Upper Zone

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In the permeable portion of the basin, the model represents the initial soil moisture storage identified as Upper Zone Tension as a volume which must be totally filled before moisture becomes available to enter other storages.
Upper Zone Tension represents that volume of precipitation which would be required under dry soil conditions to meet all interception requirements, and to provide sufficient moisture to the upper soil mantel so that percolation to deeper zones, and sometimes horizontal drainage can begin.

Before water can move through the soil to wet dry soil below, the moisture content of the shallow soil must be raised to the minimum value at the wetting front of that soil. When the Upper Zone Tension volume has been filled, excess moisture above the Upper Zone Tension Water capacity is temporarily accumulated in the Upper Zone Free Water. Free water is that water which is not bound to soil particles and is free to move. Although free water is present during filling of Upper Zone Tension Water, it is not necessary to account for it, as it is rapidly transformed into Uper Zone Tension Water until tension water requirements are satisfied. Once the Upper Zone Tension Water is satisfied, Upper Zone Free Water is available to descend to deeper portions of the soil mantle, or to move laterally through the soil in response to gravitational and pressure forces.

ET is mean areal Evapotranspiration