The current study extended the theory of heat flux which was normally used to estimate the mechanistic enhancement factor. The current improved heat flux theory included three additional phenomena that were excluded from the simplified heat flux equation. Those three phenomena were the sensible heat by liquid water movement, the sensible heat by water vapor movement, and the partial derivative of matric pressure head effects on relative humidity with respect to temperature. The first phenomenon was found to be an important factor in relatively wet porous media, whereas the third phenomenon was found important near-dry porous media condition. Moreover, mathematical descriptions derived to allow direct conversion of mechanistic enhancement factor to water vapor advection term. The study used basic mass balance equation, ideal gas law, and air advection to describe the water vapor advection mechanism. The water vapor advection equation was found to be able to describe the influence of soil moisture content and air pressure.