Water percolating from the Upper Zone Free Water to the lower zone may go totally to tension water, or some fraction of the percolated water may be made available to the primary and supplementary storages.
At any time that the Lower Zone Tension Water storage becomes totally filled, continued percolation is divided between these two Lower Zone Free Water storages.

The algorithm

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Runoff is thus the result of processing precipitation through an algorithm representing the uppermost soil mantle and lower soils.
This algorithm produces impervious runoff from permanent impervious areas and direct runoff from temporary impervious areas.
It also produces surface runoff which occurs when Upper Zone Free Water storage is full and the precipitation exceeds the rate of percolation as well as interflow resulting from the lateral drainage of the Upper Zone Free Water storage and primary baseflow.