• logically distribute applied moisture in various depths and energy states in the soil
• have rational percolation characteristics
• allow an effective simulation of streamflow

Basically, the SAC-SMA is a two layer conceptual model of a soil
column, with several modifications to account for the spatial
variability of certain processes.  Six types of runoff can be
generated to form a complete runoff hydrograph.  Each of the two
layers in the SAC-SMA contains a tension water and free water
component.  Rain falling on the soil column first encounters the
upper zone.  Here, rain falling on any impervious areas
generates impervious area runoff, while rain falling on the
non-impervious areas of the basin first encounters the upper
tension water storage.  After filling this reservoir, excess
soil water enters the upper zone free water.  Water in this free
water storage can percolate into the lower zone storages or flow
out as interflow.  If the upper zone free water fills completely,
then excess soil water flows out as surface runoff.  Most
percolated water flows into the lower zone tension water storage,
although some can go directly to free water storages in the
lower zone.  Upon filling the lower zone tension water storage,
all soil water moves into the two lower zone free water storages.
These two free water storages generate fast and slow responding
base flow. The combination of these two base flows is designed
to model a variety of hydrograph recessions.  The SAC-SMA also
has a variably-saturated area component from which saturation
excess runoff is generated.