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Map of North Carolina highlighting the project study area

Project Overview

Full Title
Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies - 2009 Updates

Location
 Statewide

Cooperating Agencies
Public Water Supply Section
Source Water Assessment and Protection Program

Project Chief
Silvia Terziotti

Period of Project
1/2009 - 6/2009

Team Members
Kirsten Tighe
Ramona Traynor

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Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies - 2009 Updates

This project was completed in 2009. These pages are for historical purposes only.

Methods

Methods developed for the initial project and published in the report, "Methods of rating unsaturated zone and watershed characteristics of public water supplies in North Carolina" (Eimers and all, 2000), were followed as closely as possible for updates to key GIS layers. Changes to the methods were necessary in some cases because of new data formats or differences in source datasets.

Unsaturated Zone Layers

Vertical Series Hydraulic Conductance

The vertical series hydraulic conductance (also referred to as conductance) layer is calculated for the entire thickness of the unsaturated zone. The entire thickness of the unsaturated zone includes a soil layer, and may include a saprolite and crystalline bedrock layer depending on the depth to water at any point. To calculate the conductance value, thickness of soils, depth to bedrock and soil conductance are necessary. The conductance of the saprolite and bedrock layers are also needed.

County-level soil information from the Natural Resources Conservation Service (NRCS) Soil Survey Geographic Database (SSURGO) database for North Carolina was used where available. The SSURGO data provide an estimate for conductance of the soil layer, thickness of the soil layer, and depth to the bedrock of a soil layer. This was combined with the depth to water layer, and the conductance values for saprolite and bedrock that were created for the 2000 compilation of conductance.

Detailed soils were available for 73 counties in the 2000 release of the data. For the update of the conductance layer, twenty additional detailed soil surveys became available since the initial project and were used to calculate the conductance of the soil layer statewide.

Newly released SSURGO data for 20 counties (Anson, Avery, Buncombe, Burke, Catawba, Chatham, Clay, Cleveland, Graham, Henderson, Lee, Lincoln, Madison, Montgomery, Stokes, Surry, Transylvania, Vance, Watauga, Yadkin) and a new version of Haywood County were included in this update. The counties of Caswell, Cherokee, Iredell, Rowan, Rutherford, Swain, and Warren were not available at the time of processing. The less detailed State Soil Geographic (STATSGO) database was used in these counties to compute conductance values.

In 2004, NRCS implemented a new database and access system to the SSURGO spatial and tabular data. Because of this, methods developed in 2000 for the original compilation of unsaturated zone data were modified. The fields from the SSURGO soils database used in 2000 were no longer available. Permeability of soil fields, PERML and PERMH, have been replaced by Ksat in the SSURGO database. The SSURGO Metadata table column descriptions define Ksat as “the amount of water that would move vertically through a unit area of saturated soil in unit time under unit hydraulic gradient.” (http://soils.usda.gov/survey/geography/ssurgo/). Verbal communication with Soil Scientists at NRCS confirmed that the Ksat variable should be used as a substitute for perml and permh (Roy Vick, April 30, 2009). The NRCS also distributes software to calculate one Ksat value per soil type, as a weighted average over the soil layers. This value for each soil is multiplied by .1417 to convert it to an inch per hour ratio, comparable to the values used in 2000.

Soil thickness and depth to bedrock were also updated from the SSURGO data for the 21 counties that were updated. New conductance values were calculated separately for the new county data, and merged with the 2000 layer.

One additional change was made from the 2000 compilation. The categories were rescaled to better represent the conductance through a smaller area represented by a 30 by 30 meter cell size. The new categories and ratings for Vertical series hydraulic conductance are:

Table 1 -- Vertical series hydraulic conductance categories and rating values for unsaturated zone rating
Vertical series hydraulic conductance categories (feet-squared per day) Percent of area in North Carolina Rating
≤ 50 33 1
> 50 to ≤ 100 14 2
> 100 to ≤ 200 9 3
> 200 to ≤ 400 11 4
> 400 to ≤ 800 8 5
>800 to ≤ 1,600 3 6
>1,600 to ≤ 3,200 3 7
>3,200 to ≤ 6,400 5 8
>6,400 to ≤ 12,800 2 9
>12,800 12 10

The following figures show the counties that were updated, a detailed view of the change in conductance values in an updated area, and the statewide conductance classes.

North Carolina counties with detailed soils data.

Figure 1 -- North Carolina counties with detailed soils data.

A comparison of Vertical series hydraulic conductance ratings, 2000 to 2009.

Figure 2 -- A comparison of Vertical series hydraulic conductance ratings, 2000 to 2009.

The vertical series hydraulic conductance category ratings for the unsaturated zone rating.

Figure 3 -- The vertical series hydraulic conductance category ratings for the unsaturated zone rating.

Land-Surface Slope

Since the initial compilation of land-surface slope classes in 2000, a new source of elevation data has become available. In 2001, the State of North Carolina, Floodplain Mapping Program began to produce a digital elevation model (DEM) from lidar data to produce digital flood maps. LiDAR is an airborne laser sensor used to collect elevation data at a very high degree of accuracy (20 cm vertical accuracy). The slopes derived from a lidar DEM tend to capture areas of high slope that occur over short distances, such as ridges on stream banks, that earlier mapping techniques were not able to record. This tends to increase slope values over a study area. Figure 4 illustrates changes in slope in a small area, from the original elevation source and the 2009 elevation source.

A comparison percent-slope ratings, 2000 to 2009.

Figure 4 -- A comparison percent-slope ratings, 2000 to 2009.

The slope dataset was created from the individual tiles of elevation with 20-foot horizontal post spacing. The data are freely downloadable at the North Carolina Floodplain Mapping Information System site (http://ncfloodmaps.com). The elevation tiles were merged to encompass the entire State of North Carolina, projected from a North Carolina state-plane coordinate system in feet, to a North Carolina state-plane coordinate system in meters, and resampled to a 30-meter cell size. The elevation values were also recalculated from feet to meters so that the horizontal and vertical units were the same. A 10-meter elevation dataset was used from the USGS National Elevation Dataset (NED) to fill in areas of surrounding States that were necessary to complete river basin areas. The slope values were calculated using ArcGIS slope function and were classified into the following classes:

Table 2 -- Land-surface slope categories and ratings for unsaturated zone rating.
Land-surface slope, in percent unit rise/run unit Percent of area in NC Rating
> 50 percent 3 1
> 20 percent to ≤ 50 percent 10 3
> 10 percent to ≤ 20 percent 10 5
> 5 percent to ≤ 10 percent 16 7
> 2 percent to ≤ 5 percent 17 9
≤ 2 percent 43 10

The following figure shows the updated slope classes and ratings for North Carolina.

The percent-slope category ratings for the unsaturated zone
   	    rating.

Figure 5 -- The percent-slope category ratings for the unsaturated zone rating.

Land cover

The source of land cover is the National Land Cover Dataset (NLCD) 2001 land cover product. The Multi-Resolution Land Characteristics (MRLC) data used in 1999 were an early version of the NLCD 1992 data product. The 2001 data were released over a multi-year period, beginning in about 2004 for North Carolina. Slight changes were made to the land cover classes for the NLCD 2001 data. The USGS has published a cross-walk to allow users of the 1992 data to match categories in the 2001 data. The following table is provided from the NLCD 1992-2001 Retrofit Change Product website (http://www.mrlc.gov/changeproduct.php).

NLCD 1992-2001 Anderson Level I Cross-walk Table.

NLCD 1992-2001 Anderson Level I Cross-walk Table.

A great deal of care was taken when assigning values to the land cover classes in the original project. A team of experts was used to come up with assigned ratings for each land cover class. The scope of this update effort did not allow the time to reassemble a team of outside experts to assign values to new classes in the land cover data set. An internal review team of specialists were assembled at the USGS to review the land cover classes and assign rating values to the new classes. The team was comprised of USGS, N.C. Water Science Center employees including the N.C. Water Quality Specialist, N.C. Surface Water Specialist, N.C. Groundwater Specialist, Albermarle-Pamlico Basin National Water-Quality Assessment Study Unit Chief, Hydrologic Investigations Section Chief, and the N.C. Geographic Information Systems Specialist. Team members reviewed the definitions of new classes and reviewed the cross-walk from the 1992 to 2001 data set to assign final ratings to all classes of the 2001 NLCD land cover data for the unsaturated zone, watershed characteristics and land use.

Table 3 -- Land cover classes and ratings for the unsaturated zone. (Rows with red text are new land cover classes, orange text indicates a change to the land cover class or NLCD code.)
NLCD code Class Class Description Percent area in NC Land Cover UZ Rating
24 Developed, High Intensity 24. Developed, High Intensity - Includes highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80 to100 percent of the total cover. < 1 1
11 Open Water 11. Open Water - All areas of open water, generally with less than 25% cover of vegetation or soil. 1 2
90 Woody Wetlands 90. Woody Wetlands - Areas where forest or shrubland vegetation accounts for greater than 20 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 11 2
95 Emergent Herbaceous Wetlands 95. Emergent Herbaceous Wetlands - Areas where perennial herbaceous vegetation accounts for greater than 80 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 1 2
23 Developed, Medium Intensity 23. Developed, Medium Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50-79 percent of the total cover. These areas most commonly include single-family housing units. 1 2
22 Developed, Low Intensity 22. Developed, Low Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20-49 percent of total cover. These areas most commonly include single-family housing units. 2 4
31 Barren Land (Rock/Sand/Clay) 31. Barren Land (Rock/Sand/Clay) - Barren areas of bedrock, desert pavement, scarps, talus, slides, volcanic material, glacial debris, sand dunes, strip mines, gravel pits and other accumulations of earthen material. Generally, vegetation accounts for less than 15% of total cover. < 1 6
82 Cultivated Crops 82. Cultivated Crops - Areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20 percent of total vegetation. This class also includes all land being actively tilled. 11 6
71 Grassland/ Herbaceous 71. Grassland/Herbaceous - Areas dominated by grammanoid or herbaceous vegetation, generally greater than 80% of total vegetation. These areas are not subject to intensive management such as tilling, but can be utilized for grazing. 6 8
81 Pasture/Hay 81. Pasture/Hay - Areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20 percent of total vegetation. 12 8
21 Developed, Open Space 21. Developed, Open Space - Includes areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20 percent of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes 6 8
52 Shrub/Scrub 52. Shrub/Scrub - Areas dominated by shrubs; less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation. This class includes true shrubs, young trees in an early successional stage or trees stunted from environmental conditions. 2 9
41 Deciduous Forest 41. Deciduous Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75 percent of the tree species shed foliage simultaneously in response to seasonal change. 28 10
42 Evergreen Forest 42. Evergreen Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75 percent of the tree species maintain their leaves all year. Canopy is never without green foliage. 14 10
43 Mixed Forest 43. Mixed Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. Neither deciduous nor evergreen species are greater than 75 percent of total tree cover. 3 10

The following figures show the increase in urbanization that has occurred between 1992 and 2001 in a zoomed in area of the State, and the overall rating distribution for land cover.

The percent-slope category ratings for the unsaturated zone
   	    rating.

Figure 6 -- A comparison of land cover categories from sources of data used for the 2000 unsaturated zone rating and the 2009 unsaturated zone rating.

The percent-slope category ratings for the unsaturated zone
   	    rating.

Figure 7 -- The land cover category ratings for the unsaturated zone rating.

Land Use

The 2009 Land use rating was applied using the same update methods as for land cover. As was the case in 2001, the identical values were used for unsaturated zone and watershed characteristics ratings. The new values are defined in the following table:

Table 4 -- Land use classes and ratings for the unsaturated zone and watershed characteristics rating. (Rows with red text are new land cover classes, orange text indicates a change to the land cover class or NLCD code.)
NLCD code Class Class Description Percent of area in NC Land Use UZR and WSC Rating
11 Open Water 11. Open Water - All areas of open water, generally with less than 25% cover of vegetation or soil. 1 1
90 Woody Wetlands 90. Woody Wetlands - Areas where forest or shrubland vegetation accounts for greater than 20 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 11 1
95 Emergent Herbaceous Wetlands 95. Emergent Herbaceous Wetlands - Areas where perennial herbaceous vegetation accounts for greater than 80 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 1 1
31 Barren Land (Rock/Sand/Clay) 31. Barren Land (Rock/Sand/Clay) - Barren areas of bedrock, desert pavement, scarps, talus, slides, volcanic material, glacial debris, sand dunes, strip mines, gravel pits and other accumulations of earthen material. Generally, vegetation accounts for less than 15% of total cover. < 1 2
41 Deciduous Forest 41. Deciduous Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75 percent of the tree species shed foliage simultaneously in response to seasonal change. 28 3
42 Evergreen Forest 42. Evergreen Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75 percent of the tree species maintain their leaves all year. Canopy is never without green foliage. 14 3
43 Mixed Forest 43. Mixed Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. Neither deciduous nor evergreen species are greater than 75 percent of total tree cover. 3 3
52 Shrub/Scrub 52. Shrub/Scrub - Areas dominated by shrubs; less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation. This class includes true shrubs, young trees in an early successional stage or trees stunted from environmental conditions. 2 4
71 Grassland/ Herbaceous 71. Grassland/Herbaceous - Areas dominated by grammanoid or herbaceous vegetation, generally greater than 80% of total vegetation. These areas are not subject to intensive management such as tilling, but can be utilized for grazing. 6 4
81 Pasture/Hay 81. Pasture/Hay - Areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20 percent of total vegetation. 12 5
21 Developed, Open Space 21. Developed, Open Space - Includes areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20 percent of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes 6 6
22 Developed, Low Intensity 22. Developed, Low Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20-49 percent of total cover. These areas most commonly include single-family housing units. 2 7
82 Cultivated Crops 82. Cultivated Crops - Areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20 percent of total vegetation. This class also includes all land being actively tilled. 11 7
23 Developed, Medium Intensity 23. Developed, Medium Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50-79 percent of the total cover. These areas most commonly include single-family housing units. 1 8
24 Developed, High Intensity 24. Developed, High Intensity - Includes highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80 to100 percent of the total cover. < 1 10

The areal extent is shown below.

The percent-slope category ratings for the unsaturated zone
   	    rating.

Figure 8 -- The land use category ratings for the unsaturated zone rating and watershed characteristics rating.

Unsaturated Zone Rating

As defined in the Methods paper (Eimers and all, 1999), the rating for unsaturated zone is calculated using the following weights assigned to each of the unsaturated zone factors:

Table 5 – Factors and weights that contribute to the unsaturated zone rating.
Factor Weight
Vertical series hydraulic conductance 3
Land-surface slope 2
Land cover 2
Land use 3
Table 6 -- The distribution of unsaturated zone rating values categorized into 10 equal interval classes.
Unsaturated zone rating category Percent of area in N.C. Square Miles
10 - 19 < 1 7
20 - 28 < 1 141
29 - 37 4 2,319
38 - 46 25 13,584
47 - 55 29 15,399
56 - 64 25 13,550
65 - 73 9 5,042
74 - 82 4 2,366
83 - 91 1 258
92 - 100 0 0

The areal distribution of the unsaturated zone is shown in figure 9.

The unsaturated zone ratings for North Carolina.

Figure 9 -- The unsaturated zone ratings for North Carolina.

Watershed Characteristics Rating

Average Annual Precipitation

The average annual precipitation was updated to reflect a more recent period of record. The annual average precipitation rating in 2000 was based on the years 1961 through 1990. The current data set covers the period 1971 to 2007. This reflects changes from recent hurricanes as well as two major droughts that occurred in N.C. during 1998-2002 and again in 2008 (McSwain, K.B., and Weaver, J.C., 2008, Effects of the current drought on streamflow and groundwater conditions in North Carolina: Charlotte, NC, Geological Society of America Southeastern Section Meeting [abs].; and Weaver, J.C., 2005, The drought of 1998-2002 in North Carolina—Precipitation and hydrologic conditions: U.S. Geological Survey Scientific Investigations Report 2005-5053, 88 p.) There was an increase in tropical storm and hurricane activity that affected North Carolina during this period. According to the State Climate Office of North Carolina, 38 tropical storms or hurricanes affected North Carolina during the period 1971 to 2007. There were only 19 during the period 1961 to 1990 (http://www.nc-climate.ncsu.edu/climate/hurricane.php).

The following table shows the average annual precipitation classes and ratings:

Table 7 -- The average annual precipitation category rating within North Carolina.
Average annual precipitation, in inches Percent of area in North Carolina Rating
≤ 40 inches 1 1
>40 to 45 inches 39 2
>45 to 50 inches 32 3
>50 to 55 inches 19 4
>55 to 60 inches 7 5
>60 to 65 inches 1 6
>65 to 70 inches 1 7
>70 to 75 inches <1 8
>75 to 80 inches <1 9
>80 inches 1 10

The following figure shows the average annual precipitation ratings within North Carolina:

The average annual precipitation ratings for North Carolina.

Figure 10 -- The average annual precipitation ratings for North Carolina.

Land-surface Slope

The percent slope layer described for the unsaturated zone rating are the same as for the watershed characteristics rating. The same slope layer was used, but different classes were created to which ratings were applied.

The following table describes the ratings and classes for the watershed characteristics rating:

Table 8 -- The land-surface slope category ratings within North Carolina.
Land-surface slope, in percent unit rise/run unit Percent of area in NC Rating
≤ 2 percent 43 1
> 2 percent to ≤ 5 percent 17 3
> 5 percent to ≤ 10 percent 16 5
> 10 percent to ≤ 20 percent 10 7
> 20 percent to ≤ 50 percent 10 9
> 50 percent 3 10

The figure below illustrates the slope ratings for the watershed characteristics rating for N.C.

The land-surface slope ratings for the watershed characteristics
      rating for North Carolina.

Figure 11 -- The land-surface slope ratings for the watershed characteristics rating for North Carolina.

Land cover

Land cover classes were determined in the same manner as for unsaturated zone. The following table summarizes the land cover categories and ratings for the watershed characteristics rating:

Table 9 -- Land cover classes and ratings for the watershed characteristics rating. (Rows with red text are new land cover classes, orange text indicates a change to the land cover class or NLCD code.)
NLCD code Class Class Description Percent of area in NC Land Cover WSC Rating
41 Deciduous Forest 41. Deciduous Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75 percent of the tree species shed foliage simultaneously in response to seasonal change. 28 1
42 Evergreen Forest 42. Evergreen Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75 percent of the tree species maintain their leaves all year. Canopy is never without green foliage. 14 1
43 Mixed Forest 43. Mixed Forest - Areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. Neither deciduous nor evergreen species are greater than 75 percent of total tree cover. 3 1
52 Shrub/Scrub 52. Shrub/Scrub - Areas dominated by shrubs; less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation. This class includes true shrubs, young trees in an early successional stage or trees stunted from environmental conditions. 2 2
11 Open Water 11. Open Water - All areas of open water, generally with less than 25% cover of vegetation or soil. 1 3
90 Woody Wetlands 90. Woody Wetlands - Areas where forest or shrubland vegetation accounts for greater than 20 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 11 3
95 Emergent Herbaceous Wetlands 95. Emergent Herbaceous Wetlands - Areas where perennial herbaceous vegetation accounts for greater than 80 percent of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 1 3
71 Grassland/ Herbaceous 71. Grassland/Herbaceous - Areas dominated by grammanoid or herbaceous vegetation, generally greater than 80% of total vegetation. These areas are not subject to intensive management such as tilling, but can be utilized for grazing. 6 3
81 Pasture/Hay 81. Pasture/Hay - Areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20 percent of total vegetation. 12 3
21 Developed, Open Space 21. Developed, Open Space - Includes areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20 percent of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes 6 4
31 Barren Land (Rock/Sand/Clay) 31. Barren Land (Rock/Sand/Clay) - Barren areas of bedrock, desert pavement, scarps, talus, slides, volcanic material, glacial debris, sand dunes, strip mines, gravel pits and other accumulations of earthen material. Generally, vegetation accounts for less than 15% of total cover. < 1 5
82 Cultivated Crops 82. Cultivated Crops - Areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20 percent of total vegetation. This class also includes all land being actively tilled. 11 6
22 Developed, Low Intensity 22. Developed, Low Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20-49 percent of total cover. These areas most commonly include single-family housing units. 2 7
23 Developed, Medium Intensity 23. Developed, Medium Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50-79 percent of the total cover. These areas most commonly include single-family housing units. 1 8
24 Developed, High Intensity 24. Developed, High Intensity - Includes highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80 to100 percent of the total cover. < 1 10

The following figure shows the areal distribution of rating classes for land cover for the watershed characteristics rating:

The land cover ratings for the watershed characteristics rating in North Carolina.

Figure 12 -- The land cover ratings for the watershed characteristics rating in North Carolina.

Land Use

Land use is processed and rated in identical ways for the unsaturated zone and watershed characteristics rating. Please see the land use category methods listed under unsaturated zone.

The following figure shows the land use class for the watershed characteristics rating.

The land use category ratings for the unsaturated zone rating and watershed characteristics rating.

Figure 13 -- The land use category ratings for the unsaturated zone rating and watershed characteristics rating.

Groundwater Contribution

The state-wide layer was recalculated with the 2009 unsaturated zone rating values clipped to the same buffered stream network that was used in 2000 as a means of comparing the published USGS watershed characteristics rating with the updated values.

The layer is not used as part of the deliverable to Public Water Supply section because the State calculates the groundwater contribution based on precise basin delineations of public water supply watersheds. These basin delineations are not static, so the final rating is done individually for each public water-supply watershed.

Watershed Characteristics Rating

The watershed characteristics rating for North Carolina using the same factors and weights as in 2000. They are summarized in table 10.

Table 10 – Factors and weights that contribute to the watershed characteristics rating.
Factor Weight
Average annual precipitation 3
Land-surface slope 2
Land cover 1
Land use 3
Groundwater contribution 1

The distribution of rating values throughout North Carolina is shown in table 11.

Table 11 -- The distribution of watershed characteristics rating values categorized into 10 equal interval classes.
Watershed Characteristics rating category Percent of area in N.C. Square Miles
10 - 19 5 2,396
20 - 28 43 22,444
29 - 37 29 15,191
38 - 46 16 8,378
47 - 55 7 3,914
56 - 64 1 308
65 - 73 < 1 37
74 - 82 < 1 2
83 - 91 < 1 < 1
92 - 100 < 1 < 1

The areal distribution of the watershed characteristics rating is shown in figure 14.

The watershed characteristics ratings for North Carolina.

Figure 14 -- The watershed characteristics ratings for North Carolina.

An intermediate watershed characteristics rating layer is provided to the Public Water Supply Section that does not include the groundwater contribution ratings. The intermediate layer is combined with a groundwater characteristics factor for each public water-supply watershed in North Carolina for the final watershed characteristics rating.

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