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Modeling unsaturated layer is somewhat more complicated, because it is very closely connected to dynamics of the saturated layer. As water seeps down from the unsaturated layer it adds to the saturated layer and the water table goes up. This in turn decreases the unsaturated depth and therefore the amount of water that can be stored in the unsaturated form. To build a simple model of the unsaturated water dynamics, we assume an enclosed area, that has no drainage of the saturated water. As a result all the water that is delivered by rainfall is stored there. The only loss is with transpiration. The transpiration is a function of the plant biomass growth, which can change over the simulation period. 
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SURFACE WATER
SNOW/ICE
SAT WATER
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UNSATURATED WATER - is the amount of water in
the unsaturated layer of the ground. We may think of the ground as a sponge.
If we pour water on a sponge it will hold a certain amount of water
before water will start dripping from the sponge. While water can be
still added and held by the sponge, it is in the unsaturated condition.
In the unsaturated
layer there is almost no horizontal movement of moisture, all flows are
only in the vertical dimension.