|Sauer, Thomas - Tom|
|Hernandez Ramirez, Guillermo|
Submitted to: Agricultural and Forest Meteorology Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/28/2008
Publication Date: 4/28/2008
Citation: Sauer, T.J., Ochsner, T.E., Heitman, J.L., Horton, R., Tanner, B.D., Akinyemi, O.D., Hernandez Ramirez, G., Moorman, T.B. 2008. Careful Measurements and Energy Balance Closure - The Case of Soil Heat Flux. Agricultural and Forest Meteorology Conference Proceedings. Available: ams.confex.com/ams/pdfpapers/138960.pdf.
Technical Abstract: An area of persistent concern in micrometeorological measurements is the failure to close the energy balance at surface flux stations. While most attention has focused on corrections associated with the eddy fluxes, none of the energy balance terms are measured without error. The flux plate method is the most commonly employed method for measuring soil heat flux and, although simple to use, it is also susceptible to significant errors. The objective of this research was to complete simultaneous calibration of several types of commercially-available soil heat flux plates. In order to eliminate possible errors due to different calibration media and especially thermal contact resistance, the measurements were completed with the plates embedded in agar-stabilized water. Four commercially-available flux plates with a range of thermal conductivity, face area, and thickness were evaluated. All measurements were completed in a calibration box consisting of an insulated 0.46 x 0.51 x 0.089-m cavity filled with agar-stabilized water. Calibration runs were completed at fluxes of 21, 43, 86, and 172 W m-2 for several days until steady-state conditions were achieved at each flux. One day of hourly data under steady-state conditions at each flux were used for analysis (Fisher’s Protected LSD) and interpretation. Average fluxes measured with the CN3, GHT-1C, and 610 flux plates were always less than the known flux through the agar. Agreement is significantly improved following the Philip correction for all plates using the manufacturer-specified thermal conductivity except the HFT1.1. Although all flux plates now underestimate the agar G, the disagreement is reduced to 11.4, 13.8, 15.8, and 27.8% for the GHT-1C, CN3, HFT1.1, and 610 plates, respectively. Use of measured plate thermal conductivities failed to improve agreement between the plate fluxes and the known agar flux. For the CN3, GHT-1C, and HFT1.1 plates there was actually better agreement without the Philip correction than with the Philip correction using measured plate thermal conductivity values. However, the best agreement of all plate-agar comparisons was achieved for the 610 plate and the Philip correction using measured thermal conductivity, which produced plate values statistically equal to the agar flux at 86 and 172 W m-2. Results of this study and previous research indicates that systematic errors resulting in consistent underestimates of soil heat flux when using flux plates is likely to occur. If more accurate soil heat flux values are desired, development of new measurement approaches (gradient method or perforated flux plates) will be necessary.