I have a few questions on this credit that are buggering me off. Since I'm working on a site with with existing imperviousness higher than 50%, the main objective of this credit is to create appropriate infrastructure in order to reduce the total runoff volume of a 2-yr 24-hr design storm in 25%. The project site is largely dominated by a clay pan, so even though I'm increasing perviousness through the use of permeable pavements and rain gardens, infiltration is not feasible, so I'm retaining filtered water (from pavers and bioswales) and excess water in a wet pond, including excess from cisterns. I have two questions:
1. The cisterns are capable of holding approximately 25% of the design storm, however, they won't be empty the whole time, and therefore won't guarantee its retention throughout the year. Should I apply a given measure (say, on average), and compensate for the excess in the wet pond?
2. How do I account for water retained in planters, gardens and bioswales?

Greetings from Brazil