Improving Soil Heat Flux Accuracy with the Philip Correction Technique

Soil heat flux G(s) is an important component of the surface energy balance. Soil heat flux plates (SHFPs) are widely used to measure G(s), although several errors are known to occur. The Philip correction has been applied to minimize errors in G(s) measured by SHFPs (G(p)) if the soil thermal conductivity lambda(s), SHFP thermal conductivity lambda(p), and plate geometry function H are known. The objective of this study is to evaluate the effectiveness of the Philip correction for a variety of SHFPs. The lambda(p) were determined without thermal contact resistance and differed from the manufacturer-specified lambda(p). A simplified H formulation was similar to or less than the full H equation for different SHFP shapes. The G ratio (G(p)/G(s)) was sensitive to lambda(s)/lambda(p) and H when they were relatively small. Compared with the G(s) determined by a gradient method (G(s)_grad), the G(p) measured under a full corn (Zea mays, L.) canopy in the field underestimated G(s) by 38%-62%. After applying the Philip correction, almost all G(p) agreed better with G(s)_grad. Generally, the G(p) corrected with measured plate parameters agreed better with G(s)_grad than those corrected with manufacturer-specified values. The G(p) corrected with the simplified and full H expression differed for different SHFPs. These results indicate that SHFPs always underestimate G(s) and that the performance of the Philip correction is affected by lambda(p), plate dimensions, and H. An alternative method to measure G(s) by a three-needle heat-pulse sensor or a gradient method, in which soil temperature and water content are measured at several depths, is recommended.