Storage - Subsurface
… The infiltrated water may continue downward in the vertical, may move through subsurface layers in a horizontal fashion, or a combination of the two directions.
… Movement through the subsurface system is much slower than the surface and thus there are storage delays. The water may also reach an aquifer, where it may be stored for a very long period of time.
… In the NWS River Forecast System (RFS), the subsurface storage is represented by imaginary zones or "tanks". These tanks release the stored water at a given or calibrated rate. The released water from the subsurface zones is added to the surface runoff for convolution with the unit hydrograph.