Improving Surface Energy Balance Closure By Reducing Errors in Soil Heat Flux Measurement.

The flux plate method is the most commonly employed method for measuring soil heat flux (G) in surface energy balance studies. Although relatively simple to use, the flux plate method is susceptible to significant errors. Two of the most common errors are heat flow divergence around the plate and failure to account accurately for heat storage in the soil layers between the plate and the soil surface. 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). Philip (1961) presented recommendations for flux plate design and use based upon theoretical considerations that were confirmed by Watts et al. (1990). Laboratory and field experiments were conducted under in a range of soil textures and physical conditions. Plate performance was compared to reference fluxes from a calibration device (laboratory) or the gradient method (field). The Philip correction, based on plate thermal properties and dimensions and soil thermal conductivity, was applied to improve plate performance. As predicted, plates with low thermal conductivity consistently underestimated the reference G and this error could not be completely eliminated with the Philip correction. Recommended practices for use of flux plates and alternatives are discussed.