Green Roofs modelled as LIDs


I am modelling Green roofs as LIDs. Each subcatchment represents about 40 hectares of urban area. The current situation (without Green Roofs) is that 80% of the total area is Imprevious and 20% is pervious. When I include the Green Roofs, that will occupy a certain area of the subcatchment which is indicated in the "LID Usage Editor", where does the Model place this Green Roofs' area?? Does it consider that the GRoofs will substitute part of the Pervious area of the subcatchment? Part of the Impervious area? None (in this case I would be adding area to the subcatchment...)?

I hope the question is clear...

Thanks in advance!!


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  • Can you please explain how you designed the green roof.As I am designing a green roof and I have the same doubt that you have mentioned.

  • I think I found the answer to my question...(page 54 of User's Manual version 5.0):


    "...after LID placement the subcatchment's Percent  Impervious and Width properties may require
    adjustment to compensate for the amount of original subcatchment area that has now been
    replaced by LIDs (see Figure 3-5 below). For example, suppose that a subcatchment which is
    40% impervious has 75% of that area converted  to a porous pavement LID. After the LID is
    added the subcatchment's percent imperviousness should be changed to the percent of impervious
    area remaining divided by the percent of non-LID area remaining. This works out to (1 - 0.75)*40
    / (100 - 0.75*40) or 14.3 %."

  • The green roofs get both direct rainfall and runon from the other areas of the watershed so you are not double counting.  I think the green roofs are part of the pervious area but i am not 100 percent certain - I will look for you.

    A better way would be to subtract the green roofs from the impervious area, so it gets the direct rainfall but does not have infiltration.

    • Thanks Bob!

      I'm sorry but I didn't get your suggestion... you mean that if originally I have 80% Imperv and 20% Pervious (still without GRoofs), and I want to turn 10% of the Imperv area into Green Roof, then I should set the Imperv area as 70% (80%-10%) and the Pervious area as 30% (20%-10%) ?? That would definetly make sense if the Model assumes that the Green Roofs' area is part of the Pervious area (as you think it is).


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