ARS | Publication request: IN SITU MEASUREMENT OF SOIL HEAT FLUX WITH THE GRADIENT METHOD

Submitted to: Vadose Zone Hydrol
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 21, 2003
Publication Date: November 1, 2003
Citation: COBOS, D.R., BAKER, J.M. IN SITU MEASUREMENT OF SOIL HEAT FLUX WITH THE GRADIENT METHOD. VADOSE ZONE HYDROL. 2003. v. 2. p. 589-594.

Interpretive Summary: With the well-documented increase in greenhouse gas levels in the atmosphere, there is currently much interest in understanding the effects of agricultural and land-use practices on the cycling of these trace gases. The most common technique for directly measuring surface-atmosphere CO2 (a greenhouse gas) dynamics is eddy covariance, but the eddy covariance method often underestimates the magnitude of the surface-atmosphere flux, and to correct this underestimation an accurate measurement of soil heat flux is necessary. Present techniques for measuring soil heat flux are plagued by a number of sensor-related problems. In this manuscript, we evaluate a new type of soil heat flux sensor in both the laboratory and field settings. This sensor has a number of theoretical advantages over the sensors presently used. We found that the new sensor and the older heat flux plates actually agreed closely with one another, and that there are other larger sources of error, namely the calorimetric correction for changes in surface heat storage. This information should be useful in improving the understanding of both soil thermal dynamics and surface-atmosphere greenhouse gas interactions.

Technical Abstract: The use of soil heat flux as a critical component of the surface energy balance is routine; however, its accurate quantification is not. The most common method for measuring soil heat flux uses soil heat flux plates to measure soil heat flow at a specified depth, and calorimetry to estimate the changes in heat storage in the soil above the plates. The presence of the heat flux plates causes perturbations in heat and fluid flow in the soil, which may give rise to measurement errors. We describe here the measurement of soil heat flux directly as the product of the soil thermal conductivity, measured by a transient heated needle, and the soil temperature gradient. Laboratory trials exposed errors in the accurate determination of thermal conductivity arising from poor sensor-soil thermal contact in coarse, dry media. Extended field data in a finer textured soil showed remarkable agreement among needle sensors and heat flux plates, and did not allow the identification of errors associated with fluid blockage by the heat flux plates. An analysis of the sources of variability showed that the calculations of the storage term resulted in twice the variability of that introduced by the sensors. This indicates that improvements in the calculation of the storage term may be the most effective way to improve the measurement of soil heat flux.