how do you compute for the volume of water collected in permeable pavements?

Answer 1

Permeable pavements don't store the water in them it just goes through to the soil. You can calculate how much goes through by determining the rainfall total for a year and multiplying the square footage of permeable paving laid. I'll include a link that has a bunch of details on permeable paving for you to review.

Answer 2

There is no easy way.

As the previous answer noted, permeable pavement is just that - pavement that is permeable and hence lets water through.

So the real question becomes how much does the soil under permeable pavements absorb? That depends on many factors including the nature of the soil, the nature of the surrounding terrain, and the rate at which water enters the system (sudden heavy rain vs. long period of light rain)

As for using the amount of rainfall on the pavement, that doesn't really work. For example, if you alternate strips of permeable and non-permeable pavement, some of the runoff from the non-permeable pavement may well get absorbed by the soil under the permeable pavement.