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[an error occurred while processing this directive]slopesw | |
Data format: Raster Dataset File or table name: slopesw Coordinate system: State Plane Coordinate System 1983 Theme keywords: Slope, Elevation, DEM, lidar, Watershed Characteristics, Rating |
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Abstract:
Percent-slope classes to characterize watersheds is one of 6 data
sets updated for the North Carolina Public Water Supply, Source
Water Assessment Program. These data are used to rate the susceptibility of
public water supplies in North Carolina to contamination.
The original dataset was produced in 2000. This version is updated to reflect new data sources.
This data set represents the ratings applied to the land-surface slope classes for use in the rating of the unsaturated zone for public ground-water suppliers.
Land-surface slope influences the amount of precipitation that ponds on the land surface and infiltrates to contribute to ground water, or runs off the land surface as overland flow to surface water. When all other factors are the same, precipitation infiltrates into
the subsurface in areas characterized by low slope; precipitation runs off land surface in areas characterized by high slope. For surface-water systems, the greater the likelihood that water will run off the land surface, (that is, the higher the slope), the higher the rating applied on a scale of 1 to 10. |
Metadata elements shown with blue text are defined in the Federal Geographic Data Committee's (FGDC) Content Standard for Digital Geospatial Metadata (CSDGM). Elements shown with green text are defined in the ESRI Profile of the CSDGM. Elements shown with a green asterisk (*) will be automatically updated by ArcCatalog. ArcCatalog adds hints indicating which FGDC elements are mandatory; these are shown with gray text.
Percent-slope classes to characterize watersheds is one of 6 data sets updated for the North Carolina Public Water Supply, Source Water Assessment Program. These data are used to rate the susceptibility of public water supplies in North Carolina to contamination. The original dataset was produced in 2000. This version is updated to reflect new data sources. This data set represents the ratings applied to the land-surface slope classes for use in the rating of the unsaturated zone for public ground-water suppliers. Land-surface slope influences the amount of precipitation that ponds on the land surface and infiltrates to contribute to ground water, or runs off the land surface as overland flow to surface water. When all other factors are the same, precipitation infiltrates into the subsurface in areas characterized by low slope; precipitation runs off land surface in areas characterized by high slope. For surface-water systems, the greater the likelihood that water will run off the land surface, (that is, the higher the slope), the higher the rating applied on a scale of 1 to 10.
This data set is to be used in a hydrologic analysis with other data sets to rate the unsaturated zone for public ground-water supplies and watershed characteristics for public surface-water supplies in North Carolina. For ground-water supplies, the factors used to rate susceptibility to contamination include vertical hydraulic conductance, land-surface slope, land cover, and land use. The selected factors used to devise ratings for surface-water supplies' susceptibility to contamination are average annual precipitation, land-surface slope, land cover, land use, and ground-water contribution.
The 1996 Amendments to the Safe Drinking Water Act require that each state prepare a source-water assessment for all public water supplies. States are required to (1) delineate source areas supplying wells and surface-water intakes, (2) inventory potential contaminant sources within the delineated source areas, and (3) determine the susceptibility of wells or intakes to the inventoried potential contaminant sources. In North Carolina, the lead agency charged with this task is the Public Water Supply Section (PWSS) of the Division of Environmental Health, Department of Environment and Natural Resources (DENR). North Carolina's Source Water Assessment Program (SWAP) provides assessments of each public drinking water intake in North Carolina. These assessments provide a relative susceptibility rating calculated using state-wide data. The U.S. Geological Survey (USGS) cooperated with the PWSS to provide components of the inherent vulnerability rating that was used with the contaminant rating to determine the overall susceptibility rating of source water supplies. To assist the PWSS with rating ground water inherent vulnerability, the USGS compiled the unsaturated zone rating based on the following characteristics: vertical hydraulic conductance, depth to water, land surface slope, land use and land cover. The watershed characteristic rating includes average annual precipitation, land surface slope, land use, land cover and groundwater contribution. The SWAP program has planned for updates to the initial ratings as conditions change and new data sources become available. The initial data used to calculate the unsaturated zone ratings and watershed characteristics ratings were compiled from sources from the late 1990's. Critical data layers have been updated since the ratings were calculated. These improved data layers need to be used in the ongoing update of the susceptibility ratings. Overlay and index methods for rating susceptibility to contamination of the unsaturated zone for ground-water suppplies, and watershed characteristics for surface-water supplies were derived for use by the State of North Carolina in assessing more than 11,000 public water-supply wells and approximately 245 public surface-water intakes. Factors that influence the inherent vulnerability of ground water and surface water were selected and assigned ratings on a scale of 1 to 10. These factors were then assigned weight to reflect their relative influence on inherent vulnerability and the reliability of the data. The values for each factor were obtained from geographic information system (GIS) data layers that were transformed into ARC/INFO raster data sets known as grids. These raster data sets have 30-meter by 30-meter cells, and each cell is assigned a weighted- factor value. Limitations -- The overlay and index methods of unsaturated zone and watershed characteristics ratings are broad-stroke methods that assess vulnerability on the basis of expert opinion. The methods aslo have limitations in the age and scale of the hydrologic and geographic data. But the most significant limitation of the methods used is that no statistical confirmation of the results have been performed. ELEVATION: 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 data is an airborne data collection method that collects elevation data at a very high degree of accuracy (20 cm vertical accuracy). This creates an elevation surface that captures very subtle changes in elevation, and much more detailed changes in elevation. 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. SLOPE: The slope dataset was created from the individual tiles of 20 foot elevation horizontal post spacing. The data is freely downable 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. Land-surface slope influences the amount of precipitation that runs off the land surface or infiltrates into the subsurface. In general, precipitation is more likely to infiltrate into the subsurface in areas characterized by low slope and run off the land surface in areas characterized by high slope. In North Carolina, regional slopes range from relatively flat in the Coastal Plain to steep and highly variable in the Piedmont and Blue Ridge Provinces. There are, however, local exceptions to these regional characterizations. For example, in the Coastal Plain, slopes may be steep near streams; and in the Piedmont and Blue Ridge Provinces, slopes may be flat in floodplains, valleys, and hilltops. Analysis of a statewide GIS layer depicting land-surface slopes at a resolution of 30-meter grid cells indicates that about 43 percent of the State has slopes of less than 2 percent. Slightly more than 60 percent of the State has slopes of less than 5 percent. Slopes were divided into classes and assigned ratings from 1 to 10. Low ratings were assigned to the low slopes, and high ratings were assigned to the high slopes. Surface water is more vulnerable to contamination in areas where land-surface slope is high. Land-surface slope categories and ratings for watershed characteristics rating: >Land-surface slope, Percent of area Rating >in percent unit rise/unit run in NC > >Less than or equal to 43 1 >2 percent > >Greater than 2 to 17 3 >less than or equal to >5 percent > >Greater than 5 to 16 5 >less than or equal to >10 percent > >Greater than 10 to 10 7 >less than or equal to >20 percent > >Greater than 20 to 10 9 >less than or equal to >50 percent > >Greater than 50 percent 3 10 >NOTE: Due to rounding error, percents may not total 100. > SELECTED REFERENCES: Eimers, J. L., Weaver, J. C., Terziotti, S., and Midgette, R. W., 2000, Methods of rating unsaturated zone and watershed characteristics of public water supplies in North Carolina: U. S. Geological Survey Water-Resources Investigations Report 99-4283, 31 p. North Carolina Department of Environment and Natural Resources, 1999, North Carolina source water assessment program plan: Raleigh, North Carolina Department of Environment and Natural Resources, Division of Environmental Health, Public Water Supply Section, [variously paged]. U.S. Geological Survey, 1999, National Elevation Dataset Fact Sheet: accessed January 26, 2001, at URL http://edcnts12.cr.usgs.gov/ned/factsheet.htm. DISCLAIMER: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this Federal Geographic Data Committee-compliant metadata file is intended to document the data set in nonproprietary form, as well as in ARC/INFO format, this metadata file may include some ARC/INFO-specific terminology.
ground condition
These data are not to be used for site-specific analysis
3916 Sunset Ridge Road
The authors are grateful to colleagues in the Public Water Supply Section of the Division of Environmental Health, North Carolina Department of Environment and Natural Resources (DENR) for their assistance in this collaborative project: thanks to Robert Midgette, Protection and Enforcement Branch Head; Amy Axon,hydrogeologist - Source Water Protection; and David Hammermann, Geographic Information Systems Specialist. The authors also thank the USGS report review team for their review of the update procedures: Jeanne Robbins, Surface-water Specialist; Melinda Chapman, Ground-water Specialist; Mary Giorgino, Water-quality Specialist; and Douglas Harned, ALBE-National Water Quality Assessment Project Chief.
Slopes and ratings plotted at 1:100,000 scale and visually checked for accuracy. No field verification was performed. The accuracy of the ratings is discussed in Eimers and others (2000).
Not applicable for raster data.
30 meter cells: visual spot checks for accuracy
elevation values
methods of applying ratings
Projected LiDAR 20 foot DEMS to StatePlane meters. Resampled the 20foot DEMS to 30 meters. Converted the elevation values to meters. Executed the slope function in Arc/Info GRID to convert the data set to a percent-rise data set. SLOPE_30M = slope(elev_30m, percentrise)
Remapped the values to the ratings using the following table (Using the Arc/Info GRID command, SLICE, with the TABLE option). >2 : 1 >5 : 3 >10 : 5 >20 : 7 >50 : 9 >1000 : 10
Metadata imported.
Internal feature number.
ESRI
> >Value Attribute Table, SLOPESW.VAT: > >COLUMN ITEM NAME WIDTH OUTPUT TYPE N.DEC ALTERNATE NAME > 1 VALUE 4 10 B - > 5 COUNT 4 10 B - > 9 PCT_TOT 4 8 F 2 > 13 SQMI 4 8 F 2 VALUE is the rating: valid values are integers 1, 3, 5, 7, 9, and 10. COUNT is the number of 30-meter by 30-meter cells in the entire data set that have the rating. PCT_TOT is the percent of the total area just of NC within each category. SQMI is the conversion from 30 meter by 30 meter cells to square miles for NC portion of Study are > > VALUE COUNT PCT_TOT SQMI > 1 116235497 42.92 23091.79 > 3 65232594 17.09 9197.07 > 5 65979799 15.96 8589.01 > 7 43998285 9.78 5260.34 > 9 41919065 9.57 5151.09 > 10 8721263 2.56 1379.81 > >Summary Statistics Table, SLOPESW.STA: > >COLUMN ITEM NAME WIDTH OUTPUT TYPE N.DEC ALTERNATE NAME > 1 MIN 8 15 F 3 > 9 MAX 8 15 F 3 > 17 MEAN 8 15 F 3 > 25 STDV 8 15 F 3 > MIN is the minimum value of item VALUE. MAX is the maximum value of item VALUE. MEAN is the average value of item VALUE. STDV is the standard deviation of item VALUE. > > MIN MAX MEAN STDV > 1.000 10.000 4.134 2.926
none
3916 Sunset Ridge Rd.
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Although these data have been used by the U.S. Geological Survey, U.S. Department of the Interior, no warranty expressed or implied is made by the U.S. Geological Survey as to the accuracy of the data. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey in the use of this data, software, or related materials.
3916 Sunset Ridge Road