ICRO is the net contribution of water from impervious surfaces onto pervious surfaces referred to as ‘indirectly connected impervious areas’. This net factor accounts for several physical conditions reduce the impacts of indirectly connected impervious areas on pervious areas.

  • Unlike precipitation and snowmelt, which are generally uniformly distributed over the pervious areas, runoff distribution from impervious areas are often concentrated in flow channels.
  • Indirectly connected impervious areas are not always uniformly distributed over the entire basin and only affect a fraction of the pervious surfaces.

The net effect is that water from indirectly connected areas is somewhat reduced and not available over the entire pervious surfaces. This is modeled with a reduction factor in Continuous OTTHYMO. Evaluation of typical lot layouts and grading suggest that a 0.20 multiplication factor (80% reduction) of the indirectly connected areas is a reasonable value.