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.
compute pathogen wash off
compute pathogen die-off and re-growth on foilage
compute pathogen die-off and re-growth in surface soil layer
net die_off - negative is regrowth