From email from the Colorado Division of Water Resources (Tracy Kosloff):
The Colorado Division of Water Resources is proposing a set of regional factors for Rainwater Harvesting Pilot Projects under House Bill 2015-1016 [colorado.gov]. Pilot projects may capture and use a specific amount of rainwater, referred to as historic natural depletion, out of priority without augmentation. The proposed regional factors estimate the historical natural depletion amount. The documentation and proposed accounting spreadsheet are posted for public comment during July 2019 on the Rainwater Collection [water.state.co.us] page of DWR’s website.