Field Tests of the Soil Heat Flux Plate Method and Some Alternatives

Heat flux plates are commonly used to measure soil heat flux, a component of the surface energy balance. The plate method is simple and precise, but several previous studies have demonstrated the potential for relatively large errors. Here we present the results of in situ tests of the plate method, and we describe some promising alternative methods. Summertime soil heat flux was measured with heat flux plates and with two alternative methods at each of three sites. In total, three alternative methods were used: a single probe gradient method, a three needle gradient method, and a self-calibrating plate method. The standard plate method underestimated the magnitude of the heat flux by 18 to 66% depending on the site and type of plate. Agreement between the alternative methods was good with discrepancies ranging from 2 to 6%. The plates underestimate flux apparently due to a combination of low plate thermal conductivity, thermal contact resistance, and latent heat transfer effects. The three needle gradient method for measuring heat flux performed well at all three sites, providing a good alternative to the standard plate method. The self-calibrating plate method performed well at the one site where it was tested and may also be a good alternative. Increased adoption of these methods should lead to more accurate soil heat flux and surface energy balance data.