Step 3: Select Indicators
After selecting subwatersheds to screen, users will continue designing their screening by selecting indicators that are relevant to their screening objective. Users can select among pre-calculated indicators from the RPS Indicator Database or add their own custom indicator data. As part of indicator selection, users also have the option to assign weights to the selected indicators. Weights are used in calculations of the index scores that summarize indicator data.
On this page:
Indicator Basics
RPS indicators are organized into three major categories:
- Ecological Indicators describe the condition of aquatic ecosystems and related landscape characteristics.
- Stressor Indicators describe risks to subwatershed and aquatic ecosystem health that are the focus of planning or on-the-ground activities to restore and protect water quality.
- Social Indicators describe a wide variety of societal or programmatic factors that influence watershed management approaches, planning and priority-setting.
Indicators are further divided into subcategories within the Ecological, Stressor and Social categories. Subcategories and example indicators are provided on the RPS Indicators webpage.
Indicators in the RPS Tool
The Web RPS Tool and Excel RPS Tool each provide access to the RPS Indicator Database with hundreds of pre-calculated indicators for HUC12 subwatersheds in the conterminous U.S. Custom Excel RPS Tool files are also available for Alaska, Hawaii and territories with pre-loaded indicator data. All versions of the RPS Tool contain descriptions of the available pre-calculated indicators and features for exploring indicator data. Resources for learning more about indicators include:
- The RPS Indicator Database and Map Service webpage.
- Indicator Reference Sheets for many of the most frequently used indicators.
- RPS Scenario Fact Sheets highlighting example screening scenarios and associated indicator selections.
Identify Candidate Indicators
While hundreds of pre-calculated indicators are available in the RPS Tool, a screening will use just a smaller group that align with the screening objective. A group of candidate indicators for a screening can be identified by reading indicator descriptions and noting which indicators are most relevant to the screening objective. Additional considerations for each category are described below.
- Ecological Indicators: One option available when screening subwatersheds in the conterminous U.S. is to choose the Preliminary Healthy Watersheds Assessment (PHWA) Watershed Health Index as the single Ecological Indicator for the screening. The PHWA Watershed Healthy Index was designed to provide a basic but reasonably comprehensive measure of watershed health across all HUC12 subwatersheds in the conterminous U.S. It can serve as a default Ecological Index for users interested in a general gradient of condition among all the subwatersheds being compared.
Alternatively, it may be desirable to select a group of Ecological Indicators that characterize one or more key ecological attributes that are especially crucial for the screening purpose. For example, a screening objective that is focused on aquatic life could concentrate on Ecological Indicators that are most relevant to biological condition. - Stressor Indicators: When choosing Stressor Indicators, users typically do not attempt to select a comprehensive group that reflects all stressors across their area of interest. Instead, Stressor Indicator selections are tailored to the specific pollutants, sources of pollution and other threats to subwatershed and aquatic ecosystem health in the screened subwatersheds that are relevant to the screening objective. For example, users interested in identifying priority subwatersheds for stormwater management could focus on indicators related to impervious surface cover and flood risk, and supplement those with indicators that characterize any other critical stressors that influence restoration and protection decisions.
- Social Indicators: Like the Stressor category, the group of Social Indicators selected for a screening is very case-specific. The indicators in this category cover a wide range of topics such as: local involvement and historic investment in watershed management efforts (e.g., number of conservation easements, nonpoint source management projects or extent of public lands); availability of existing information or plans (e.g., number of Total Maximum Daily Loads or extent of waters assessed for attainment of surface water quality standards); potential human health and community benefits offered by restoration and protection (e.g. drinking water sources or recreational resources); or community context (e.g., demographic characteristics). To identify candidate indicators, users must think critically about which Social Indicators are important for their priority setting effort.
The RPS Tool requires at least one indicator to be selected from each category (Ecological, Stressor and Social). If a category is not relevant to a screening, users can select the ‘neutral indicator’ for that category. If the ‘neutral indicator’ is selected, all screened subwatersheds will be assigned the same index score for that category and users can focus on results for other categories only.
Explore Indicator Data
The RPS Tool includes built-in features for exploring data to further inform and refine indicator selection. Through data exploration, users can better understand indicator values and evaluate potential data issues.
A basic evaluation of indicator data can be completed by examining the summary statistics, histogram plot and correlation coefficients displayed in the RPS Tool:
- Evaluate the range of indicator values. If the difference between the minimum and maximum values of an indicator is very small, then all subwatersheds have similar values and the indicator may not be helpful for distinguishing priority watersheds. A threshold for describing the data range as “too small” for meaningful comparisons will depend on the specific indicator.
- Check for outliers. Outlier subwatersheds have very different values of an indicator compared to the rest of the group. Outliers can skew RPS results. For example, a screening may result in high Stressor Index scores for a small group of subwatersheds with very extreme values of stressor indicators and low Stressor Index scores for all other subwatersheds. Because of the outliers, subwatersheds with high (but not extreme) stressor indicator values could be misinterpreted as having minimal stressor presence. Look for outliers in histograms by reviewing the low and high ends of the chart to determine if extreme values are present.
- Evaluate the number of blanks. When indicator data are not available for a subwatershed, the RPS Tool stores a blank cell for the subwatershed in the indicator data table. If the proportion of subwatersheds with blank values of an indicator is high, users may elect to remove the indicator from the screening and find indicators with better coverage.
- Consider indicator correlation. Correlation is a measure of the similarity between two indicators. Highly correlated indicators may introduce redundancy and bias to RPS results. RPS users can consider correlation when choosing between two or more indicators to include in a screening. When the correlation statistics (e.g. coefficient of determination, or R squared) displayed on the correlation tab of the data exploration menu are very high, users may choose to only use one indicator for the screening in order to streamline indicator selections and reduce redundancy.
Add Custom Indicator Data
After reviewing lists and definitions of available indicators, RPS users sometimes decide that they have additional data and can compile new indicators that will improve their screening. Although it can be time-consuming to compile additional indicators, local or state-specific data sources often include key parameters (such as bioassessment datasets) that aren’t available in the RPS Indicator Database.
Assign Indicator Weights
Indicator weights determine the relative influence of each indicator on the index scores calculated by the RPS Tool. Indicators with higher weights will have a greater influence on index scores than indicators with lower weights. The screening setup process within the RPS Tool defaults to equally weighted indicators, but weights can be adjusted before every screening run. Users may change the default weights based on their screening objective or any expert insights they may have. Weights must be numeric but any set of numeric values can be used. A typical approach is to set a certain number of weight categories (e.g., 3 = high, 2 = medium, 1 = low) and assign weights to indicators based on their relevance to the purpose of the screening and data quality considerations.