Base Indicators

Base Indicators

This web page describes example indicators for Recovery Potential Screening’s Base Indicators category. Unlike the Ecological, Stressor, or Social Indicators, this category’s metrics do not have a positive or negative influence on RPS index scores – they are value-neutral. Base Indicators are not used in any index calculations. These are reference parameters that are useful for tracking watersheds and providing the organizational structure for relating indicator values to specific watersheds. The most essential Base Indicators are Watershed ID and Watershed Name. 

The Base Indicators category is also sometimes a source of indicators that actually can influence watershed condition positively as ecological or social features or negatively as stressors. Their 'storage' in the Base category keeps them available to add to the RPS Tool’s appropriate category only when they are appropriate to the screening purpose at hand. For example, Watershed Area is normally value-neutral and a typical Base Indicator, but it could be used as a Social Indicator in a screening that intends to prioritize larger watersheds for an initiative that seeks partnerships to collaborate on restoring larger and more challenging impaired watersheds. In order to use a Base Indicator in another category, follow the RPS Tool directions for ADD INDICATORS and copy (do not move or delete) the Base Indicator and its values over into the new category location.

Nearly all of these example indicators have been compiled for the conterminous US on HUC12 watershed units and are either already embedded in state-specific RPS Tools or publicly available through the Watershed Index Online (WSIO) Indicator Data Library. Base Indicators that weren't compiled nationally may be available at state or local scales, and these can and should be added to the RPS Tool by the user if they would support their screening objectives.

On this page:

Watershed ID  

  • Description: A unique numeric or alphanumeric identifier for the watershed unit.
  • Example metrics: HUC12 ID; HUC8 ID
  • Why relevant: One of the few mandatory metrics without which the RPS Tool will not function. This field must be included in the RPS Tool’s data table because it is the primary basis for linking an individual watershed to its many indicator attributes.
  • Data sources and measurement: The US Geological Survey (USGS) Hydrologic Units Codes (HUCs) provide the best-known, widely used existing source of watershed ID numbers at a national scale through the Watershed Boundary Dataset (WBD). RPS, Watershed Index Online (WSIO) and many other EPA projects extensively use HUC12 scale watershed IDs based on this system. Because the USGS allows continual minor updating of HUC IDs and boundaries, however, RPS, WSIO, the EPA EnviroAtlas and many other projects use a WBD snapshot from 2014 that does not undergo the minor updates, thereby ensuring that the HUC12 data are fully consistent across all users' projects. The WBD snapshot is identical to the current WBD in most states, but users are advised to check for minor inconsistencies due to state-level minor updates in some areas.

Watershed Name 

  • Description: The name of the watershed unit, typically describing the water body located at the watershed outlet.
  • Example metrics: Name HUC12 Watershed; Name HUC8 Watershed
  • Why relevant: Also considered a mandatory RPS metric, although the RPS Tool will function without this field. Name is most useful in providing a more familiar informal reference to actual locations than the ID number and can be used in RPS bubble plots to label specific watersheds on the graph.
  • Data sources and measurement: The Watershed Boundary Dataset (WBD) includes watershed name as an attribute for each hydrologic unit in the dataset. RPS, Watershed Index Online and many other EPA projects extensively use HUC12 scale watershed names based on this system. Because the WBD allows continual minor updating of HUCs, RPS and other projects use a WBD snapshot from 2014 that does not undergo the minor updates, thereby ensuring that the HUC12 name data are fully consistent across projects.

Watershed Total Area 

  • Description: The total area of the watershed unit, including all land and water encompassed by the watershed unit’s boundary. Note that, unless specifically identified as a “cumulative” metric that includes the full watershed area up-gradient of the watershed unit, RPS and WSIO indicator data represent only the “local” watershed unit (e.g., the individual HUC12 alone).
  • Example metrics: Area of HUC12 Watershed
  • Why relevant: Many indicators are calculated as a percent of total watershed area, thus area is an important Base indicator to have available for future indicator development. In addition, some projects may target larger or smaller groups of watersheds as a subset of interest.
  • Data sources and measurement: Watershed total area can be measured from a geospatial dataset of watershed boundaries in polygon or gridded (raster) format.

Watershed Land Area 

  • Description: The area of land within the watershed, not including open water area.
  • Example metrics: % Land in HUC12 Watershed; Land Area in Watershed
  • Why relevant: Some indicators may be calculated as a percent of land area, thus land area is an important Base indicator to have available for future indicator development.
  • Data sources and measurement: Measured as watershed total area minus the area of open water in the watershed.

Watershed Water Area 

  • Description: The area of lakes, reservoirs, and other open water features in the watershed.
  • Example metrics: NHDPlus2 Waterbody Area; NHD High Resolution Waterbody Area; % Water in Watershed
  • Why relevant: Many RPS indicators involve calculating an open waters attribute as a percentage of the total water body area within the watershed, thus it is an important Base indicator to have available for future indicator development. Some projects may target watersheds with higher or lower than average open water area as a subset of interest.
  • Data sources and measurement: The USGS National Hydrography Dataset (NHD) provides a national source of information on the location of surface waters in the US. The NHD includes a separate medium-resolution version (1:100,000 scale) and high-resolution version (1:25,000 scale). Open water is also mapped in the National Land Cover Dataset. Other statewide hydrography datasets may also be available. The NHD or alternatives can be intersected with watershed boundaries using geospatial analysis software to determine watershed water area.

Watershed Stream Length 

  • Description: The linear length of streams and rivers in the watershed.
  • Example metrics: NHDPlus2 Streamlength; NHD High Resolution Streamlength
  • Why relevant: Many RPS indicators involve calculating a stream attribute as a percentage of the total stream length within the watershed, thus stream length is an important Base indicator to have available for future indicator development. Some projects may target watersheds with higher or lower than average stream mileage as a subset of interest.
  • Data sources and measurement: The USGS National Hydrography Dataset (NHD) provides a national source of information on the location of surface waters in the US. The NHD includes a separate medium-resolution version (1:100,000 scale) and high-resolution version (1:25,000 scale). Other statewide hydrography datasets may also be available. The NHD or alternatives can be intersected with watershed boundaries using geospatial analysis software to determine watershed stream length. Because hydrography datasets typically also include ditches, canals, and other areal water features, a pre-processing step may be necessary to remove features that are not streams/rivers.

Percent Watershed in Riparian Zone (RZ) 

  • Description: Percent of the watershed that is in the Riparian Zone (RZ), defined as the corridor of land adjacent to surface waters.
  • Example metrics: % Riparian Zone (RZ) in Watershed
  • Why relevant: Many indicators measure conditions within the Riparian Zone because of the influence of Riparian Zone processes on water body health. The percent of the watershed in the Riparian Zone can be informative for evaluating its extent within your study watersheds.
  • Data sources and measurement: The Riparian Zone dataset in WSIO is determined using grid analysis to delineate an approximate 100 meter buffer around surface water features in the contiguous US. The 2016 National Land Cover Database (NLCD) was used to initially map surface waters as areas classified as 'Open Water' (code 11), 'Woody Wetlands' (code 90) and 'Emergent Herbaceous Wetlands' (code 95). Next, flowline and waterbody features in the NHDPlus Version 2 hydrography dataset were added. The combination of these two datasets represents surface water and is referred to as the ‘Water Mask’. The Riparian Zone was then created using the ArcMap Spatial Analyst Euclidean Distance tool. All cells with a distance of 108 meters or less to the Water Mask are included in the Riparian Zone.

Percent Watershed in Hydrologically Connected Zone (HCZ) 

  • Description: Percent of the watershed that is in the Hydrologically Connected Zone (HCZ), defined as areas with high runoff potential that are contiguous to surface water.
  • Example metrics: % Hydrologically Connected Zone (HCZ) in Watershed
  • Why relevant: Many indicators measure conditions within the Hydrologically Connected Zone because of the influence of Hydrologically Connected Zone processes on water body health. The percent of the watershed in the Hydrologically Connected Zone can be informative for evaluating its extent within your study watersheds.
  • Data sources and measurement: The Hydrologically Connected Zone (HCZ) dataset in WSIO is determined using grid analysis to delineate areas with high runoff potential that are contiguous to surface waters in the contiguous US. The 2016 National Land Cover Database (NLCD) was used to initially map surface waters as areas classified as 'Open Water' (code 11), 'Woody Wetlands' (code 90) and 'Emergent Herbaceous Wetlands' (code 95). Next, flowline and waterbody features in the NHDPlus Version 2 hydrography dataset were added. The combination of these two datasets represents surface water and is referred to as the ‘Water Mask’. A topographic Wetness Index grid was then derived from a Digital Elevation Model (DEM) based on the slope gradient and upstream drainage area of each grid cell. Finally, the Hydrologically Connected Zone was created by combining the Water Mask and the Wetness Index grid to identify areas with a high Wetness Index value (550 or greater) that are also contiguous to surface water.

Percent Watershed in Hydrologically Active Zone (HAZ) 

  • Description: Percent of the watershed that is in the Hydrologically Active Zone (HAZ), defined as the combined area of the Riparian Zone and Hydrologically Connected Zone.
  • Example metrics: % Hydrologically Active Zone (HAZ) in Watershed
  • Why relevant: Many indicators measure conditions within the Hydrologically Active Zone because of the influence of Hydrologically Active Zone processes on water body health. The percent of the watershed in the Hydrologically Active Zone can be informative for evaluating its extent within your study watersheds.
  • Data sources and measurement: The Hydrologically Active Zone is determined using grid analysis by overlaying the Riparian Zone grid and the Hydrologically Connected Zone grid. It therefore includes both a distance-based buffer around surface waters and areas with high runoff potential that are adjacent to surface waters.

Dominant Ecoregion  

  • Description: The ecoregion with the largest area in the watershed.
  • Example metrics: Level 3 Ecoregion with Largest Area
  • Why relevant: Ecoregions are commonly used to identify zones in the landscape that share many terrain, vegetative, and hydrologic similarities. Dominant ecoregion can be an important basis for identifying and targeting subsets of watersheds with similarities.
  • Data sources and measurement: Determined by percent of total watershed area in Omernik Level 3 Ecoregions.

States in Watershed 

  • Description: Various indicators describing all of the states that the watershed intersects, whether the watershed only intersects a single state, or the majority state in the watershed.
  • Example metrics: All States in HUC12; Single State in HUC12 Flag; Majority State in HUC12
  • Why relevant: State or states occurring per watershed can be an important basis for sorting either in-state watersheds for a specific state’s projects, or targeting interstate watersheds specifically.
  • Data sources and measurement: The US Census Bureau TIGER geospatial dataset provides a national source of state boundaries. Indicators can be calculated by intersecting watershed polygons with state boundary polygons.

Counties in Watershed 

  • Description: List of all counties that the watershed intersects or the majority county in the watershed.
  • Example metrics: All Counties in HUC12, Majority County in HUC12
  • Why relevant: Counties can be an important basis for subsetting watersheds to include in a recovery potential screening.
  • Data sources and measurement: The US Census Bureau TIGER geospatial dataset provides a national source of county boundaries. The counties intersecting a watershed can be compiled as a comma-delimited list. The majority county is defined as the county with the largest area in the watershed.

EPA Region 

  • Description: List of all EPA Regions that the watershed intersects.
  • Example metrics: EPA Region in Watershed
  • Why relevant: EPA Regions can be an important basis for subsetting watersheds to include in a recovery potential screening.
  • Data sources and measurement: EPA Region can be determined based on the list of states that the watershed intersects.

Percent Tribal Lands 

  • Description: Percent of the watershed that is within areas constituting Tribal lands.
  • Example metrics: % Tribal Lands in Watershed; Adjacency to Watershed with Tribal Lands
  • Why relevant: This indicator can enable Tribes to identify subsets of watersheds that contain any amount of Tribal lands, and then run RPS Tool screenings on just those watersheds if desired.
  • Data sources and measurement: EPA maintains a geospatial dataset of Tribal lands, including all lands associated with Federally-recognized tribal entities (Federally recognized Reservations, Off-Reservation Trust Lands, and Census Oklahoma Tribal Statistical Areas).

Watershed Climate Parameters 

  • Description: Indicators describing precipitation, air temperature, or other climate attributes in the watershed.
  • Example metrics: Mean Annual Precipitation in Watershed; Mean Annual Air Temperature in Watershed
  • Why relevant: These indicators can be used to characterize watershed conditions for interpreting other RPS indicator data and results. Watershed climate can also be used as a basis for subsetting watersheds to include in a screening.
  • Data sources and measurement: The PRISM Climate Group distributes geospatial datasets of average historical climate conditions across the US. Other state-specific climate datasets may be available.

Watershed Topographic Setting 

  • Description: Indicators describing the topographic setting of the watershed, such as elevation, slope, or other topographic characteristics.
  • Example metrics: Slope, Mean Value in Watershed; Elevation, Mean Value in Watershed; Flow Accumulation, Mean in Watershed
  • Why relevant: These indicators can be used to characterize watershed conditions for interpreting other RPS indicator data and results. Watershed topography can also be used as a basis for subsetting watersheds to include in a screening.
  • Data sources and measurement: The USGS 3D Elevation Program (3DEP) provides data for calculating watershed elevation and slope indicators. Higher resolution elevation data may be available for state-specific projects. Elevation datasets can also be used to calculate flow accumulation, a topographic measure of the upstream drainage area of a location.

Position on Drainage Network 

  • Description: Indicators describing the total upstream area of a watershed, upstream-downstream watershed connections, or the presence/extent of headwater areas in the watershed.
  • Example metrics: Upstream HUC12 Count; Upstream HUC12 Area; Downstream HUC12 ID
  • Why relevant: The position of a watershed unit such as a HUC12 on a full watershed’s drainage network can be important for understanding management opportunities and challenges. For example, restoration and protection of headwater watersheds may be important for improving and maintaining downstream water quality. Conversely, water quality issues in watershed units with large drainage areas above them may be significantly affected by upstream rather than local factors.
  • Data sources and measurement: These indicators require a watershed boundary dataset in which upstream-downstream network connections are defined.

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Last updated on March 25, 2024