a typical Open-Path Eddy-Covariance station. Point the Eddy-Covariance sensors into the prevailing wind to minimize the flow distortion from the tower, mounting hardware, and other sensors. 2