Step 2: Decide Which Watershed Units to Screen
After defining the screening objective, users will begin to design their screening by selecting subwatersheds to screen. Subwatershed selection involves two stages, defining an initial area of interest and then selecting a subset of subwatersheds within that area.
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Define the Geographic Area of Interest
The geographic area of interest establishes the initial boundary for selecting subwatersheds to include in a screening. The choice of an area interest will depend on the version of the RPS Tool used and the location that users would like screen. Users of the Web RPS Tool can choose one or more states, major river basins or subbasins as their area of interest. Excel RPS Tool users are limited to a single state.
Select a Subset of Subwatersheds
Screenings do not need to include all subwatersheds in the selected area of interest. Focusing on a subset - a user-defined set of subwatersheds that share some key traits of interest - provides a chance to compare within a smaller and more similar group of subwatersheds with higher relevance to the screening objective.
For example, a user may be interested in comparing subwatersheds that contain impaired waters only, subwatersheds with greater than 5% urban land, subwatersheds that contain coldwater fisheries, or other characteristics of interest. Subsets can be geographically continuous (e.g., all HUC12s within a HUC8) or dispersed (e.g., HUC12s statewide that contain pathogen-impaired streams).
Additional subsetting concepts and examples are described in the RPS Scenario Factsheet Series.