Soil water thermal liquid diffusivity
The background of the thermal liquid diffusivity (TLD) concept was reviewed. Application of TLD to matric-head-based equations for water flow when a thermal gradient is present have been contradictory to the original development by Philip and de Vries (PdV). Beginning from Darcy's law for liquid water transport and Fick's law for water vapor transport was the foundational and well-known approach of PdV. The end result for an equation system based on water content and temperature gradients as driving forces includes a TLD coefficient. In a system based on gradients of water potential and temperature, no TLD follows, but one has appeared in several derivative works that have claimed PdV as their starting point. Typical calculated liquid water transport values attributable to TLD as a fraction of pure Darcy law transport may be about 5%. For isohydraulic conditions (zero hydraulic potential gradient) the relative difference is, however, unbounded.