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United States Department of Agriculture

Agricultural Research Service

Title: SOIL HEAT FLUX PLATES: HEAT FLOW DISTORTION AND THERMAL CONTACT RESISTANCE

Authors
item Sauer, Thomas
item Ochsner, Tyson
item Horton, Robert - IOWA STATE UNIVERSITY

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 6, 2006
Publication Date: N/A

Interpretive Summary: When energy from the sun reaches the earth's surface, it evaporates water, warms the air, or warms the soil. How solar radiation is divided between these processes is determined by properties of the surface including the height and density of the plant canopy, air temperature, and soil wetness. With global climate change, it is important to know how to manage the soil surface layers to increase warming or cooling to suit crop management. The amount of energy flowing in the soil is most often measured with sensors called soil heat flux plates. Although these plates are commonly used, they produce errors because the plates do not measure all the heat that is flowing through the soil. Laboratory and field experiments were conducted to determine the size of these errors to evaluate possible corrections. It was found that flux plates consistently underestimate the actual heat flow in soil, especially in dry sands. The errors were smaller in moist, finer-textured soils. Coating the plates with a heat-conducting grease proved to not be a practical solution to this problem. The results of this study are important to scientists trying to make accurate measurements of the surface energy balance and soil thermal regimes as they show how much flux plates may underestimate soil heat flow.

Technical Abstract: 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 were completed with flux plates embedded in air-dry clay soil or quartz sand. Individual experiments determined plate thermal conductivities and measured the effect of air gaps and thermal heat sink compound on plate performance. Use of measured instead of manufacturer-specified thermal conductivity and plate dimensions in a heat flow distortion correction improved the consistency but not the average disagreement between imposed G in sand and corrected plate heat flux density (Gm). Consistent underestimates of G in sand by 23.4±2.5% was attributed to thermal contact resistance between sand particles and plate surfaces. Subsequent experiments showed that a convex air gap 0.1 to 1.32 mm-thick over 5.9% of the plate face area reduced Gm by up to 9.7%. Application of a thermal heat sink compound with ' 0.18 W m**-1 K**-1 greater than the plate thermal conductivity (1.0 W m**-1 K**-1) did not increase Gm in the clay soil but increased Gm by ~6% in quartz sand. A similar but statistically-insignificant increase (6.5%) was observed for plates treated with the same heat sink compound in a silt loam soil under field conditions. Results indicate that thermal contact resistance can lead to relatively small but systematic errors and is difficult to detect and/or prevent.

Last Modified: 9/1/2014
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