Nodes of different colours represent the following:
Solid arrows point from a submodule to the (sub)module which it is
descended from. Dashed arrows point from a module or program unit to
modules which it uses.
Where possible, edges connecting nodes are
given different colours to make them easier to distinguish in
large graphs.
set channel drainage areas
order channels by drainage area - set linked list
set the sorted object- aq_ch
save total channel length in aquifer
compute length of channel left when current channel dries up