A detailed description of SAC-SMA can be found in Burnash et al,
[1973] and Burnash et al,[1995].  The basic design of the SAC-SMA 
model centers on a two layer structure: a relatively thin upper
layer, and usually a much thicker lower layer which supplies
moisture to meet the evapotranspiration demands.  Each layer
consists of tension and free water storages that interact to
generate soil moisture states and five runoff components.  The
free water storage of the lower layer is divided into two
sub-storages: the LZFSM which controls supplemental (fast) base
flow, and the LZFPM which controls primary (slow) ground water
flow.  Partitioning of rainfall into surface runoff and
infiltration is constrained by the upper layer soil moisture
conditions and the percolation potential of the lower layer.
No surface runoff occurs before the tension water capacity of the
upper layer, UZTWM, is filled.  After that, surface runoff
generation is controlled by the content of the upper layer free
water storage, UZFWM, and the deficiency of lower layer tension
water, LZTWM, and free water storages.  Each free water reservoir
can generate runoff depending on a depletion coefficient of the
reservoir, namely the UZK coefficient for the upper layer, and
LZSK and LZPK for the lower layer supplemental and primary free
water storages, respectively.