There are two impervious ratios required, the amount of directly connected imperviousness, XIMP, and the total imperviousness, TIMP. XIMP must be less than or equal to TIMP.

TIMP is a function of the land use of the catchment. Land use is a planning term that describes the approved, or proposed, use for the catchment (e.g. residential, commercial, industrial). Water resources studies are generally tied to planning applications and depending on the level of planning application, (i.e. Secondary Plan, Official Plan Amendment, Draft Plan), the modeler will have a little or a lot of information about the land use. Therefore, it is important to select a conservative value for the imperviousness when performing more macro level studies so that when the subsequent more detailed studies are completed, the more refined land use calculations will still be valid in the overall model.

The following table gives examples of suggested TIMP and XIMP values, based on land use, for the macro-level studies. These values can be used with the information supplied by the planner to determine area weighted values for the catchment of interest.

Estate Residential 20 40
Low Density Residential (e.g. Single Units) 25 50
Medium Density Residential (e.g. Semi-detached Units) 35 55
High Density Residential (e.g. Townhouse Units) 50 60
School 55 55
Commercial 85 85
Park 0 0

For more detailed level studies (i.e. Site Plan), there should be more information available so that the XIMP and TIMP can be calculated.