"Impervious surfaces have a perviousness of less than 50% and promote runoff of water instead of infiltration into the subsurface. Examples include parking lots, roads, sidewalks, and plazas."

"Perviousness is the percentage of the surface area of a paving system that is open and allows moisture to soak into the ground below."

Perviousness refers only to paving systems, therefore imperviousness refers only to paving systems...rubbish!

I have an existing site. The new building will be built on an underground parking garage which is currently covered by a layer of turf...so what is the pre-construction "perviousness" of this area? What units is it measured in?

I can calculate pre and post developement run-off rates and volumes, but what does that help if I can't determine into which of the 2 cases it falls into.

Does anyone have a quantifiable definition of perviousness?