Thank you for your fast reply. Yes, I use the Site:Precipitation object. First I had following inputs: Model: ScheduleAndDesignLevel, Design Level: 0.437 (my annual sum), Schedule Name: my schedule name, Average annual sum: 0.437. The output cumulative sum was 24.6 higher than my input, so I divided now 0.437/24.6 = 0.018 which I use as new input for the Design Level. I now get the correct annual sum as output but the rainfall pattern is totally wrong: in the first half of the year it is much less and in the second much higher than my data values. Any help with that?

To rephrase my question: is it possible that EnergyPlus just uses my precipitation data as they are without modulating the data somehow?

Interesting to note is that the modelled moisture ratio of the substrate corresponds quite well with my measurements, when I don't lower the value of the design level input (and badly when I do so). Might something be wrong with the precipitation reporting?

The Average Total Annual Precipitation should equal the sum of the schedule. You can report an output variable called Schedule Value to check the values of your schedule pattern.

I have correct values now (schedule values = input values). The Environment:Green roof cumulative precipitation depth are still not correct, but I am now very sure that the rest is working correctly. Thank you for your help!