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### 2.4. Total moisture transport

In the previous sections, the transport equations for the liquid and the vapor phases were considered separately (Eqs. (27) and (47)). To get a complete description of the transport, both equations should be added together:

 (48)

As the density of water vapor has a much lower value than the one of liquid water and θGθL, Eq. (48) can be simplified as:

 (49)

Let

 (50)

Substituting Eq. (50) into (49) gives:

 (51)

This is Richards' equation. As can be seen, the equation fully describes the movement of both vapor and liquid water on the basis of a single variable θL. The influence of both phases is taken into account through the function Dθ, which contains a term associated to the vapor transport and a second one related to the transport of the liquid phase. The demonstration also indicates that the variable θ of the original Eq. (4) stands for the volumetric liquid water content, which is expressed in cubic meter of water per cubic meter of material.
Next: Determination of the moisture Up: Water transport Previous: Transport of water