Deeper within the soil mantle, tension and free water must also be considered.
The Lower Zone Tension water is that remaining volume of tension water, in excess of the upper zone tension water component, that is necessary to fully satisfy moisture requirements based on the molecular attraction between dry soils and moisture, but does not include any free water in the interstices between the soil molecules.

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The problem now becomes one of utilizing these storages to define the moisture distribution and runoff process which accompanies storms of varying magnitudes occurring over a particular river basin.
The transfer of these concepts to a river basin, and the development of the peripheral logic needed for simulating the percolation and runoff processes, follow from these concepts.

The reader is referred to NWSRFS Users Manual II.3 (Burnash and Ferral) for a more complete conceptual description.