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Accumulation of dirt and its effect on the system performance is an uncertainty which strongly depends on the environment of the system, raining conditions, etc.

In medium-rainy climates (like middle of Europe) and in residential zones, this is usually low and may be neglected (less than 1%).

In rural environments with agricultural activity, it may be important during some seasonal activities. In industrial zones, one can observe not negligible effects of the order of several percents.  Häberlin reports the effect of metallic dusts near to a railway line, which may initiates further pollution and mosses.  

The accumulation of dusts and the growth of mosses and lichens along the frame of the modules produces partial shadings on the bottom cells, and tend to retain more dust. Moreover these pollutions are not removed by the rainfalls. Therefore with low tilts, it is recommended to use frameless modules when possible.  

Birds droppings represent a serious problem, as they are usually not removed by rainy events. But their impact is reported as relatively small (less than 2%).  

The soiling losses are strongly dependent on the rainfalls of course. Therefore PVsyst allows the definition of soiling loss factors in monthly values. During the simulation, the soiling loss is accounted for as an irradiance loss.

We can propose some reference articles about soiling, and also a review in Thevenard (2010), p.22. But they don't provide definitive answers...  

Snow

Snow is not part of the meteo data in PVsyst, and its effect is very difficult to foresee (when is it leaving the array ?).  In very concerned conditions, you can take it into account by defining partial or complete soiling attenuation during some months.