ERRORS IN SOIL HEAT FLUX MEASUREMENT: EFFECTS OF FLUX PLATE DESIGN AND VARYING SOIL THERMAL PROPERTIES

Authors: SAUER, THOMAS; HARRIS, ALLISON - DRAKE UNIVERSITY; OCHSNER, TYSON - IA STATE UNIVERSITY; HORTON, ROBERT - IA STATE UNIVERSITY

Submitted to: Agricultural and Forest Meteorology Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/24/2002
Publication Date: 5/25/2002
Citation: SAUER, T.J., HARRIS, A.R., OCHSNER, T.E., HORTON, R. ERRORS IN SOIL HEAT FLUX MEASUREMENT: EFFECTS OF FLUX PLATE DESIGN AND VARYING SOIL THERMAL PROPERTIES. AGRICULTURAL AND FOREST METEOROLOGY CONFERENCE PROCEEDINGS. 2002. P. 11-12.

Interpretive Summary: The energy from sunlight that enters and warms the soil is called soil heat flux. Soil heat flux is important because the amount of heat that moves into a soil affects the soil temperature, and soil temperature affects many biological and chemical processes like the decomposition of crop residues. Most measurements of soil heat flux are made using a sensor called a heat flux plate. The material that the plate is made of and the plate dimension affect the accuracy of the measurements. Four different plate styles were compared in a laboratory and a field experiment. It was found that the plates that conducted heat more slowly had larger errors. A method already available that corrects for this error did not work well. It was also found that accounting for the amount of heat stored in the soil above the plates was very important. These findings are important for resource managers interested in optimizing soil temperature for crop growth or explaining how tillage or crop residue affects soil temperature.

Technical Abstract: The flux plate method is the most commonly employed method for measuring soil heat flux (G) in surface energy balance studies. Nonetheless, significant errors in G measured with flux plates can occur unless proper installation techniques are used and necessary corrections made. The objective of this research was to quantify potential errors in measured G when using soil heat flux plates of contrasting designs (thermal properties and dimensions) and under varying environmental conditions. Four different designs of flux plates with thermal conductivity ranging from 0.23 to 1.0 W/m K, area from 4.9 to 27 sq. cm., and thickness from 2.6 to 7 mm were evaluated. Laboratory and field experiments were completed to compare plate performance under conditions of varying G and soil water content. Results indicate that plates with low thermal conductivities had measured fluxes up to 45% less than the actual flux. The Philip correction technique did not fully compensate for low plate thermal conductivity. Correction for heat storage above the flux plate significantly affected peak G, the timing of peak G, and increased the cumulative daily G by over 20%. Direct measurement of C with the dual source probe agreed with de Vries' C estimate and can be used for real-time heat storage corrections without measuring soil water content.