U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 99–4283

Methods of Rating Unsaturated Zone 

and Watershed Characteristics of Public 

Water Supplies in North Carolina

Prepared in cooperation with the

Public Water Supply Section of the Division of Environmental Health, 

North Carolina Department of Environment and Natural Resources

By Jo Leslie Eimers, J. Curtis Weaver, Silvia Terziotti, and Robert W. Midgette

Raleigh, North Carolina

2000
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information write to: Copies of this report can be purchased

from:

U.S. Geological Survey

Information Services

Box 25286, Federal Center

Denver, CO 80225

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

District Chief

U.S. Geological Survey

3916 Sunset Ridge Road

Raleigh, NC 27607
Contents III

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction ........................................................................................................................................................................... 1

Purpose and scope ....................................................................................................................................................... 2

Acknowledgments ....................................................................................................................................................... 3

Methods of rating unsaturated zone and watershed characteristics of public water supplies ............................................... 3

Mechanisms of inflow to public water supplies .......................................................................................................... 3

Method for rating vulnerability of a water supply....................................................................................................... 3

Factors used to determine an unsaturated zone rating................................................................................................. 5

Vertical hydraulic conductance of the unsaturated zone.................................................................................... 5

Land-surface slope............................................................................................................................................. 7

Land cover ......................................................................................................................................................... 8

Land use............................................................................................................................................................. 10

Example of an unsaturated zone rating.............................................................................................................. 11

Factors used to determine a watershed characteristics rating...................................................................................... 11

Average annual precipitation ............................................................................................................................. 13

Land-surface slope............................................................................................................................................. 15

Land cover ......................................................................................................................................................... 15

Land use............................................................................................................................................................. 16

Ground-water contribution ................................................................................................................................ 18

Example of a watershed characteristics rating................................................................................................... 19

Limitations............................................................................................................................................................................. 19

Summary................................................................................................................................................................................ 21

References ............................................................................................................................................................................. 23

Appendix—Documentation on Geographic Information System (GIS) data sources .......................................................... 27

FIGURES

1. Diagram showing mechanisms for delivery of precipitation to ground water and surface water............................... 4

2. Map showing physiographic provinces and availability of Soil Survey Geographic (SSURGO) data 

in North Carolina, August 1999 ................................................................................................................................. 7

3. Diagram showing (A) an unnamed well encircled by source water assessment area and (B) a portion of the 

source water assessment area overlain by 60-meter by 60-meter cells to illustrate the calculation 

of the unsaturated zone rating .................................................................................................................................... 11

4. Maps showing (A) average annual precipitation and (B) a 24-hour, 25-year rainfall event in North Carolina.......... 14

5. Diagram showing an unnamed watershed upstream from a surface-water supply intake showing basin 

outline, stream network, and 1,000-foot buffered area around streams ..................................................................... 19

TABLES

1. Factors that contribute to the unsaturated zone rating ................................................................................................ 5

2. Vertical hydraulic conductance categories and ratings for the unsaturated zone........................................................ 7

3. Land-surface slope categories and ratings for the unsaturated zone........................................................................... 8

4. Land-cover categories and ratings for the unsaturated zone, 1990–93....................................................................... 9

5. Land-use categories and ratings for the unsaturated zone and watershed characteristics, 1990–93 .......................... 10

6. Example determination of an unsaturated zone rating for an unnamed water-supply well........................................ 12

7. Factors that contribute to the watershed characteristics rating ................................................................................... 12

8. Average annual precipitation categories and ratings for watershed characteristics.................................................... 13

9. Land-surface slope categories and ratings for watershed characteristics ................................................................... 15

10. Land-cover categories and ratings for watershed characteristics, 1990–93................................................................ 17

11. Example determination of a watershed characteristics rating for part of an unnamed watershed upstream 

from a water-supply intake......................................................................................................................................... 20
IV Contents

CONVERSION FACTORS, TEMPERATURE, VERTICAL DATUM, AND 

ACRONYMS

Temperature: In this report, temperature is given in degrees Fahrenheit (°F), which can be 

converted to degrees Celsius (°C) by using the following equation:

°C = 5/9 (°F – 32)

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929), a geodetic datum derived from a general adjustment of the first-order level nets of both the

United States and Canada, formerly called Sea Level Datum of 1929.

Acronyms used in this report:

AMC ARC/INFO macro language

BFI base-flow index

DEM Digital Elevation Model

DENR Department of Environment and Natural Resources

DLG digital line graph

EDC EROS Data Center

GIS Geographic Information System

HMK harmonic mean hydraulic conductivity

LANDSAT Land Remote Sensing Satellite

MLRC Multi-Resolution Land Characteristics

MUIR Map Unit Interpretation Record

NAWQA National Water-Quality Assessment

NOAA National Oceanic and Atmospheric Administration

NRCS Natural Resources Conservation Service

NWI National Wetlands Inventory

PRISM Parameter-Elevation Regressions on Independent Slopes Model

PWSS Public Water Supply Section

SCS Soil Conservation Service

SDWA Safe Drinking Water Act

SSURGO Soil Survey Geographic data base

STATSGO State Soil Geographic data base

SWAP Source Water Assessment Program

TM Thematic Mapper

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Multiply by To obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

Flow

foot squared per day (ft2

/d) 0.0929 meter squared per day
Abstract 1

Methods of Rating Unsaturated Zone and Watershed 

Characteristics of Public Water Supplies in North 

Carolina

By Jo Leslie Eimers, J. Curtis Weaver, Silvia Terziotti, and Robert W. Midgette

ABSTRACT

Overlay and index methods were derived for 

rating the unsaturated zone and watershed 

characteristics 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. The rating of the 

unsaturated zone and watershed characteristics 

represents a practical and effective means of 

assessing part of the inherent vulnerability of 

water supplies to potential contamination. Factors 

that influence the inherent vulnerability of the 

drinking water supply to potential contamination 

were selected and assigned ratings (on a scale of 

1 to 10) to cover the possible range of values in 

North Carolina. These factors were assigned 

weights of 1, 2, or 3 to reflect their relative 

influence on the inherent vulnerability of the 

drinking water supply. The factor values were 

obtained from Geographic Information System 

data layers, and were transformed into grids 

having 60-meter by 60-meter cells, with each cell 

being assigned a value. 

Identification of factors, the development of 

ratings for each, and assignment of weights were 

based on (1) a literature search, which included 

examination of potential factors and their effects 

on the drinking water; and (2) consultation with 

experts in the science and engineering of 

hydrology, geology, forestry, agriculture, and 

water management.

Factors selected for rating the inherent 

vulnerability of the unsaturated zone are vertical 

hydraulic conductance, land-surface slope, land 

cover, and land use. Vertical hydraulic conductance is a measure of the capacity of unsaturated 

material to transmit water. Land-surface slope 

influences whether precipitation runs off land 

surfaces or infiltrates into the subsurface. Land 

cover, the physical overlay of the land surface, 

influences the amount of precipitation that 

becomes overland flow or infiltrates into the 

subsurface. Land use describes activities that 

occur on the land surface and influence the 

potential generation of nonpoint-source 

contamination.

Factors selected for rating the watershed 

characteristics upstream from surface-water 

intakes are average annual precipitation, landsurface slope, land cover, land use, and groundwater contribution. The average annual 

precipitation represents the mass of water that 

becomes available for transport in a watershed. 

Land-surface slope, land cover, and land use have 

similar influences in watersheds as those identified 

for the unsaturated zone. Ground-water 

contribution represents the part of streamflow that 

is derived from ground-water discharge. 

INTRODUCTION

The need for high-quality drinking water 

supplies in North Carolina has become more critical in 

recent years as population growth and economic 
2 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

development have become widespread. The Federal 

Safe Drinking Water Act (SDWA) Amendments of 

1996 emphasize pollution prevention as an important 

strategy for the protection of ground-water and surfacewater resources. This new focus in the SDWA 

promotes the prevention of drinking water 

contamination as a cost-effective means of ensuring 

reliable, long-term, and safe drinking water sources for 

public water-supply systems (North Carolina 

Department of Environment and Natural Resources, 

1999a). Specifically, Section 1453 of the SDWA 

Amendments requires that States develop and 

implement a Source Water Assessment Program 

(SWAP) to delineate source water areas, inventory 

potential contaminants in these areas, and determine 

the susceptibility of each public water supply to 

contamination. Guidance in developing a SWAP plan is 

provided by the U.S. Environmental Protection Agency 

(USEPA; U.S. Environmental Protection Agency, 

1997). North Carolina’s source water protection 

strategy is to build upon existing programs and 

activities with a program that is non-regulatory, state 

implemented, and incentives driven, (North Carolina 

Department of Environment and Natural Resources, 

1999a).

The agency charged with the task of 

susceptibility assessment in North Carolina is the 

Public Water Supply Section (PWSS) of the 

Department of Environment and Natural Resources. 

The U.S. Geological Survey (USGS) is directed under 

the Clean Water Action Plan, funded by Congress in 

1999, to assist States with water-quality monitoring 

and susceptibility determinations. 

In February 1999, the PWSS submitted a SWAP 

plan to the USEPA describing a method for assessing 

the susceptibility to contamination of more than 11,000 

wells and approximately 245 surface-water intakes in 

North Carolina. To develop the method and procedures 

described in the SWAP plan, the PWSS convened a 

Technical and Citizens Advisory Committee. The 

committee reviewed draft proposals prepared by PWSS 

staff, discussed alternatives, and provided recommendations. This process resulted in the decision that the 

determination of overall susceptibility of each public 

ground-water supply well and surface-water intake 

should be based on two key components—a 

contaminant rating and an inherent vulnerability rating. 

The contaminant rating is determined by the 

PWSS from an inventory of existing data bases of 

potential contaminant sources. Additional factors 

include the density of contaminant sources in the 

delineated area, proximity to the intake, and the relative 

risk of the contaminants to the public water supply. 

The inherent vulnerability rating is a measure of 

the potential for contaminants within a delineated 

source area to reach the ground-water or surface-water 

supply. The inherent vulnerability of a ground-water 

source of public water supply is determined by 

combining an aquifer rating and an unsaturated zone 

rating (North Carolina Department of Environment and 

Natural Resources, 1999a). The inherent vulnerability 

of a surface-water source of public water supply is 

determined by combining a watershed classification, 

intake location, raw water quality (water plant data), 

North Carolina Division of Water Quality Use Support 

rating, and watershed characteristics rating (North 

Carolina Department of Environment and Natural 

Resources, 1999a). In cooperation with the PWSS, the 

USGS developed methods to rate unsaturated zones for 

public ground-water systems and watershed 

characteristics for public surface-water intakes. All 

other components of inherent vulnerability were 

compiled by the PWSS.

Developing methods for rating unsaturated zone 

and watershed characteristics required identification of 

factors affecting the transport of water through the 

unsaturated zone or watershed. These factors were used 

to construct the ratings by an overlay and index method 

(National Research Council, 1993). The specific 

unsaturated zone and watershed characteristics ratings 

used in this report are not necessarily transferable to 

other regions; however, the methods used to develop 

the ratings are transferable.

Purpose and Scope

The purpose of this report is to present methods 

to rate the unsaturated zone for public ground-water 

supplies and watershed characteristics for public 

surface-water supplies in North Carolina, and to show 

an example calculation for both unsaturated zone and 

watershed characteristics ratings. Factors contributing 

to the ratings of the unsaturated zone and watershed 

characteristics are presented, and limitations of this 

statewide overlay and index rating system are 

discussed.

For ground-water supplies, the selected 

contributing factors include vertical hydraulic 

conductance, land-surface slope, land cover, and land 

use. The selected factors contributing to watershed 
Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies 3

characteristics ratings are average annual precipitation, 

land-surface slope, land cover, land use, and groundwater contribution. Methods for determining 

unsaturated zone and watershed characteristics ratings 

are presented for an unnamed well and an unnamed 

surface-water intake. 

Acknowledgments

The authors are grateful to colleagues in the 

Public Water Supply Section of the North Carolina 

Division of Environmental Health, Department of 

Environment and Natural Resources (DENR), for their 

assistance in this collaborative project—Elizabeth 

Morey, Hydrogeologist; Gale Johnson, 

Hydrogeologist; and Rajpreet Butalia, Geographic 

Information Systems Coordinator.

The authors also gratefully acknowledge the 

following scientists and engineers who provided 

additional technical review of this work:

• Richard Burns, Watershed and Forest Hydrologist, 

U.S. Forest Service, U.S. Department of 

Agriculture

• Ronald Coble, Professional Geologist (retired 

USGS)

• Ed Holland, Orange Water and Sewer Authority

• Beth McGee, Clean Water Management Trust 

Fund

• Ted Mew, Groundwater Section, Division of 

Water Quality, North Carolina DENR

• Joe Rudek, Environmental Defense Fund

• Henry Wade, Pesticides Section, North Carolina 

Department of Agriculture

• Steve Zoufaly, Division of Water Quality, North 

Carolina DENR

METHODS OF RATING UNSATURATED 

ZONE AND WATERSHED CHARACTERISTICS OF PUBLIC WATER SUPPLIES

Mechanisms of Inflow to Public Water 

Supplies

Precipitation recharges ground water by 

infiltrating the subsurface to reach the upper surface of 

the saturated zone, referred to as the water table 

(Winter and others, 1998). Vertical hydraulic 

conductance of the unsaturated zone is a quantitative 

measure of the ease of water movement through 

unsaturated materials. Precipitation also can travel 

overland to nearby surface water. Land-surface 

characteristics, such as land use, land cover, and slope, 

affect the amount of infiltration versus overland flow. 

T th, (1963) characterized ground-water flow as 

local, intermediate, and regional. The local flow system 

recharges at interstream uplands and to adjacent 

lowlands or streams. Local flow systems are the most 

dynamic and shallowest flow systems and have the 

greatest interchange with surface water (Winter and 

others, 1998).

Local flow systems can be underlain by 

intermediate and regional flow systems. Intermediate 

ground-water flow is discharged from the ground-water 

system into nearby, larger streams, and regional 

ground-water flow is discharged from the ground-water 

system into distant streams. 

Inflow to surface-water bodies includes both 

overland flow and discharge from the ground-water 

system. Precipitation flows through a thin unsaturated 

zone adjacent to streams and lakes, which causes the 

water table to rise quickly adjacent to the surface-water 

body (fig. 1; Winter and others, 1998). When the rate of 

precipitation exceeds the infiltration capacity of the 

subsurface, precipitation ponds at land surface or 

travels to streams as overland flow. The mechanisms 

for delivery of precipitation to the ground water and 

surface water are depicted in figure 1.

The procedures that are presented for rating 

unsaturated zone and watershed characteristics 

represent the potential for water, with or without 

contaminants, to (1) travel through the unsaturated 

zone to reach the water table in a ground-water supply 

source water assessment area, or (2) travel overland or 

through the shallow subsurface of a watershed to reach 

a surface-water supply intake. This study does not 

attempt to characterize how water transport processes 

might effect contaminants. 

Method for Rating Vulnerability of a Water 

Supply

Although the vulnerability of drinking water 

supply is a concept and not a measurable property, 

inherent vulnerability can be inferred from surrogate 

information that is measurable (National Research 

Council, 1993). Methods for assessing vulnerability 

are of three types—overlay and index (such as those 

used in this report), process-based simulation models, 

o′
4 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

and statistical (National Research Council, 1993; Vrba 

and Zaporozec, 1994). Process-based simulation 

models require analytical or numerical solutions to 

equations that represent coupled processes governing 

contaminant transport. Statistical methods incorporate 

data on known areal contaminant distributions and 

characterize the contamination potential for specific 

geographic areas from which data were drawn. 

The overlay and index method used in this report 

combines maps of various factors that influence 

inherent vulnerability. Each factor is categorized over 

the range of its possible values; these categories are 

then rated according to their relative influence on 

water-resource vulnerability. Statewide data were 

developed for each factor by using a Geographic 

Information System (GIS).

Because shallow ground-water vulnerability is 

strongly influenced by the spatial distribution of 

hydrogeologic and geographic features, mapping 

commonly is used to determine and display groundwater vulnerability around a well screen (Johnson and 

Van Driel, 1978; Aller and others, 1987; Whittemore 

and others, 1987; de Mulder and Hillen, 1990; Adams 

and Foster, 1992). Surface-water quality also is 

influenced by the spatial distribution of surficial 

features, and mapping hydrogeologic and geographic 

features is a tool used to determine surface-water 

vulnerability (Miller and Mattraw, 1982; McMahon 

and Lloyd, 1995; Nolan and Clark, 1997).

For this investigation, geologic, hydrologic, 

climatic, physiographic, and cultural factors were 

assigned weights that reflect their influence on water 

resources. Ratings and weights were assigned after 

review of pertinent literature and after seeking broadbased expert opinion. An initial estimate of ratings and 

weights was derived from literature and the expert 

opinions of the authors and other members of the 

PWSS. Eight scientists and engineers representing the 

2

3

4

5

1

Surface

water

{

{Saturated zone

Unsaturated zone

Water table

OVERLAND FLOW

WATER TABLE FOLLOWING FOCUSED RECHARGE

LOCAL GROUND-WATER FLOW

INTERMEDIATE GROUND-WATER FLOW

5 REGIONAL GROUND-WATER FLOW

4

3

2

1

EXPLANATION Precipitation

Figure 1. Mechanisms for delivery of precipitation to ground water and surface water 

(modified from Freeze and Cherry, 1979; Winter and others, 1998).
Factors Used to Determine an Unsaturated Zone Rating 5

U.S. Forest Service, the Orange Water and Sewer 

Authority, the Clean Water Management Trust Fund, 

the Division of Water Quality of North Carolina DENR, 

the Environmental Defense Fund, and the Pesticides 

Section of the North Carolina Department of 

Agriculture reviewed these ratings and weights. The 

USGS and PWSS selected these individuals because of 

their expertise in the science and engineering of 

hydrology, geology, forestry, agriculture, and water 

management across the State of North Carolina. Six 

USGS reviewers with technical expertise in North 

Carolina hydrology examined the ratings and weights 

that characterize the influence of the factors on waterresource vulnerability. 

Factors Used to Determine an Unsaturated 

Zone Rating

The unsaturated zone rating is based on a 

combination of factors that contribute to the likelihood 

that water, with or without contaminants, will reach the 

water table by following the path of aquifer recharge. 

The selected factors, which are represented by GIS 

spatial-data layers, include vertical conductance of the 

unsaturated zone, land-surface slope, land cover, and 

land use (table 1). The values of each of these four 

factors are categorized, and the categories are assigned 

a rating on a scale of 1 to 10. A rating of 1 reflects a low 

contribution to inherent vulnerability and 10 reflects a 

high contribution. For example, the rating for landsurface slope is low (1) in areas of high slope (greater 

than 50 percent slope) and high (10) in areas of low 

slope (less than 2 percent slope) because increased 

infiltration potential in flat terrain leads to an increased 

likelihood of ground-water contamination.

With the exception of land use, these factors 

influence the physical transport of water. The land-use 

factor is included as a measure of the potential for 

generating nonpoint-source contamination at land 

surface and is included to fulfill requirements for the 

SWAP plan to consider nonpoint-source contaminants 

(North Carolina Department of Environment and 

Natural Resources, 1999a). For the purpose of 

calculating the unsaturated zone rating, each of these 

four factors is weighted on the basis of the importance 

of the factor in determining vulnerability. Vertical 

hydraulic conductance and land use are weighted more 

heavily than are land-surface slope and land cover, 

because expert opinion determined that vertical 

hydraulic conductance and land use are more important 

influences on ground-water supplies than are landsurface slope and land cover. 

Vertical Hydraulic Conductance of the Unsaturated 

Zone

In order to measure the capacity of the entire 

sequence of materials that overlie the saturated zone to 

transmit water, the thickness of the unsaturated zone 

and the hydraulic conductance of unsaturated material 

must be determined. At selected sites throughout the 

State, depth to the water table and hydraulic 

conductance of the unsaturated zone were estimated. As 

the methods of rating the unsaturated zone and 

watershed characteristics described in this report are 

implemented statewide, estimates of the depth to the 

water table and the hydraulic conductance of a variety 

of geologic materials will be needed. 

For a given component of the series of materials 

composing the unsaturated zone, the vertical hydraulic 

conductance value, C, is calculated by using the 

following equation:

Table 1. Factors that contribute to the unsaturated zone 

rating

Factor Relevance of the factor Weight

Vertical hydraulic

conductance of 

the unsaturated 

zone

The harmonic mean of a series 

of layers of unsaturated 

material provides a single 

value for the capacity of the 

entire sequence of the 

unsaturated zone to transmit 

water, with or without 

contaminants, from the land 

surface to the water table.

3

Land-surface slope The inclination, or change in 

elevation, of the land surface 

indicates the likelihood that 

precipitation will infiltrate or 

run off.

2

Land cover The type of material covering 

the land surface influences 

the likelihood that 

precipitation will infiltrate or 

run off.

2

Land use The type of land use influences 

the likelihood of potential 

nonpoint-source 

contamination.

3
6 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

(1)

where:

Kunsat = vertical hydraulic conductivity of the 

unsaturated zone,

A = cross-sectional area, and 

L = length of vertical flow, or depth from land 

surface to the water table. 

Kunsat is a function of moisture content, porosity, 

and other textural aspects of the material (O'Hara 

1996). Saturated hydraulic conductivity, Ksat, is the 

upper bound of possible Kunsat values (Freeze and 

Cherry, 1979) and, as such, is used in this study as a 

conservative estimate of Kunsat. That is, vertical 

hydraulic conductance derived by substituting Ksat for 

Kunsat is Cmax, the maximum possible vertical 

hydraulic conductance:

(2)

Because Kunsat is difficult to estimate, this 

substitution of Ksat for Kunsat is commonly made 

(O'Hara, 1996). In this report, Cmax is referred to 

simply as C, and it can be inferred that Ksat has been 

substituted for Kunsat in the computation of C.

Hydraulic conductance over the entire thickness 

of the unsaturated zone, C, is calculated for layers in 

series:

(3)

where Ci

 represents the hydraulic conductance of each 

layer, i, of the unsaturated zone. Depending on depth to 

water in any given location, the Blue Ridge and Piedmont Provinces can include estimates of vertical conductance for layers of soil, saprolite, and(or) fractured 

rock. The Coastal Plain Province can include estimates 

of vertical conductance for layers of soil and(or) sedimentary formations. In other words, the determination 

of vertical hydraulic conductance is reduced to estimating the thickness and vertical Ksat of each component 

of the unsaturated zone. 

Thickness and vertical Ksat estimates were 

obtained for soil from the Natural Resources 

Conservation Service (NRCS) Soil Survey Geographic 

data base (SSURGO), State Soil Geographic data base 

(STATSGO), and the associated Map Unit 

Interpretation Record data base (MUIR) (Soil Survey 

Division Staff, 1993; U.S. Department of Agriculture, 

1994, 1995). Estimates of soil thickness and vertical 

Ksat are explained below. 

Soil Ksat values are available statewide. Soil 

types, by county, are identified in the SSURGO data 

base (U.S. Department of Agriculture, 1995). The 

NRCS developed the SSURGO data base at a scale of 

1:24,000, primarily for use in planning and managing 

farm and ranch, landowner/user, township, or county 

natural resources. At present, SSURGO data are 

available for 70 of 100 North Carolina counties (fig. 2).

Where SSURGO data are unavailable, 

STATSGO data are used. The STATSGO data base was 

developed at a scale of 1:250,000 for use in regional, 

multistate, State, multicounty, and river basin resource, 

planning, management, and monitoring (U.S. 

Department of Agriculture, 1994). STATSGO data are 

available for every part of North Carolina. STATSGO 

map units can have multiple soils, each with specific 

soil characteristics. The area-weighted mean was used 

to calculate values for each mapping unit. 

In North Carolina, up to six layers of a soil type 

are identified in both SSURGO and STATSGO data. 

Soil Ksat and thickness are identified for each soil layer. 

Depths to the upper and lower soil-layer boundaries are 

recorded in inches and are stored as dephi and depli, 

respectively. The maximum and minimum values for 

the range in soil Ksat, expressed in inches per hour, are 

stored in the MUIR data base as permhi and permli

, 

respectively. 

Ksat is distributed lognormally (Freeze and 

Cherry, 1979). Therefore, a log transformation is used 

in the determination of mean saturated Ksat in soil layer 

i, μi (O'Hara, 1994):

(4)

where:

μi = log mean saturated Ksat.

C

Kunsat × A

L = -------------------------

Cmax

Ksat × A

L = --------------------

1

C

--- 1

C1

------ 1

C2

------ 1

C3

= + + ------

μi 10

permhi ( ) permli log + log( )

2 ---------------------------------------------------------------------

=

6 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina
Factors Used to Determine an Unsaturated Zone Rating 7

SSURGO data available

SSURGO data not available

PHYSIOGRAPHIC PROVINCE LINE

EXPLANATION

BLUE RIDGE

PIEDMONT COASTAL PLAIN

0

0

25 50 75 100 MILES

25 50 75 100 KILOMETERS

Atlantic Ocean

To estimate the effective Ksat of a series of soil 

layers—μ1, μ2, μ3, μ4, μ5, and μ6—the harmonic mean 

described by Collins (1961), McDonald and Harbaugh 

(1988), and O'Hara (1996) is used. This harmonic mean 

Ksat (HMK) is defined as:

(5)

The SSURGO spatial data include up to two soil 

types per map unit. The STATSGO spatial data include 

up to 21 soil types within each map unit. It is necessary 

to area-weight HMK within each map unit. The 

analytical technique detailed above was performed by 

using an ARC/INFO macro language (AML) program 

run in an ARC/INFO GIS environment (Environmental 

Systems Research Institute, Inc., 1994).

Vertical hydraulic conductance categories were 

divided into the same classes used in a previous study 

(O'Hara, 1996) and assigned ratings from 1 to 10 

(table 2). Low ratings were assigned to the low conductance, and high ratings were assigned to the high 

conductance; areas characterized by low vertical 

hydraulic conductance contribute the least to the 

inherent vulnerability of ground-water supplies, and 

areas characterized by high vertical hydraulic 

conductance contribute the most to the inherent 

vulnerability of ground-water supplies.

HMK

dephi depli –

i = 1

i = 6

∑

dephi depli –

μi

-----------------------------------

i = 1

i = 6

∑

= ---------------------------------------------

Table 2. Vertical hydraulic conductance categories and 

ratings for the unsaturated zone (after O’Hara, 1996)

Vertical hydraulic conductance,

in feet squared per day Rating

Less than or equal to 500 1

Greater than 500 to less than or equal to 1,000 2

Greater than 1,000 to less than or equal to 2,000 3

Greater than 2,000 to less than or equal to 4,000 4

Greater than 4,000 to less than or equal to 8,000 5

Greater than 8,000 to less than or equal to 16,000 6

Greater than 16,000 to less than or equal to 32,000 7

Greater than 32,000 to less than or equal to 64,000 8

Greater than 64,000 to less than or equal to 128,000 9

Greater than 128,000 10

Land-Surface Slope

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. The land-surface slope rating scheme was 

developed from literature about the effects of landsurface slope on surface water. The land-surface slope 

rating for surface water is the inverse of the rating used 

for ground water. The reader is referred to the section 

entitled "Factors Used to Determine a Watershed 

Characteristics Rating" for a more complete discussion 

of the derivation of this rating. 

Figure 2. Physiographic provinces and availability of Soil Survey Geographic (SSURGO) data in North 

Carolina, August 1999.
8 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

Demek and others (1972) suggested that slope 

categories should be based on slope frequency, but 

cautioned that categories may vary significantly from 

one region to another. No single slope-rating scheme is 

applicable in disparate geographic areas. In North 

Carolina, regional slopes range from relatively flat in 

the Coastal Plain Province to steep and highly variable 

in the Piedmont and Blue Ridge Provinces. However, 

local exceptions to these regional characterizations 

occur. For example, slopes in the Coastal Plain may be 

steep near streams, and in the Piedmont and Blue Ridge 

Provinces, slopes may be flat in flood plains, in valleys, 

and on hilltops. Analysis of a statewide GIS layer 

depicting land-surface slopes at a resolution of 60-

meter grid cells indicates that nearly 57 percent of the 

State has slopes less than 2 percent (table 3). Slightly 

more than 85 percent of the State has slopes less than 10 

percent. 

infiltration of rainfall. Soil compaction promotes runoff 

and decreases infiltration.

Land-cover information was obtained from the 

USEPA and USGS Multi-Resolution Land 

Characteristics (MRLC) land-cover data base. This 

coverage was developed from remotely sensed data that 

were collected by using the Landsat Thematic Mapper 

(TM) sensor from 1990 through 1993, primarily during 

the spring seasons of 1991, 1992, and 1993 (Vogelmann 

and others, 1998). Data were stored at a 30-meter 

resolution. 

Information was processed into nine general 

land-cover classes, which were further subdivided into 

23 land-cover categories that were established for the 

eventual development of a consistent and generalized 

land-cover data base for all of the United States 

(Vogelmann and others, 1998). Within North Carolina, 

15 land-cover categories from among six of the nine 

general classes of land cover are represented (table 4) 

and are used in the rating scheme developed for the 

land-cover factor. 

Runoff coefficients (Viessmann and others, 1977; 

Chow and others, 1988; Lindeburg, 1992) and Soil 

Conservation Service (SCS) curve numbers (U.S. 

Department of the Interior, Bureau of Reclamation, 

1973; Overton and Meadows, 1976; Lindeburg, 1992) 

were used as general guidelines in assigning ratings for 

land cover. The reader is referred to the discussion 

entitled "Factors Used to Determine a Watershed 

Characteristics Rating" for a more thorough discussion 

of surface-water studies that were used to make a 

preliminary estimate of land-cover ratings. 

Land-cover categories are rated from 1 to 10 

according to their contribution to the inherent 

vulnerability of ground-water supplies. Land covers 

that impede infiltration and contribute least to the 

inherent vulnerability of ground-water supplies are 

assigned a low rating. Land covers that permit 

infiltration and contribute the most to the inherent 

vulnerability of ground-water supplies are assigned a 

high rating. For example, where asphalt and structures 

dominate land cover, such as commercial/industrial 

areas, very little rainfall infiltrates into the subsurface; 

the land-cover rating for this category is 1. Where the 

land cover is forested, the surface is pervious and 

vegetation impedes runoff; the land-cover rating for this 

category is 10.

Table 3. Land-surface slope categories and ratings for the 

unsaturated zone

[<, less than]

Land-surface slope, in percent

Area in 

North 

Carolina, 

in percent

Rating

Greater than 50 percent <1 1

Greater than 20 to less than or equal to 50 percent 8 3

Greater than 10 to less than or equal to 20 percent 6 5

Greater than 5 to less than or equal to 10 percent 10 7

Greater than 2 to less than or equal to 5 percent 18 9

Less than or equal to 2 percent 57 10

Slopes were divided into classes and assigned 

ratings from 1 to 10 (table 3). Low ratings were 

assigned to high slopes, and high ratings were assigned 

to low slopes. Ground water is more vulnerable to 

contamination in areas where land-surface slope is low 

and infiltration is likely. 

Land Cover

Land cover, which describes the physical overlay 

of the land surface, influences the amount of 

precipitation that infiltrates into the ground. Infiltration 

occurs where land cover is pervious. When 

precipitation falls directly onto the ground, the amount 

of infiltration depends on such factors as vegetative 

cover and soil compaction. Vegetation impedes runoff, 

increases temporary surface storage and, thus, increases 
Factors Used to Determine an Unsaturated Zone Rating 9

Table 4. Land-cover categories and ratings for the unsaturated zone, 1990–93

[<, less than]

Land-cover 

category General description or example

Area in 

North 

Carolina, 

in percent

Rating

Commercial/

industrial

Land used for the manufacture of products or sale of goods. Includes all 

highly developed lands not classified as residential, most of which are 

commercial, industrial, or transportation.

1 1

Water All areas of open water, generally with less than 25 percent vegetative cover. 9 2

Woody wetland Areas of forested or shrubland vegetation where the soil or substrate is 

periodically saturated or covered with water.

11 2

Emergent wetland Non-woody, vascular, perennial vegetation where the soil or substrate is 

periodically saturated or covered with water.

1 2

High-intensity 

residential

Residential development. Densely built urban centers, apartment complexes, 

and row houses. Vegetation occupies less than 20 percent of the landscape. 

Constructed materials account for 80 to 100 percent of the total area.

<1 2

Low-intensity 

residential

Residential development. Constructed materials account for 30 to 80 percent 

of the total area. Most commonly single-family housing areas, especially 

suburban neighborhoods.

2 4

Transitional Areas dynamically changing from one land cover to another, often because of 

changes in land-use activities.

<1 5

Quarries/strip mines/

gravel pits

Areas of extractive mining activities with significant exposure of land surface. <1 6

Row crops Areas dominated by vegetation that is planted and(or) used for the production 

of crops, such as corn, soybeans, vegetables, tobacco, and cotton.

15 6

Barren land Bare rock, sand, silt, gravel, or other earthen material with little or no 

vegetation regardless of its inherent ability to support life. 

<1 7

Other grass Vegetation planted in developed settings for recreation, erosion control, or 

aesthetic purposes. Examples include parks, lawns, and golf courses.

<1 8

Hay/pasture Areas dominated by vegetation, which is planted and(or) maintained for the 

production of food or feed. Grasses, legumes, or mixtures planted for 

livestock grazing. 

6 8

Deciduous forest Areas dominated by trees where 75 percent or more of the tree species shed 

foliage simultaneously.

24 10

Mixed forest Areas dominated by trees where neither deciduous nor evergreen species 

represent more than 75 percent of the cover present.

10 10

Evergreen forest Areas dominated by trees where 75 percent or more of the tree species retain 

their leaves all year. Canopy is never without green foliage.

19 10
10 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

Land Use

Land use describes activities that occur on the 

land surface. This factor represents the potential for 

generation of nonpoint-source contamination that 

might result from these activities. Land use is rated 

identically for the unsaturated zone and watershed 

characteristics (table 5). The reader is referred to the 

discussion entitled "Factors Used to Determine a 

Watershed Characteristics Rating” for a discussion 

relating land use to surface-water quality.

The effect of land use on ground-water quality 

has been the subject of many data-collection and 

interpretive investigations (Corwin and others, 1997). 

In 1984, the USGS began studies to evaluate quantitatively the effects of human activities, expressed as land 

use, on regional ground-water quality (Helsel and 

Ragone, 1984). One of these studies was performed on 

Long Island, New York. Recent work in this area 

(Eckhardt and Stackelberg, 1995) demonstrated that 

logistic regression equations based on explanatory 

variables of land use and population density 

characterize the probability of contaminants. The 

factors that most directly control the contaminant 

loadings at the water table, especially in unreactive 

surficial deposits, are the type, strength, and number of 

contaminant sources at land surface. Eckhardt and 

Table 5. Land-use categories and ratings for the unsaturated zone and watershed characteristics, 1990–93

[<, less than]

Land-use 

category General description or example

Area in 

North 

Carolina, 

in percent

Rating

Water All areas of open water, generally with less than 25 percent vegetative cover. 9 1

Emergent wetland Non-woody, vascular, perennial vegetation where the soil or substrate is 

periodically saturated or covered with water.

1 1

Woody wetland Areas of forested or shrubland vegetation where the soil or substrate is 

periodically saturated or covered with water.

11 1

Barren land Bare rock, sand, silt, gravel, or other earthen material with little or no 

vegetation regardless of its inherent ability to support life. 

<1 2

Deciduous forest Areas dominated by trees where 75 percent or more of the tree species shed 

foliage simultaneously.

24 3

Evergreen forest Areas dominated by trees where 75 percent or more of the tree species retain 

their leaves all year. Canopy is never without green foliage.

19 3

Mixed forest Areas dominated by trees where neither deciduous nor evergreen species 

represent more than 75 percent of the cover present.

10 3

Quarries/strip mines/ 

gravel pits

Areas of extractive mining activities with significant exposure of land surface. <1 5

Hay/pasture Areas dominated by vegetation, which is planted and(or) maintained for the 

production of food or feed. Grasses, legumes, or mixtures planted for 

livestock grazing. 

6 5

Other grass Vegetation planted in developed settings for recreation, erosion control, or 

aesthetic purposes. Examples include parks, lawns, and golf courses.

<1 6

Transitional Areas dynamically changing from one land cover to another, often because of 

changes in land-use activities.

<1 7

Row crops Areas dominated by vegetation that is planted and(or) used for the production 

of crops, such as corn, soybeans, vegetables, tobacco, and cotton.

15 7

Low-intensity 

residential

Residential development. Constructed materials account for 30 to 80 percent 

of the total area. Most commonly single-family housing areas, especially 

suburban neighborhoods.

2 7

High-intensity 

residential

Residential development. Densely built urban centers, apartment complexes, 

and row houses. Vegetation occupies less than 20 percent of the landscape. 

Constructed materials account for 80 to 100 percent of the total area.

<1 8

Commercial/

industrial

Land used for the manufacture of products or sale of goods. Includes all 

highly developed lands not classified as residential, most of which are 

commercial, industrial, or transportation.

1 10
Factors Used to Determine a Watershed Characteristics Rating 11

Stackelberg (1995) stated that characterization of 

contaminant sources can be statistically quantified 

through the surrogate variable, land use.

Also of note is a series of ground-water-quality 

studies in large river basins across the United States 

conducted by the USGS as part of the National WaterQuality Assessment (NAWQA) Program, which began 

in 1991. Investigations, such as Saad (1997), have 

focused on relating ground-water quality to land use 

and other factors. 

The source of data for the land-use factor is 

identical to the source of data for land cover, both of 

which are derived from the same land-use and landcover GIS layer (see Appendix). Although land-use and 

land-cover categories use the same data source and 

terminology, they are considered separate factors in the 

unsaturated zone ratings. The land-use factor measures 

the potential for generating nonpoint-source 

contamination at land surface; the land-cover factor 

influences the amount of precipitation that infiltrates 

the ground. These factors are treated separately to 

highlight the influence of nonpoint-source 

contaminants in the unsaturated zone rating. 

Example of an Unsaturated Zone Rating

An unsaturated zone rating will be calculated for 

source water assessment areas around each public 

water-supply well in North Carolina. The PWSS will 

determine source water assessment areas by using a 

delineation method specified in the State's approved 

Wellhead Protection Program, where the area is a 

function of the amount of water pumped from the well 

and the approximate average rate of ground water 

recharged in the region (Heath, 1994; North Carolina 

Department of Environment, Health, and Natural 

Resources, 1995). The source water assessment area 

can be truncated by the presence of substantial surfacewater bodies. In the method of determining unsaturated 

zone ratings, the source water assessment area is 

divided into discrete 60-meter by 60-meter cells 

(fig. 3). Only cells with more than 50 percent of their 

area in the source water assessment area are included in 

the calculation.

The four contributing factors—vertical hydraulic 

conductance, land-surface slope, land cover, and land 

use (table 1)—are assigned weights in the final 

calculation of the unsaturated zone rating. Weights are 

subjective measures (1, 2, or 3) that reflect the relative 

importance of factors that are used to determine 

ground-water vulnerability to contamination (table 1). 

The factor weights are multiplied by factor ratings and 

summed, resulting in an unsaturated zone rating that 

ranges from 10 to 100 for each cell (table 6). The 

unsaturated zone rating for a delineated source water 

assessment area is the average value over all the cells in 

the area; for the cells used in this example, the 

unsaturated zone rating is 57.8.

Factors Used to Determine a Watershed 

Characteristics Rating

The watershed characteristics rating is based on 

a combination of factors that contribute to the 

likelihood that water, with or without contaminants, 

1234

5678

A. B.

Figure 3. (A) An unnamed well encircled by source water assessment area and 

(B) a portion of the source water assessment area overlain by 60-meter by 

60-meter cells to illustrate the calculation of the unsaturated zone rating. Only 

cells with more than 50 percent of their area in the source water assessment 

area are included in the calculation [in this example, cells 1, 2, 3, 5, 6, 7, 8].
12 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

will reach a public surface-water supply intake by 

following the path of overland flow or the path of 

shallow subsurface flow. The selected factors, which 

can be represented in the form of GIS spatial-data 

layers, include average annual precipitation, landsurface slope, land cover, land use, and ground-water 

contribution (table 7). The values of each of these five 

factors are categorized, and the categories are assigned 

a rating on a scale of 1 to 10. A rating of 1 reflects a low 

contribution to inherent vulnerability and 10 reflects a 

high contribution. For example, the rating for landsurface slope is low (1) in areas where the slope is low 

and high (10) in areas where the slope is steep. Runoff 

potential increases in steeper terrain, which leads to an 

increased likelihood of surface-water contamination.

With the exception of land use, these factors 

influence the physical transport of water. The land-use 

factor is included as a measure of the potential 

nonpoint-source of contamination caused by activities 

occurring at the land surface and is included to fulfill 

requirements of the SWAP plan to consider nonpointsource contaminants (North Carolina Department of 

Environment and Natural Resources, 1999a). To 

determine the watershed characteristics rating, the five 

Table 6. Example determination of an unsaturated zone rating for an unnamed water-supply well

[For each cell, the product of the factor weights and ratings are summed to determine the total rating for the cell. The overall rating is the average for all 

the cell ratings; possible values range from 10 to 100]

Cell 

number i

(fig. 3)

Vertical hydraulic 

conductance 

[weight (w1)=3]

Land-surface 

slope 

[weight (w2)=2] Land-cover 

and land-use 

category

Land 

cover 

[weight 

(w3)=2]

Land

use 

[weight 

(w4)=3]

Grid cell rating

In feet

squared 

per day

Rating 

(r1)

(table 2)

In 

percent

Rating 

(r2)

(table 3)

Rating 

(r3)

(table 4)

Rating 

(r4)

(table 5)

1 2,500 4 3 9 Row crop 6 7 63

2 2,000 3 4 9 Hay/pasture 8 5 58

3 2,100 4 3 9 Low-intensity 

residential

4 7 59

5 1,300 3 4 9 Hay/pasture 8 5 58

6 1,200 3 6 7 Hay/pasture 8 5 54

7 1,000 2 1 10 Low-intensity 

residential

4 7 55

8 1,100 3 2 10 Low-intensity 

residential

4 7 58

Unsaturated zone rating for n (7) selected cells (fig. 3) 

Ri wi ri ( ) ×

i = 1

4

= ∑

Ri

i = 1

7

∑

n -------------- 4057 = = -------- 57.8

Table 7. Factors that contribute to the watershed 

characteristics rating

Factor Relevance of the factor Weight

Average annual 

precipitation

The source of water that 

travels overland to streams 

or lakes.

3

Land-surface 

slope

The inclination, or change in 

elevation, of the land 

surface indicates the 

likelihood that precipitation 

will infiltrate or run off.

2

Land cover The type of material covering 

the land surface influences 

the likelihood that 

precipitation will infiltrate 

or run off.

1

Land use The type of land use influences 

the likelihood of potential 

nonpoint-source 

contamination.

3

Ground-water 

contribution

The portion of surface water 

derived from ground water.

1

factors are weighted on the basis of importance of the 

factor relative to other factors in affecting public watersupply vulnerability. Ratings are computed for 
Factors Used to Determine a Watershed Characteristics Rating 13

delineated source water assessment areas upstream 

from each intake, which are portions of the basin 

defined in accordance with the State's Water Supply 

Watershed Protection program, (North Carolina 

Department of Environment and Natural Resources, 

1999b, 1999c).

Average Annual Precipitation

Precipitation is the source of water transported 

overland to a stream or lake. In this study, two measures 

of precipitation were evaluated for use in the rating 

scheme—rainfall intensity and average annual 

precipitation. Rainfall intensity is a meaningful 

measure of precipitation because streamflows peak 

during intense rainfalls (Viessmann and others, 1977). 

Concerns about the quality of raw water at surfacewater intakes commonly are related to intense rainfall 

occurring in 24 hours or less. In a statewide 

investigation of sediment characteristics of streams in 

North Carolina, Simmons (1993) reported that rainfall 

magnitude and intensity are the most important 

precipitation factors affecting sediment load. When 

substantial amounts of rain fall in relatively short 

periods of time, such as a few hours, large drops of 

water hitting the ground surface commonly will break 

down soil particles. These particles in turn may be 

transported via overland flow, especially after soil 

infiltration rates are exceeded. Selected chemical 

constituents, such as trace metals and some nutrients 

that attach to soil may, therefore, become subject to 

transport. Concerns related to the resolution and 

accuracy of available rainfall-intensity information, 

however, resulted in the selection of average annual 

precipitation as the measure of precipitation for the 

watershed characteristics rating scheme. 

Several issues limit the use of available 

information for rainfall intensity in North Carolina. 

Current information on rainfall intensity, presented in 

the rainfall-frequency atlas for the United States 

(Hershfield, 1961), is based on 30 years of rainfall data 

collected through 1958. In North Carolina, approximately 110 rainfall-observation stations with data for 

24-hour intervals were used to map the magnitudes and 

intensity of rainfall events. Since 1961, no update of the 

frequency data has been completed for North Carolina. 

Changes to rainfall-intensity maps are likely to occur as 

a result of additional data at many observation stations 

and improved statistical analysis methods developed 

since 1961 (Dr. Leslie Julian, Hydrometeorological 

Design Studies Center, National Oceanic and 

Atmospheric Administration, oral commun., April 28, 

1999). Another limitation of rainfall intensity is the 

relatively coarse scale of the existing rainfall-frequency 

maps (approximately 1:10,000,000) as compared to the 

map scales of the other coverages (1:250,000) used in 

the watershed characteristics ratings. In the future, if 

updates are made to rainfall intensity maps, rainfall 

intensity data could be re-examined as a possible 

replacement for average annual precipitation. An 

appropriate choice would be the 24-hour, 25-year storm 

because of its common use in engineering design of 

hydraulic overflow-prevention structures. Selection of 

average annual precipitation for the watershed 

characteristics rating in North Carolina is consistent 

with its use in other States where annual or seasonal 

precipitation is included as a factor in the rating 

(California Department of Health Services, Division of 

Drinking Water and Environmental Management, 

January 1999; R.L. Joseph, U.S. Geological Survey, 

Water Resources Division, oral commun., March 8, 

1999; Texas Natural Resource Conservation 

Commission, April 30, 1999). 

In North Carolina, average annual precipitation 

varies from about 40 inches to more than 80 inches 

(table 8; fig. 4); however, two-thirds of the State 

receives between 40 and 50 inches of average annual 

rainfall. Most of the variation occurs in the Blue Ridge 

Mountains, with the highest and lowest average 

amounts occurring in this area. Giese and Mason (1993) 

reported that more of the precipitation that falls in the 

mountains is converted to streamflow than in the 

Table 8. Average annual precipitation categories and 

ratings for watershed characteristics

[<, less than]

Average annual precipitation, 

in inches

Area in 

North 

Carolina, 

in percent

Rating

Less than or equal to 40 < 1 1

Greater than 40 to less than or equal to 45 20 2

Greater than 45 to less than or equal to 50 48 3

Greater than 50 to less than or equal to 55 23 4

Greater than 55 to less than or equal to 60 5 5

Greater than 60 to less than or equal to 65 2 6

Greater than 65 to less than or equal to 70 1 7

Greater than 70 to less than or equal to 75 < 1 8

Greater than 75 to less than or equal to 80 < 1 9

Greater than 80 1 10
14 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

AVERAGE ANNUAL PRECIPITATION,

IN INCHES

EXPLANATION

A.

B.

< 5

> 5 to < 5.5

> 9.0

RAINFALL, IN INCHES

EXPLANATION

> 5.5 to < 6.0

> 6.5 to < 7.0

> 6.0 to < 6.5

> 7.0 to < 7.5

> 7.5 to < 8.0

> 8.0 to < 8.5

> 8.5 to < 9.0

< 40

> 40 to < 45

> 45 to < 50

> 50 to < 55

> 55 to < 60

> 60 to < 65

> 65 to < 70

> 70 to < 75

> 75 to < 80

> 80

Figure 4. (A) Average annual precipitation and (B) a 24-hour, 25-year rainfall event in North Carolina.
Factors Used to Determine a Watershed Characteristics Rating 15

Piedmont and Coastal Plain areas of the State. Among 

the factors likely to contribute to the occurrence of 

higher streamflow in the mountains are higher relief, 

cooler temperatures, and a shorter growing season 

(lower evapotranspiration). McMahon and Lloyd 

(1995) reported that evapotranspiration rates within the 

Albemarle-Pamlico drainage basin vary from about 30 

inches in the Blue Ridge Province to about 36 inches in 

the Coastal Plain Province. These rates constitute 

approximately 55 and 70 percent, respectively, of the 

average annual precipitation in these areas.

Average annual precipitation is derived from the 

Parameter-Elevation Regressions on Independent 

Slopes Model (PRISM), which uses a regression model 

relating land-surface elevation to precipitation in order 

to interpolate between weather observation stations 

(Daly, 1996). The average annual precipitation values 

used in PRISM are based on data collected from 1961 

to 1990 at about 140 observation stations in North 

Carolina. 

Average annual precipitation was categorized in 

increments of 5 inches, from less than 40 inches to more 

than 80 inches (table 8). The areas of the State receiving 

between 40 and 50 inches of rainfall during the year 

have a precipitation rating of 2 or 3 (table 8; fig. 4). 

Average annual precipitation amounts exceeding 80 

inches are rated 10 because very few water-supply 

systems have watersheds located solely in the highest 

elevations of North Carolina where rainfall exceeds this 

amount.

Land-Surface Slope

Land-surface slope influences the amount of 

precipitation that either runs off the land surface as 

overland flow and contributes to surface water or ponds 

on the land surface and contributes to ground water. The 

reader is referred to the previous section entitled 

"Factors Used to Determine an Unsaturated Zone 

Rating" for a more complete discussion of the 

derivation of this rating.

The relation between slope and the occurrence of 

overland flow is underscored by its effects on water 

quality in regionalization studies for predicting 

streamflow quantity and quality. Sauer and others 

(1983) used slope as one of the explanatory variables in 

regression models developed in a hydrologic 

investigation of urban runoff. Harned and others (1995) 

noted higher suspended-sediment concentrations in a 

river in the Piedmont than in the Coastal Plain, which 

generally has lower topographic relief and lower stream 

gradients than the Piedmont. Nutrient and trace metal 

constituents can attach to sediment particles; thus, 

steeper slopes result in higher vulnerability of surfacewater supply intakes to contaminant transport 

(Simmons, 1993). Giese and Mason (1993) report that 

among the factors likely to contribute to the occurrence 

of higher streamflow in the mountains is the existence 

of steep slopes that result in more rapid runoff. Chow 

and others (1988) report that the percentage of rainfall 

that is translated into overland flow to the streams is 

based on a combination of factors, including landsurface slope. 

Slopes were divided into six categories and 

assigned ratings from 1 to 10 (table 9). These are the 

same categories used in unsaturated zone ratings (table 

2), but rating values are reversed. Low ratings are 

assigned to the low slopes, and high ratings are assigned 

to the high slopes. Surface-water supplies are more 

vulnerable to contamination in areas where landsurface slopes are high. 

Table 9. Land-surface slope categories and ratings for 

watershed characteristics

[<, less than]

Land-surface (basin) slope, in percent

Area in 

North 

Carolina, 

in 

percent

Rating

Less than or equal to 2 percent 57 1

Greater than 2 to less than or equal to 5 percent 18 3

Greater than 5 to less than or equal to 10 percent 10 5

Greater than 10 to less than or equal to 20 percent 6 7

Greater than 20 to less than or equal to 50 percent 8 9

Greater than 50 percent < 1 10

Land Cover

Land cover, which describes the physical overlay 

of the land surface, influences the amount of 

precipitation that runs off. Runoff predominates where 

land cover is impervious. For developed areas where 

asphalt and structures dominate the surface, most of the 

rainfall runs off as overland flow. Where rain falls 

directly onto the ground, the level of infiltration 

depends, in part, on the soil characteristics and 

vegetative cover. The reader is referred to the previous 

section entitled "Factors Used to Determine a 

Watershed Characteristics Rating" for more discussion 

of the land-cover factor.
16 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

Simmons (1993) discussed the effects of 

vegetative cover as an important factor in erosion, 

sediment disintegration, and transport by overland flow 

or wind. Vegetative cover impedes erosion in a number 

of ways—by reducing splash erosion, increasing 

evapotranspiration, reducing runoff potential, and 

increasing infiltration as precipitation falls on and is 

held by decayed matter. Simmons (1993) cited average 

annual erosion rates from the U.S. Department of 

Agriculture (1977) for rural areas—0.1 ton per acre 

from forests, 1.3 tons per acre from grassland pastures, 

and 7.5 tons per acre from croplands. Disturbing 

cropland areas and clearing vegetative cover results not 

only in more soil material available for transport, but 

less impedance to overland flow.

Peak flow, or maximum discharge, also may be 

used as a measure of the degree of infiltration potential 

provided by a particular land cover. Hydraulic 

equations used in the prediction of peak flows 

commonly include a variable that represents the land 

cover in the basin upstream from the structure.

The Rational Method equation for estimating 

peak discharge includes a variable referred to as the 

runoff coefficient. Chow and others (1988) reported 

that the runoff coefficient implies a fixed ratio of the 

peak discharge rate to the rainfall rate in the basin. The 

percentage of rainfall that is translated into overland 

flow to the streams, however, is based on a combination 

of factors, including percentage of imperviousness, 

ponding characteristics, and soil condition. Tables of 

runoff characteristics used with the Rational Method 

commonly are found in applied hydrology, hydraulic, 

and civil engineering manuals (Viessmann and others, 

1977; Chow and others, 1988; Lindeburg, 1992).

Predicting peak flows by using methods 

developed by the U.S. Department of Agriculture SCS 

(now the Natural Resources Conservation Service) 

relies on a runoff variable known as a curve number, 

which is based on land cover and other factors (U.S. 

Department of the Interior, Bureau of Reclamation, 

1973; Overton and Meadows, 1976; Lindeburg, 1992). 

Overton and Meadows (1976) developed a 

comprehensive table of curve numbers with respect to 

land-cover categories.

As with the land-cover ratings for the 

unsaturated zone, ratings were assigned to the landcover categories by using information about the runoff 

coefficients and SCS curve numbers as general guidelines. High ratings are associated with land cover that 

presents low impedance to overland flow (table 10).

Land Use

Land use describes activities that occur on the 

land surface. This factor represents the potential for 

generation of nonpoint-source contamination that 

might result from these activities. Land use is rated 

identically for watershed characteristics and the 

unsaturated zone (table 5). The reader is referred to the 

previous section entitled "Factors Used to Determine 

an Unsaturated Zone Rating” for a discussion relating 

land use to ground-water quality.

The effect of land use on surface-water quality 

has been the subject of many data-collection and 

interpretive investigations. Of note is a series of 

surface-water-quality studies in large river basins 

across the United States conducted by the USGS as part 

of the NAWQA Program. Mueller and others (1995) 

developed nationwide comparisons of findings from 

individual NAWQA river basins. In general, nutrient 

concentrations downstream from agricultural areas 

were higher than concentrations downstream from 

undeveloped areas. In the water-quality investigation of 

the Albemarle-Pamlico drainage basin located in North 

Carolina and Virginia, Harned and others (1995) noted 

that the highest nitrogen and phosphorus concentrations were observed in more developed basins and 

areas having a large percentage of agricultural and 

farm-animal operations.

In the Albemarle-Pamlico drainage basin, 

McMahon and Lloyd (1995) confirmed that 

agricultural and developed areas tend to have the 

greatest negative effect on water quality. These land 

uses generally introduce high quantities of nutrients, 

sediments, and other chemical constituents into the 

hydrologic system. Within agricultural areas, the 

effects on water quality vary, depending on the use of 

lands for crop production or livestock grazing. 

McMahon and Lloyd (1995) also noted that while 

runoff from forested areas may be expected to have the 

least impact on water quality, runoff from lands used 

for silviculture (tree production) could contain 

pesticides used for weed and insect control. However, 

McMahon and Lloyd (1995) also noted that wetlands 

can act as natural water-treatment systems because the 

slower water velocities allow suspended sediments and 

adsorbed chemical constituents to settle out. 

Omernik (1977) reported some general relations 

between land use and nitrogen and phosphorus loads in 

surface water. Using land-use categories expressed as a 

function of percentage forest, cleared, agriculture, and 

urban uses, basins with high percentages of urban and 
Factors Used to Determine a Watershed Characteristics Rating 17

agricultural land uses produced higher loadings of total 

nitrogen. Surface waters draining entirely agricultural 

or urban areas had a nearly tenfold increase in nitrogen 

concentration compared to forested drainage basins. 

Similar trends for total phosphorus were observed; 

however, the differences in concentration between 

urban/agricultural basins and forested basins were not 

as pronounced.

The source of data for the land-use factor is 

identical to the source of data for land cover, and both 

are derived from the same land-use and land-cover GIS 

layer (see Appendix). Although land-use and landcover categories use the same data source and 

terminology, they are considered separate factors in the 

watershed characteristics ratings. The land-use factor 

measures the potential for generating nonpoint-source 

contamination at land surface; the land-cover factor 

influences the amount of precipitation that runs off as 

overland flow. These factors are treated separately to 

highlight the influence of nonpoint-source 

contaminants in the watershed characteristics rating. 

Table 10. Land-cover categories and ratings for watershed characteristics, 1990–93

[<, less than]

Land-cover 

category General description or example

Area in 

North 

Carolina, 

in percent

Rating

Deciduous forest Areas dominated by trees where 75 percent or more of the tree species shed 

foliage simultaneously.

24 1

Evergreen forest Areas dominated by trees where 75 percent or more of the tree species retain 

their leaves all year. Canopy is never without green foliage.

19 1

Mixed forest Areas dominated by trees where neither deciduous nor evergreen 

species represent more than 75 percent of the cover present.

10 1

Water All areas of open water, generally with less than 25 percent vegetative cover. 9 3

Emergent wetland Non-woody, vascular, perennial vegetation where the soil or substrate is 

periodically saturated or covered with water.

1 3

Woody wetland Areas of forested or shrubland vegetation where the soil or substrate is 

periodically saturated or covered with water.

11 3

Hay/pasture Areas dominated by vegetation, which is planted and(or) maintained for the 

production of food or feed. Grasses, legumes, or mixtures planted for 

livestock grazing. 

6 3

Other grass Vegetation planted in developed settings for recreation, erosion 

control, or aesthetic purposes. Examples include parks, lawns, and golf 

courses.

< 1 4

Barren land Bare rock, sand, silt, gravel, or other earthen material with little or no 

vegetation regardless of its inherent ability to support life. 

< 1 5

Transitional Areas dynamically changing from one land cover to another, often because of 

changes in land-use activities.

< 1 5

Quarries/strip mines/

gravel pits

Areas of extractive mining activities with significant exposure of land surface. < 1 5

Row crops Areas dominated by vegetation that is planted and(or) used for the 

production of crops, such as corn, soybeans, vegetables, tobacco, and 

cotton.

15 6

Low-intensity 

residential

Residential development. Constructed materials account for 30 to 80 percent 

of the total area. Most commonly single-family housing areas, especially 

suburban neighborhoods.

2 7

High-intensity 

residential

Residential development. Densely built urban centers, apartment 

complexes, and row houses. Vegetation occupies less than 

20 percent of the landscape. Constructed materials account for 

80 to 100 percent of the total area.

< 1 8

Commercial/

industrial

Land used for the manufacture of products or sale of goods. Includes all 

highly developed lands not classified as residential, most of which are 

commercial, industrial, or transportation.

1 10
18 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

Ground-Water Contribution

Surface water and ground water are parts of one 

system. In North Carolina, streamflows are, in part, 

derived from underlying aquifers, particularly aquifers 

where the upper boundary is the water table (fig. 1). 

The effect of ground-water contribution to surface 

water is included in the assessment of watershed 

characteristics to address the influence that ground 

water has on surface-water quantity and quality. In this 

study, ground-water contribution is derived from the 

unsaturated zone rating described in the previous 

section entitled "Example of an Unsaturated Zone 

Rating."

The portion of streamflow derived from ground 

water is known as base flow, which is affected by a 

number of factors. Sear and others (1999) reported that 

whereas base flow to a stream depends on the nature of 

the soils and lithology in the upstream catchment, 

recharge is controlled by precipitation, evaporation, 

and temperature. 

To obtain a general snapshot of ground-water 

contribution in streams across the State, streamflow 

records at more than 200 active and discontinued 

gaging stations across North Carolina were used to 

determine the base-flow index (BFI) for each site by 

using available period of record (through the 1997 

water year for active sites). The BFI is the ratio of the 

mean annual ground-water discharge (base flow) to the 

mean total annual streamflow (Wahl and Wahl, 1988; 

Nathan and McMahon, 1990; Rutledge and Mesko, 

1996). BFI values across the State ranged from about 

13 to nearly 73 percent (mean of about 49 percent) of 

the total annual streamflow. 

Base flow generally composes more of the total 

streamflow in the Coastal Plain Province than in the 

Piedmont Province. The lower base flow in the 

Piedmont reflects the fact that Piedmont topography 

has higher relief than Coastal Plain topography, and 

that Piedmont formations are, on the whole, less 

transmissive than Coastal Plain formations. In the 

Coastal Plain Province, BFI values ranged from about 

29 to 71 percent of the total streamflow with a mean of 

48 percent. Wilder and others (1978) assessed the water 

resources for northeastern North Carolina and noted 

that the ground-water contribution accounted for about 

two-thirds of the total streamflow in that region. Bales 

and Pope (1996) reported that base flow at a gaging 

station on the Waccamaw River averaged 53 percent of 

the total annual streamflow from 1940 to 1994. Base 

flow averaged nearly 71 percent at a site on the nearby 

Lumber River (Bales and Pope, 1996). 

The BFI values for sites in the Piedmont 

Province ranged from about 13 to 68 percent of total 

annual streamflow with a mean of 37 percent. In the 

Blue Ridge Province, BFI values ranged from 30 to 73 

percent of the total annual streamflow with a mean of 

62 percent. The relatively high mean is consistent with 

previous regional assessments, which indicated higher 

levels of base flow in the Blue Ridge Province 

compared to the Piedmont Province (Giese and Mason, 

1993; Rutledge and Mesko, 1996). Factors influencing 

the higher levels of base flow include higher 

precipitation amounts and greater topographic relief, 

lower evapotranspiration, shorter growing seasons as 

well as shallow soils and fractured bedrock, which can 

serve as conduits for the subsurface flow to streams. 

The inclusion of a ground-water contribution 

factor in this study addresses the influence that ground 

water has on surface waters. The method previously 

described for determining unsaturated zone ratings is 

applied to an area 1,000 feet on either side of streams 

within the delineated basins. Unsaturated zone ratings 

can range from 10 to 100. Ground-water contribution 

ratings for the cells in these areas are calculated exactly 

as for unsaturated zone ratings, then divided by 10 to 

scale the values in the range from 1 to 10, like other 

factors used in other watershed characteristics ratings.

The use of a 1,000-foot buffer is consistent with 

the buffer being used by the PWSS to inventory and 

rate point-source discharges near streams (North 

Carolina Department of Environment and Natural 

Resources, 1999a). By restricting ground-water 

contribution to an area of about 1,000 feet on either 

side of streams, the ground-water contribution factor 

emphasizes focused recharge (a local water table rise 

caused by stormflow) and local ground-water flow, the 

most dynamic and shallowest flow system that has the 

greatest interchange with surface water (Winter and 

others, 1998). 

In this study, ground-water contribution 

originating outside of this buffer is not considered, 

although sub-surface flow does occur over much longer 

distances. Beyond the 1,000 feet from surface-water 

bodies, the rating for ground-water contribution is zero.
Limitations 19

Example of a Watershed Characteristics Rating

For a given watershed, ratings for each of the five 

factors (precipitation, slope, land cover, land use, and 

ground-water contribution) are multiplied by 

respective factor weights, and summed to create a 

unique rating for each grid cell. Weights (1, 2, or 3) are 

subjective measures that reflect the relative importance 

of factors used to determine ground-water vulnerability 

to contamination (table 7). The overall rating for the 

watershed is determined by averaging the ratings for 

the grid cells. The range of possible ratings is 10 to 100.

An example watershed characteristics rating is 

presented. Ratings are computed for delineated source 

water assessment areas upstream from each intake, 

which are portions of the basin defined in accordance 

with the State’s Water Supply Watershed Protection 

program (North Carolina Department of Environment 

and Natural Resources, 1999b, 1999c). The watershed 

has grid cells (60 meters by 60 meters) throughout the 

basin (fig. 5). Only cells with more than 50 percent of 

their area contained within the watershed are included 

in the calculation. Ratings for all grid cells were 

calculated (table 11) and averaged to produce the 

overall watershed characteristics rating (47.9 for the 

cells used in this example). 

LIMITATIONS

The overlay and index methods of unsaturated 

zone and watershed characteristics ratings that were 

used here are broad-brush-stroke methods that assess 

an aspect of inherent vulnerability on the basis of 

expert opinion. Given that the task of susceptibility 

assessments is to rate hundreds of surface-water 

intakes and thousands of wells, the statewide approach 

is a practical and effective step. These methods of 

rating the unsaturated zone and watershed 

characteristics have limitations, which include the 

following:

• The data used to represent land use and land cover 

have limitations. The land-use and land-cover data 

base was gathered during 1990–93; in some areas, 

land use and land cover have changed since the 

early 1990's. Substantial development has 

occurred in North Carolina around urban centers. 

Furthermore, the land-use and land-cover data 

1 2

3 4

A.

B.

C.

Figure 5. (A) An unnamed watershed upstream from a surface-water supply 

intake showing basin outline, stream network, and 1,000-foot buffered area around 

streams; inset (B) 60-meter by 60-meter cells overlaid on a portion of the watershed; and inset (C) a subset of four cells used to illustrate the calculation of a 

watershed characteristics rating.
20 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

tend to underestimate the extent of urban centers, 

possibly assigning heavily wooded urban land as 

forested land. The land-use and land-cover data 

base relied upon the National Wetlands Inventory 

(NWI) to identify wetland areas. The 1:24,000 

NWI maps are inconsistent across quadrangle 

boundaries because of differing dates of mapping 

and technician error. The land-use and land-cover 

data in North Carolina are limited to 15 categories. 

It is not possible to extract categories that have 

been grouped together. The data do not distinguish 

orchards or Christmas tree farms, but lump 

silviculture in with forests or row crops. Also, the 

data do not distinguish quarries from strip mines, 

or bare rock from sand. 

• The data used to calculate vertical hydraulic 

conductance have limitations. The primary layer 

for this factor was soils data. The soil data are 

from two data bases with quite different scales 

(1:24,000 and 1:250,000). The differences 

between STATSGO and SSURGO data are 

noteworthy; SSURGO data are much more 

detailed and informative. Additionally, some soil 

types were not assigned permeability and 

thickness values; these soils are assigned the 

permeability and thickness values derived from 

STATSGO data (see Appendix). In all cases, 

unsaturated hydraulic conductivity of the 

unsaturated zone is replaced by saturated 

hydraulic conductivity, which means that 

estimated vertical hydraulic conductance is 

always the maximum estimated value. Another 

limitation of the vertical hydraulic conductance 

layer is that statewide estimates of depth to water 

are not available, neither are statewide estimates 

available for vertical hydraulic conductance of 

geologic units underlying soil.

• The primary limitation of the land-surface slope 

factor was introduced in the development of the 

source —the digital elevation model. To develop a 

continuous elevation surface, a connected stream 

and river network was created in North Carolina. 

The centerline of water bodies was used to create 

the stream network. This created a surface 

representative of stream channels beneath 

reservoirs or lakes, thus assigning higher slopes 

within lakes and reservoirs than now exist. For the 

final statewide slope computation, elevation data 

that incorporate existing water-body outlines 

should be used.

• The ground-water contribution factor to watershed 

characteristics rating has limitations. First, it is not 

just the 1,000-foot buffered area that contributes to 

surface-water bodies, but rather the entire 

interstream area. Second, the method used to 

identify the ground-water contribution area is 

flawed. Buffered areas from all surface-water 

Table 11. Example determination of a watershed characteristics rating for part of an unnamed watershed upstream from a 

water-supply intake

Cell 

number i

(fig. 5)

Precipitation 

[weight (w1)=3]

Land-surface 

slope 

[weight (w2)=2] Land-cover 

and land-use 

category

Land 

cover 

[weight 

(w3)=1]

Land 

use 

[weight 

(w4)=3]

Ground-water 

contribution 

[weight (w5)=1] Grid cell rating

In 

inches

Rating 

(r3)

(table 8)

In 

percent

Rating 

(r2)

(table 9)

Rating 

(r3)

(table 

10)

Rating 

(r3)

(table 5)

Unsaturated 

zone 

rating 

value

Rating 

(r3)

1 56 5 52 10 Forest 1 3 46 4.6 49.6

2 56 5 47 9 Forest 1 3 0 0 43.0

3 56 5 48 9 Forest 1 3 50 5.0 48.0

4 56 5 45 9 Hay/pasture 3 5 0 0 51.0

Watershed characteristics rating for n (4) selected cells (fig. 5) 

Ri wi ri ( ) ×

i = 1

5

= ∑

Ri

i = 1

4

∑

n -------------- 191.64 = = ------------- 47.9
Summary 21

bodies are used in calculations, even if the surfacewater body is located in an adjacent watershed.

Other contributing factors may be important in 

determining inherent vulnerability to specific 

contaminants. Organic content of soil may have been 

used to predict areas through which certain 

contaminants, such as pesticides, would not likely be 

transported but rather bound to the soil. Organic 

content was not selected because vulnerability 

assessments in this study are not specific to particular 

contaminants. Vegetative cover, which also might have 

been used to predict areas through which some 

contaminants would not likely be transported but rather 

bound to plants, and which also would influence 

overland flow, was not selected for lack of a map 

showing average annual vegetative cover. Also not 

considered is the effect of airborne contaminants, 

notably nitrogen, from livestock and fertilizer that are 

generated outside the watershed or source water 

assessment area (Rudek, 1997). Anthropogenic effects 

also can be a substantial source of airborne nitrogen. 

This factor was not selected because insufficient 

statewide ambient air-quality monitoring data are 

available to evaluate this potential source of 

contamination.

• Other data may have been used to represent the 

contributing factors. Storm intensity would have 

been selected in place of average annual 

precipitation had more recent, finer-scale data 

been available. 

• The methods derived for rating unsaturated zone 

and watershed characteristics use 60-meter by 60-

meter cells. However, the data used to represent 

the factors would support a finer grid of 30-meter 

by 30-meter cells.

• The assigned rates and weights were determined 

by expert opinion and selected to reflect 

contributions to water-supply vulnerability. A 

statistical analysis of the factors and ambient 

water-quality data could be performed in the 

future to determine if assigned weights and rates 

could be improved.

The strength of the overlay and index method is 

its appeal to common sense. The weakness is that 

consensus among experts does not imply veracity; the 

hypothesis that selected factors influence water quality 

was not tested empirically. Several studies to determine 

statistical relations between contributing factors and 

specific ground-water-quality parameters have been 

performed (Grady, 1994; Eckhardt and Stackelberg, 

1995; Rupert, 1998, 1999; Eric Vowinkel, USGS, oral 

commun., February 5, 1999; Mike Sweat, USGS, oral 

commun., August 3, 1999). It would be a meaningful 

contribution to investigate the statistical relation 

between contributing factors and particular water 

contaminants in North Carolina. The greatest expense 

of such a study would be gathering sufficient waterquality data through a monitoring program; the benefit 

would be increased capability to protect source waters.

SUMMARY

The Source Water Assessment Program (SWAP), 

established by the SDWA Amendments of 1996, is 

designed to promote pollution prevention as a costeffective means of providing reliable, long-term, safe 

drinking water sources for public water-supply 

systems. In North Carolina, the lead agency 

responsible for developing and implementing this 

pollution prevention program is the Public Water 

Supply Section (PWSS). To assist the PWSS in its 

efforts to rate the inherent vulnerability of more 

than 11,000 public water-supply wells and approximately 245 public surface-water intakes, the USGS 

developed methods to rate the unsaturated zone around 

public ground-water supplies and watershed 

characteristics of public surface-water intakes. 

The PWSS will complete the inherent 

vulnerability analysis by further considering aquifer 

characteristics and watershed classification, intake 

location, and raw-water quality. Additionally, the 

PWSS will consider known point sources of 

contamination to describe the susceptibility of public 

water supplies to contamination.

Overlay and index methods were applied by the 

USGS to rate unsaturated zone and watershed 

characteristics. Factors were selected that influence the 

inherent vulnerability of drinking water sources. The 

distribution of values for each factor was classified over 

the possible range of values in North Carolina. 

Categories were rated on a scale of 1 to 10, where 1 

indicates a minimal influence on the inherent 

vulnerability of a water supply, and 10 indicates a 

maximal influence on inherent vulnerability. The GIS 

data layers were divided into 60-meter by 60-meter 

cells, with one class of each contributing factor 

assigned to each cell. 
22 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

Each factor was weighted in terms of its 

influence on the inherent vulnerability. Factor weights 

sum to 10. Multiplying the rates and weights and 

summing for each factor produces an index of the 

inherent vulnerability of the unsaturated zone and 

watershed characteristics for each cell. Inherent 

vulnerability values for all cells in the delineated 

source water protection areas are averaged to yield a 

single index characterizing the ground- or surfacewater supply.

Selection of factors and subsequent assignment 

of final rates and weights for every category of 

contributing factors was based initially on a literature 

search. The literature search was followed by 

consultation with experts in hydrology, geology, 

forestry, agriculture, and water management.

Factors contributing to the inherent vulnerability 

of the unsaturated zone are the vertical hydraulic 

conductance, land-surface slope, land cover, and land 

use. Factors contributing to the inherent vulnerability 

of the watershed are the average annual precipitation, 

land-surface slope, land cover, land use, and groundwater contribution. These factors influence the physical 

transport of water, with or without contaminants. In 

addition to influencing water transport, land use effects 

the likelihood for generation of nonpoint-source 

contamination.

Vertical hydraulic conductance measures the 

capacity of the entire sequence of unsaturated material 

to transmit water. In the western and central parts of 

North Carolina, the unsaturated zone can be soil, 

saprolite, and(or) fractured rock. In the eastern part of 

North Carolina, the unsaturated zone can be soil, 

sedimentary rock, or unconsolidated sediments. 

Estimates of vertical hydraulic conductivity and 

thickness of these unsaturated zone components are 

used to derive the vertical hydraulic conductance. Soil 

data are from one of two data bases—SSURGO or 

STATSGO. Up to six soil layers may exist in any 

location; harmonic mean hydraulic conductivity for 

these six layers is the central value of hydraulic 

conductivity for all six layers combined. Vertical 

hydraulic conductivity of saprolite, fractured rock, 

sedimentary rock, and unconsolidated sediments are 

estimated from literature.

Land-surface slope influences whether 

precipitation runs off the land surface or infiltrates into 

the subsurface. Regional slopes range from relatively 

flat in the Coastal Plain Province to steep, highly 

variable slopes in the Blue Ridge and Piedmont 

Provinces. Locally, slopes in the Coastal Plain can be 

steep near streams. About 85 percent of the State is 

characterized by slopes less than 10 percent; about 57 

percent of the State is characterized by slopes less than 

2 percent. 

Land cover, the physical overlay of the land 

surface, influences the amount of precipitation that 

infiltrates into the ground or runs off as overland flow. 

The more impervious the surface, the more 

precipitation runs off; the more pervious the surface, 

the more precipitation infiltrates into the subsurface. 

Commercial/industrial land cover has the least amount 

of pervious surface; forests have the most amount of 

pervious surface. 

Land use describes activities that occur on the 

land surface and influence the potential generation of 

nonpoint-source contamination. Agricultural and 

developed areas (commercial/industrial and highintensity residential areas) tend to have the greatest 

contribution of nutrients, sediment, and other chemical 

constituents. Forested areas are rated as having 

minimal impact on water quality, except where the land 

is used for silviculture and pesticide use can be high. 

Wetlands and barren lands also are rated low in terms 

of adverse impact on water quality. 

Ground-water contribution represents the part of 

streamflow that is derived from ground-water 

discharge. In this study, ground-water contribution is a 

factor in areas within about 1,000 feet of streams. By 

restricting ground-water contribution to an area of 

about 1,000 feet on either side of streams, groundwater contribution emphasizes focused recharge and 

local ground-water flow. In this scenario, ground-water 

contribution is not considered to have originated 

outside of this buffer, though much of the flow in a 

stream experiencing low-flow conditions will have 

originated in these interstream areas. Base flow was 

assessed at 61 streams throughout North Carolina. 

Base flow ranges from about 29 to 71 percent in the 

Coastal Plain Province, from about 13 to 68 percent in 

the Piedmont Province, and from about 30 to 73 

percent in the Blue Ridge Province.

The overlay and index methods described in this 

report are based on expert opinion concerning the 

relative importance of selected factors on source water 

quality, not on scientific experimentation. The relative 

rating of unsaturated zone and watershed characteristics is a practical and effective method for assessing 

one aspect of the inherent vulnerability of water 

supplies to contamination. 
References 23

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o′
26 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

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Appendix 27

APPENDIX
28 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

Land Use and Land Cover

The source data for both the land-use and landcover components is the Multi-Resolution Land 

Characteristics (MRLC) data set. The MRLC data set is 

a product of the MRLC Consortium which consists of 

the Ecological Monitoring and Assessment Program of 

the U.S. Environmental Protection Agency (USEPA), 

U.S. Forest Service Remote Sensing Application 

Center, Gap Analysis Program of the U.S. Geological 

Survey (USGS) Biological Resources Division, 

Coastal Change Analysis Program of the National 

Oceanic and Atmospheric Administration (NOAA), 

and the National Water-Quality Assessment (NAWQA) 

Program and EROS Data Center (EDC) of the USGS. 

The mechanisms for collaboration were formalized 

with the signing of a Memorandum of Understanding 

in 1995.

The main objective of the MRLC consortium 

was to generate a generalized and consistent (seamless) 

land-cover data layer for the entire conterminous 

United States (Bara, 1994). The North Carolina portion 

of the data set was created as part of the land-cover 

mapping activities for Federal Region IV (the States of 

Kentucky, Tennessee, Mississippi, Alabama, North 

Carolina, South Carolina, Georgia, and Florida). The 

development of the Region IV data set was initiated 

during the spring of 1997, and a first draft product was 

completed during summer 1997. This data set was 

developed by personnel at the EDC, Sioux Falls, S.D.

The primary source of data for the MRLC data 

set was leaves-off (primarily spring) Landsat Thematic 

Mapper (TM) data, acquired during 1990–93, 

primarily during the spring seasons of 1991, 1992, and 

1993 (Vogelmann and others, 1998). Additionally, 

leaves-on (summer) TM data sets were acquired and 

referenced. In total, 24 TM scenes were analyzed. 

These data sets were referenced to Albers Conical 

Equal Area coordinates, but projected to the North 

Carolina State Plane coordinate system for this project.

The general procedure used for processing the 

MRLC data set was to (1) mosaic multiple leaves-off 

TM scenes and classify them by using an unsupervised 

classification algorithm, (2) interpret and label classes 

into land-cover categories by using aerial photographs 

as reference data, (3) resolve confused classes by 

using the appropriate ancillary data source(s), and 

(4) incorporate land-cover information from leaves-on 

TM data, National Wetlands Inventory (NWI) data, and 

other data sources to refine and augment the "basic" 

classification developed above. More detailed 

information about the background and production 

process of the MRLC data sets can be obtained at the 

USEPA web site http://www.epa.gov/mrlc. 

To test the methods, the MRLC data set was 

resampled from a 30-meter by 30-meter grid to a 60-

meter by 60-meter grid, shifted, and snapped to match 

the lower-left corner of the other contributing-factor 

data sets. Since the MRLC is categorical data, a nearest 

neighbor algorithm was used to maintain the 

classification scheme. This resampling technique 

assigns a value to each cell in the coarser data set that 

is the value of the cell in the original source closest to 

the center of the larger cell. Resampling by using the 

nearest neighbor algorithm from a 30-meter by 30-

meter to a 60-meter by 60-meter grid implies that the 

value from only 1 cell out of each 4-cell neighborhood 

will be represented in the output data set. However, the 

overall representation of land-cover and land-use 

classes is the same—the percentage of area within each 

land-use and land-cover class is the same statewide for 

the 30-meter by 30-meter and 60-meter by 60-meter 

data sets.

When rating methods are applied statewide, cell 

size will be retained at 30 meters by 30 meters. A 1999 

release of the MRLC land-use and land-cover data (still 

the 1990–93 data, but with better distinctions among 

some categories) should be used.

Elevation and Slope

The source of the elevation data set used to 

derive the slope components is a Digital Elevation 

Model (DEM) developed by the USGS and North 

Carolina State University. The ARC/INFO version 

7.1.1 TOPOGRID command was used to process the 

elevation surface model. TOPOGRID incorporates the 

software package, developed by Michael Hutchinson at 

Australian National University, known as "ANUDEM" 

(abbreviated form of Australian National University 

Digital Elevation Model) to produce the DEM. Four 

DOCUMENTATION ON GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA 

SOURCES 
Appendix 29

types of input data were used for the production of the 

DEM—hypsography (land-surface elevation) contour 

lines, hypsography points, hydrography, and shoreline. 

Following ANUDEM processing, a "fill" procedure 

(Jenson and Domingue, 1988) was used to remove 

remaining depressions. Each of the pre-processing 

steps is described briefly below.

Hypsography pre-processing:

The USGS 1:100,000-scale digital line graph 

(DLG) hypsography files were downloaded from the 

USGS GeoData web site (http://edcwww.cr.usgs.gov/

doc/edchome/ndcdb/ndcdb.html). The DLG files were 

converted into a point GIS layer and a contour line GIS 

layer. Only the elevation and depression contours were 

used.

Hydrography pre-processing:

The 1:100,000-scale hydrography data are an 

early release of the River-Reach File (RF-3) distributed 

by the USEPA. Cataloging units that include any part 

of North Carolina were processed. Several changes 

were made in the RF-3 data set before its use in 

TOPOGRID. First, many small water bodies and 

streams that were not connected to the main stream 

network were eliminated. Larger unconnected streams 

were retained. Second, centerlines were generated for 

all large lakes, wide streams, and other water bodies. 

The polygons forming the water bodies were removed. 

Because the stream centerlines were used in the 

creation of the DEM rather than water-body polygons, 

the DEM is not flat in the areas covered by water. Third, 

TOPOGRID requires that all streams point 

downstream, so all lines pointing upstream were 

flipped. Finally, the RF-3 data were incomplete in 

several places. Large parts of several rivers and lakes 

were missing. Corrections were made by using data 

extracted from the USGS 1:100,000-scale hydrography 

DLG.

Shoreline pre-processing:

The shoreline of North Carolina at 1:24,000 

scale was combined with the shoreline of adjacent 

states at 1:70,000 scale. The shoreline data were 

processed into a GIS data layer that defined water and 

land. The shoreline arc also was entered as a contour 

elevation with a zero-meters elevation value. Shoreline 

areas were examined to verify that no overlap of 

contour lines with shorelines occurred.

Once the entered data sets were finalized, the 

data were processed through the TOPOGRID function 

in half-degree by 1-degree geographic blocks. A 

6-kilometer area of overlap was included around each 

block to minimize edge effects. The blocks were then 

mosaiced together to create a seamless DEM for the 

State. The final resolution of the DEM tested in several 

areas of the State is 60-meter by 60-meter cells, stored 

as a floating-point, raster grid. A percentage slope data 

layer was derived by using the SLOPE function in 

ARC/INFO GRID module. 

When rating methods are applied statewide, 

GIS grid-cell size will be reduced to 30 meters by 

30 meters. Improved slope data (based on 1:24,000 

DEM data base) will be used.

Soils

Two sources of soil data were used for this 

report—county level and state level. Soil types by 

county were identified in the SSURGO data base of the 

Natural Resources Conservation Service (NRCS). The 

NRCS developed the SSURGO data base at a scale of 

1:24,000, primarily for use in the natural-resource 

planning and management of farms and ranches, 

townships, or counties and by landowners/users. At the 

time of this report, county-level data have been 

processed for Alamance, Beaufort, Brunswick, 

Cabarrus, Currituck, Durham, Edgecombe, Granville, 

Guilford, Halifax, Hyde, Mecklenburg, Nash, Orange, 

and Stanly Counties in North Carolina. 

Where county-level soil information was not 

available, the STATSGO data base for North Carolina 

was used. STATSGO is a digital, general-soils 

association map developed by the NRCS. It consists of 

a broad inventory of soil and non-soil areas that occur 

in a repeatable pattern on the landscape and that can be 

cartographically shown at the scale mapped. The soil 

maps for STATSGO are compiled by generalizing 

more detailed soil survey maps. Where more detailed 

soil survey maps are not available, data on geology, 

topography, vegetation, and climate are assembled, 

together with Land Remote Sensing Satellite 

(LANDSAT) images. Soils of like areas are studied, 

and the probable classification and extent of the soils 

are determined. STATSGO maps are at the 1:250,000 

scale and are designed primarily for regional, 

multicounty, river basin, State, and multistate resource 

planning, management, and monitoring.
30 Methods of Rating Unsaturated Zone and Watershed Characteristics of Public Water Supplies in North Carolina

To test the rating methods, the STATSGO and 

SSURGO soil layers were compiled into one layer with 

a cell size of 60 meters by 60 meters. The SSURGO 

data were superimposed on the STATSGO data so that 

the best available data are always used. When rating 

methods are applied statewide, a cell size of 30 meters 

by 30 meters will be used.

Information about soil permeability and 

thickness was obtained from the Map Unit 

Interpretation Record (MUIR) attribute data base that 

is linked to the SSURGO soil-unit delineation and the 

STATSGO mapping unit. MUIR contains information 

about soils and individual layers within soils. Some 

problems were encountered with the attribute data for 

the SSURGO data. Certain soil series were not 

assigned permeability or thickness values, including 

dams, gullied lands, pits, mines, quarries, stony lands, 

udorthents, urban lands, dunes, and water. For 

statewide evaluation, missing values should be 

assigned the STATSGO value for the area. 

ARC/INFO programs were written to process 

the MUIR data to extract thickness and permeability by 

layer for each soil unit. For SSURGO and STATSGO 

data, the weighted average by percentage of each soil 

component was applied to each mapping unit for 

thickness and harmonic mean permeability. The body 

of the report defines the equations used to calculate the 

harmonic mean permeability values.

More information on STATSGO, SSURGO, and 

the MUIR data bases can be obtained from the U.S. 

Department of Agriculture, NRCS, National Soil 

Survey Center, National Soil Data Access Facility web 

site, http://www.statlab.iastate.edu/soils/nsdaf/

index.html.

Precipitation

The mean monthly precipitation estimates were 

generated by the Parameter-Elevation Regressions on 

Independent Slopes Model (PRISM) (Daly and others, 

1994, 1997). PRISM is an analytical tool that uses 

point data, a DEM, and other spatial data sets to 

generate estimates of monthly, yearly, and event-based 

climatic parameters, such as precipitation, temperature, 

snowfall, degree days, and dew point. PRISM-derived 

data sets have been used in applications of climatology, 

hydrology, natural resources, global climate change, 

land use, planning, relocation, education, and 

geography. PRISM is uniquely designed to map 

climate in the most difficult situations, including high 

mountains, rain shadows, temperature inversions, 

coastal regions, and other complex climatic regimes. 

PRISM uses a DEM to estimate the elevations of 

precipitation stations at the proper orographic scale, 

and uses the DEM and a windowing technique to group 

stations onto individual topographic facets. For each 

DEM grid cell, PRISM develops a weighted 

precipitation/elevation (P/E) regression function from 

nearby stations, and predicts precipitation at the cell's 

DEM elevation with this function. In the regression, 

greater weight is given to stations with location, 

elevation, and topographic positioning similar to that of 

the grid cell. Whenever possible, PRISM calculates a 

prediction interval for the estimate, which is an 

approximation of the uncertainty involved. By relying 

on many localized, facet-specific P/E relations rather 

than a single domain-wide relation, PRISM continually 

adjusts its frame of reference to accommodate local 

and regional changes in orographic regime with 

minimal loss of predictive capability.

Data entry into the national model consisted of 

1961–90 mean monthly precipitation data from more 

than 8,000 NOAA cooperative sites, snow telemetry 

(SNOTEL) sites, and selected State network stations. 

Data-sparse areas were supplemented by a total of 

about 500 short-term stations. A station was included in 

this data set if it had at least 20 years of valid data, 

regardless of its period of record. PRISM software was 

used to minimize "seams" along State and regional 

boundaries. The North Carolina portion of the data set 

is distributed separately. 

The DEM data for the model are 1:250,000-

scale, distributed by the USGS. These data and their 

associated metadata are available from the USGS web 

site http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/

ndcdb.html.

Summing 12 monthly maps for the country 

created the national mean annual precipitation maps. 

The annual maps underwent extensive peer review by 

many State climatologists and other experts. This is 

part of a national effort by the NRCS and Oregon State 

University to develop state-of-the-art precipitation 

maps for each State in the United States.

Precipitation estimated for each grid cell is an 

average over the entire area of that cell; thus, point 

precipitation can be estimated at a spatial precision no 

better than half the resolution of a cell. For example, 

the precipitation data were distributed at a resolution of 

approximately 4 kilometers (km). Therefore, point 

precipitation can be estimated at a spatial precision no 
Appendix 31

better than 2 km. However, the overall distribution of 

precipitation features is thought to be accurate. For 

further information, the online PRISM homepage can 

be accessed at the Oregon State University’s “Climate 

Mapping with PRISM” web site—http://

www.ocs.orst.edu/prism/prism_new.html. 

Ground-Water Contribution

The ground-water contribution component was 

derived from applying the unsaturated zone ratings 

within a 305-meter area around all streams identified in 

the early release of the RF-3 distributed by the USEPA.

First, 305-meter polygons were drawn around all 

streams by using the ARC/INFO “BUFFER”

command. The streams were processed within 8-digit 

hydrologic cataloging unit areas. Occasionally, the 

buffered areas extended over the ridgelines defined by 

the cataloging units. The areas that extended over the 

cataloging unit boundaries were removed.

Next, the buffered streams were adjusted to 

include water bodies that overlapped the buffered 

stream areas. This ensured that the middle of lakes 

wider than 305 meters were included in the analysis of 

ground-water contribution.

Finally, the unsaturated-zone component of 

inherent vulnerability was applied to the buffered zones 

within the six pilot sites by using overlay analysis. A 

raster layer with 60-meter by 60-meter cells was 

created for testing the rating methods in several areas of 

the State; however, 30-meter by 30-meter cells will be 

used when the method is applied statewide.
ERRATA

Please attach to the PDF version of USGS WRIR 99-4283

“Methods of rating unsaturated zone and watershed characteristics of public water supplies in 

North Carolina,” by J.L. Eimers, J.C. Weaver, Silvia Terziotti, and R.W. Midgette

1. Page IV, Acronyms used in this report—The acronym for ARC/INFO macro language was 

incorrectly listed as AMC; the correct acronym is AML. The acronym for Multi-Resolution Land 

Characteristics was incorrectly listed as MLRC; the correct acronym is MRLC.

2. Page 20, Table 11—The column subheadings under “Precipitation,” “Land use,” and “Ground-water 

contribution,” were incorrectly shown as “Rating (r3)”; the subheadings should read as circled below:

3. Page 23, References—The URL for California Department of Health Services, Division of Drinking 

Water and Environmental Management, was incorrectly shown as http://www.dhs.ca.gov/ps/ddwem/

dswapdoc/appc. The correct URL is:

http://www.dhs.ca.gov/ps/ddwem/dwsap/dwsapdoc/appc.htm.

Table 11. Example determination of a watershed characteristics rating for part of an unnamed watershed upstream 

from a water-supply intake

Cell 

number i

(fig. 5)

Precipitation 

[weight (w1)=3]

Land-surface 

slope 

[weight (w2)=2] Land-cover 

and land-use 

category

Land 

cover 

[weight 

(w3)=1]

Land 

use 

[weight 

(w4)=3]

Ground-water 

contribution 

[weight (w5)=1] Grid cell rating

In 

inches

Rating 

(r1)

(table 8)

In 

percent

Rating 

(r2)

(table 9)

Rating 

(r3)

(table 

10)

Rating 

(r4)

(table 5)

Unsaturated 

zone 

rating 

value

Rating 

(r5)

1 56 5 52 10 Forest 1 3 46 4.6 49.6

2 56 5 47 9 Forest 1 3 0 0 43.0

3 56 5 48 9 Forest 1 3 50 5.0 48.0

4 56 5 45 9 Hay/pasture 3 5 0 0 51.0

Watershed characteristics rating for n (4) selected cells (fig. 5) 

Ri wi ri ( ) ×

i = 1

5

= ∑

Ri

i = 1

4

∑

n -------------- 191.64 = = ------------- 47.9