This is the surface water runoff regime from a site before development, or the existing site conditions for brownfield redevelopment sites.

To maintain the natural equilibrium of a site, the surface water discharge from a developed site should not exceed the natural greenfield run-off rate.  Environment Agency Wales will advise on allowable discharge rates to watercourses for sites that are to be developed.  However, where possible it is better that surface water is managed at source.