Dividing the soil mantle

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It is also necessary to divide the soil mantle and consequently, the tension and free water, into upper and lower zones.
Although an infinite number of zones could be established, the goal in any parameterization is to use no more that is necessary to effectively describe the physical system.

If we assume that our soil box represents a simple river basin, then the upper zone tension water is restricted to that volume of water which can be applied to the dry soil before any component of leakage takes place from our soil box or basin boundaries.

Another component of moisture in the upper soil mantle of which we must be aware, is that volume of water which readily moves laterally through the soil to provide interflow or vertically into deeper levels of the soil as the supply for the wetting front.
The volume of water utilized for this purpose is a portion of the free water and is identified by its physical position as Upper Zone Free Water.