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.

Most catchments are comprised of more than one type of land cover. For example, a roadside drainage inlet may accept flow from the paved roadway, the curbside grass, and a nearby wooded area.

The Subareas Collection dialog allows you to model this, using a collection of subareas. Each subarea, or row in the table, provides for 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.

Select the “Area Defined By” choices of either Multiple Subareas or Land Cover Areas for the Catchment to be defined by the weighted average of several subareas. The option of Multiple Subareas provides for the user supplied subarea proportions and details. The Land Cover Areas choice will compute the subarea proportions and populate their runoff terms based on the Land Use Area polygons that intersect the Catchment.

Computing the Subareas Collection with Land Use Areas

After stacking Land Use Area polygons atop one or catchments, simple right click and select “Compute Catchment Subareas” to populate the rows of the Subareas Collection. A row of Default will appear in the table to represent the proportion of the catchment that is not overlapped by any Land Use Area. When multiple Land Use Areas collide within the intersection space of the Catchment, the highest value of Priority on the Land Use Area determines which is represented in the Subareas Collection. After making adjustment to your Catchments and Land Use Areas simply Compute Hydrology (or right click Compute Catchment Subareas) to refresh the Subareas Collection to all the applicable catchments.