On the Measurement of Soil Heat Flux to Improve Estimates of Energy Balance Closure

Measurements of soil heat flux (G) are required to test for energy balance closure at many fluxnet locations. Data quality is often evaluated by comparing total available energy, composed of net radiation (Rn) and G, to the sum of latent (LE) and sensible (H) heat fluxes as measured by eddy covariance. Unfortunately, G often receives minimal attention during an experiment because other flux terms (e.g., Rn and LE) are larger or require more complex instrumentation. However, G at certain time periods can account for 10 to 30 percent of Rn, especially beneath short vegetation. Thus, errors in G can confound energy closure tests and lead to erroneous conclusions about of eddy covariance performance. Research was conducted to evaluate difference methods for automating and improving measurements of G. Automated soil heat capacity sensors were combined with heat flux plates to measure conduction and changes in heat storage at different position in the soil profile. The heat capacity sensors were also used to measure soil water content and soil temperature. Data were collected beneath turfgrass, a tallgrass prairie and a juniperous forest. Field results and numerical models were used to study the best sensor configuration and data analysis algorithms. Practical guidelines for measuring and calculating soil heat flux will be presented.