Improving Soil Heat Flux Accuracy with the Philip Correction Technique

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