The volume which is percolated to the lower zone is divided among
three significant soil moisture storages.
The first of these, lower zone tension, represents that volume of moisture in the lower soil which will be claimed by dry soil particles when moisture from a wetting front reaches that depth.
Tension water deficiences are associated with a suction force, so they tend to absorb all percolating water until these deficiencies are satisfied.
However, variations in soil conditions and rainfall amounts over a drainage basin cause variations from the average condition.
The effect of these variations is approximated in the model by diverting a fraction of the percolated water into Lower Zone Free Water storages before lower zone tension water deficiencies are fully satisfied.
A constant PFREE is used to express the fractional split.
