Percolation
The percolation mechanics of this system have been designed to correspond with observed characteristics of the motion of moisture through the soil mantle, including the formation and transmission characteristics of the wetting front.
The mechanics of transfer from Upper Zone Free Water to lower zone volumes is based upon the computation of a lower zone percolation demand.

When the lower zone is totally saturated, then the percolation into the lower zone must be limited to that water which is draining out of the lower zone.
This limiting drainage rate is computed as the sum of the products to the two Lower Zone Free Water storages and their respective drainage rates.

This limiting rate of drainage from the combined saturated lower zone storages is defined as PBASE.

PBASE = LZFSM * LZSK + LZFPM * LZPK
   where
      LZFSM = Lower Zone Free Water maximum supplementary storage,
            which is the maximum storage capacity for faster draining baseflow
      LZSK = Lower Zone supplementary storage depletion coefficient
      LZFPM = Lower Zone Free Water maximum primary storage,
            which is the maximum storage capacity for slower draining baseflow
      LZPK = Lower Zone primary storage depletion coefficient