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Title: ERRORS IN HEAT FLUX MEASUREMENT BY FLUX PLATES OF CONTRASTING DESIGN AND THERMAL CONDUCTIVITY

Author
item Sauer, Thomas
item Meek, David
item OCHSNER, TYSON - IOWA STATE UNIVERSITY
item HARRIS, ALLISON - DRAKE UNIVERSITY
item HORTON, ROBERT - IOWA STATE UNIVERSITY

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2003
Publication Date: 11/3/2003
Citation: SAUER, T.J., MEEK, D.W., OCHSNER, T.E., HARRIS, A.R., HORTON, R. 2003. ERRORS IN HEAT FLUX MEASUREMENT BY FLUX PLATES OF CONTRASTING DESIGN AND THERMAL CONDUCTIVITY. VADOSE ZONE JOURNAL. 2:580-588.

Interpretive Summary: Soil temperature affects many processes in the soil including decay of organic materials, chemical reactions, and growth of bacteria and plants. The amount of heat that moves through soil is called soil heat flux. Soil heat flux is commonly estimated by measuring the heat flow through a small plate buried in the soil. The plate has a fixed ability to transfer heat but the ability of a soil to transfer heat changes as the soil water content changes. This leads to an error in the plate estimates of soil heat flux. To determine how large this error is, this study compared several different types of plates currently used. Errors up to 75% were found with the largest errors occurring when the soil was wet and the plate had a low ability to transfer heat. A correction technique was used but didn't work very well, which suggests either that the correction itself has errors or other errors are occurring. This research is important to other scientists interested in making accurate soil heat flux measurements because it shows that errors when using plates to estimate soil heat flux can be large and some plates are more accurate than others.

Technical Abstract: The thermal conductivity of soils (k) may vary by a factor of ~4 over a range of field soil water contents. Measurement of soil heat flux (G) using a heat flux plate with a fixed k distorts heat flow through the plates and in the adjacent soil. The objectives of this research were to quantify heat flow distortion errors for soil heat flux plates of widely contrasting designs and to evaluate the accuracy of a previously reported correction. Six types of commercially available heat flux plates with varying thickness, face area, and thermal conductivity (km) were evaluated. Steady-state laboratory experiments at flux densities from 20 to 175 W m-2 were completed in a large box filled with dry or saturated sand having k of 0.36 and 2.25 W m-1 K-1. A field experiment compared G measured with pairs of four plate types buried at 6 cm in a clay soil with G determined using the gradient technique. The flux plates underestimated G in the dry sand by 2.4 to 38.5%, in saturated sand by 13.1 to 73.2%, and in a moist clay soil by 8.7 to 75%. Application of the correction generally improved agreement between plate estimates and independent G measurements, especially when k > km, although most plate estimates were still significantly lower than the actual G. Theoretical limitations of the correction procedure, inability to accurately characterize plate thermal properties, and factors such as thermal contact resistance likely limit the effectiveness of the correction procedure.