The simplest way to visualize the basic components of the Sacramento model is to start NOT with streamflow,
but with the soil model.
If a block of soil mantle were to be carefully isolated in a container, some important characteristics could be observed as the soil block goes through successive cycles of drying and being rewetted.
If the soil vegetation were allowed to dry naturally, a certain weight of material would remain within the imaginary container.
This material would contain a small amount of residual moisture.
It would be virtually impossible to remove the remaining moisture in any ordinary atmospheric condition, and it is apparent that a small level of residual moisture is natural to any particular environment.
This level represents a lower limit of mositure.
It is so tightly bound to the soil molecules that it is generally unavailable for plant use or for evaporation into the atmosphere.

If moisture is slowly added to the soil block, there is a substantial capability of absorption without any leakage.
This volume of water generally is about 15% by weight, although it will vary depending upon soil types.
This additional volume represents moisture which is attracted to moisture-deficient soil particles so strongly that it can be removed only by evaporation or evapotranspiration.
In the Sacramento model, as in most soil analysis, this volume is identified as tension water.

However, once all tension water demands of the soil have been met, it apparent that there are many voids in the interstices between soil particles which could temporarily be filled with water which, unless artificially obstructed, would eventually drain out of the box.
The volume of water not bound to the soil molecules is identified as free water.

These components, tension water and free water, are the basic building blocks of the Sacramento model.