Upper Zone Free Water has two functions:
- the horizontal potential to generate interflow
- more significantly, a vertical potential which varies with the amount of water available in the upper zone.
Interflow is proportional to the available free water volume remaining after percolation.
Interflow = UZK * UZFWC
where
UZK = the Upper Zone Free Water storage depletion coefficient
UZFWC = that volume of free water presently stored in the upper zone
Lower Zone
Lower Zone Tension Water capacity is that depth of water held by the lower zone soils after wetting and drainage which is generally available for evapotranspiration.
The two Lower Zone Free Water storages, Primary and Supplemental, represent those volumes which are available for drainage as baseflow or subsurface outflow not appearing in the channel.
These free water storages fill simultaneously from percolated water and drain independently at different rates, generating a variable ground water recession.
