November 2003
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Overview

Sample of NHESP Living Waters Layers
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Critical Supporting Watersheds (CSWs) are those areas with more immediate hydrologic contributions to Living Waters Core Habitats. As such, they represent the areas with the highest potential to sustain or degrade Core Habitats. The CSWs were produced through the AQUALAND grid-based watershed model at a 30 x 30 m resolution. The AQUALAND model was created through the combined efforts of the Natural Heritage Program and the University of Massachusetts’ Landscape Ecology Program. The model created individual CSW grids - one CSW for every Core Habitat. The model also calculated several threat assessment metrics for each CSW, taking into account such factors as impervious surfaces, road density, road crossings, potential point sources, agricultural intensity, dam intensity and public water withdrawals.

MassGIS stores this polygon layer in ArcSDE as LWCSW_POLY.

Production

See the Additional References section below for information on where to find more detail about the creation, functioning and source datasets of the AQUALAND model and threat assessment.The following parameters for the AQUALAND model were set using the options under a ‘Model’ drop-down menu (visible in the menu bar when the model is first initiated):

Watershed Delineator: the watershed type was set to “constrained”. The CSWs were constrained using the spread value of 667. Most land cover types were set with resistance values of 20, except for some hydrologic features:  wetlands were given a resistance of 4, lakes and ponds a resistance of 1, largest rivers (> 3000 km2) a resistance of 1, medium rivers (200-3000 km2) a resistance of 2, and the smallest streams were given a resistance of 4. Also, large dams had a resistance of 1 and small dams had a resistance of 2. The option to ‘multiply resistance by slope’ was selected with x = 0.5 and y = 0.75. Slope was derived from a 30-meter digital elevation model prepared by UMass; land cover comprised the MassGIS Land Use datalayer.

Model: the CSWs were created with all metrics turned on and a functional distance weighting factor enabled.

Run: the model was run for all of the Core Habitat IDs simultaneously.

A total of 428 CSW grids (one for each input Core Habitat), with a 30 m cell size, were created by the model. The resulting grids were processed using a combination of automated and manual ArcInfo procedures. Each grid was extracted from the model results folder, converted to integer format and named for its corresponding Core Habitat (LW_ID). The grids were converted to polygon coverages and their boundaries were generalized with a 100m weed tolerance in order to smooth out the ‘blocky’ grid appearance.  To remove possible artifacts of the grid to vector conversion, the CSW extents were checked to ensure they completely enclosed the Core Habitats. CSW polygons less than 3.8 acres, which were deemed to be fragments at the outer CSW boundary, were deleted such that only one polygon existed for every CSW. Additionally, interior polygons (not CSW) less than or equal to 15 acres were dissolved into the surrounding CSW. All of the polygons were then appended into one final coverage and clipped to the state boundary.

Due to limitations in the flow grid in very flat areas, such as floodplains and ponds, some CSW boundaries needed manual adjustment (mainly along the Connecticut River and in Southeastern Massachusetts). The first round of adjustments was based on the Federal Emergency Management Agency’s Q3 Flood data from MassGIS. (Note there were no FEMA data available for Franklin County). The FEMA polygons attributed as being inside the floodplain (SFHA = ‘IN’) and intersecting CSW boundaries were pulled out and individually checked to determine whether inclusion was appropriate. These FEMA polygons were merged into the CSW polygons. Each CSW was then examined in reference to the digital USGS Topographic Quadrangle datalayer from MassGIS. The CSW extents were adjusted wherever they were incorrect due to inaccuracies within the model’s flow grid.

The AQUALAND model also created a spreadsheet containing threat assessment metric scores for every CSW, which included scores for: impervious surfaces, road density, road crossings, potential point sources, agricultural intensity, dam intensity, and public water withdrawals. Each metric was given a potential threat rating of low, medium, or high based on its score relative to other scores for the same metric across the state (see Table 1). The resulting table was joined to the final CSW coverage. Please note that these potential threat ratings are intended to highlight possible threats to Living Waters Core Habitats, but each site must be evaluated individually based on the characteristics of the aquatic habitat and on the needs of the species present.

Table 1. System for assigning a potential threat rating
 

Potential Threat Rating
 		Relative Rank of Metric Source
 
Low
 		0-25%
 
Medium
 		25-75%
 
High
 		>75%
 

Attributes

The LWCSW_POLY polygon attribute table contains the following items:
LW_ID 
 		5 
 		5 
 		C 
 		Living Waters Core Habitat ID
CSW_ID 6 6 C Living Waters Critical Supporting Watershed ID
IMPERV_SUR 6 6 C Potential threat rating for impervious surfaces
ROAD_DENS 6 6 C Potential threat rating for road density
ROAD_CROSS 6 6 C Potential threat rating for road crossings
POINT_SRC 6 6 C Potential threat rating for potential point sources
AG 6 6 C Potential threat rating for agricultural intensity
DAM 6 6 C Potential threat rating for dam intensity
PUB_WAT_WD 6 6 C Potential threat rating for public water withdrawals

USAGE

  • The legend that MUST accompany this datalayer on ALL maps is:
  • "NHESP Living Waters Critical Supporting Watershed"
  • The Critical Supporting Watershed polygons were designed for use at a regional or town scale. For accurate portrayal, the data should be displayed at scales of less than 1:25,000 (e.g. 1:30,000).
  • Please note that the AQUALAND model generated Critical Supporting Watershed polygons within the Massachusetts boundary only, and so CSWs along the border will likely appear cut-off.
  • This datalayer is intended for conservation planning purposes only- it has no regulatory purpose. The NHESP layers designed for regulatory use are produced in the Natural Heritage Atlas.
Users should bear in mind that Critical Supporting Watersheds (CSWs) are those areas with the more immediate hydrologic contributions to Living Waters Core Habitats, but usually do not represent the full watersheds of Core Habitats. Furthermore, because of the resolution at which the underlying data was produced, the actual boundaries are meant to highlight general areas, not discrete linear boundaries.

Maintenance

Questions about this datalayer should be directed to NHESP at 508-792-7270 x200.

This layer is one of two Living Waters layers from NHESP. Also see the NHESP Living Waters Core Habitats datalayer description.

ADDITIONAL REFERENCES

Last Updated 12/5/2003