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Soil Heat Flux Plates

Heat Flow Distortion and Thermal Contact Resistance

^a USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-3120
^b USDA-ARS, Soil and Water Management Research Unit, St. Paul, MN 55108
^c Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

^* Corresponding author (sauer{at}nstl.gov)

Received for publication February 2, 2006. Persistent concern regarding surface energy balance closure encourages increased scrutiny of potential sources of error. Laboratory and field experiments addressed heat flow distortion and thermal contact resistance errors during measurement of soil heat flux (G) using the flux plate technique. Steady-state, one-dimensional heat flow experiments determined flux plate thermal conductivities (_m) and measured the effect of air gaps and thermal heat sink coatings on plate performance. Use of measured instead of manufacturer-specified _m and plate dimensions in a heat flow distortion correction improved the consistency but not the average disagreement between imposed sand G and corrected plate heat flux density (G_m). Consistent underestimates of G in dry sand by 20 to 25% after heat flow distortion correction was attributed to thermal contact resistance effects. A convex air gap 0.1 to 1.32 mm thick across 5.9% of the plate face area reduced G_m by up to 9.7%. A thin layer of a thermal heat sink compound with 0.18 W m^–1 K^–1 greater than the plate _m (1.0 W m^–1 K^–1) did not increase G_m in a clay soil but increased G_m by 6% in quartz sand. A 6.5% increase in G_m was also observed for plates treated with the same heat sink compound in a silt loam soil under field conditions. Thermal contact resistance errors are probably <10% in moist, medium-textured soils and can be minimized by careful plate installation. Relatively greater errors in G_m may occur due to thermal contact resistance in dry sand and due to heat flow distortion when soil >> _m.