Has anyone managed to achieve this credit for an urban site in Colorado, with pre-development imperviousness greater than 50%? our water laws prevent any water harvesting. We will provide retention which significantly decreases the release rate for a 2-year 1-hour event (the local design criteria). My civil engineer also tells me that reductions due to infiltration and evotranspiration "not easily quantifiable", so we have left that out in computing run-off, which basically shows that while the run-off rate is greatly improved, we have not reduced the total run-off quantity.