Most catchments are comprised of more than one type of ground cover. For example, a roadside drainage inlet may accept flow from the paved roadway, the curbside grass, and a nearby wooded area. This dialog allows you to model this, using a collection of subareas, each with their own characteristics. You can define the percentage of the total catchment area, a surface description, and a runoff coefficient. Note that the percentages must add up to 100. Surface description is a text field that you can use to further describe the subarea, so you could use descriptions like "Paved Roadway Surface" or "Curbside Grass" for example.