This is the volume of air or water free pore space in the soil profile after the free or gravitational water has moved down to the water table.  Values of air volume (XVOL) corresponding to various water table depths (WTD) are input in the Drain Vol - Upflux tab of the Soil input screen.  DRAINMOD uses the volume drained relationship to determine rise and fall of the water table when a given amount of water is removed or added.  Drainage volume can be derived from the soil water characteristic values using the soil preparation program discussed in the Appendix.  The last entry for XVOL MUST be 100.0 and the last entry for WTD MUST be 1000 for the volume drained relationship.

 

The slope of the XVOL-WTD relationship is the drainable porosity.  Therefore this relationship can be obtained very simply from drainable porosity if it is known or if it can be estimated.  See the Reference Report (Skaggs, 1980) Figure 5.5 for details.