Accumulation of dirt and its effect on the system performance is an uncertainty, which strongly depends on the environment of the system, rain conditions, etc.  The accumulation of dust and the growth of mosses and lichens along the frame of the modules may produce partial shadings on the bottom cells, and tend to retain more dust.
Moreover, rainfall does not remove these accumulations.  A solution to this potential issue is the use of frameless modules. Bird droppings are usually not removed by rain and may represent a persistent issue. But their impact is typically relatively small.
With the rain in Canada, dust is not an issue.  However, bird droppings and potential moss needs to be cleaned once or twice a year.
Typical soiling performance factor in Canada is around 0.5%.