Hydrologic Conditions and Simulation of Groundwater and 

Surface Water in the Great Dismal Swamp of Virginia and 

North Carolina

Scientific Investigations Report 2018–5056

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the U.S. Fish and Wildlife Service
Cover: Photo of the area burned during the 2011 Lateral West fire, Great Dismal Swamp, Virginia and North Carolina. Ground view looking northeast from Corapeake 

Ditch, photograph taken by Frederic Wurster, U.S. Fish and Wildlife Service, June 2014.
Hydrologic Conditions and Simulation of 

Groundwater and Surface Water in the 

Great Dismal Swamp of Virginia and North 

Carolina

By Jack R. Eggleston, Jeremy D. Decker, Jason S. Finkelstein, Frederic C. Wurster, 

Paul E. Misut, Luke P. Sturtevant, and Gary K. Speiran

Prepared in cooperation with the 

U.S. Fish and Wildlife Service

Scientific Investigations Report 2018-5056

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Eggleston J.R., Decker, J.D., Finkelstein, J.S., Wurster, F.C., Misut, P.E., Sturtevant, L.P., and Speiran, G.K., 2018, 

Hydrologic conditions and simulation of groundwater and surface water in the Great Dismal Swamp of Virginia and 

North Carolina: U.S. Geological Survey Scientific Investigations Report 2018-5056, 67 p., https://doi.org/10.3133/

sir20185056.

ISSN 2328-0328 (online)

ISSN 2328-031X (print)
iii

Acknowledgments

The authors acknowledge the generous and expert contributions of the Great Dismal Swamp 

Wildlife Refuge staff, particularly Karen Balentine and Chris Lowie, and of numerous managers, 

researchers, and others who contributed data and expertise to the study, including Owen Reece 

and Joel Scussel of the U.S. Army Corps of Engineers, Jin Woo Kim and Zhong Lu of Southern 

Methodist University, and John Jones of the U.S. Geological Survey.

v

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Previous Investigations........................................................................................................................3

Description of the Study Area .....................................................................................................................4

Topography ............................................................................................................................................4

Climate....................................................................................................................................................4

Land Use.................................................................................................................................................8

Vegetation .....................................................................................................................................8

Peat.............................................................................................................................................9

Fire..................................................................................................................................................9

Water Use........................................................................................................................................ 9

Withdrawals................................................................................................................................10

Wastewater ................................................................................................................................10

Hydrogeology.......................................................................................................................................10

Aquifers .......................................................................................................................................10

Aquifer Properties .....................................................................................................................10

Groundwater Levels and Flow Patterns.................................................................................11

Precipitation and Evapotranspiration.....................................................................................12

Surface-Water Hydrology .................................................................................................................14

Ditches and Roads.....................................................................................................................14

Water Control Structures .........................................................................................................16

Surface-Water Inflows and Outflows.....................................................................................16

Water Management ..................................................................................................................24

Inundation ...................................................................................................................................24

Geospatial Analysis of Land-Surface Elevations and Peat Thickness ...............................................27

Land-Surface Elevation Observations by Lidar .............................................................................27

Peat Thickness....................................................................................................................................27

Conceptual Hydrologic Model...................................................................................................................29

Numerical Model Development ................................................................................................................30

Block C1 Preliminary Model..............................................................................................................30

Cross-Sectional Block C1 Model ............................................................................................30

Block C1 Model Sensitivity Analysis ......................................................................................33

Block C1 Model Simulation Results........................................................................................33

Spatial Discretization of the Three-Dimensional Model ..............................................................34

Time Periods Represented by Simulations.....................................................................................37

Boundary Conditions..........................................................................................................................37

Lateral Inflows............................................................................................................................38

Specified Head Boundaries .....................................................................................................38

Evapotranspiration ....................................................................................................................38

Roads..................................................................................................................................... 39

Surface Water............................................................................................................................39
vi

Weirs and Culverts ....................................................................................................................40

Calibration Approach .........................................................................................................................41

Model Parameterization ...........................................................................................................41

Observations and Weighting....................................................................................................41

Groundwater-Level Observations ..................................................................................42

Inundation Observations .................................................................................................42

Surface-Water Level and Flow Observations ..............................................................42

Calibration and Error-Based Calibration Criteria..................................................................47

Calibration Results.....................................................................................................................47

Sensitivity....................................................................................................................................48

Simulated Hydrology and Water Management ......................................................................................48

Average Spring Hydrologic Conditions ...........................................................................................49

Wet and Dry Simulation Results.......................................................................................................52

Water Management Simulation Results .........................................................................................59

Simulation of Proposed Water Control Structures...............................................................59

Flood the Swamp Wet/Dry........................................................................................................61

Drain the Swamp Wet/Dry........................................................................................................61

Model Limitations.......................................................................................................................61

Summary and Conclusions.........................................................................................................................64

References Cited .........................................................................................................................................65
vii

Figures

1. Map showing location of the Great Dismal Swamp study area, Virginia and

North Carolina ...............................................................................................................................2

2. Map showing land-surface elevations based on lidar surveys, Great Dismal

Swamp, Virginia and North Carolina .........................................................................................5

3. Photographs showing the area burned during the 2011 Lateral West fire, Great

Dismal Swamp, Virginia and North Carolina: A, aerial view, August 2011, and B,

ground view looking northeast from Corapeake Ditch, June 2014.......................................6

4. Graph showing profile of land-surface elevations observed by using lidar in

2012, across the Lateral West Burn Area, Great Dismal Swamp, Virginia and

North Carolina ...............................................................................................................................7

5. Graph showing average monthly precipitation, from 1931 to 2015, and

temperature, from 1997 to 2015, at meteorological station USC00448837

operated by the U.S. Army Corps of Engineers on Lake Drummond, Great

Dismal Swamp, Virginia and North Carolina............................................................................7

6. Graph showing spring (April–June) precipitation for the years 1986 to 2015 at

meteorological station USC00448837 on Lake Drummond, Virginia, operated by

the U.S. Army Corps of Engineers..............................................................................................8

7. Photograph showing water overtopping Weyerhauser Ditch road after wet

weather, June 2014, Great Dismal Swamp, North Carolina.................................................11

8. Graphs showing response of the water table in well 59A 39 to precipitation, A

water-level response to selected precipitation events, B water-level response

to 12 inches of precipitation from August 26 to 28, 2011, Great Dismal Swamp

National Wildlife Refuge, Virginia and North Carolina.........................................................13

9. Graph showing mean observed temperature at meteorological station

USC00448837 on Lake Drummond operated by the U.S. Army Corps of

Engineers, total calculated potential evapotranspiration (PET), and mean

calculated PET, determined by using Thornthwaite’s equation for April, May,

and June, 2005 through 2015, Great Dismal Swamp, Virginia and North Carolina ..........14

10. Map showing ditches and roads in the Great Dismal Swamp, Virginia and

North Carolina .............................................................................................................................15

11. Cross-sectional diagrams of groundwater and surface-water flow A, before

ditching and B, after ditching, C, photo of road and ditch in Great Dismal

Swamp, Virginia and North Carolina, and D, diagram of road and ditch ..........................17

12. Map showing locations of water control structures in the Great Dismal Swamp,

Virginia and North Carolina.......................................................................................................18

13. Photographs showing representative water control structures managed by the

Great Dismal Swamp National Wildlife Refuge, Virginia and North Carolina: A,

installed structures and B, a new combined culvert and riser on route to

installation....................................................................................................................................22

14. Graph showing mean observed surface-water levels along Corapeake Ditch,

Great Dismal Swamp, Virginia and North Carolina, April 2013–June 2015 .......................23

15. Maps showing inundated land areas in the Great Dismal Swamp, Virginia and

North Carolina, determined by A, simulated water levels above land surface

from model Scenario 1, B, Dynamic Surface Water Extent (Jones, 2015), and C,

Synthetic Aperture Radar backscatter analysis (Kim and others, 2015)...........................26

16. Map showing interpolated peat thickness and locations of elevation

observation points, Great Dismal Swamp, Virginia and North Carolina............................28

17. Map showing Block C1 with line of section X–X’, Great Dismal Swamp, Virginia and 

North Carolina .............................................................................................................................31
viii

18. Cross-sectional views of the Block C1 model showing A, grid spacing and

boundary conditions B, simulated heads and flow paths under baseline

average conditions, and C, simulated heads and flow paths when water levels

in Sycamore Ditch are raised by 2 feet...................................................................................32

19. Map showing hydrologic model grid and boundary conditions for the Great

Dismal Swamp, Virginia and North Carolina..........................................................................35

20. Representative cross-sectional view of a ditch and water-table elevations in

the 3-dimensional hydrologic simulation model of the Great Dismal Swamp,

Virginia and North Carolina.......................................................................................................36

21. Graph showing the ratio of actual evapotranspiration (ET) to potential

evapotranspiration (PET) for a forested land type over sandy clay soils using

exponential and linear functions..............................................................................................39

22. Map showing groundwater-level observation sites in the Great Dismal Swamp,

Virginia and North Carolina, and simulation errors ..............................................................43

23. Map showing surface-water-level observation sites at water control structures

and simulation errors, Great Dismal Swamp, Virginia and North Carolina.......................44

24. Graph showing simulated and observed groundwater and surface-water levels

and the ideal 1:1 line...................................................................................................................47

25. Maps showing simulated groundwater levels in the Great Dismal Swamp,

Virginia and North Carolina, and dry cells in A, Layer 1, B, Layer 2, C, Layer 3,

and D, Layer 4..............................................................................................................................50

26. Map showing simulated vertical head gradients between the water table and

Layer 4 in the three-dimensional model of Great Dismal Swamp, Virginia and

North Carolina .............................................................................................................................51

27. Map showing subbasins and generalized flow directions that are based on

simulated baseline hydrologic conditions, model Scenario 1, in the

three-dimensional model of Great Dismal Swamp, Virginia and North Carolina.............53

28. Map showing simulated flow paths under baseline conditions, model Scenario

1, in the three-dimensional model of Great Dismal Swamp, Virginia and North

Carolina ........................................................................................................................................54

29. Map showing simulated flow paths at the foot of the scarp near the

intersection of West Ditch and South Ditch, Great Dismal Swamp, Virginia and

North Carolina, under baseline conditions, model Scenario 1, of the

three-dimensional model...........................................................................................................55

30. Map showing simulated depth to water table under baseline conditions, model

Scenario 1, of the three-dimensional model of Great Dismal Swamp, Virginia

and North Carolina .....................................................................................................................56

31. Maps showing simulated groundwater levels in model Layer 4 of the

three-dimensional model of Great Dismal Swamp, Virginia and North Carolina,

under A, baseline (model Scenario 1), B, wet (model Scenario 2), and C, dry

(model Scenario 3) climatic conditions...................................................................................57

32. Maps showing change in simulated groundwater levels relative to baseline

conditions for A, wet conditions (model Scenario 2 levels minus Scenario 1

levels) and B, dry conditions (model Scenario 3 levels minus Scenario 1 levels)

from the three-dimensional model of Great Dismal Swamp, Virginia and North

Carolina ........................................................................................................................................58

33. Map showing changes in simulated groundwater levels caused by the addition

of proposed water control structures and raising of weir levels at existing water

control structures (model Scenario 4 levels minus Scenario 1 levels) from the

three-dimensional model of Great Dismal Swamp, Virginia and North Carolina.............60
ix

34. Maps showing changes in simulated groundwater levels caused by raising

weir levels to their highest possible elevations at existing and proposed water

control structures under A, wet conditions (model Scenario 5 levels minus

Scenario 2 levels) and B, dry conditions (model Scenario 6 levels minus

Scenario 3 levels) in the three-dimensional model of Great Dismal Swamp,

Virginia and North Carolina.......................................................................................................62

35. Maps showing changes in simulated groundwater levels caused by lowering

weir levels to their lowest possible elevations at existing and proposed water

control structures under A, wet conditions (model Scenario 7 levels minus

Scenario 2 levels) and B, dry conditions (model Scenario 8 levels minus

Scenario 3 levels) of the three-dimensional model of Great Dismal Swamp,

Virginia and North Carolina.......................................................................................................63

Tables

1. Description of water control structures in the Great Dismal Swamp, Virginia

and North Carolina. ...................................................................................................................19

2. Estimated surface-water inflows to the Great Dismal Swamp, Virginia and

North Carolina .............................................................................................................................23

3. Observed surface-water outflows from the Great Dismal Swamp, Virginia and

North Carolina .............................................................................................................................24

4. Ground survey control elevation datasets and error analysis of lidar-derived

elevations and interpolated top of sand elevation values ...................................................27

5. Observation frequency, by decade, of elevation and peat extent observations

used for interpolating peat thickness over the entire study area ......................................29

6. Calibrated parameters of the Block C1 cross-sectional model..........................................33

7. Results of parameter sensitivity analysis for the Block C1, Great Dismal

Swamp, Virginia and North Carolina, cross-sectional model .............................................33

8. Simulated water budgets from the Block C1 model for the steady-state

baseline scenario and for the Sycamore Ditch raised water level scenario,

Great Dismal Swamp, Virginia and North Carolina...............................................................34

9. Thickness and hydraulic properties of Layers 2 and 3 of the three-dimensional

hydrologic simulation model of the Great Dismal Swamp, Virginia and North

Carolina ........................................................................................................................................36

10. Climatic conditions and representative time periods for model scenarios for

the three-dimensional hydrologic simulation model of the Great Dismal

Swamp, Virginia and North Carolina .......................................................................................37

11. Assigned boundary conditions for representative climatic conditions in the

three-dimensional model for Great Dismal Swamp, Virginia and North Carolina ...........37

12. Parameters used in model calibration and composite normalized parameter

sensitivities ..................................................................................................................................40

13. Observation group weight multiplier and contributions to the total objective function 

(Φ) at the end of calibration of the three-dimensional model.............................................41

14. Average observed surface-water levels during 2005–15 and surface-water

levels during 2015 wet conditions and 2011 dry conditions at 55 observation

sites in the Great Dismal Swamp, Virginia and North Carolina, used to

calibrate the three-dimensional model...................................................................................45
x

15. Simulation errors for different observation groups ..............................................................47

16. Description of hydrologic simulation scenarios for the three-dimensional

model of the Great Dismal Swamp, Virginia and North Carolina........................................49

17. Simulated water budget for the calibrated baseline three-dimensional

hydrologic model, Scenario 1, of the Great Dismal Swamp, Virginia and North

Carolina ........................................................................................................................................49

18. Simulated flow rates at major water control structures in Great Dismal

Swamp, Virginia and North Carolina, using a three-dimensional model ..........................59

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

foot (ft) 12.0 inch (in.)

mile (mi) 5280.0 feet (ft)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

cubic foot per second (ft3

/s) 86,400.0 cubic foot per day (ft3/d)

cubic foot per second (ft3

/s) 0.53817 million gallons per day (Mgal/d)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

 °C = (°F – 32) / 1.8.

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.
xi

Abbreviations

DSWE Dynamic Surface Water Extent

EPA U.S. Environmental Protection Agency

ET Evapotranspiration

GDSNWR Great Dismal Swamp National Wildlife Refuge

InSAR Interferometric Synthetic Aperture Radar

K Hydraulic conductivity

lidar Light detection and ranging

NCDENR North Carolina Department of Environment and Natural Resources

NCDSSP North Carolina Dismal Swamp State Park

NOAA National Oceanic and Atmospheric Administration

PET Potential evapotranspiration

RMSE Root mean square error

SAR Synthetic Aperture Radar

SMWA South Mills Water Association 

USACE U.S. Army Corps of Engineers

USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey

VADEQ Virginia Department of Environmental Quality

WCS Water control structure

Hydrologic Conditions and Simulation of Groundwater and 

Surface Water in the Great Dismal Swamp of Virginia and 

North Carolina

By Jack R. Eggleston11, Jeremy D. Decker1, Jason S. Finkelstein1, Frederic C. Wurster2, Paul E. Misut1, 

Luke P. Sturtevant1, and Gary K. Speiran1

Abstract

The U.S. Geological Survey (USGS), in cooperation with 

the U.S Fish and Wildlife Service, has investigated the hydrology of the Great Dismal Swamp (Swamp) National Wildlife 

Refuge (Refuge) in Virginia and North Carolina and developed a three-dimensional numerical model to simulate groundwater and surface-water hydrology. The model was developed 

with MODFLOW-NWT, a USGS numerical groundwater flow 

modeling program, in combination with the Surface-Water 

Routing Process, a software package that simulates dynamic 

surface-water flows, water control structure management, and 

groundwater/surface-water interactions.

The steady-state model was calibrated to average spring 

conditions by using automated parameter estimation software 

(PEST) to reduce simulation errors and assess model parameter sensitivity. The model was then used to simulate wet and 

dry climatic conditions and a variety of hypothetical scenarios 

in which water levels in the Swamp were raised and lowered 

by simulated management of water control structures. Results 

of the model simulations indicate that, under average spring 

conditions, precipitation is the primary water input (92%); 

surface-water (5%) and groundwater (3%) inflows make up 

the remainder. The primary outflow (or loss) is evapotranspiration (55%), with surface outflows (about 41%) and groundwater outflow (about 4%) making up the remainder.

Simulated adjustment of water control structure weir levels demonstrates that groundwater levels are affected by water 

levels in adjacent ditches and that surface-water and groundwater levels can be controlled through management of water 

control structures, allowing the Refuge to better manage fire 

risks and preserve forested-wetland ecosystems in the Refuge. 

The 13 water control structures proposed in the simulated scenario representing possible future conditions effectively raised 

simulated water levels in the northeastern corner of the study 

area, a goal of the Refuge management.

1

 U.S. Geological Survey.

2

 U.S. Fish and Wildlife Service.

Results of this study demonstrate use of MODFLOW 

with the Surface-Water Routing Process for simulating water 

management options in peat wetlands and will help Refuge 

managers to better understand existing hydrologic conditions, 

assess the hydrologic effects of planned changes to water 

control structures, and apply the new simulation tool to guide 

water management on the Refuge.

Introduction

The Great Dismal Swamp is one of the iconic wetlands 

of the United States and home of the largest National Wildlife 

Refuge in the U.S. Fish and Wildlife Service northeast region 

(Dennis, 1988; Simpson, 1990; Mitsch and Gosselink, 2007). 

The Great Dismal Swamp (Swamp) is a peatland historically 

dominated by several types of ecologically important and 

sensitive forested-wetland ecosystems (fig. 1). The Swamp is 

estimated to have originally covered nearly 1,500,000 acres in 

southeastern Virginia and northeastern North Carolina (Shaler, 

1890). Since colonial times, however, the forest ecosystems 

have been altered by timber harvesting, wildfires, and hydrologic modifications caused by the construction of the Dismal 

Swamp Canal and numerous drainage ditches with adjacent 

spoil piles. The first ditches in the Swamp were constructed 

between 1763 and 1768 by a company formed by George 

Washington and several associates to facilitate timber harvesting and prepare the land for agriculture (Hansen, 2010). 

The 32-mile-long Dismal Swamp Canal, on the east side of 

the Great Dismal Swamp National Wildlife Refuge (Refuge), 

is the largest canal in the Swamp. Construction commenced 

in 1793 and ended in 1805, and the canal has been dredged 

and expanded since. The canal was built to allow commerce 

between Albemarle Sound (not shown) to the south and 

Chesapeake Bay to the north (not shown) and is the oldest 

continuously used man-made waterway in the United States 

(Trout, 1998). Additional ditches have been constructed 

within the Swamp through the years, resulting in the current 

144-mile (mi) ditch network.
2 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

North Carolina

Dismal Swamp

State Park

2011 Burn

Area

Great Dismal Swamp

National Wildlife Refuge

Inflow Rt. 158

Dismal Swamp Canal

Pasquotank River

Pocosin

Swamp

Moss

Swamp

Cypress

Swamp

Daniels Road

Swamp

Folly

Swamp

Goose Creek

and Acorn Hill

LAKE

DRUMMOND

Shingle

Creek

Deep

Creek

A A'

VIRGINIA

NORTH

CAROLINA

64

13

32

58

17

158E

158

32

13

58

17

32N

-76°40' -76°30' -76°20'

36°40'

36°30'

CSX Railroad

USC00448837

Feeder Ditch

CHESAPEAKE BAY

VIRGINIA

MARYLAND NJ

DE

WV DELAWARE BAY

0 2.5 5 KILOMETERS

0 2.5 5 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

National Wildlife Refuge

North Carolina State Park

Burn Area

Basin discharging to the 

 swamp

Major road

Model grid extent

A-A’ Line of section

Railroad

Ditch

Stream or river

0204382800 Surface-water 

 station and number

USC00448837 Meteorological 

 station and number

EXPLANATION

City of

Suffolk

City of

Chesapeake

Isle of

Wight Plain

Suffolk

Scarp

Block

C1

Corapeake

Ditch

Pasquotank

River

0204382800

South

Mills

NORTH

CAROLINA

ATLANTIC

OCEAN

Figure 1. Location of the Great Dismal Swamp study area, Virginia and North Carolina. (Cross section shown in figure 4)
Introduction 3

In 1974, the U.S. Congress established the Refuge with 

the U.S. Fish and Wildlife Service (USFWS) responsible for 

managing 49,100 acres in southeastern Virginia and northeastern North Carolina (U.S. Fish and Wildlife Service, 2006) 

(fig. 1). The Refuge has since expanded to its current 

112,000 acres, making it the largest National Wildlife Refuge 

in the USFWS northeast region. An additional 14,432 acres 

of wetland forest is protected by the North Carolina Dismal 

Swamp State Park (NCDSSP), adjacent to the Refuge to the 

southeast. The combined 126,432 acres protected by the Refuge and NCDSSP are bounded by the Dismal Swamp Canal 

to the east and the Suffolk Scarp, a Pleistocene-age, marine 

shoreline (Oaks and Coch, 1973), to the west.

Although streams flow across the Suffolk Scarp into the 

Swamp and ditches extend throughout the Swamp, groundwater is a major part of the hydrologic system. The high permeability of the peat and the good hydraulic connection between 

the peat and the ditch network make the ditches major sinks 

for groundwater discharge, lowering groundwater levels and 

making much of the Swamp drier than pre-ditching conditions. Spoil piles adjacent to ditches were converted to logging 

roads and form barriers to water flow, causing land upgradient 

from roads to be wetter than pre-ditching conditions (Shaler, 

1890). The road and ditch network has fundamentally changed 

the Swamp’s hydrology, leading to tree stress and mortality, 

lowering of the land-surface elevation through oxidation and 

compaction of peat, and increasing the risk of severe wildfire 

(USFWS, 2006).

The USFWS has worked to mitigate the hydrologic 

effects of ditch and road construction by installing more 

than 49 water control structures (WCSs) in ditches since the 

Refuge was established. WCSs were installed in order to 

manage Swamp water levels to better preserve, re-establish, 

and maintain the Swamp’s unique forested-wetland ecosystems (USFWS, 2006). In addition, the USFWS is working 

to improve its understanding of the Swamp’s hydrology by 

regularly monitoring water levels and flows. In response to 

the South One fire which burned nearly 5,000 acres of forest 

and peat in 2008, an intensive water monitoring program was 

undertaken by Refuge staff to collect data for better describing 

Swamp hydrology. Data collected since 2008 have provided a 

valuable foundation for the research described in this report.

The U.S. Geological Survey (USGS), in cooperation with 

the USFWS, has developed a numerical, steady-state simulation model of water in the Swamp, which is described in this 

report. The model can simulate the effects of WCS management on surface-water and groundwater flows and levels. This 

enables compilation of water balances and mapping of contributing areas to important WCSs. Through development of the 

model and running simulations, a better understanding of the 

Swamp’s complex hydrology has been obtained. The model 

can be used in the future to guide water-resource management 

strategies and answer additional questions about WCS management in the Swamp, such as where WCSs should be added 

or removed and how weir elevations should be set.

Purpose and Scope

This report describes the development of a two-dimensional (2-D), steady-state, numerical simulation model of a 

portion of the Swamp; development of a three-dimensional 

(3-D), steady-state model of the entire Swamp; and simulation of selected climatic conditions and water control structure 

management alternatives. The three-dimensional numerical 

model developed in this study simulates water levels and flows 

within the study area under steady-state conditions. Once calibrated, the model is used to determine water balances, groundwater and surface-water flow fields, and contributing areas 

to major flow outlets and WCSs. The model then is applied 

to simulate swamp hydrology under various climatic conditions and alternative WCS management strategies. Simulation 

results are then analyzed to assess the effects of water-resource 

management decisions on inundation, depth to water table, and 

flows at major WCSs. Results of this study will help Refuge 

managers to better understand existing hydrologic conditions, 

to assess the hydrologic effects of planned changes to WCSs, 

and to apply the new simulation tool to guide water management on the Refuge. The results also have broader application 

to other drained Coastal Plain peatlands of the mid-Atlantic 

and southeastern United States with hydrologic conditions 

similar to those of the Swamp. A data product associated with 

this report has been published (Eggleston and others, 2018), 

which is available online and contains a model archive and 

associated data files.

Previous Investigations

The hydrologic effects of the 144-mi ditch network have 

long been assumed (Lichtler and Walker, 1974; USFWS, 

2006), and restoring the hydrology of the Swamp by partly 

blocking ditches has been a goal of the USFWS since Refuge 

establishment (USFWS, 2006). The strategy of the USFWS 

for hydrologic restoration is to implement controlled drainage practices that have been successfully used elsewhere in 

the Virginia and North Carolina Coastal Plain on agricultural 

land and managed forests. Adjustable-elevation WCSs were 

installed in ditches to reduce and slow drainage and to set 

desired ditch water levels (Dukes and others, 2003; Evans 

and others, 2007). Forty-nine structures were installed on 

the Refuge between 1974 and 2015, and more structures 

are planned. The effectiveness of this approach for restoring peatland hydrology is uncertain because, until recently, 

there has been little long-term hydrologic monitoring on the 

Refuge and therefore limited understanding among Refuge 

staff of hydrologic response to WCS management (USFWS, 

2006). Ongoing studies are increasing knowledge of hydraulic 

characteristics of the Swamp’s peat, the connection between 

the ditches and groundwater, and the hydrologic budget of the 

Swamp. In spite of this and other continuing research, significant gaps in our knowledge of the Great Dismal Swamp’s 

hydrology remain.
4 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Successful wetland restoration often hinges on an understanding of the hydrologic processes that control a wetland’s 

structure and function. The National Research Council (2001) 

concluded many wetland mitigation projects fail because predisturbance hydrologic conditions are not well understood or 

could not be recreated at mitigation sites. Middleton (1999) 

highlights the importance of re-creating a wetland’s predisturbance water regime (annual and long-term fluctuations in 

water levels in wetlands) when designing restoration strategies. In peatlands like the Great Dismal Swamp, restoring a 

hydrologic regime that supports peat accumulation is considered the most important aspect of a successful restoration 

project (Chimner and others, 2016). Therefore, to improve the 

success of wetland restoration projects, many researchers first 

conduct hydrologic investigations to inform restoration design 

(for example, Cooper and others, 1998; Gorham and Rochefort, 2003; Wilcox and others, 2006; Wosten and others, 2008; 

Bonsel and Sonneck, 2011).

The importance of investigating groundwater and 

surface-water interactions at Great Dismal Swamp was first 

proposed by Lichtler and Walker (1974). They recommended 

quantifying water-budget parameters in a portion of the 

Swamp affected by existing drainage ditches. This present 

report builds on those recommendations and supports ongoing 

studies by providing a detailed and quantitative description of 

Swamp hydrology and describing results from simulations of 

selected WCS management alternatives.

Description of the Study Area

The study area is in southeastern Virginia and northeastern North Carolina and is defined by the boundary of 

the hydrologic model developed in this study (fig. 2). The 

boundary of the hydrologic model was chosen to align with 

surface-water and groundwater features and encompass most 

of the Refuge and the NCDSSP. The study area extends from 

the eastern part of the Isle of Wight Plain west of the Suffolk 

Scarp to the Dismal Swamp Canal to the east and from the 

CSX railroad and Deep Creek in the north (fig. 1) to highway 

U.S. Route 158 and the Pasquotank River to the south (fig. 2).

Topography

Three main, natural, topographic features set the landscape on which other natural and man-made features are 

superimposed: the Isle of Wight Plain (higher elevation) and 

the Dismal Swamp terrace (lower elevation) are separated by 

the Suffolk Scarp (Wentworth, 1930; Oaks and Coch, 1973) 

(figs. 1 and 2). The Isle of Wight Plain is dissected by streams 

that drain across the Suffolk Scarp onto the Dismal Swamp 

terrace. The Dismal Swamp terrace is a relatively flat surface gently sloping to the north, east, and south toward Deep 

Creek, the Dismal Swamp Canal, and the Pasquotank River 

drainages, respectively.

Elevations, referenced to the North American Vertical 

Datum of 1988 (NAVD 88), range from 0 feet (ft) at the tidal 

Pasquotank River and Deep Creek, at the southeastern and 

northeastern boundaries of the study area, respectively, to a 

maximum of about 65 ft on the Isle of Wight Plain to the west 

of the Suffolk Scarp (scarp) (Oaks and Coch, 1973), which 

forms the western boundary of the study area (fig. 2). The 

scarp is a remnant Pleistocene-age, marine shoreline from 

a past high sea-level stand. Elevations are 40–60 ft at the 

top of the scarp and about 25 ft along the eastern foot of the 

scarp where the Swamp begins. The scarp is incised by small 

streams draining into the Swamp from the west. From the foot 

of the scarp, land surface generally slopes to the east, north, 

and south. Local elevation differences within the Swamp 

are caused by natural hummocks and hollows that create up 

to about 3 ft of local relief in the peat surface. Hummocks 

typically form around the roots of trees and shrubs, whereas 

hollows form between the trees and shrubs.

Man-made local differences in topography include roads 

and ditches across the study area. Roads were built on spoil 

material (peat and sand) dug out to construct the ditches and 

typically deposited on one side of each ditch. In many cases, 

the land-surface elevation shifts up or down across road and 

ditch boundaries. This likely is due to water being impounded 

on the road side of a combined road and ditch boundary, causing peat to accumulate or perhaps degrade more slowly on the 

road side, whereas water levels on the ditch side are at lower 

elevations, contributing to peat degradation that decreases 

the land-surface elevation. Other known human actions that 

have altered the local topography include the construction of 

railroads and fire breaks.

Fire can affect swamp topography by burning peat 

(fig. 3). Fires in 2008 and 2011 lowered surface elevations to 

the west and southwest of Lake Drummond (fig. 1). Although 

the amount of peat loss from the 2008 fire could not be 

determined, the 2011 Lateral West fire burned about 1.5 ft 

(47 centimeters [cm]) of peat in the Burn Area, as documented 

by light detection and ranging (lidar) observations (Reddy and 

others, 2015) and seen in the lower elevation profile (fig. 4).

Climate 

The study area has a humid to subtropical climate 

(Trewartha and Horn, 1980). Average monthly temperatures 

observed from 1986 to 2015 at a climate station operated 

by the U.S. Army Corps of Engineers (USACE) on Lake 

Drummond (WALLACETON LK DRUMND, GHCND: 

USC00448837) ranged from about 5 degrees Celsius (oC) 

in January to about 26 oC in July and August (fig. 5). Mean 

annual precipitation observed at meteorological station 

USC00448837 from 1931 through 2015 is 50.7 inches (in.). 

Although average monthly precipitation does not vary much 

throughout the year, periods of little precipitation occasionally 

cause very dry conditions, and wet conditions occasionally 

cause floods. Observed monthly precipitation for 1986 to 2015 
Description of the Study Area 5

Dismal Swamp Canal

Pasquotank River

LAKE

DRUMMOND

Shingle

Creek

64

13

32

58

158E

32

13

58

32N

VIRGINIA

NORTH

CAROLINA

-76°40' -76°30' -76°20'

36°40'

36°30'

17

17

Deep

Creek

158

0 2.5 5 KILOMETERS

0 2.5 5 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

12.01 to 15.00

15.01 to 18.00

18.01 to 21.00

21.01 to 24.00

<0.00

>75.00

Active model extent

Major road

Model grid extent

Stream or river

Ditch and Lake Drummond 

 boundary

EXPLANATION

City of

Suffolk

City of

Chesapeake

Isle of

Wight Plain

Lidar land-surface elevation,

 in feet referenced to NAVD 88

75.00

24.01

12.00

0.00

Figure 2. Land-surface elevations based on lidar surveys, Great Dismal Swamp, Virginia and North Carolina.
6 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

B

A

Photo by: Frederic Wurster, U.S. Fish and Wildlife Service, June 2014 

Photo by: U.S. Fish and Wildlife Service, August 2011 

Figure 3. Photographs of the area burned during the 2011 Lateral West fire, Great Dismal Swamp, Virginia and North Carolina: A, aerial 

view, August 2011, and B, ground view looking northeast from Corapeake Ditch, June 2014.
Description of the Study Area 7

-30

-20

-10

0

10

20

30

0

4

5

6

7

8

10

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Temperature, in degrees Celsius

Precipitation, in inches

Month

Mean precipitation

Mean temperature

EXPLANATION

9

3

2

1

0

5

10

15

20

25

30

35

40

45

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000

Elevation, in feet referenced to NAVD 88

West to East Distance, in feet

Suffolk

Scarp

Burn

Area

Lake

Drummond

Myrtle

Ditch

Laurel

Ditch

Cedar

Railroad

Ditch

Dismal

Swamp

Canal

Land 

Surface

Land-Surface Elevation for Cross A-A’

A A′

Lidar observed

 land-surface 

 elevation

EXPLANATION

Figure 4. Profile of land-surface elevations observed by using lidar in 2012, across the Lateral West Burn Area, Great Dismal Swamp, 

Virginia and North Carolina. (Section line shown in figure 1)

Figure 5. Graph of average monthly precipitation, from 1931 to 2015, and temperature, from 1997 to 2015, at meteorological station 

USC00448837 operated by the U.S. Army Corps of Engineers on Lake Drummond, Great Dismal Swamp, Virginia and North Carolina. 

(Location shown in fig. 1)
8 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

at station USC00448837 allows characterization of average, 

typical wet, and typical dry conditions during spring (April, 

May, and June; fig.6). Observed precipitation totals for spring 

varied from 7.04 inches (in.) to 20.16 in.

Droughts and floods occur periodically, affecting swamp 

hydrology and presenting challenges for Refuge managers. 

Tropical hurricanes, which occur in the summer and fall, have 

historically caused flooding that raised water levels in the 

Swamp, damaged WCSs, and inundated populated areas in 

South Mills, Suffolk, and Chesapeake Counties.

Land Use

Prior to establishment of the Refuge in 1974, human 

use of land in the study area included forestry, and crop and 

animal agriculture. Historical land use and the associated 

canal construction are implicated in the reduction of the species diversity of vegetation in the study area (USFWS, 2006). 

Since 1974, land use in the study area has changed to conservation of wildlife habitat and restoration of forests. Although 

most of the study area is uninhabited, people live along the 

southern, western, and northern edges of the study area.

Vegetation

The Swamp is characterized primarily as a seasonally flooded forested wetland. Vegetation in the Swamp has 

changed substantially since humans introduced agriculture and 

19861987198819891990

199119921993199419951996199719981999

200020012002200320042005200620072008

200920102011201220132014

Year

Total spring

 precipitation

Precipitation for spring months (April−June), in inches

EXPLANATION

0

5

10

15

20

25

2015

Figure 6. Graph showing spring (April–June) precipitation for the years 1986 to 2015 at meteorological station USC00448837 on Lake 

Drummond, Virginia, operated by the U.S. Army Corps of Engineers. (Location shown in fig. 1)

timber harvesting in the late 18th century. Agriculture, timber 

harvesting, altered hydrology, and fires have changed the types 

of forested-wetland ecosystems from the predominantly Taxodium distichum (bald cypress), Pinus serotina Michx (pocosin 

pine), and Chamaecyparis thyoides (Atlantic white cedar) 

(Shaler, 1890), for which the Swamp was known, to predominantly Acer rubrum (red maple)/Nyssa sylvatica (black 

gum) plant communities. This transition from a diverse forest 

community of bald cypress, Nyssa aquatica (water tupelo), 

black gum, Atlantic white cedar, and pocosin pine habitat 

to one dominated by red maple and black gum (Carter and 

Gammon, 1976; Levy, 1991; USFWS, 2006) is evident from 

pollen in peat cores (Stevens and Patterson, 1998). Pollen data 

near Block C1, an area southeast of Lake Drummond (fig. 1), 

reveal a vegetation history of an Atlantic white cedar swamp 

until around the time of colonial settlement about 400 years 

before present. The vegetation then shifted to that characteristic of a pocosin pine community with an increase in pine pollen to 58 percent and red maple pollen to 2 percent, the highest 

level in the pollen record (Stevens and Patterson, 1998). This 

time coincides with a major increase in the charcoal content of 

the peat, indicating an increase in the occurrence of fire.

The South Atlantic Coastal Plain of the United States 

is home to extensive forested wetlands on peat soils known 

as pocosins (Richardson, 1991). Also described as evergreen 

(or southeastern) shrub bogs, the pocosin vegetation community consists of an open-pine canopy underlain by a dense 

shrub layer. Pocosins are found chiefly on peatlands with low 

topographic relief, near the edge of estuarine wetlands, or in 
Description of the Study Area 9

depressions with poor drainage (Brinson, 1991). Pocosins are 

found on the South Atlantic Coastal Plain from Virginia to 

Florida; the highest concentration is in coastal North Carolina 

(Richardson, 1991). Pocosins are at the northern extent of their 

range in the Swamp (USFWS, 2006).

Peat

The land surface across most of the Swamp is covered 

by leaf litter and an underlying root mat. In the root mat and 

below, peat forms the surface soil type across most of the 

Swamp (Oaks and Coch, 1973). The peat is composed of 

partially decomposed material from woody plants and has 

little mineral content. Peat in the Swamp can be divided into 

two horizons, the upper peat and lower peat, on the basis of 

textural characteristics observed in the field. It is common to 

encounter layers of buried trees, branches, and roots at varying depths below land surface. Maximum total peat thickness 

exceeds 10 ft in places but is typically 3–5 ft over much of the 

Swamp (Oaks and Coch, 1973). Further description of peat 

thickness is given in the “Geospatial Analysis of Peat Thickness” section below.

Total peat thickness shows spatial patterns corresponding to dendritic erosion patterns in the underlying sand, silt, 

and clay of the Tabb and Yorktown Formations (Oaks and 

Coch, 1973, fig. 33; Peebles and others, 1984). A convincing 

description of the formation of these spatial patterns in the 

peat thickness is given by Heath (1975, p. 16–19). In summary, peat accumulated over the past 9,000 years in low-lying 

areas, shallow depressions, and stream channels (Harrison and 

others, 1965, p. 217–221). Peat accumulated preferentially 

in stream courses where wet conditions prevailed. As peat 

accumulated in low-lying areas, it obstructed streams, causing 

wet conditions to spread and the peat to form over wider areas, 

expanding across valleys and covering uplands. Therefore, 

the greatest modern-day peat thickness is found aligned with 

former stream channels, which can be seen in stratigraphic 

contours of the top of a sand unit (Oaks and Coch, 1973)

Peat in the Great Dismal Swamp does not appear to have 

a true acrotelm and catotelm, on the basis of peat composition 

as described by Ingram (1978) for boreal peatlands. Instead, 

the composition of the peat is more characteristic of Ingram’s 

catotelm throughout its depth but with variations in structure 

near the land surface. On the basis of field observations, the 

upper peat is very porous and highly decomposed, as is the 

lower peat. The lower peat likely has lower permeability and 

specific yield than the upper peat because of its finer texture 

and cohesive, clay-like consistency described as “mucky” 

(Henry, 1970). The upper peat transitions gradually downward 

to the lower peat. Various sizes of decomposing tree and shrub 

parts are present throughout most peat in the study area, and 

large branches and trunks are not uncommon. In some areas, a 

laterally discontinuous and thin layer of sticky clay separates 

the peat from the underlying sand.

Fire

Small fires occur frequently in the Swamp during dry 

periods as a result of lightning strikes and, in some instances, 

human activity. These small fires typically have minimal effect 

and are less common during wet periods. Large wildfires are 

less frequent, but can have extensive effects, and are known 

to have occurred in 1806, 1839, 1923–6, 1930, 1941–42, 

1955, 1967, 2008, and 2011 (Simpson, 1990; USFWS, 2006; 

Stevens and Patterson, 1998). Such wildfire, which is required 

for the existence of some habitats in the Great Dismal Swamp, 

can alter other habitats, burn deeply into the peat releasing 

large amounts of carbon dioxide (a greenhouse gas contributing to climate change), change inundation patterns (fig. 3), 

cause health problems, be expensive to fight, and cause economic loss across the area (USFWS, 2006).

Fire is considered one of the most important controls on 

vegetation distribution, contributing to the existence of pocosins, a type of pine and shrub peatland (Richardson, 1991). 

Drainage caused by installed ditches tends to increase the 

frequency and severity of ground fires in peatlands (Turetsky 

and others, 2015). Fire suppression techniques developed for 

pocosin focus on attempting to douse ground fire by flooding 

the burning area because dousing is more effective than spraying when peat is smoldering below the land surface.

The wildfires of 2008 and 2011 are good examples of the 

difficulty of fighting peat fires and avoiding their effects. The 

fires occurred during dry summer periods in an area being prepared for regeneration of a cedar forest. Because groundwater 

and ditch water levels were low, water was pumped from lowelevation sources through the ditch network to extinguish the 

peat fires at higher elevations. Temporary dams were coupled 

with the existing network of WCSs to raise and direct water to 

the burning areas. Because groundwater levels were low and 

conditions were dry, the fire burned deeply into the peat and 

continued to smolder for more than 3 months. An average peat 

thickness of 1.5 ft and 1.10 teragrams of carbon were lost to 

the atmosphere in the 2011 Lateral West fire (Reddy and others, 2015). Collectively, these fires cost about $25 million to 

fight. A large part of the previously forested area remains open 

water and marsh (fig.3).

Water Use 

Human withdrawal of groundwater and surface water, 

and discharge of wastewater, have the potential to affect 

Swamp hydrology. Although the study area is sparsely populated, water use occurs on all sides of the study area. Small 

areas along the northern, western, and southeastern boundaries of the study area are served with publicly supplied water. 

Wastewater from these small areas is collected and piped out 

of the study area for treatment in the cities north of the study 

area. Elsewhere in the study area, wastewater is disposed via 

private septic systems that drain to groundwater. Because 
10 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

developed areas in the southeast, northeast, and northwest are 

at lower elevations than the rest of the study area and receive 

groundwater and surface-water discharge from the Swamp, 

water use in these areas is unlikely to have much effect on 

groundwater and surface water within the Swamp. Because 

areas to the west drain to the Swamp, withdrawal in these 

areas can reduce the flow of water to the Swamp.

Withdrawals

Water supplies for the approximately 500 households in 

South Mills in the southeastern part of the study area (fig. 1) 

are withdrawn from wells by the South Mills Water Association (SMWA). On the basis of records from the North 

Carolina Department of Environment and Natural Resources 

(NCDENR) obtained from the online North Carolina Ground 

Water Data Map Interface (available through http://www.

ncwater.org/), SMWA has two wells (South Mills wells 9 

and 10) within the study area, both of which withdraw groundwater from screened intervals at 40–60 ft depths. In 2014, 

annual withdrawals from wells 9 and 10 totaled 0.05 million 

gallons per day (6,684 cubic feet per day [ft3/d]). Well 9 was 

used only part of the year, whereas well 10 was reserved for 

emergency use and had no withdrawals. Although the SMWA 

wells are screened in, and therefore withdraw from, the shallow surficial aquifer, they are unlikely to have much effect on 

groundwater within the Refuge boundaries because the Pasquotank River and Dismal Swamp Canal provide fixed head 

boundaries less than 1 mi away, and most of the pumped water 

is likely returned nearby to the aquifer via septic discharge. No 

groundwater-level monitoring data for that part of the study 

area were available to this study, so the effects of withdrawals 

from SMWA supply wells on groundwater levels is unknown.

An estimated 1,050 households are within the northwestern part of the study area in the City of Suffolk, whereas an 

estimated 1,750 households are within the northeastern part of 

the study area within the City of Chesapeake. These households are served with public water that is not withdrawn from 

within the study area. Although a small amount of pumping 

of groundwater for lawn irrigation and landscaping likely 

occurs in these areas, no groundwater withdrawal records for 

such use were available for this study. Records obtained from 

the Virginia Department of Environmental Quality (VADEQ; 

Curt Thomas, VADEQ, written commun., April 2016) indicate 

that there were no permitted groundwater withdrawals in the 

Virginia portion of the study area from 2005 to 2015.

Wastewater

Only developed areas in the northeastern and northwestern parts of the study area have public wastewater collection, 

which is provided by the City of Chesapeake and the City of 

Suffolk, respectively. Wastewater is routed to the regional collection system operated by Hampton Roads Sanitation District 

and treated outside the study area. Elsewhere in the study area, 

households discharge wastewater to septic systems that drain 

to the underlying water table.

Hydrogeology

Groundwater is present throughout the study area, usually 

just a few feet or less below the land surface. During wet periods, the water table rises above the land surface, inundating 

much of the study area (fig. 7). Groundwater likely discharges 

to ditches over most of the study area most of the time and, 

during drought or seasonal dry conditions, is the only source 

of water to the ditches.

Aquifers

At the land surface, most of the study area is covered 

with a thin layer of peat, as described in the “Geospatial 

Analysis of Land-Surface Elevations and Peat Thickness” 

section. In some parts of the Swamp, the peat is underlain by 

a sticky clay layer up to 3 ft thick. Below the lower peat and 

sticky clay is mineral sediment consisting of fine-grained sand, 

silt, and clay (Oaks and Coch, 1973; Oaks, 1965). The mineral 

sediments are part of the Yorktown-Eastover confining unit 

and the underlying upper portion of the Yorktown-Eastover 

aquifer (McFarland and Bruce, 2006). Immediately underlying 

the Yorktown-Eastover aquifer is the Saint Marys confining 

unit, a low-permeability clay and silt layer found at a depth of 

about 130 ft below land surface in the study area. Overall, the 

study area is underlain by about 2,000 feet of layered sediments that are part of the regional North Atlantic Coastal Plain 

aquifer system, a major water-supply source for residents of 

the Coastal Plain in Virginia and North Carolina (Masterson and others, 2016a). The aquifers beneath the study area 

include the Potomac, Virginia Beach, Aquia, and Piney Point 

found at depths of about 600, 450, 350, and 300 ft, respectively (McFarland and Bruce, 2006, Attachment 1, boreholes 

58A 75 and 58A76).

Pumping in these deeper aquifers of the North Atlantic 

Coastal Plain aquifer system, especially the Potomac, has 

caused groundwater-level drawdowns across the Coastal Plain, 

including underneath the study area. However, the shallow 

aquifer system in the study area appears to be hydraulically 

isolated from, and therefore not much affected by, the deeper 

water-supply aquifers. The apparent hydraulic separation is 

caused by the Saint Marys confining unit, a fine-grained unit 

with very low hydraulic conductivity (K) that is about 120 ft 

thick in the study area and lies below the Yorktown-Eastover 

aquifer (McFarland and Bruce, 2006, Attachment 1, boreholes 

58A 75 and 58A76).

Aquifer Properties

The most important aquifer property controlling 

groundwater flows and levels that is considered in this study 

is hydraulic conductivity (K). Because only steady-state 
Description of the Study Area 11

Photo by: Frederic Wurster, U.S. Fish and Wildlife Service, June 2014 

Figure 7. Photograph of water overtopping Weyerhauser Ditch road after wet weather, June 2014, Great Dismal Swamp, North 

Carolina.

conditions are considered in this study, storage coefficients are 

not used as model parameters. The steady-state analysis was 

designed to ignore the effects of storage change in this system; 

therefore, the ability for the Swamp to accumulate and release 

water from storage was not considered in this study. Hydraulic 

conductivity is an empirically derived value expressing the 

ease with which water can move through an aquifer or other 

porous material under the presence of a hydraulic pressure 

gradient.

No measurements of hydraulic conductivity in the study 

area were available for this study. Hydraulic conductivity of 

the upper peat layer is thought to be very high because it is a 

coarsely textured highly porous material. Hydraulic conductivity of the underlying lower peat and clay layer is expected 

to be lower. Hydraulic conductivity of the underlying sand of 

the Yorktown-Eastover aquifer likely varies within the study 

area depending on the relative percentages of clay, silt, and 

sand making up the aquifer material. Verry and others (2011) 

summarize relations between hydraulic conductivity and peat 

physical properties, such as fiber content, bulk density, and 

degree of decomposition. In general, peat hydraulic conductivity is higher near the land surface where peat density tends to 

be lower and organic material is less decomposed compared 

to deeper peat. Hydraulic conductivity in peat near the land 

surface can be 2–3 orders of magnitude greater than hydraulic 

conductivity of the deeper, highly decomposed peat (Verry and 

others, 2011). Other researchers have noted similar patterns in 

peatlands in Indonesia (Wosten and others, 2008; Dommain 

and others, 2010), the United Kingdom (Ramchunder and others, 2009), and Canada (Landry and Rochefort, 2012).

Values of horizontal K for the Yorktown-Eastover aquifer 

reported in other studies range from 1 to 100 feet per day (ft/d) 

(Smith, 2003; Heywood and Pope, 2009). As described later 

in the “Model Development” section, horizontal K values for 

the upper peat, lower peat, and sand layers are 13,200; 24; and 

10 ft/d, respectively.

Groundwater Levels and Flow Patterns

Groundwater-level observations were compiled from a 

variety of sources for this study and are the foundation for 

understanding patterns of groundwater levels and inferred 

directions of flow. The compiled dataset includes periodic 

observations collected and provided by Refuge staff plus 

periodic and continuous observations collected by the USGS 

and downloaded from the USGS National Water Information System (NWIS) online database (http://nwis.usgs.gov). 

Groundwater-level observations known to have high uncertainties were removed from the dataset. The complete set of 

1,999 groundwater-level observations from 131 wells spans 
12 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

1987 through 2016. Water-level data were collected during 

all months of the year and over a wide range of hydroclimatic 

conditions. April, May, and June are the target period for this 

study, as described in the “Model Development” section, and 

further description of water levels in this report focuses on 

those months. Hydrologic conditions during spring were identified by Refuge staff as critical to water management in the 

Refuge because most tree growth occurs then and is sensitive 

to water-table elevations. Dry conditions in the spring usually 

precede increased fire risk during the summer. To describe 

average spring conditions, water levels observed in the spring 

between April 1, 2005, and June 30, 2015, were averaged for 

each monitoring location.

With few exceptions, groundwater-flow rates and directions cannot be directly observed and must instead be inferred 

from groundwater-level and surface-water-level observations. 

Groundwater discharge to the land surface can be observed in 

some locations at the bottom of the scarp along the western 

side of the study area, where it is expected that groundwater 

flows eastward from the scarp.

Precipitation and Evapotranspiration

Precipitation and evapotranspiration (ET) make up a 

large fraction of the total water budget of the Swamp. A portion of precipitation falling on the Swamp percolates down to 

the water table and recharges the groundwater system. ET is 

the flux of groundwater, soil water, and surface water to the 

atmosphere either by evaporation or by plant transpiration. 

Both precipitation and ET occur everywhere in the study area. 

Because the water table is within a few feet or less below the 

land surface in most locations, groundwater is widely available to plants for transpiration, and the water table responds 

rapidly to precipitation and ET. Where the water table is above 

land surface, water can evaporate directly to the atmosphere.

Water-table responses in well 59 A39 (USGS station 

identifier 363320076261101) to 11 different precipitation 

events that occurred from November 2009 to September 2011 (fig. 8A) demonstrate that the first 0.39–0.79 in. 

(10–20 millimeters [mm]) of precipitation during a rainfall 

event is intercepted by vegetation or wets unsaturated peat and 

does not cause a measurable rise in the water table. But nearly 

all additional precipitation during a rainfall event percolates 

to the water table. The water-table response to precipitation 

varies depending on the water-table position relative to the 

land surface. Water-table response to a storm event, which 

occurred from August 26 to 28, 2011 and had total precipitation of 12 in. (0.3 m), shows that porosity or specific yield 

(Sy) increases from about 7 percent below a depth of 2.3 ft 

(0.70 meter [m]) to about 60 percent above a depth of about 

1.5 ft (0.45 m) (fig. 8B). For example, 1 in. of recharge will 

raise the water table by 1.7 in. when the water table is near 

land surface and Sy is 60 percent but by as much as 14 in. 

when the water table is below 2.3 ft and Sy is 7 percent.

Potential evapotranspiration (PET) rates were calculated for April–June from observed mean monthly 

temperature data at the Lake Drummond meteorological 

station (USC00448837) by using Thornthwaite’s equation 

(eq. 1–3). Calculated spring PET ranges from 9.7 to 13.0 in. 

for the years 2005 through 2015 (fig. 9) with the average 

PET rate for that period equaling 11.0 in., or 44.2 inches per 

year (in/yr) when converted to an annual rate. For the years 

2011 and 2015, which are used to define dry and wet climatic 

conditions in the model scenarios discussed in the “Simulated 

Hydrology and Water Management” section of this report, 

spring PETs are calculated to be 11.2 in. (45.1 in/yr) and 

11.7 in. (47.1 in/yr), respectively.

PET = L N Ta

I

16

12 30

 10



 









 









 







(1)

a I I

I

= 6.75x10 - 7.71x10 +

1.792x10 + 0.49239

-7 3 -5 2

-2

   

  (2)

I =

Tai

i=1 5

12

1.514 



 

   (3)

where

PET is estimated potential evaporation (mm/

month),

L is mean day length (hours) of the month being 

calculated, 

N is the number of days in month being 

calculated,

Ta is mean daily temperature in degrees Celsius 

of the month being calculated, 

	 α	 is an empirical exponent coefficient that 

varies with location, and 

I is the heat index dependent on the 12 monthly 

mean temperatures.

Other data are available to describe ET. The MODISMOD16 dataset (Mu and others, 2011) provides calculated 

actual ET at 3,218-ft (1-kilometer [km]) grid spacing for 

the years 2000–14. Monthly actual ET values are based on 

the Penman-Monteith equation using daily meteorological 

reanalysis data and 8-day remotely sensed vegetation property dynamics from MODIS satellite (hereafter MODIS) as 

inputs. The MODIS ET values give a mean rate of 50.7 in. 

(0.01158 ft/d) for April, May, and June from 2005 through 

2014. Gridded ET estimates at the 3,281-ft (1-km) scale for 

the years 1971–2000 are also available from Sanford and 

Selnick (2013), who used a water-balance method combined 

with a climate and land-cover regression equation. Average ET 

rates from the Sanford and Selnick (2013) dataset for the study 

area show a mean annual ET rate of 27.6 in/yr (0.0062 ft/d). 
Description of the Study Area 13

Total precipitation, in inches

0 2 4 6 8 10 12 14

0

5

10

15

20

25

30

35

40

45

50

Fine-grained peat interval

of low specific yield (0.07)

Transitional grained peat

interval of moderate specific

yield (0.31)

Coarse-grained peat interval

of high specific yield (0.72)

B

Depth to water, in inches below land surface

EXPLANATION

Observed depth to 

 water, in inches

-1.5

-1.0

-0.5

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0.0001 0.001 0.01 0.1 1 10 100

Precipitation required for Land surface

interception and wetting peat

High

yielding

peat

Transition

peat

Low yielding

peat

(A)

Cumulative precipitation, in inches (log scale)

A

Depth to water, in feet below land surface

Figure 8. Graphs showing response of the water table in well 59A 39 to precipitation, A water-level response to selected precipitation 

events, B water-level response to 12 inches of precipitation from August 26 to 28, 2011, Great Dismal Swamp National Wildlife Refuge, 

Virginia and North Carolina.
14 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

0

2

4

6

8

10

12

14

10

15

20

25

2004 2006 2008 2010 2012 2014 2016

Year

Average

Temperature, April−June, in 

degrees Celsius

Potential evapotranspiration, 

April−June, in inches

Mean temperature, in degrees 

 Celsius

Total calculated potential 

 evapotranspiration, in inches

Mean calculated potential 

 evapotranspiration for 2005−15, 

 in inches

EXPLANATION

Figure 9. Graph showing mean observed temperature at meteorological station USC00448837 on Lake Drummond operated by the 

U.S. Army Corps of Engineers, total calculated potential evapotranspiration (PET), and mean calculated PET, determined by using 

Thornthwaite’s equation for April, May, and June, 2005 through 2015, Great Dismal Swamp, Virginia and North Carolina. (Location of 

station USC00448837 shown in fig. 1)

These various ET rates are calculated by using different methods and data collected at different times, so the ET rates are 

examples of possible values rather than a defined single rate. 

These values are used as comparison for ET rates calculated 

by the model, as discussed in the “Calibration Approach” and 

“Calibration Results” sections.

Surface-Water Hydrology

Surface water is present across the study area as standing 

water on the land surface during both wet and dry conditions 

and as surface-water bodies with defined banks. The surfacewater bodies include streams draining across the scarp and 

into the Swamp, streams and ditches within the Swamp, the 

Dismal Swamp Canal, and Lake Drummond (fig. 1). Lake 

Drummond is a 3,108-acre shallow lake near the center of the 

Swamp (USFWS, 2006) that, although originally naturally 

occurring, has been impounded since at least 1830 (Shaler, 

1890; Trout, 1998). The maximum depth of the lake is about 

6 ft, and mean depth is about 4.5 ft, based on bathymetry data 

(USACE, 1970). The observed mean daily water-level elevation is 15.92 ft for January 1, 2005, through June 30, 2015, 

recorded by the USACE. The Lake Drummond Spillway keeps 

water in Lake Drummond about 5 ft higher than in the Feeder 

Ditch and passes flow from the lake into the Feeder Ditch. 

The USACE manages the dam, Feeder Ditch, and Dismal 

Swamp Canal to provide navigation for watercraft between the 

Chesapeake Bay and Albemarle Sound. Other surface-water 

bodies in the study area include natural freshwater drainages 

flowing across the Suffolk Scarp into the Swamp and streams 

at the boundaries including the freshwater Shingle Creek at 

the northwest corner and the tidal Deep Creek and Pasquotank 

River at the northeast and southwest corners, respectively. The 

most widely occurring surface-water features are ditches that 

have been dug throughout the Swamp.

Most ditches have a predominant flow direction, but 

because of the very flat topographic gradients, flow directions 

can reverse temporarily in some ditches under changing climatic and hydrologic conditions, or under changed weir level 

settings. Clogging of culverts and construction of beaver dams 

can also change flow directions and rates.

Ditches and Roads

Ditches were dug historically to lower water levels, 

provide fill for road construction, and carry water and goods 

from the interior of the Swamp to waterways to the north, 

east, and south. Although an unknown number of ditches have 

been abandoned and filled, many currently (2017) continue to 

act as primary discharge pathways for surface water flowing 

out of the Swamp. Actively flowing ditches are described and 

mapped (fig. 10) on the basis of data provided by Refuge staff.
Description of the Study Area 15

North Carolina

Dismal Swamp

State Park

2011 Burn

Area

Great 

Dismal 

Swamp

National 

Wildlife 

Refuge Dismal Swamp Canal

Pasquotank River

LAKE

DRUMMOND

Shingle

Creek

Deep

Creek

VIRGINIA

NORTH

CAROLINA

64

32

58

17

158E

158

32

13

58

17

32N

-76°30' -76°20'

36°40'

36°30'

HARDWOOD

HUDNELL

EAST

EAST

PORTSMOUTH

CORAPEAKE

WEST

JERICHO

HUDNELL

MYRTLE

LAUREL

BIG ENTRY

SOUTH

CROSS CANAL

WASHINGTON

RAIL ROAD

WILLIAMSON

CAMP

WEYERHAUSER

KIM SAUNDERS

MIDDLE

INSURANCE

COUNTY LINE

BULL BOULEVARD AND LEG

SHERRILL

ROSEMARY

PERSIMMON

NORTHEAST

INTERIOR

JUNIPER

LYNN

RIDDICK

BADGER

SHEPARD

FOREST LINE

NEW

DIKE

LEG REDWATER

JERICHO

CIRCLE

PASQUOTANK

CUT

SOUTH 

MILLS

SPILLWAY

OUTFALL

CEDAR RAILROAD

ARBUCKLE

NORTH MARTHA

WASHINGTON

PAW-PAW

SOUTH MARTHA

WASHINGTON

WESTERN BOUNDARY

NEWLAND DIKE

SYCAMORE

FIVE

MILE

LATERAL

NORTHEAST

LEG

DEEP

CREEK

LOCK

SOUTH

MILLS

LOCK

National Wildlife 

 Refuge

North Carolina State 

 Park

Burn Area

Basin discharging to 

 the swamp

Major road

Railroad

Ditch

Stream or river

Canal lock

EXPLANATION

0 2.5 5 KILOMETERS

0 2.5 5 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

FEEDER

CSX Railroad

Figure 10. Ditches and roads in the Great Dismal Swamp, Virginia and North Carolina.
16 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Roads built on top of spoil piles parallel most ditches 

(fig. 11). Spoil-bank roads are a barrier to surface-water flow 

because, unless water is higher than the road surface, water 

must divert to a culvert or pipe before flowing around or 

underneath the road. Roads are also barriers to groundwater 

flow because roads compact peat and reduce its hydraulic 

conductivity. However in a few locations, groundwater flows 

through roadbeds of corduroy construction. Corduroy construction is the creation of a roadbed base from tree trunks 

lain side-by-side across the width of the road. Dirt, sand, or 

gravel is then spread over the tree trunks. The tree trunk layer 

can provide a relatively low-resistance path for water to flow 

through the roadbed, particularly as the wood rots or degrades. 

Geospatial data describing road locations were provided by 

Refuge staff.

Water Control Structures

The strategy of the USFWS for hydrologic restoration of the Swamp centers on installation of engineered 

structures with weirs to control ditch water levels and flows 

(figs. 12 and 13, table 1). WCSs and their management follow the techniques of controlled drainage practices used on 

agricultural land in southeast Virginia and northeast North 

Carolina (Dukes and others, 2003; Evans and others, 2007). 

The Refuge manages smaller structures on interior ditches, 

termed “WCSs,” which in this report describes all engineered 

structures controlling surface water in the Swamp. The most 

common WCS type is made of corrugated aluminum culvert material (fig. 13B) and consists of a horizontal culvert 

(typically 4–6 ft diameter) with a one-half section of culvert 

material welded vertically to the upstream end. Two or three 

structures are installed at some sites to provide extra flow 

capacity. Structures are typically placed in ditches at road 

crossings using heavy excavating equipment. Headwalls of 

aluminum, stone, and wood on the upstream and downstream 

faces of structures were installed to reduce erosion. Once a 

structure is in place, 0.5-ft stop-logs are added to the riser section to control water-surface elevations. The stop-logs function as rectangular weirs, and Refuge staff control ditch water 

levels by adjusting the elevation of weirs at 49 WCSs (fig. 12) 

(Wurster and others, 2016). Depending on the position of a 

WCS within the ditch network and elevation relative to the 

surrounding peat surface, a single WCS can affect water levels 

in several miles of adjacent ditches. WCSs that are currently 

(2017) actively managed to control water levels were installed 

starting in the mid-1980s (table 1).

The USACE manages water levels in the Dismal Swamp 

Canal, Feeder Ditch, and Lake Drummond (fig. 12). Water 

levels in Lake Drummond are controlled by the spillway on 

the Feeder Ditch (#28 on fig. 12), whereas water levels in the 

Dismal Swamp Canal are controlled by navigational locks and 

associated spillways and weirs at South Mills, N.C., and Deep 

Creek, Va. (#70 and #1 in fig. 12). These control structures are 

managed as described in the “Water Management” section.

Most head loss in the surface-water flow network occurs 

at structures, rather than as friction loss along the ditches. 

This can be seen in the mean observed surface-water levels of 

Corapeake Ditch from April 2013 to June 2015 (fig. 14). Data 

show that 4.3 ft of the total head drop of 6.3 ft, or 69 percent, 

occurred at the three WCSs (Western Boundary Ditch , 0.6 ft; 

Laurel Ditch, 1.5 ft; and Weir 1, 2.2 ft), whereas the remaining 

2.0 ft, or 31 percent, occurred as frictional head loss along the 

length of Corapeake Ditch.

Surface-Water Inflows and Outflows

Surface water flows into the Swamp from the west and 

south. From the west, small streams drain into the Swamp 

through channels eroded across the Suffolk Scarp. From the 

south, constructed channels drain water from forested and 

agricultural land through culverts under Route 158 into the 

drainage ditch on the north side of Route 158, which carries flow east to the Pasquotank River. Rates of inflow to the 

Swamp (table 2) were estimated (eq. 4) on the basis of the 

inflowing streams’ contributing drainage areas, and observed 

precipitation and calculated evapotranspiration for the periods 

representing wet, dry, and average climatic conditions. As 

described in the “Model Development” section, the inflows 

were later assigned as input to the model. Precipitation totals 

during the spring (April–June) for the wet, dry, and average 

climatic conditions are 17.81, 7.06, and 12.86 in., whereas 

evaporation totals are 11.75, 11.24, 11.03 in., also described 

in the “Model Development” section. Although a small area to 

the east of Route U.S. 17 drains to the Dismal Swamp Canal 

through culverts, this inflow is not expected to be significant 

and is not included in the model.

Inflow = Drainage area x (Precipitation -

 Potential evapotranspiration) (4)

At the foot of the scarp, water from streams flowing into 

the Swamp from the west spreads out and moves as diffuse 

groundwater or surface drainage until captured by drainage ditches in the interior of the Swamp. Exceptions to this 

are streams in Taylor Swamp and Pocosin Swamp, which 

after entering the Swamp are intercepted by Cross Canal and 

Washington Ditch, respectively (fig. 10). On the basis of field 

observations, groundwater is expected to discharge mostly to 

land surface at the toe of scarp and into ditches running parallel to the scarp, such as Lynn Ditch, West Ditch, and Sherrill 

Ditch (fig. 10).

Outflows from the Swamp are to Shingle Creek from 

Jericho Ditch at the northwestern corner of the study area; 

to Deep Creek from Portsmouth Ditch and from the Dismal 

Swamp Canal at the northeastern corner; and to the Pasquotank River from various ditches and the Dismal Swamp Canal 

at the southeastern corner (fig. 10). Some of these outflows 

are monitored (table 3). The USGS has operated a streamflow 

gaging station (0204382800 Pasquotank River near South 

Mills, N.C.) in the southeast corner of the study area since 
Description of the Study Area 17

Ditch Spoil bank 

road

Peat

surface

Peat

surface

Ditch

Spoil bank

road

Sand

Sand

West East

Ditch

Water ponds upstream

from roads

Peat

Surface-water flow

Groundwater flow

Graphic vertically exaggerated

Groundwater flow in

peat is diverted under

or around roads

Peat

A

B

C

D

Photo by: U.S. Fish and Wildlife Service 

Ditch

Before ditching

After ditching

Road Roads impede

groundwater movement Road

Ditches capture water

and remove from swamp

Figure 11. Cross-sectional diagrams of groundwater and surface-water flow A, before ditching and B, after ditching, C, photo of road 

and ditch in Great Dismal Swamp, Virginia and North Carolina, and D, diagram of road and ditch.
18 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

North Carolina

Dismal Swamp

State Park

2011 Burn

Area

Great 

Dismal 

Swamp

National 

Wildlife 

Refuge Dismal Swamp Canal

Pasquotank River

LAKE

DRUMMOND

Shingle

Creek

Deep

Creek

VIRGINIA

NORTH

CAROLINA

64

32

58

17

158E

158

32

13

58

17

32N

13

-76°30' -76°20'

36°40'

36°30'

National Wildlife 

 Refuge

North Carolina State 

 Park

Burn Area

Basin discharging to 

 the swamp

Major road

Railroad

Ditch

Stream or river

EXPLANATION

Water control structure 

 and number

Adjustable 

 weir–existing

Fixed 

 weir–existing

Adjustable 

 weir–proposed

Fixed 

 weir–prooposed

Culvert

Gate

Canal lock

Surface-water 

 observation 

0 2.5 5 KILOMETERS

0 2.5 5 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

Feeder Ditch

CSX Railroad

4

9

17

102

103

23

22

15

2

26 27

32 31

34

28

29

35

39 38 37 40

44

45 43 42 41

50

47

49 51 48 46

53

56 55 54 52

58

59 57

62

63

65

64

67

71

17

66

7

100 101

3

5

6

8

11 12

13 14

16

19

18 20

21

10

1

25

24

30 33

36

61

60

68

72

69

70

Figure 12. Locations of water control structures in the Great Dismal Swamp, Virginia and North Carolina. (Structure information listed 

in table 1)
Description of the Study Area 19

Table 1. Description of water control structures in the Great Dismal Swamp, Virginia and North Carolina.[Locations of sites are shown in figure 12. WCS, water control structure; NA, not applicable; NE, Northeast; N, North; E, East; NW, Northwest; S, South; W, West; SE, Southeast]

Structure name

Site 

map 

number

Site in model Structure type

Typical 

flow 

direction

Year 

installed Gage

Weir invert elevation, in feet, according to model scenario

1, 2 

Baseline, 

wet

3 

Dry

4 

Future 

WCS 

added

5, 6 

Flood the 

swamp 

(wet, dry)

7, 8 

Drain the 

swamp 

(wet, dry)

Deep Creek Locks 1 Yes Canal Lock NE 1941 Yes 7.9 7.9 7.9 7.9 7.9Deep Creek Spillway 2 Yes Adjustable Weir N 1941 Yes 7.9 7.9 7.9 7.9 7.9Williamson/East 3 Yes, proposed Adjustable Weir E Proposed NA None None 20.5 22.0 16.0Jericho North 4 Yes Fixed Weir NW 2009 Yes 19.0 19.0 19.0 19.5 16.5Big Entry Fixed Weir 2 5 Yes, proposed Fixed Weir NA Proposed NA None None 14.0 14.0 14.0Portsmouth/Big Entry 6 Yes, proposed Adjustable Weir NE 1994/Proposed NA 7.4 7.4 12.0 13.6 7.4Jericho @ Williamson 7 Yes, proposed Adjustable Weir NW NA/Proposed No 18.0 18.0 20.0 23.0 15.0Rosemary/Big Entry 8 Yes, proposed Adjustable Weir NA Proposed NA None None 11.0 14.5 6.5Lynn/Jericho 9 Yes Adjustable Weir N 1984 Yes 16.2 16.2 16.2 22.0 16.2Rosemary @ Road 10 Yes, proposed Adjustable Weir NA Proposed NA None None 12.0 15.5 9.0Rosemary @ Portsmouth 11 Yes, proposed Adjustable Weir NE 1984 Yes 16.1 16.1 14.5 16.7 11.2Portsmouth @ Rosemary 12 Yes, proposed Adjustable Weir E Proposed NA 14.3 14.3 14.3 17.0 12.0Juniper @ Road 13 Yes, proposed Adjustable Weir N Proposed NA None None 15.0 17.0 13.0Portsmouth @ Juniper 14 Yes, proposed Adjustable Weir NE Proposed NA None None 15.5 17.0 12.0Northeast 15 Yes Adjustable Weir NE 1984 Yes 13.5 13.5 13.5 15.0 9.0Portsmouth/Northeast 16 Yes, proposed Adjustable Weir NA Proposed NA 16.1 16.1 15.0 16.5 11.1Hudnell/East 17 Yes Adjustable Weir E 2012 Yes 17.6 17.6 17.6 21.3 15.5Camp/East 18 Yes, proposed Adjustable Weir NA NA/Proposed NA 17.3 17.3 20.0 21.0 16.0Portsmouth/Hardwood 19 Yes, proposed Adjustable Weir W NA/Proposed NA 14.0 14.0 16.5 18.5 14.5Portsmouth Dam 20 Yes, proposed Adjustable Weir N Proposed NA 9.0 9.0 16.5 18.5 14.5Jericho/Camp 21 Yes, proposed Adjustable Weir E 1990/Proposed NA 17.0 17.0 20.0 20.0 16.0Lynn @ Badger 22 Yes Culvert S NA No 19.0 19.0 19.0 19.0 19.0Lynn/Middle 23 Yes Adjustable Weir E or W 1984 Yes 18.6 18.6 18.6 24.0 18.6Lynn South 24 Yes Gate N or S 1984 Yes 17.0 17.0 17.0 17.0 17.0Washington/Lynn 25 Yes Adjustable Weir SE 1984 Yes 21.0 21.0 21.0 24.0 18.0Washington/Jericho 26 Yes Adjustable Weir S 1977 Yes 17.2 17.2 17.2 18.3 14.0Porstmouth Unknown 27 Yes Fixed Weir NA NA NA 17.7 17.7 17.7 17.7 17.7Lake Drummond Spillway 28 Yes Adjustable Weir E 1935 Yes 15.1 15.1 15.1 15.1 15.1
20 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C. Table 1. Description of water control structures in the Great Dismal Swamp, Virginia and North Carolina.—Continued [Locations of sites are shown in figure 12. WCS, water control structure; NA, not applicable; NE, Northeast; N, North; E, East; NW, Northwest; S, South; W, West; SE, Southeast] Structure name Site map number Site in model Structure type Typical flow direction Year installed Gage Weir invert elevation, in feet, according to model scenario 1, 2 Baseline, wet 3 Dry 4 Future WCS added 5, 6 Flood the swamp (wet, dry) 7, 8 Drain the swamp (wet, dry) Myrtle North 29 Yes Adjustable Weir

N 2006 Yes 12.8 13.3 12.8 15.5 11.4

Paw Paw/Myrtle 30 Yes, proposed Adjustable Weir

W Proposed No 16.0 16.0 13.0 16.0 11.0

Interior to Drummond 31 Yes Fixed Weir

E 2011 NA 16.2 16.2 16.2 16.2 16.2

Interior @ mid point 32 Yes Fixed Weir NA 2011 NA 17.1 17.1 17.1 17.1 17.1Laurel Weir @ Persimmon 33 Yes, proposed Adjustable Weir

N Proposed No None None 14.5 17.0 13.0

Riddick Plug 34 Yes Fixed Weir

N 2011 No 19.3 19.3 19.3 19.3 19.3

Myrtle South 35 Yes Adjustable Weir

N 2012 Yes 14.0 14.5 14.0 17.5 12.0

S. Martha Weir @ Persimmon 36 Yes, proposed Adjustable Weir

N 2015 No None None 14.5 17.0 13.3

S. Martha/Persimmon 37 Yes Adjustable Weir

N 2012 Yes 14.8 14.8 14.8 17.3 13.3

Persimmon/Laurel 38 Yes Culvert

W NA No 10.0 10.0 10.0 10.0 10.0

Myrtle/Persimmon 39 Yes Adjustable Weir E or W 1984 Yes 14.5 14.5 14.5 18.7 10.7Rock Weir 40 Yes Fixed Weir

N 1990 No 17.1 17.1 17.1 17.1 17.1

Sycamore/South Martha Washington 41 Yes Adjustable Weir S 1984 Yes 14.9 15.3 14.9 18.6 10.0Sycamore/Laurel 42 Yes Adjustable Weir

E 2013 Yes 15.3 15.7 15.3 18.7 13.8

Sycamore/Myrtle 43 Yes Culvert

E NA No 12.5 12.5 12.5 12.5 12.5

Sycamore/West 44 Yes Adjustable Weir

E 2012 Yes 16.0 16.5 16.0 19.5 14.3

South/Riddick 45 Yes Adjustable Weir

E 1990 Yes 17.6 17.6 17.6 17.7 13.0

Corapeake/Laurel 46 Yes Adjustable Weir

E 1985 Yes 15.0 15.5 15.0 18.5 12.0

Weir 1 47 Yes Adjustable Weir S 2012 Yes 13.5 14.5 13.5 21.0 8.5Corapeake/West Bndry 48 Yes Adjustable Weir

E 1995 Yes 16.0 16.0 16.0 18.3 12.3

Forestline North 49 Yes Adjustable Weir N or S 1994 Yes 18.2 18.2 18.2 21.0 16.3Sherrill North 50 Yes Adjustable Weir

N 1984 Yes 17.0 17.5 17.0 19.5 15.0

Weir 2 51 Yes Adjustable Weir

N 2013 No 13.8 13.8 13.8 16.8 9.5

Kim Saunders/Laurel 52 Yes Adjustable Weir

E 2015 Yes 13.3 14.0 13.3 18.0 13.0

Forestline Culvert North 53 Yes Culvert S NA No 14.7 14.7 14.7 14.7 14.7Kim Saunders/Myrtle 54 Yes Culvert

E NA No 10.5 10.5 10.5 10.5 10.5

Kim Saunders/Western Boundary 55 Yes Adjustable Weir

E 2015 Yes 13.0 14.0 13.0 18.0 13.0

Forestline 2 56 Yes Adjustable Weir S 2014 No 15.7 15.7 15.7 21.0 15.7
Description of the Study Area 21

Table 1. Description of water control structures in the Great Dismal Swamp, Virginia and North Carolina.—Continued[Locations of sites are shown in figure 12. WCS, water control structure; NA, not applicable; NE, Northeast; N, North; E, East; NW, Northwest; S, South; W, West; SE, Southeast]

Structure name

Site 

map 

number

Site in model Structure type

Typical 

flow 

direction

Year 

installed Gage

Weir invert elevation, in feet, according to model scenario

1, 2 

Baseline, 

wet

3 

Dry

4 

Future 

WCS 

added

5, 6 

Flood the 

swamp 

(wet, dry)

7, 8 

Drain the 

swamp 

(wet, dry)

Forestline South 57 Yes Adjustable Weir S 1984 Yes 16.6 17.0 16.6 22.0 14.8Cross Canal/Weyerhauser 58 Yes Adjustable Weir E 1994 Yes 15.0 15.5 15.0 20.0 13.5County Line/Weyerhauser 59 Yes Adjustable Weir SE 1994 Yes 15.0 15.5 15.0 20.0 12.5Sherrill South 60 Yes Gate N 1985 Yes 15.0 15.0 15.0 15.0 15.0Cross Canal/Sherrill 61 Yes Adjustable Weir E 1985 Yes 18.0 18.0 18.0 21.0 15.0Bull Boulevard/Insurance 62 Yes Adjustable Weir SE 1995 Yes 14.3 14.3 14.3 17.5 10.5Head of River 1 63 Yes Adjustable Weir SE NA Yes 12.0 13.0 12.0 16.5 10.8Insurance/Countyline 64 Yes Adjustable Weir NE 1995 Yes 7.7 7.7 7.7 15.0 7.7County Line/Insurance 65 Yes Adjustable Weir SE 1995 Yes 12.4 12.8 12.4 15.0 8.7Bull Boulevard South 66 Yes Culvert S NA No 5.8 5.8 5.8 5.8 5.8Weyerhauser 67 Yes Adjustable Weir S 1984 Yes 16.8 16.8 16.8 18.8 14.0Newland Slide Gate 68 Yes Gate E NA No 13.0 13.0 13.0 13.0 13.0South Mills Spillway 69 Yes Adjustable Weir W 1941 Yes 7.9 7.9 7.9 7.9 7.9South Mill Locks 70 Yes Canal Lock S 1941 Yes 7.9 7.9 7.9 7.9 7.9Weyerhauser 158 culverts 71 Yes Culvert E NA Yes 10.0 10.0 10.0 10.0 10.0Newland Dike Screwgate 72 Yes Gate E NA No 12.0 12.0 12.0 12.0 12.0Williamson/East - Williamson 100 No Observation NA NA No NA NA NA NA NAWilliamson/East - East 101 No Observation NA NA Yes NA NA NA NA NAWilliamson/New 102 No Culvert NW NA Yes NA NA NA NA NAJericho/Hudnell 103 No Culvert NW NA Yes NA NA NA NA NA
22 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Corapeake Ditch at Laurel

Map Number 46

SycamoreDitch

Map Number 42

Adjustable Stop

Logs / Bays 

South Martha

Washington Ditch

Map Number 47 

Photo by: Frederic Wurster, U.S. Fish

and Wildlife Service, April 2011 

Photo by: Frederic Wurster, U.S. Fish and Wildlife Service, March 2013

Photo by: Frederic Wurster, U.S. Fish and Wildlife Service, January 2014

Photo by: Frederic Wurster, U.S. Fish and Wildlife Service, November 2012

A

B

Figure 13. Photographs of representative water control structures managed by the Great Dismal Swamp National Wildlife Refuge, 

Virginia and North Carolina: A, installed structures and B, a new combined culvert and riser on route to installation.
Description of the Study Area 23

10

12

14

16

18

20

0 10,000 20,000 30,000 40,000 50,000

22

24

Elevation, in feet referenced to NAVD 88

West to east distance, in feet

Sherrill

Ditch Weir

Map # 50

Burn

Area

Forest

Line

Ditch Weir

Map # 49

Western

Boundary

Ditch Weir

Map # 48

Laurel

Ditch Weir

Map # 46

Weir 1

Map # 47

Corapeake Ditch Water Levels (Mean) April 2013 to June 2015

Land

surface

Surface-water

level

EXPLANATION

Lidar observed land-surface 

 elevation

Estimated surface-water 

 level in Corapeake Ditch

Mean observed surface-water 

 level

Figure 14. Mean observed surface-water levels along Corapeake Ditch, Great Dismal Swamp, Virginia and North Carolina, April 

2013–June 2015.

Table 2. Estimated surface-water inflows to the Great Dismal Swamp, Virginia and North Carolina.

[mi2, square miles; ft3/s, cubic feet per second]

Inflow area Total area (mi2) Estimated inflow (ft3

/s)

Dry Wet Average

Cypress Swamp 30.0 0 53.7 16.3

Daniels Road Swamp 22.8 0 40.8 12.3

Folly Swamp 6.5 0 11.6 3.5

Goose Creek/Acorn Hill 7.2 0 12.9 3.9

South of Rt. 158 13.0 0 23.2 7.0

Moss Swamp 5.0 0 8.9 2.7

Pocosin Swamp 8.8 0 15.7 4.8
24 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Table 3. Observed surface-water outflows from the Great Dismal Swamp, Virginia and North Carolina.

[USACE, U.S. Army Corps of Engineers; USFWS, U.S. Fish and Wildlife Service; USGS, U.S. Geological Survey; WCS, water control structure; fig., figure; 

ft3/s, cubic feet per second; --, no data; Avg, average]

Station Data 

source

Location

Condition and average period flow rate, in ft3/s

Dry: 

April–June 2011

Wet: 

April–June 2015

Avg: 

April–June, 2005–15

South Mills USACE WCS map numbers 73 and 74 in fig. 12 55.0 154.7 131.3

Deep Creek USACE WCS map numbers 1 and 2 in fig. 12 56.4 158.6 113.8

Jericho USFWS WCS map number 4 in fig. 12 4.7 11.8 8.5

Pasquotank USGS USGS station 0204382800 in fig. 1 56.8 127.8 105.3

Portsmouth None No structure -- -- --

Shingle Creek None No structure -- -- --

Total Outflow 1172.9 + 1452.9 + 1361.4 +

1

Sum of measured flow is less than total flow.

1994 (fig. 1). The USACE has recorded daily water levels 

since 1923 and flows since 1953 at the Lake Drummond 

spillway (#28 in fig. 12) and, for this study, provided daily 

records of levels and flows for 2010 through 2015 at the Deep 

Creek and South Mills locks (#1 and #70 on fig. 12). At the 

northwestern corner of the study area, Jericho Ditch and other 

smaller unnamed ditches and drainages discharge to Shingle 

Creek (fig. 10). Flow is not observed in Shingle Creek, but 

Refuge staff make periodic flow observations at the last WCS 

on Jericho Ditch, which on the basis of field observations 

constitutes most of the outflow to Shingle Creek.

Water Management 

Water management by the USFWS has focused on 

“restoring” the hydrology of the Swamp (USFWS, 2006) and 

has the additional operational goal of maintaining water levels 

for fire suppression (Wurster and others, 2016). By installing 

WCSs across canals and ditches, Refuge managers are able to 

re-wet drained peat areas and raise or lower water-table elevations to encourage the growth of particular tree species and 

ecologies and to mitigate peat loss. 

The USACE manages the Feeder Ditch and Dismal 

Swamp Canal to maintain water at levels that can support boat 

traffic, while also managing occasional floods. In addition, the 

USACE works with the Refuge to honor an informal agreement with the USFWS, in place since 1977, to manage the 

Dismal Swamp Canal so as to not adversely affect the Refuge 

(USFWS, 2006). The USACE manages the dam between Lake 

Drummond and Feeder Ditch, and the locks at Deep Creek 

and South Mills (J. Scussel, USACE, oral commun., 2016), 

according to the following operational goals:

• Maintain water levels in Lake Drummond at 

15.98 ft,

• Maintain water levels in the Dismal Swamp Canal at 

Deep Creek at 8.41 ft,

• Maintain water levels in the Dismal Swamp Canal at 

South Mills at 8.33 ft,

• Reduce the number of boat lockings when the Lake 

Drummond water level is less than 15.28 ft and 

dropping, and

• Stop boat lockings when Lake Drummond water 

levels fall to 14.38 ft or below.

Drought and dry summer conditions can make it difficult 

to maintain water levels in the lake and canal. As a result of 

drought, the canal has been closed to boat traffic for as much 

as several months in recent years. On the basis of USACE 

data, the mean daily water-level elevation at Lake Drummond 

from January 1, 2005, to June 30, 2015, was 15.92.

Several other agencies manage water in the study area. 

The Newland Drainage District manages dikes and WCSs to 

facilitate farming on land southeast of the Swamp near the 

Pasquotank River. The City of Suffolk and the City of Chesapeake both manage stormwater drainage networks in populated areas in the northern part of the study area.

Inundation

Water inundates the land surface in many areas of the 

Swamp, particularly during wet periods. Because access is 

difficult in many areas of the Swamp, remote sensing can be 

an effective tool for identifying the extent of inundation. Two 

different remote sensing products were used in the study as 

indicators of inundation:

1. Dynamic Surface Water Extent (DSWE) (Jones, 2015; 

http://remotesensing.usgs.gov/ecv/SWE_pp.php) and

2. Interferometric Synthetic Aperture Radar (InSAR) 

coherence and Synthetic Aperture Radar (SAR) doublebounce backscatter analysis (Kim and others, 2015).
Description of the Study Area 25

The DSWE product is derived from Landsat satellite imagery and shows areas of standing water. DSWE is 

well suited for analysis of the Dismal Swamp because it is 

effective in areas that have tree cover and are occasionally 

inundated and relatively inaccessible on the ground. The 

DSWE product comes in raster format with spatial resolution 

of 98 ft (30 m). Relatively clear skies at the time of satellite 

overpass are needed to collect data that can be used to successfully generate the DSWE product. DSWE methods were 

used to estimate the percent of time a given location was 

inundated using LANDSAT data scenes covering the Great 

Dismal Swamp (path 14, row 35) during April, May, and 

June 2005 through 2015.

The other remote sensing product used is derived from 

InSAR coherence and SAR double-bounce backscattering 

(Kim and others, 2015). The SAR/InSAR data product indicates whether a land area is inundated at the time of satellite 

flyover. For this study the inundation estimate was developed 

from L-band data from the ALOS PALSAR mission (Rosenqvist and others, 2004) and acquired on April 1, 2010, and 

May 17, 2010. The SAR/InSAR product is a binary raster 

describing inundation or lack of inundation, with a spatial resolution of about 75 ft (23 m) in the study area.

Both inundation observation values, the spatially averaged 

DSWE and SAR/InSAR inundation measures, were scaled up 

to the model grid cell size of 500 ft for use in calibrating the 

model, as described in the section “Observations and Weighting” subsection “Inundation Observations.” The approximately 

45 SAR/InSAR raster values within each model cell were 

averaged to produce a measure of percent area inundated for 

the April–May 2010 period, and for the active model area, the 

values averaged 4.7 percent inundated. The approximately 

26 DSWE raster values within each model cell were averaged 

to produce a measure of percent area inundated, and for the 

active model area, the values averaged 2.5 percent inundated. 

Because these DSWE raster values are already averaged temporally, giving for each raster the percent time inundated during 

April–June 2005 through 2015, the resulting observed DSWE 

value for each model cell was averaged over space and time. 

Both the inundation observation values, the spatially averaged 

DSWE and SAR/InSAR inundation measures, ranged smoothly 

from 0 to 100 percent, with most values close to 0 (fig. 15).
26 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C. LAKE DRUMMOND VIRGINIA NORTH CAROLINA -76°30' -76°20' 36° 40' 36° 30' 64 32 58 17 158 E 158 32 13 58 17 32N

0 2.5 5 KILOMETERS0 2.5 5 MILES

Base from U.S. Geological Survey National Elevation Data 1/3 arc-secondLambert Conformal Conic projectionStandard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

EXPLANATION

Simulated head

Above land–surface elevationBelow land–surface elevationInactive cellActive model extentMajor roadLake Drummond boundary

A. Simulated

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°40'36°30'

64

32

58

17

158E

158

32

13

58

17

32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-secondLambert Conformal Conic projectionStandard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

0 2.5 5 KILOMETERS0 2.5 5 MILES

Inactive cellActive model extentMajor roadLake Drummond  boundary

Average observed  inundation, as  percent

0.0

0.1025.099.9100.0

EXPLANATION

B. Dynamic Surface Water Extent (DSWE)

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°40'36°30'

64

32

58

17

158E

158

32

13

58

17 32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-secondLambert Conformal Conic projectionStandard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

0 2.5 5 KILOMETERS0 2.5 5 MILES

Inactive cellActive model extentMajor roadLake Drummond  boundary

Average observed  inundation, as  percent

0.0

0.1025.099.9100.0

EXPLANATION

C. Synthetic Aperture Radar (SAR)

Figure 15. Inundated land areas in the Great Dismal Swamp, Virginia and North Carolina, determined by 

A, simulated water levels above land surface from model Scenario 1, 

B, Dynamic Surface Water Extent (Jones, 2015), and 

C, Synthetic Aperture Radar backscatter analysis (Kim and others, 2015).
Geospatial Analysis of Land-Surface Elevations and Peat Thickness 27

Geospatial Analysis of Land-Surface 

Elevations and Peat Thickness

Because land-surface topography and peat stratigraphy 

are important to swamp hydrology, geospatial analysis was 

used to determine land-surface elevations and peat thickness as accurately as possible. Land-surface elevations were 

derived from multiple lidar datasets. Peat thickness was determined for the entire active model area by spatial interpolation 

of point values and subsequent processing with spatial filters. 

These peat thickness values were then used to assign layering 

in the groundwater model (“Model Development” section). 

Land-Surface Elevation Observations by Lidar

Three lidar datasets describing land-surface elevations in 

the study area and one bathymetric dataset describing elevations of the bottom of Lake Drummond were compiled into a 

6.6-ft (2-m) raster grid, digital-elevation model (DEM). The 

DEM was then used as the basis for vertical layering in the 

model and for describing topography of the study area. In 

2010, lidar data were collected for the Great Dismal Swamp 

NWR and NCDSSP. The 2010 data were combined in a 

mosaic with a 2006 lidar dataset covering the City of Chesapeake, a 2012 lidar dataset covering the area burned during the 

Lateral West Fire, and a bathymetric DEM of the bottom of 

Lake Drummond. The Lake Drummond DEM was developed 

by digitizing lake-bottom-elevation data from a bathymetric 

map (USACE, 1970) and interpolating the elevations across a 

6.6-ft (2-m) grid covering the lake (fig. 2). The mosaic DEM 

was then tested for error.

The vertical accuracy of the mosaicked DEM was tested 

by comparing DEM elevation values to ground survey point 

elevations collected by Courtney and Associates in 2013, by 

Woolpert Inc. in 2010 and 2012, and by USFWS staff in 2010 

(Wurster, 2014). Survey points that were located on sloped 

road embankments were removed from the dataset following 

vertical checkpoint standards (American Society for Photogrammetry and Remote Sensing, 2015). Root mean square 

error (RMSE) values for the difference between surveyed 

elevations and lidar values were then calculated (table 4). The 

RMSE for the elevation dataset was 0.55 ft (16.7 cm) with 

a 95-percent confidence interval of 1.1 ft (37.7 cm), which 

meets the standard for the National Elevation Dataset at Quality Level 3 (Heidemann, 2014) 

The mapped lidar elevation surface (fig. 2) reveals both 

natural and man-made topographic features. A low-relief 

dendritic drainage pattern shows a natural pattern of flow from 

Isle of Wight Plain eastward into the Swamp, from the interior 

of the Swamp towards Lake Drummond, towards the Pasquotank River in the southeast, towards Shingle Creek and Deep 

Creek in the north, and towards the Dismal Swamp Canal on 

the eastern boundary. 

Table 4. Ground survey control elevation datasets and error 

analysis of lidar-derived elevations and interpolated top of sand 

elevation values.

[USFWS, U.S. Fish and Wildlife Service; IDW, inverse distance weighted; 

RMSE, root mean square error; ft, foot]

Dataset

Number 

of 

points

Mean 

error 

(ft)

Mean 

percent 

error

RMSE 

(ft)

Lidar

Woolpert 25 0.01 1.34 0.10

USFWS 48 0.00 2.85 0.27

Courtney 20 0.27 7.24 0.51

All 93 0.06 3.39 0.31

Top of sand1

Topo to Raster 294 −0.03 0.03 0.43

IDW 294 0.00 0.46 0.56

Kriging 292 0.28 4.27 1.59

1

Data were checked against measured points, where the elevation was 

surveyed or estimated from lidar.

Peat Thickness

Because peat is important to Swamp hydrology, total peat 

thickness was estimated for the entire study area. Observations of peat depth and thickness were compiled from a variety 

of data sources from January 1973 through October 2015. 

These data provide good coverage in the northern two-thirds 

of the study area but generally sparse coverage in the southern 

one-third of the study area (fig. 16). Thickness of the upper 

peat layer, a portion of total peat thickness, was recorded in 

60 locations by the USFWS and USGS. The mean upper peat 

thickness from these 60 values is 1.47 ft. Additional data 

were compiled from previous studies (Oaks and Coch, 1973; 

USACE, 1970), which had greater uncertainty associated with 

the horizontal locations.

Estimated peat thickness values were added to improve 

interpolation in areas where observed values were not available. Estimated point values, based on field reconnaissance, 

were added along the western side of the study area (Suffolk 

Scarp) and in the south of the study area near the Pasquotank River. Estimated point values were also used to limit 

the extent of peat in areas where peat is known to be absent, 

in which case they were assigned a thickness of 0. In total, 

2,494 estimated point values were used in the interpolation 

(fig. 16 and table 5).

Because peat thickness changes over time, sometimes 

rapidly as seen for the 2008 and 2011 fires (Reddy and others, 

2015), the resulting interpolated peat thicknesses represent 

general conditions between 1973 and 2015 rather than conditions during any particular year. Some observations of peat 

thickness gave minimum peat thickness rather than absolute 

thickness because peat was thicker than the probe length or 
28 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

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Pasquotank River

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

58

32

58

13

17

64

32

32N

158E

17

13

158

EXPLANATION

Peat Thickness, 

 in feet

0.0

>12.0

0.1

12.0

Active model 

 extent

Major road

Ditch and Lake

 Drummond 

 boundary

Burn Area 

 boundary

Source of peat 

 observation

Measured; 

 elevation 

 surveyed

Measured; 

 elevation 

 from lidar 

Oaks and Coch

 (1973); interpo-

 lated from 

 contours

Oaks and Coch 

 (1973); peat 

 extent

USACE; Bathy-

 metry

Artificial 

 observation

#

#

"

"

!

X

Figure 16. Interpolated peat thickness and locations of elevation observation points, Great Dismal Swamp, Virginia and North Carolina. 

(USACE, U.S. Army Corps of Engineers)
Conceptual Hydrologic Model 29

Table 5. Observation frequency, by decade, of elevation and 

peat extent observations used for interpolating peat thickness 

over the entire study area.

[USACE, U.S. Army Corps of Engineers]

Source 

of peat 

observation

Observation frequency per decade Total per 

1970s 1980s 1990s 2000s 2010s source

Measured; 

elevation 

surveyed

47 0 0 8 10 65

Measured; 

elevation 

from lidar

15 0 0 0 222 237

Oaks and Coch 

(1973); 

interpolated 

from contours

742 0 0 0 0 742

Oaks and Coch 

(1973); 

peat extent

419 0 0 0 0 419

USACE; 

Bathymetry 204 0 0 0 0 204

Artificial 

observation 0 0 0 0 827 827

Total per decade 1,427 0 0 8 1,059 2,494

borehole depth. In such cases, the observation was not used 

as a basis for interpolation but was instead used as an error 

check. 

Point values of peat thickness were interpolated using 

the Topo to Raster tool in ESRI ArcMap, which applies spline 

methods adapted for terrain modeling (Hutchinson and others, 

2011). Different interpolation methods were tested, but Topo 

to Raster yielded the smallest percent error at locations that 

had an associated observed value. The resulting 6.6-ft (2-m) 

resolution raster was clipped to a known peat extent (Oaks 

and Coch, 1973), excluding areas where there is no peat (such 

as Lake Drummond and developed areas). The resulting map 

(fig. 16) shows some interpolation artifacts but honors the 

observed point values well and was used successfully as input 

to the hydrologic simulation model.

Peat thickness was used to calculate top-of-sand elevations by subtracting peat thickness from lidar land-surface-elevation values. In parts of the study area where peat was absent, 

lidar land-surface elevations were used to generate a continuous top of sand elevation raster for the model area.

Conceptual Hydrologic Model

A conceptual model of swamp hydrology can be developed from previous studies and observations. Although water 

budgets for neither groundwater nor surface water have been 

previously quantified in much detail, major water exchanges 

in the Swamp are generally known. Groundwater and surfacewater hydrology are closely linked throughout the study 

area, so much so that understanding the water budget for one 

component requires a full understanding of the water budget 

for other component. Groundwater discharges to ditches, 

streams, and the land surface at the foot of the western scarp. 

During wet periods, the water table rises and inundates land 

surface in many areas. Although most surface-water bodies 

are expected to receive groundwater discharge, surface water 

likely recharges the underlying aquifer in areas where surface water is ponded behind roads or WCSs. On the basis of 

expected high porosity and permeability of the upper peat 

layer, there likely is low resistance to lateral groundwater flow 

in the upper peat. 

The primary water input to the Swamp is precipitation, 

which upon entering the Swamp gets divided between surface 

water, evapotranspiration, and groundwater infiltration. A second major input of water to the Swamp is flow from streams 

and drainages to the west and south. Although these surface 

inflows are generally unmonitored, the area of contributing watershed is about 39 percent of the area of the Swamp. 

The third known water input to the Swamp is groundwater 

discharge that originates along the scarp and areas west of the 

Swamp, then flows eastward to discharge at the bottom of the 

scarp slope near the western boundary.

A major outflow (or loss) of water from the Swamp is 

evapotranspiration, discussed in more detail in the “Recharge 

and Evapotranspiration” section. The other major outflow of 

water from the Swamp is surface-water discharge from ditches 

and the Dismal Swamp Canal to tidal water bodies. Surface 

water flows from the study area to the Pasquotank River outflow in the southeast, to Shingle Creek in the northwest, and to 

Deep Creek in the northeast, as well as from both ends of the 

Dismal Swamp Canal at South Mills and Deep Creek (fig. 1). 

Although groundwater almost certainly discharges to the 

tidal water bodies bordering the Swamp, such as Deep Creek 

or Shingle Creek to the north or the Pasquotank River to the 

south, these presumed groundwater discharges have not been 

previously observed or modeled. 

The active groundwater system of the Swamp is probably limited to the shallow subsurface. Although the shallow 

groundwater system of the Swamp includes the upper part of 

the Yorktown-Eastover aquifer, little groundwater exchange 

with deeper parts of the formation is expected because, as 

discussed in the “Hydrogeology” section, the Saint Marys confining unit that underlies the Yorktown-Eastover aquifer likely 

isolates the Swamp hydrologic system from deeper aquifers of 

the Coastal Plain.
30 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Numerical Model Development

Numerical simulation models were developed to simulate water levels and flows in the study area. The first model 

is a preliminary 2-D, cross-sectional, steady-state model of 

Block C1 (fig. 17) that simulates groundwater levels and flows 

between Corapeake Ditch and Sycamore Ditch. The second 

model is a 3-D steady-state model of the entire study area that 

simulates groundwater and surface water. The 2-D model was 

used to conceptualize vertical layering, assess likely flow patterns, develop experience applying the modeling software to 

the study area, and develop initial hydraulic parameter values 

for use in the 3-D model. Because the 3-D model simulates 

both groundwater and surface-water levels and flows within 

the study area, it can be used to answer questions about management of WCSs and their effect on water levels. This report 

section first describes the 2-D model, then describes the 3-D 

model. Where the term “model” is used by itself, it refers to 

the 3-D model (fig. 17). The input, output, and execution files 

for the numerical model, as well as documentation on use of 

the model, are publicly available (Eggleston and others, 2018).

The 2-D and 3-D models use the hydrologic modeling 

software package MODFLOW-NWT (Niswonger and others, 2011), which is based on simulation of groundwater but 

has many add-on capabilities including simulation of surface 

water. MODFLOW-NWT is a finite-difference groundwater 

model that uses a Newton-Raphson formulation to improve the 

solution of unconfined groundwater flow problems. From the 

multiple MODFLOW packages capable of simulating surface 

water, the Surface-Water Routing (SWR1) Process (Hughes 

and others, 2012) was selected for its simulation capabilities, 

previous success simulating flooded environments (Hughes 

and White, 2016), and available technical support. The SWR1 

Process is well suited to simulate Swamp hydrology because 

of its capability to simulate surface-water features as boundary 

conditions for the groundwater system and to simulate complex surface-water features, including dynamic surface flows, 

flow in both directions within a reach, and detailed control 

structure and weir management options. 

The 2-D and 3-D models were used only for steady-state 

simulations to simplify model development and reduce model 

run times. In steady-state simulations, hydrologic conditions 

remain constant over time and changes in storage are not 

accounted for. A steady-state model does not have the capability to simulate transient events such as individual storms or 

changing hydrologic conditions; a transient model is needed 

for that. Construction of a transient SWR1 model was beyond 

the scope of this project. The steady-state model still has 

significant value because it can simulate average water balances, flow directions, and rates. The steady-state models were 

developed and calibrated so that in future studies they can be 

readily modified to simulate transient conditions. 

Three climatic scenarios were developed to represent 

three different steady-state, hydrologic conditions: (1) average spring conditions, which is the baseline scenario; (2) 

dry spring conditions; and (3) wet spring conditions. Spring 

(April, May, and June) conditions were simulated because 

these months are critical in the Great Dismal Swamp for 

water-management decisions. Low water levels resulting from 

particularly dry periods result in increased fire risk, whereas 

high levels following particularly wet periods negatively 

affect vegetation growth rates and can lead to forest stress and 

mortality. April–June is the most active period of WCS management as Refuge staff adjust weir levels to strike a balance 

between preventing wildfires and avoiding tree stress.

Groundwater flow paths were simulated by using 

MODPATH version 6 software (Pollock, 2012) to illustrate 

the groundwater flow field and determine watershed boundaries within the study area. Using MODPATH, a tracking 

particle was placed in every other active model cell, and the 

path of each particle from its origin to its model exit point 

was simulated. Simulated groundwater particles can leave 

the groundwater system by evapotranspiration, by discharge 

to head dependent boundaries (SWR1 reaches) representing 

surface water, or by discharge to specified-head boundaries 

(general head boundary [GHB] cells) representing tidal boundary waters. 

Block C1 Preliminary Model

As a first step towards building a model of the entire 

study area, a 2-D preliminary flow model of Block C1 (fig. 17) 

was constructed. The Block C1 model aided development of 

the full-scale 3-D model by improving hydrogeologic understanding, testing the use of the SWR1 Process and assignment 

of boundary conditions, and improving assignment of hydraulic parameter values in the 3-D model. The Block C1 area 

was chosen for preliminary modeling because it has relatively 

simple hydrologic boundaries and a relatively high density of 

groundwater-level observations.

Cross-Sectional Block C1 Model

A 2-D cross-sectional model was developed to quantify 

local-scale groundwater discharge patterns along a transect 

from Corapeake Ditch (south) to Sycamore Ditch (north) 

(fig. 17). The axis of the model is aligned north-south, parallel 

to Myrtle and Western Boundary Ditches and perpendicular 

to Corapeake and Sycamore Ditches. Vertically the model is 

divided into three layers: the lowest layer is sand of uniform 

thickness of 21 ft, overlain by a 4-ft thick, low-permeability 

layer, overlain by a 3-ft thick high-permeability peat layer that 

extends to the land surface/water table (fig. 18A). Horizontal 

discretization varies from 20 to 200 ft. 

In the Block C1 model, Corapeake and Sycamore Ditches 

are represented by GHB conditions, with stages set to average observed levels (14.74 ft for Sycamore Ditch and 16.88 ft 

for Corapeake Ditch). Average water levels are the mean of 

periodic observations collected approximately monthly from 

June 2009 to March 2011. During this period, water levels in 

Sycamore Ditch were about 2 ft lower than those in Corapeake 
Numerical Model Development 31

Great Dismal

Swamp National

Wildlife Refuge

North Carolina

Dismal Swamp

State Park

SYCAMORE DITCH

WESTERN BOUNDARY DITCH

CORAPEAKE DITCH

MYRTLE DITCH

CORAPEAKE ROAD

WESTERN BOUNDARY ROAD

SYCAMORE ROAD

MYRTLE ROAD

X

X’

59A 27

59A 28

59A 29

59A 30

59A 31

59A 32

59A 33

59A 34

59A 35

59A 77

59A 76

59A 75

59A 72

59A 73

59A 74

59A 71

59A 60

59A 69

59A 67

59A 66

59A 68

59A 36

59A 37

59A 38

59A 39

59A 40

59A 41

59A 63

59A 64

59A 65

59A 57

59A 58

59A 59

59A 54

59A 55

59A 56

59A 60

59A 61

59A 62

59A 42

59A 43

59A 44

CA-097

CA-098

CA-099

CA-094

CA-095

CA-096

CA-100

CA-101

CA-102

" A@

A@

A@

A@

A@

A@

A@

A@

A@A@A@ A@ A@ A@A@A@

A@

National Wildlife Refuge

North Carolina State Park

Ditch

Road

X-X’ Model line of section

Monitoring well cluster

 (59A 27, 59A 28, 59A 29)

Water Control Structure

Weir

Adjustable weir

Culvert

A@

")

EXPLANATION

VIRGINIA

NORTH

CAROLINA

Suffolk Chesapeake

-76°27'

36°34'

36°33'30"

36°33'

36°

32'

30"

-76°26'30” -76°26' -76°25'30”

0 150 300 METERS

0 0.1 0.2 MILES

VIRGINIA

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

NORTH

CAROLINA

Figure 17. Block C1 with line of section X–X’, Great Dismal Swamp, Virginia and North Carolina. (Cross sections shown in 

figures 18A–C)
32 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

EXPLANATION

Water table

Flow path

Ditch

Grid cell

Unsaturated cell

Well

Bottom

Shallow

Flow direction

17.4

16.5

16.9

17.2

16.7

Simulated Head, 

 in feet

Flow 

divide

South North

X’ X

X’ X

Flow 

divide

South North

Water table

South 5,240 Feet North

Elevation, in feet N

AVD 88

11

-10

15

20

Impermeable bottom assumed

Layer 3: Sand

K = 100 feet/day

Layer 2: Clay and Peat K = 24 feet/day

Layer 1: Peat

K = 13,220 feet/day

Corapeake

Ditch

Sycamore

Ditch

Monitoring

wells

Sycamore

Road

Sycamore Road

K = 23 feet/day

(Specified Head) (Specified Head)

Land surface

Vertical Exaggeration about 50x

A

B

C

Figure 18. Cross-sectional views of the Block C1 model showing A, grid spacing and boundary conditions B, simulated heads and flow 

paths under baseline average conditions, and C, simulated heads and flow paths when water levels in Sycamore Ditch are raised by 2 

feet. (Line of section X–X’ shown in fig. 17. K, hydraulic conductivity)
Numerical Model Development 33

Ditch because surface-water flow between the two ditches was 

limited by WCSs. The bottom of the model was assigned a noflow boundary, and the water-table surface was calculated during model simulations. Recharge of 0.89 foot per year (ft/yr) 

was assigned on the basis of long-term average annual 

precipitation of 4.22 ft/yr recorded at the Lake Drummond 

meteorological station for 1931 through 2015, minus average annual evapotranspiration from MODIS-MOD16 dataset 

(Mu and others, 2011), averaging 3.33 ft/yr over the study area 

for 2003 through 2013.

Hydraulic conductivity is the only assigned hydraulic 

property in the Block C1 model. Four hydraulic conductivity 

values were assigned, 1 for each of the 3 layers and 1 for cells 

representing Sycamore Road (table 6). Horizontal to vertical anisotropy was initially set at 10:1 for all units but was 

allowed to vary during sensitivity analysis.

Sycamore Road, lying between Block C1 and Sycamore 

Ditch, impedes groundwater flow as a result of compaction of 

the upper high-permeability peat layer. In the model, Sycamore Road is represented by lower permeability assigned to 

the upper two layers, which represent peat. Corapeake Road is 

on the far side of Corapeake Ditch from Block C1 and is not 

represented in the model. 

The Block C1 model was manually calibrated to groundwater levels at monitoring wells located near the model 

transect (fig. 17). Target groundwater levels were assigned 

equal to average annual present conditions. Hydraulic conductivity parameter values for Layers 1, 2, and 3 and for the 

road bed were adjusted to improve the fit between simulated 

and observed groundwater-level values. The mean absolute 

error between 18 observed and simulated head values is 0.25 ft 

or about 10 percent of the head difference between the two 

ditches (2.14 ft).

Table 6. Calibrated parameters of the Block C1 cross-sectional 

model.

[Horizontal to vertical anisotropy fixed at 10:1 for all units. Kh, horizontal 

hydraulic conductivity; ft/d, foot per day; in/yr, inch per year]

Parameter Value Calibration method

Peat Kh 13,220 ft/d Least squares minimization

Sand Kh 100 ft/d Least squares minimization

Clay Kh 24 ft/d Least squares minimization

Road Kh 23 ft/d Least squares minimization

Precipitation/recharge rate 44.24 in/yr Assigned

Block C1 Model Sensitivity Analysis

A sensitivity analysis was performed to evaluate the 

effects of the different aquifer parameters and boundary conditions on the simulated groundwater levels. For each sensitivity 

run, a single parameter or boundary condition was manually 

adjusted from the values used in the calibrated baseline model. 

Parameters were then qualitatively assigned either a small, 

moderate, or large effect on the basis of magnitude of changes 

in the simulated heads. In addition, groups of parameters were 

identified that are highly correlated and potentially result in an 

equally matched comparison of simulated and observed targets 

(table 7). Highly correlated parameter pairs with horizontal 

hydraulic conductivity (Kh) are peat Kh/recharge, peat Kh/

road Kh, and road Kh/recharge.

Table 7. Results of parameter sensitivity analysis for the Block 

C1, Great Dismal Swamp, Virginia and North Carolina, crosssectional model.

[Kh, horizontal hydraulic conductivity]

Parameter

Composite 

scaled 

sensitivity

Ratio to 

maximum

Qualitative 

effect

Peat Kh 1.35E−01 3.91E−01 Moderate

Road Kh 2.77E−02 8.00E−02 Small

Clay Kh 1.99E−02 5.75E−02 Small

Sand Kh 1.03E−01 2.96E−01 Moderate

Precipitation/recharge rate 3.46E−01 1.00E+00 Large

Block C1 Model Simulation Results

Results from the Block C1 baseline model show that 

84 percent of groundwater outflow discharges to Sycamore 

Ditch; the remaining 16 percent discharges to Corapeake 

Ditch. Most of the recharge to the water table traveled laterally through the upper peat nearly to the road before turning 

downward across the clay and then into the sand to flow under 

the road and discharge to Sycamore Ditch. Correspondingly, 

the groundwater flow divide is located 84 percent of the 

cross-sectional distance towards Corapeake Ditch (fig. 18B). 

Most of the flow travels to Sycamore Ditch even though the 

low-permeability road and compacted peat beneath the road 

act as a barrier for flow towards Sycamore Ditch. This result 
34 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

demonstrates the importance of ditch water levels in controlling groundwater flow. 

To further assess the effect of ditch stage control on 

groundwater flow patterns, an additional model scenario was 

simulated in which the water-level in Sycamore Ditch was 

raised by 2 ft, from 14.74 to 16.74 ft, whereas the 16.88-ft 

water level in Corapeake Ditch was unchanged. The higher 

assigned ditch stage caused the water table to rise over the 

whole cross section and the groundwater divide to shift to 

the north closer to Sycamore Ditch. Despite the higher stage 

in Corapeake Ditch, 63 percent of groundwater flowed to 

Corapeake Ditch (fig. 18C). This is caused by the low permeability of the road bed and underlying compacted peat between 

Sycamore Ditch and the interior of the block, increasing 

resistance to flow. These results show that ditch levels and the 

presence of intervening roads have a strong effect on groundwater levels and groundwater flow directions, and indicate that 

Refuge staff can affect groundwater levels by managing water 

levels in ditches.

Water budgets for the baseline and raised ditch-stage 

scenarios were scaled to percent of total flow (table 8) to 

provide general insight into the Swamp flow system. The 

water budget results also show that the water level in Sycamore Ditch strongly affects groundwater flow. Raising the 

stage in Sycamore Ditch caused the flow divide to shift north 

(fig. 18B and C) and the percent of total inflow (recharge) discharging to Sycamore Ditch to decrease from 84 to 37 percent. 

Raising the stage in Sycamore also caused the percent of 

recharge penetrating to the bottom sand layer to decrease from 

69 to 30 percent of total recharge. This can be attributed to 

the combined effects of less water flowing to Sycamore Ditch 

because water was forced to flow beneath Sycamore Road 

to discharge to Sycamore Ditch, as seen in the flow paths 

(fig 18B and C). 

Table 8. Simulated water budgets from the Block C1 model for 

the steady-state baseline scenario and for the Sycamore Ditch 

raised water level scenario, Great Dismal Swamp, Virginia and 

North Carolina.

[Values shown are flow as a percent of recharge]

Model boundary conditions

Baseline case Raised stage

Outflow destination Percent of 

total flow

Percent of 

total flow

Corapeake Ditch 16 63

Sycamore Ditch 84 37

Percent flow through 

aquifer materials

Percent of 

total flow

Percent of 

total flow

Peat 100 100

Clay 80 75

Road 23 14

Sand 69 30

Spatial Discretization of the Three-Dimensional 

Model

For the 3-D model (model), the study area was divided 

into a rectilinear grid of 500-ft by 500-ft square cells (fig. 19) 

with axes oriented east-west and north-south. Horizontal 

spacing of 500 ft was chosen as a compromise between a 

cell size small enough to simulate hydrologic conditions at a 

useful scale for Refuge managers and a cell size large enough 

to allow reasonable model run times. The model grid has 

154 columns and 256 rows for a total of 39,424 cells per layer, 

of which 26,391 are active in Layer 1. Groundwater conditions 

and hydrogeologic properties are uniform within each model 

cell, so the model does not simulate differences in groundwater levels or flows over horizontal distances of less than 500 ft.

Land surface, as represented by the lidar elevation dataset 

described previously, is the reference elevation for vertical 

layering in the model. Because lidar elevations are in a 6.6-ft 

raster grid, and the model has a 500-ft grid, land-surface elevations were scaled up for the model, with about 5,800 lidar raster cells in each model grid cell. The land-surface elevation for 

each model cell was assigned a value equal to the mean minus 

one standard deviation of the lidar elevation values falling 

within that model cell. This approach sets land-surface elevations in the model to relatively low values so that hummocks 

and local elevation high points have less effect on land-surface 

elevation in the model, and therefore less effect on simulated 

ET from the water table. In contrast, lower lidar elevations in 

depressions and shallow channels would have a greater effect 

on model land-surface elevations and simulated ET from the 

water table. Under the assumption that lidar elevations are normally distributed, this assigns the local 16 percent land-surface 

elevation value to each model cell.

Vertically the model is divided into four layers 

(fig. 20 and table 9) that allow hydrologic conditions and 

hydraulic properties to vary with depth. Layer 1 holds water 

that inundates the land surface. Layer 2 represents the upper, 

more permeable and porous peat, whereas Layer 3 represents 

the underlying less permeable peat. Layer 4 represents the 

sand of the Yorktown-Eastover aquifer underlying the peat.

Layer 1 contains water above the land surface under 

inundated conditions. The choice was made to use a high-K 

Layer 1 to allow water on the land surface to move laterally with little resistance, rather than routing the water flow 

through SWR1 reaches, to improve model stability and reduce 

model run times. Hydraulic conductivity of Layer 1 is set to 

a high value to establish rapid lateral flow, as described in the 

“Calibration Approach” section. The top of Layer 1 for each 

cell is assigned an elevation equal to the land-surface elevation 

plus 20 ft; the bottom elevation represents the land surface and 

is based on the lidar DEM as described above.

Layer 2 has variable thickness and, because it represents 

the coarse upper peat, is assigned a relatively high hydraulic conductivity value. The elevation of the top of Layer 2 

for each cell is the land-surface elevation. The thickness of 

Layer 2 is assigned cell by cell, with values between 1.0 and 
Numerical Model Development 35

Lake

Drummond

Pasquotank River

Dismal Swamp Canal

Shingle

Creek

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

Inflow Rt. 158 Goose Creek and

Acorn Hill

Folly

Swamp

Daniels 

Road

Swamp

Moss 

Swamp

Cypress

Swamp

Pocosin

Swamp

58

32

58

13

17

64

32

32N

158E

17

13

Deep

Creek

158

EXPLANATION

Active model cell

Inactive model cell

GHB flow boundary

Groundwater inflow

Basin discharging to the swamp

Active model extent

Major road

HFB boundary

SWR (ditch, stream or river)

Figure 19. Hydrologic model grid and boundary conditions for the Great Dismal Swamp, Virginia and North Carolina. (GHB, general 

head boundary; SWR, surface-water feature simulated with the MODFLOW-SWR1 Process; HFB, Hydraulic Flow Boundary)
36 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Layer 2: Upper peat

Layer 1: Inundation water storage Land surface Water table SWR 1-dimensional reach

representing a ditch, 

canal, or stream

Water 

in the 

ditch

Layer 4: Sand

Layer 3: Lower peat and clay

Impermeable bottom

500 ft 500 ft 500 ft 500 ft 500 ft 500 ft 500 ft 500 ft 500 ft 500 ft

Figure 20. Representative cross-sectional view of a ditch and water-table elevations in the 3-dimensional hydrologic simulation model 

of the Great Dismal Swamp, Virginia and North Carolina. (ft, feet; SWR, surface-water reach simulated with the MODFLOW-SWR1 

Process)

Table 9. Thickness and hydraulic properties of Layers 2 and 3 of the three-dimensional hydrologic simulation model of the Great 

Dismal Swamp, Virginia and North Carolina.

[ft, feet; <, less than; >, greater than]

Model layer Total peat thickness in swamp (ft)

Thickness of layers in model (ft)

Number Description 0–0.5 >0.5–1.0 >1.0–1.5 >1.5–2.0 >2.0–2.5 >2.5

2 Upper peat 1 1 Total peat thickness 1.5 1.5 1.5

3 Lower peat 1 1 1 1 1 Layer 2 Bottom elevation - Total Peat thickness + 1.5

Property assignment

Upper peat

Lower peat

Sand

Thickness rules

Minimum thickness for all layers is 1 ft

Maximum thickness for Layer 2 is 1.5 ft

Where peat is < 0.5 ft, properties of the 

underlying sand are assigned
Numerical Model Development 37

1.5 ft (table 9) that are based on land-surface elevations and 

peat thicknesses described in the “Peat Thickness” section 

(fig. 16). Peat thickness for each model cell was assigned as 

the mean of all peat thickness elevations from the lidar DEM 

that fall within the model cell. In parts of the study area where 

total peat thickness is less than 1.0 ft, Layer 2 is assigned a 

minimum thickness of 1 ft to prevent numerical instabilities 

in the model (table 9). In areas where total peat thickness is 

between 1.0 and 1.5 ft, Layer 2 is assigned a thickness equal 

to the total peat thickness. In areas where total peat thickness is greater than 1.5 feet, Layer 2 is assigned a thickness 

of 1.5 feet. The bottom elevation of Layer 2 is determined by 

subtracting the assigned Layer 2 thickness from the landsurface elevation.

Layer 3 has variable thickness and represents the lower 

peat, which has relatively lower porosity and permeability 

than the upper peat (table 9). The top elevation of Layer 3 

is equal to the bottom elevation of Layer 2. In areas of the 

Swamp where total peat thickness is less than 2.5 feet, Layer 3 

is assigned a minimum thickness of 1 ft. Where total peat 

thickness is greater than 2.5 feet, Layer 2 is assigned a thickness equal to total peat thickness minus 1.5 ft. The bottom 

elevation of Layer 3 is determined by subtracting the assigned 

Layer 3 thickness from the Layer 2 bottom elevation.

Layer 4 has variable thickness and represents the relatively permeable sand layer underlying the peat. The top 

elevation of Layer 4 is equal to the bottom elevation of 

Layer 3. The bottom elevation of Layer 4 is constrained to 

have a maximum elevation value of 0 ft to prevent problems 

with layer discontinuity. Layer 4 is assigned a thickness of 

21 ft everywhere, except along the western side of the model 

where the combination of higher surface elevations and the 

Layer 4 maximum bottom elevation of 0 ft causes the thickness of some Layer 4 cells to be greater than 21 ft. The bottom 

elevation for each Layer 4 cell is determined by subtracting 

the thickness from the elevation of the top of Layer 4.

Time Periods Represented by Simulations

The model is steady-state, so simulated conditions do not 

change over time. Three periods were selected to represent 

baseline, wet, and dry climatic conditions. The baseline simulation represents average climatic and hydrologic conditions 

during the spring growing months of April, May, and June 

for 2005 through 2015. Observations collected during this 

period are used to determine average conditions, which were 

used to assign boundary conditions (table 10) and calibrate 

the baseline model. The period representing dry conditions is 

spring (April, May, and June) 2011, which had a total rainfall 

of 7.06 in. The representative wet period is spring 2015, which 

had a total rainfall of 17.81 in. Because the model is steadystate rather than transient, the simulations do not represent 

hydrologic conditions in the spring of 2011 and 2015, but 

rather are a generic representation of dry and wet conditions.

Table 10. Climatic conditions and representative time periods for 

model scenarios for the three-dimensional hydrologic simulation 

model of the Great Dismal Swamp, Virginia and North Carolina.

Condition Range Representative period

Start End

Dry Average conditions 

Spring 2011 April 1, 2011 June 30, 2011

Wet Average conditions 

Spring 2015 April 1, 2015 June 30, 2015

Baseline

11-year average, 

April, May, and 

June only

April 1, 2005 June 30, 2015

Boundary Conditions

Head or flow conditions are assigned to all external 

model boundaries (top, bottom, and sides). Assignments of 

boundary conditions in the baseline model include precipitation/recharge rates, evapotranspiration rates, lateral inflows, 

and specified head boundaries, which are based on average 

observed values during the spring of 2005 through 2015 

(tables 10 and 11). For simulations of wet and dry climatic 

conditions, different boundary conditions are assigned to 

describe the representative periods (tables 2 and 11). The bottom of the model is assigned as a no-flow boundary.

Table 11. Assigned boundary conditions for representative 

climatic conditions in the three-dimensional model for Great 

Dismal Swamp, Virginia and North Carolina.

[ET, evapotranspiration]

Assigned boundary 

condition Unit

Climatic conditions

Average Wet Dry

Surface inflows, total inches/year 2.42 4.97 0

Groundwater inflow, total inches/year 1.48 4.89 1.48

Recharge inches/year 51.61 71.44 28.32

Maximum ET rate inches/year 44.24 47.13 45.08
38 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Recharge is assigned equal to observed precipitation 

rates under the three climatic scenarios simulated (table 11). 

Recharge is not readjusted during calibration of the baseline 

model. Steady-state recharge rates for the three scenarios 

are assigned equal to observed precipitation rates during the 

spring months of 2011, 2015, and 2005–15: 7.06, 17.81, and 

12.86 in. (0.00647, 0.01631, and 0.01178 ft/d) for the dry, wet, 

and average conditions, respectively (table 11). Unlike other 

groundwater modeling studies in which ET is subtracted from 

precipitation prior to assignment of recharge (for example, 

Masterson and others, 2016b), 100 percent of precipitation is 

assigned as recharge to the water table. Water is then removed 

from the water table by evapotranspiration as described in the 

“Evapotranspiration” and “Surface Water” sections. Simulated ET rates are determined by the model during simulation on the basis of assigned calibrated parameter values. 

Recharge to Lake Drummond is assigned as an inflow in the 

SWR1 Process.

Lateral Inflows 

Lateral inflows to the model for each scenario simulated 

(table 2) are assigned along the western and southern boundaries to represent groundwater and surface-water flows into 

the Swamp (fig. 1). Inflows from Daniels Road Swamp and 

Pocosin Swamp discharge primarily to Cross Canal Ditch and 

Washington Ditch (fig. 19), respectively, so those two inflows 

are assigned as specified flow to the most upstream SWR1 

reaches representing those ditches. The other five inflows 

from Cypress Swamp, Folly Swamp, Goose Creek/Acorn 

Hill, South of Route 158, and Moss Swamp disperse diffusely 

into the Swamp, so these inflows are assigned as groundwater 

inflows to Layer 4 in the model (fig. 19) using the MODFLOW well package (WEL) (Harbaugh and others, 2000). 

These inflow rates (table 2) were estimated as described above 

in the “Surface-Water Inflows and Outflows” section.

Specified Head Boundaries

Specified heads are assigned to model boundaries formed 

by Deep Creek, Shingle Creek, and the Pasquotank River 

(fig. 19). Model cells are assigned a specified head of 2 ft 

along Deep Creek and Shingle Creek and a specified head of 

1 ft along the Pasquotank River.

Evapotranspiration 

ET occurs over the entire active model area and is 

simulated with the MODFLOW EVT package (Harbaugh and 

others, 2000). Simulated evapotranspiration for a given cell 

depends on three parameters and on the simulated water-table 

elevation. The three parameters specified to control ET rates 

are (1) ET surface elevation, (2) maximum ET rate, and (3) 

extinction depth. All three parameters are assigned initial 

values that are based on likely assumptions, then adjusted 

during model calibration. The ET surface elevation for each 

cell is assigned an initial value equal to the land-surface 

elevation minus 4.76 ft. The maximum ET rate is assigned an 

initial value of 44.2 in/yr, as calculated with the Thornthwaite 

equation and described previously in the “Precipitation and 

Evapotranspiration” section. The extinction depth is the depth 

below the ET surface where the ET rate is zero and is assigned 

an initial value of 3 ft (McDonald and Harbaugh, 1988). The 

simulated water-table elevation is calculated by the model.

Given the assigned parameter values, ET is calculated 

by the model according to equations 4a–c. Paraphrasing from 

McDonald and Harbaugh (1988), the groundwater ET from 

a given model cell equals the maximum ET rate (ETmax) 

when the water-table elevation (Hwt) is at or above the ET 

surface (Hes). When the depth of the water table below Hes

(D) is greater than the extinction depth (Dex), then ET equals 

zero. When Hwt is below Hes but not down as far as Dex, ET

decreases linearly with depth between zero and ETmax (eq. 4)

 ET = ET max Hwt > Hes (4a)

 ET = 0 Hwt < Hes - Dex (4b)

 ET = ETmax

Dex - D / Dex

x

( )

Hes - Dex <=

Hwt <= Hes

(4c)

The initial assigned values of Hes and Dex were assigned 

on the basis of results obtained by Shah and others (2007) and 

by numerically simulating the ET process with the one-dimensional (1-D) Richards’ equation for different soil and land 

cover types. Results show that for forested land cover, the full 

PET was removed from the soil until groundwater levels fell 

below a range of depths from 1.28 to 6.10 ft beneath the land 

surface, depending on soil type. On the basis of these results, 

a Hes value of 4.76 ft for a sandy clay soil type was chosen to 

represent the soil of the Great Dismal Swamp for all modeling 

scenarios. 

Shah and others (2007) fit an exponential decay function 

to observation data and calculate the ratio of ET to ETmax for 

groundwater levels below Hes. In MODFLOW EVT a linear 

function is used to scale the ET rate, so parameter values are 

chosen that would roughly match the results of Shah and 

others (2007). Extinction depth is assigned to result in similar 

numerically integrated values of the ratio of ET to PET for 

the range of depths (fig. 21). For a forested land cover type 

over sandy clay soils, this extinction depth value was calculated to be 8.86 ft from the surface elevation. The extinction 

depth used within the EVT package is entered as the depth 

below the ET surface, which is 4.76 ft below land surface, and 

corresponds to a value of 4.1 ft, which is used for all model 

scenarios. 

Because recharge values in the model are assigned equal 

to 100 percent of precipitation, simulated ET from the water 

table is intended to include all types of ET, including ET of 
Numerical Model Development 39

Function

EXPLANATION

Linear

Exponential

0 1 2 3 4 5 6 7 8 9

Ratio of ET to PET

1.25

1.00

0.75

0.50

0.25

0.00

10 11 12 13 14 15 16

Depth below land surface

Figure 21. The ratio of actual evapotranspiration (ET) to potential evapotranspiration (PET) for a forested land type over sandy clay 

soils using exponential and linear functions.

water intercepted by vegetation or stored on the land surface. 

This approach to simulation of recharge and ET was chosen 

because, in many parts of the swamp, the water table is above 

land surface and because actual ET rates vary spatially and are 

poorly constrained by observation values. Simulated ET can 

occur anywhere in the study area that the simulated water table 

is within 8.86 ft of the land surface. Of the 131 wells with 

groundwater-level observations, only 2 wells have any observations showing depth to water table of more than 8.86 ft, 

and both of these wells are at locations with relatively high 

elevations. One well is on the western scarp, and the other is at 

a spoils pile next to the Great Dismal Swamp Canal. In these 

areas, assigned model recharge rates equal to 100 percent of 

precipitation will be greater than actual net recharge rates (Precipitation-Evapotranspiration-Runoff), which are discussed 

further in the “Limitations” section.

Roads

The Hydraulic Flow Boundary (HFB) package (Harbaugh 

and others, 2000; Hsieh and Freckleton, 1993) of MODFLOW 

is used to represent the effects of roads on groundwater flow in 

the Swamp. The HFB package reduces conductance between 

pairs of model cells to simulate barriers to flow. Because 

roads in the Swamp are known to compact underlying peat, 

HFB barriers are placed in the model coincident with roads to 

represent peat compaction and reduce horizontal conductance 

between cells in Layers 2 and 3. On the basis of geospatial 

data processing that identified 5,499 cell pairs containing 

roads, HFB boundaries are assigned to reduce conductance 

between the cell pairs. The HFB package requires assignment 

of a hydraulic characteristic of the flow barrier, which in this 

model is equal to K of material beneath the road divided by 

the width of the road. Initial values for the hydraulic characteristic were set to 1.53, calculated as the estimated K of 

the roadbed material from the cross-sectional model (23 ft/d) 

divided by a representative road width of 15 ft. The hydraulic 

characteristic value was then modified during calibration of 

the model. 

Surface Water 

Surface water in ditches, streams, canals, and Lake 

Drummond is simulated using the SWR1 process of MODFLOW (Hughes and others, 2012). SWR1 features are simulated as boundary conditions that interact with aquifer cells; 

however, SWR1 features also interact with each other, with 

WCS, and with external boundaries (fig. 20). Spatial discretization and time steps for the SWR1 component of the model 

are specified separately from the groundwater component of 

the model. Canals, ditches, and streams are represented by 

1-D “reaches” with assigned cross-sectional geometry and 

hydraulic characteristics; Lake Drummond is represented with 

a 2-D reach. To reduce model run times, SWR1 was not used 

to simulate overland surface runoff from the interiors of the 

blocks to the ditches. Instead this overland flow was simulated 

as groundwater flow through Layer 1, as described previously, 

using a high hydraulic conductivity value to permit water to 

move laterally with little resistance. 

Because ditches are dug down through the peat layer and 

into the underlying sand (fig. 20), SWR1 reaches representing ditches and canals in the model are assigned to penetrate 
40 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

the entire thickness of model Layers 2 and 3 and a portion of 

model Layer 4. SWR1 reaches that represent ditches exchange 

water with all layers and all adjacent SWR1 reaches. To 

reduce model run times, each reach cell is assigned to a levelpool reach group, which constrains water levels to be the same 

for every reach in the group. This assignment is considered to 

be realistic because most head loss occurs at WCSs rather than 

along ditches, as discussed previously in the “Water Control 

Structures” section (fig. 14). All reaches are assigned to have 

rectangular cross-sections. Ditch bottom widths are assigned 

on the basis of data provided by the Refuge, with most ditches 

having a bottom width of 25 ft and larger ditches having 

greater widths (for example 50 ft for the Feeder Ditch and 

100 ft for the Dismal Swamp Canal; Lake Drummond SWR1 

reaches have assigned widths of 500 ft, equal to cell widths). 

Weirs and Culverts

All weirs, culverts, and other WCSs in the Swamp that 

are known to affect flow (fig. 12) are explicitly simulated in 

the model as SWR1 structures. Weir invert elevations must be 

assigned in the SWR1 Process, so for each model scenario, 

elevations were assigned that are based on survey data provided by Refuge staff (table 1). Any culvert or weir classified 

as “failed” in the geospatial data provided by the Refuge was 

not included in the model. 

Table 12. Parameters used in model calibration and composite normalized parameter sensitivities.

[ET, evapotranspiration; Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; HFB, Hydrologic Flow Boundary; ft/d, foot per day; ft, foot; 

s/m1/3, second per meter1/3]

Parameter Description Parameter 

group

Units Lower 

bounds

Upper 

bounds

Calibrated 

value Sensitivity

etratemax Maximum ET rate etrate ft/d 0.005 0.012 0.007 35,354,150

etextdepth ET extinction depth (below ET surface) et ft 1 5 3.19 5,511.95

etsurfdepth ET surface depth et ft 1 6 3.50 5,109

hk2 Kh of Layer 2 aquiferk ft/d 1,000 1,000,000 13,224 0.64

hk3 Kh of Layer 3 aquiferk ft/d 10 1,000 24 99.81

hk4 Kh of Layer 4 aquiferk ft/d 10 1,000 100 114.99

vk2 Kv anisotropy of Layer 2 aquiferk unitless 1 10 1 5,113

vk3 Kv anisotropy of Layer 3 aquiferk unitless 0.10 1 0.224 585

vk4 Kv anisotropy of Layer 4 aquiferk unitless 0.10 1 0.504 2,954

leaktypical Leakance of ditches swrleak 1/day 1 10 5.05 3,292

leakcanal Leakance of Dismal Swamp Canal swrleak 1/day 1 10 4.94 2,712

leakpasquo Leakance of Pasquotank River swrleak 1/day 1 10 4.97 1,129.53

manngtypical Manning’s n for a typical ditch swrmnng s/m1/3 0.02 0.05 0.05 249,830

manngcanal Manning’s n for the Dismal Swamp Canal swrmnng s/m1/3 0.02 0.05 0.05 248,994

manngpasquo Manning’s n for the Pasquotank River swrmnng s/m1/3 0.02 0.05 0.05 72,163

hfbcond Hydraulic characteristic of HFB boundaries hfb 1/d 0.10 10 1.58 2,465

inqmult Multiplier for lateral surface inflow inqgroup unitless 0.50 2 1.11 16,765

Fixed-crest weirs specified in the SWR1 Process represent WCSs at Deep Creek, South Mills, and the spillway from 

Lake Drummond to the Feeder Ditch. Weir widths are set 

equal to 48 ft for Deep Creek and South Mills and to 30 ft for 

the Lake Drummond Spillway, following data provided by the 

USACE (J. Scussel, oral commun., 2015). Weir invert elevations were assigned separately for both Deep Creek and South 

Mills structures and were adjusted during model calibration to 

match simulated upstream stages with mean observed values. 

Mean upstream stage observations for average January–April 

are 8.50 and 8.49 ft (NAVD 88) for Deep Creek and South 

Mills, respectively. An invert elevation of 7.88 ft resulted in 

simulated upstream stages of 8.50 and 8.50 for Deep Creek 

and South Mills, respectively, and was used for all simulations. The invert elevation specified for the Lake Drummond 

Spillway weir was adjusted so that simulated lake stage 

matched the mean observed lake stage of 15.95 ft (NAVD 88). 

An invert elevation of 15.11 ft resulted in a lake stage of 

15.95 ft and was used for all subsequent simulations.

Weir elevations at WCSs change over time as Refuge 

staff add or remove boards. Weir elevations can also change 

for unintended reasons such as debris accumulation and 

failure of WCS components including boards, which occasionally cause flow to bypass a WCS. Weir elevations used 

in the model are equivalent to board elevations, which were 

observed regularly by Refuge staff at most active WCSs from 

2010 to 2015 but were mostly unobserved from 2005 to 2010. 
Numerical Model Development 41

Board and invert elevation values for each WCS, provided by 

Refuge staff, were then averaged to match the period of each 

model scenario (table 10). 

Calibration Approach 

The model was calibrated so that it would accurately 

simulate water levels and flows. To calibrate the model, 

parameter values were adjusted as described below until simulated water level and flow values adequately matched observed 

values. Initial calibration was performed manually to produce 

a model that was stable and gave a first approximation of 

hydrologic conditions. Final calibration was automated, using 

parameter estimation (PEST) software (Doherty, 2010) to fine 

tune the model, improve accuracy by optimizing parameters, 

and determine sensitivity of model results to various parameters. Parameter sensitivity was evaluated simultaneously with 

optimization of parameter values.

Model Parameterization 

Model parameters were assigned on the basis of best 

available field data, prior studies, and results from the crosssectional 2-D model. Some model parameters were then 

adjusted during calibration (table 12). Boundary conditions 

that are well constrained by observational data, such as 

precipitation rates, were assigned fixed values and were not 

calibrated. Other boundary conditions that are less well constrained by observational data, such as lateral surface-water 

inflows and ET, were adjusted during calibration. Hydraulic 

conductivity and conductance of the ditch bottoms are not well 

known, so values were adjusted during calibration (table 12), 

except for horizontal hydraulic conductivity of Layer 1, which 

was fixed at 1,000,000 ft/d. Hydraulic parameters for the 

aquifer are assigned by layer and are not subdivided laterally 

into smaller zones because available data do not indicate clear 

zonation across the study area. A closer model fit between 

observed and simulated water levels and flows could be 

obtained; however, locally adjusting parameters within smaller 

zones to match observations without a supporting conceptual 

basis does not improve the understanding of hydrologic processes and may not result in a model that accurately represents 

the hydrologic system.

Observations and Weighting 

Values used to calibrate the model are observations of 

groundwater levels in wells; surface-water levels in ditches, 

the Dismal Swamp Canal, and Lake Drummond; observations 

of surface-water flows at WCSs and at a few open channel 

flow sites, such as in the Pasquotank River; and remote-sensing-derived observations of land-surface inundation (table 13). 

Observations were filtered so that only those made during the 

months of April, May, and June from 2005 through 2015 were 

Table 13. Observation group weight multiplier and contributions 

to the total objective function (Φ) at the end of calibration of the 

three-dimensional model.

[Percentages do not sum to 100 as a result of rounding. %, percent; Φ, objective function]

Type of 

observation

Group 

weight 

multiplier

Number of 

observations 

in group

Φ contribution in 

calibrated model

Inundation 200 2 21%

Surface-water 

outflow

1.3 5 15%

Surface-water 

internal flow

1 11 28%

Groundwater 

level

400,000 42 10%

Surface-water 

level upstream from 

structure

500,000 36 24%

Surface-water 

level downstream from 

structure

200,000 19 3%

used for calibration of the baseline model. All groundwater 

and surface-water monitoring sites have multiple observations 

made over time. To calibrate the steady-state model, mean 

values for each site were calculated and used as the observation target values.

An additional date filter was applied to groundwaterlevel- and surface-water-level observations made in the Blocks 

area, roughly that area bordered on the west by Western 

Boundary Ditch, on the east by South Martha Washington 

Ditch, on the north by Persimmon Ditch, and on the south by 

Kim Saunders Ditch (fig. 10). In the Blocks area, groundwater 

levels were affected by a number of WCSs installed between 

2011 and 2013 that caused ditch water levels to rise. Because 

of this disturbance, observations made in the Blocks area were 

used for model calibration only if they were made in spring 

2014 or 2015. 

Each observation target was assigned a weight, and 

weighted observations were used in calculating the objective 

function value (Φ) that is minimized during automated calibration. Weights serve several purposes during calibration: 

• emphasize observation types that are most important to 

match,

• scale observations of different magnitudes, for example, scaling values of flow (ft3

/d) relative to values of 

water level (ft) to account for magnitude differences, 

and 

• emphasize observations with lower observation error or 

uncertainty. 
42 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Group weights were assigned (table 13) to emphasize 

observation types containing the most important information 

for steering the model towards correct simulation of water 

flows, and groundwater and surface-water level patterns, at 

the Refuge scale. Inundation observations, were assigned 

a relatively high group weight because they were the only 

hydrologic observations with complete spatial coverage of the 

entire study area. Surface-water flow observations received 

a moderate relative weight because, although matching total 

water balances was important, there were no observations 

for flow out of Portsmouth Ditch, and flows for Pasquotank 

River were uncertain owing to the tidal nature of the station. 

In addition, the best known outflows, at Deep Creek and South 

Mills locks, were extremely sensitive to the invert elevation of the SWR1 weirs representing the locks. For example, 

lowering the weir elevation at Deep Creek by 0.074 ft, less 

than 1in., caused Deep Creek outflow to increase by 5 percent 

(from 111 to 116 ft3/s). Therefore, the parameters adjusted 

during the calibration process were relatively less important to 

outflows at Deep Creek and South Mills than some assigned 

parameters. Internal flow observations were given high relative weights because they had wide spatial coverage and were 

important for accurately simulating the catchment areas for 

individual WCSs. Groundwater-level observations were given 

relatively low weight because they do not have wide spatial 

coverage. Surface-water-level observations collected upstream 

from WCSs were given relatively high weights because they 

had good spatial coverage, accurate simulation of water levels 

in ditches is important, and ditch levels are expected to have 

strong control over groundwater levels. Surface-water-level 

observations made downstream from WCSs were given relatively low weight because these levels are often affected by 

debris and by turbulence when water free falls over the WCS.

To de-emphasize observations with high error, the 

weight of an observation is typically determined by its variance, which includes observation error. All observations 

are assumed to represent steady-state conditions, which is a 

simplifying assumption for numerical representation because 

average observation values represent different sample sizes 

of observations collected at different times. With an accurate 

numerical representation of evenly distributed observations, 

the assigned weights would be based on observation variance 

and observation error (Hill and Tiedeman, 2007). However, 

other factors, such as representing time-varying water levels 

at a well or WCS with a single average value and representing land elevations within a 250,000-square foot (ft2

) model 

cell with a single elevation, likely introduce more error into 

an observation than observation error itself. Therefore, the 

weights assigned to each observation group may be somewhat 

subjective and can be used to reflect existing knowledge of the 

hydrogeologic system. However, parameter values are generally not very sensitive to moderate changes in the weights used 

(Hill, 1998), and observation may be weighted as a group on 

the basis of relative magnitudes. 

Groundwater-Level Observations

Average spring groundwater levels for 42 wells in the 

Swamp (fig. 22), described previously in the “Groundwater 

Levels and Flow Patterns” section, were used for calibration. 

Criteria for inclusion of a well in the groundwater observation 

calibration dataset were (1) land-surface elevation accuracy 

of 0.5 ft or less, (2) sample size of two or more, and (3) well 

depth reported.

Inundation Observations

Inundation observations, described previously in the 

“Inundation” section, were included as targets during model 

calibration. For comparison to the remotely sensed inundation, 

a model cell is considered inundated if the simulated water 

table was above land surface. Simulated inundation for each 

active cell in Layer 1, not including Lake Drummond cells, 

was therefore assigned a value of 1 (inundated) if the simulated water table was in Layer 1; otherwise it was assigned a 

value of 0 (not inundated). 

Whereas simulated inundation has binary values 0 or 1, 

observed inundation values for each model cell, the spatially 

averaged DSWE and SAR/InSAR inundation measures, had 

values ranging continuously from 0 to 1, with most values 

close to 0. For use in calibration, a sum of square error values 

was calculated for the entire active model area from observed 

and simulated inundation values and separately for DSWE and 

SAR inundation observations (eq. 5–6).

DSWE Inundation Error Sum of Squares

 = Simulated DSWE Inundationi

 Observed Inundation

Number of 

 



 



  i i

2

1

active

Layer 1 Cells



(5)

SAR Inundation Error Sum of Squares

 = Simulated SAR Inundationi

 Observed Inundation

Number of ac

 



 



  i i

2

1

tive

Layer 1 Cells



(6)

Surface-Water Level and Flow Observations

Surface-water-level observations made at 37 sites 

(WCSs) were used to calibrate the model, and at 18 of these 

sites, observations were made just upstream and just downstream from the WCS. Therefore, a total of 55 mean surfacewater-level values was used to calibrate the model. Because 

more observations give averaged values more certainty, 

sites with less than 5 surface-water-level observations were 

excluded as calibration targets; 55 stations met this requirement (fig. 23 and table 14). 

Flow-monitoring stations were required to have at least 

three flow observations. Sixteen stations met this criteria, and 
Numerical Model Development 43

North Carolina

Dismal Swamp

State Park

2011 Burn

Area

Great 

Dismal 

Swamp

National 

Wildlife 

Refuge Dismal Swamp Canal

Pasquotank River

LAKE

DRUMMOND

Shingle

Creek

Deep

Creek

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°40'

36°30'

64

32

58

17

158E

158

32

13

58

17

32N

EXPLANATION

!

Groundwater-level

 simulation error, 

 in feet

-1.57 to -1.00

-0.99 to 0.00

0.01 to 1.00

1.01 to 2.00

2.01 to 3.00

3.01 to 3.29

Basin discharging 

 to the swamp

Active model extent

Major road

Ditch

Not used to

 calibrate model

0 2.5 5 KILOMETERS

0 2.5 5 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

!!!

!

!

! !! ! ! !!

!

!

!

!

!! !!

!

!

!

!

!

!

! !

!

!

!

!!

!!

!

!!! !

!

!

!

! ! !!!

!!!

Figure 22. Groundwater-level observation sites in the Great Dismal Swamp, Virginia and North Carolina, and simulation errors.
44 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

North Carolina

Dismal Swamp

State Park

2011 Burn

Area

Great 

Dismal 

Swamp

National 

Wildlife 

Refuge Dismal Swamp Canal

Pasquotank River

LAKE

DRUMMOND

Shingle

Creek

Deep

Creek

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°40'

36°30'

64

32

58

17

158E

158

32

13

58

17

32N

EXPLANATION

Surface-water-level

 simulation error, in 

 feet

-1.09 to -1.00

-0.99 to 0.00

0.01 to 1.00

1.01 to 2.00

2.01 to 3.00

3.01 to 4.00

4.01 to 4.91

Basin discharging 

 to the swamp

Active model extent

Major road

Ditch

Water control structure

U Upstream

D Downstream

26 Map 

number

0 2.5 5 KILOMETERS

0 2.5 5 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

4

6 102

100

101

103

9

11U 11D

17

18U 18D

24

25U

23

26

37U

37D

41U

28 29U

35U 35D

39

42U

41D

44U

44D

45U

45D 48U 47U

42D

46U

48D 47D

49

50U

50D

57

59U

59D

58U

58D

46D

60

61U

61D 62U

62D

63U

64U

64D

67U

67D

71U

1

70

Figure 23. Surface-water-level observation sites at water control structures and simulation errors, Great Dismal Swamp, Virginia and 

North Carolina.
Numerical Model Development 45

Table 14. Average observed surface-water levels during 2005–15 and surface-water levels during 2015 wet conditions and 2011 dry conditions at 55 observation sites in the Great Dismal Swamp, Virginia and North Carolina, used to calibrate the three-dimensional model.[NA, not available]

Water level (feet)

Climate scenario—spring growing conditions

Water control structure Dry observed Wet observed Average

Name

Water control 

structure 

number1

April–June, 2011 Number of observations April–June, 2015 Number of observations April–June, 2005–15 Number of observations

Camp/East Upstream 18 NA 0 19.61 4 19.69 15Camp/East Downstream 18 NA 0 19.42 4 19.45 15County Line/Weyerhauser Upstream 59 16.71 6 16.07 9 16.37 96County Line/Weyerhauser Downstream 59 14.04 6 14.41 9 14.85 35Portsmouth/Rosemary Upstream 11 14.58 5 14.50 9 14.31 45Portsmouth/Rosemary DownStream 11 NA 0 13.69 7 13.87 11Sherrill North Upstream 50 17.82 6 17.79 11 18.09 58Sherrill South 60 17.34 4 NA 0 18.08 21Cross Canal/Sherrill Upstream 61 17.93 5 18.33 4 18.15 70Cross Canal/Sherrill Downstream 61 18.22 2 17.47 3 17.88 24Cross Canal/Weyerhauser Upstream 58 16.71 6 16.09 7 16.71 50Cross Canal/Weyerhauser Downstream 58 14.52 3 14.94 6 15.58 33County Line/Insurance Upstream 64 12.58 4 13.38 5 13.34 24County Line/Insurance Downstream 64 10.74 2 11.69 3 11.39 13Head of River 1 Upstream 63 12.26 3 12.99 2 12.82 14Weyerhauser 158 culverts Upstream 71 14.04 3 NA 0 15.55 12Bull Boulevard/Insurance Upstream 62 12.62 3 13.66 2 13.41 15Bull Boulevard/Insurance Downstream 62 12.07 1 NA 0 12.27 9Weyerhauser Upstream 67 16.85 3 16.44 4 16.40 53Weyerhauser South Downstream 67 14.02 2 15.38 3 15.70 16Forestline South 57 17.47 8 17.06 11 17.07 100Forestline North 49 17.59 6 17.44 15 17.50 68Corapeake/West Boundary Upstream 48 16.77 2 17.08 12 17.41 39Sycamore/West Upstream 44 NA 0 16.79 9 17.15 34Sycamore/West Downstream 44 NA 0 16.57 6 16.69 23Sycamore/Laurel Upstream 42 NA 0 15.71 16 16.03 45Sycamore/Laurel Downstream 42 NA 0 15.45 9 15.38 33Corapeake/Laurel Upstream 46 16.49 5 15.66 15 16.24 80Corapeake/Laurel Downstream 46 11.04 2 14.81 8 14.33 20
46 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C. Table 14. Average observed surface-water levels during 2005–15 and surface-water levels during 2015 wet conditions and 2011 dry conditions at 55 observation sites in the Great Dismal Swamp, Virginia and North Carolina, used to calibrate the three-dimensional model.—Continued [NA, not available] Water level (feet) Climate scenario—spring growing conditions Water control structure Dry observed Wet observed Average Name Water control structure number1 April–June, 2011 Number of observations April–June, 2015 Number of observations April–June, 2005–15 Number of observations Weir 1 Upstream 47 NA 0 14.00 10 14.56 17 Weir 1 Downstream 47 NA 0 11.52 3 11.54 8 Sycamore/South Martha Washington Upstream 41 13.86 2 15.30 11 15.09 32 Sycamore/South Martha Washington Downstream 41 10.16 2 14.91 6 13.95 18 S. Martha/Persimmon Upstream 37 NA 0 15.10 5 15.29 18 Myrtle/Persimmon 43 13.68 2 13.92 11 14.34 38 Myrtle South Upstream 35 NA 0 13.80 8 14.49 25 Myrtle South Downstream 35 NA 0 13.34 8 13.59 21 Myrtle North Upstream 29 13.89 3 13.29 4 13.81 47 Lynn/Middle 23 21.20 3 20.69 2 21.53 21 Washington/Lynn Upstream 25 21.21 3 20.62 4 21.50 79 Lynn South 24 20.35 2 NA 0 20.98 17 Washington/Jericho 26 17.04 2 17.62 1 17.39 36 Jericho North 4 19.36 3 19.14 4 18.95 22 Jericho/Hudnell 103 19.76 1 20.72 3 21.03 12 Lynn/Jericho 9 20.27 2 20.78 3 21.11 48 Hudnell/East 17 NA 0 19.75 3 19.82 13 Williamson/East - Williamson2 100 19.57 1 NA 0 20.70 10 Williamson/East - East2 101 18.57 1 19.06 3 19.11 13 Williamson/New 102 NA 0 20.66 2 20.97 10 Portsmouth/Big Entry 6 10.50 4 11.55 9 11.19 31 South/Riddick Upstream 45 17.43 3 17.72 2 17.65 13 Lake Drummond Spillway 28 12.59 91 12.65 91 12.55 987 South Mills Lock 70 8.47 91 8.56 91 8.44 987 Deep Creek Lock 1 8.41 91 8.58 91 8.45 987 Pasquotank River (0204382800)3 NA 0.69 86 0.79 91 0.70 987

1Water control structure number shown in Figure 12.2Surface-water-level observation.3Shown in figure 1.
Numerical Model Development 47

mean values from these stations were used to calibrate the 

model. Flow observations made during the simulation periods were averaged to provide mean flow target values for the 

baseline simulation period.

Calibration and Error-Based Calibration Criteria 

The model was calibrated with the automated parameter 

estimation software (PEST) (Doherty, 2010). Calibration 

targets included groundwater-level data from 42 monitoring wells (fig. 22), surface-water-level data from 55 sites 

(fig. 23), surface-water-flow data from 16 WCSs, and surface 

inundation data from remote sensing (fig. 15). The automated 

calibration was run in estimation mode, adjusting parameters 

to minimize the objective function Φ (eq. 7).

Φ = Σ(wiri)2 (7)

where

Φ is the objective function to be minimized, 

ri is residual for observation i (simulated value 

– observed value), and

wi is weight for observation i.

Calibration Results

Simulated surface-water-level and groundwater-level 

values match well with observed values at most monitoring 

sites (table 15 and figs. 22, 23, and 24), indicating that the 

model provides a reasonable steady-state representation of 

swamp hydrology. In general, simulated values that are within 

2 ft of average observed spring values are considered reasonable. Simulated water-level values that exceed observations by 

3 ft are less desirable and are indicative of uncertainties in the 

model conditions at those locations. Simulated groundwater 

Table 15. Simulation errors for different observation groups.

[SW, surface water; GW, groundwater]

Observation 

group

Number of 

observations Mean error

Mean 

absolute 

error

Mean 

percent 

error

Inundation 2 −13,630.32 13,630.30 −100.00

Major SW 

outflows

5 −117,477.60 8,182,940.00 −1.42

Internal SW 

flows

11 216,821.18 1,665,354.50 14.97

GW levels 42 0.18 17.10 1.04

SW levels up 36 −0.66 16.40 −3.99

SW levels 

down

19 −1.04 14.40 −7.21

EXPLANATION

Groundwater level

Surface-water level

1:1 line

Simulated water level, in feet

Observed water level, in feet

25

20

15

10

10 15 20 25

Figure 24. Simulated and observed groundwater and surfacewater levels and the ideal 1:1 line.

levels have a mean error (simulated minus observed value) of 

+0.18 ft, and simulated surface-water levels have mean errors 

of -0.66 ft and -1.04 ft, upstream and downstream from a WCS 

respectively.

Simulated flows and levels at the three structures managed by the USACE (Deep Creek Locks, South Mills Locks, 

and Lake Drummond Spillway) match well with observed 

mean values for average April–June conditions. The simulated upstream stage and flow at Deep Creek are 8.49 ft and 

111.3 ft3/s, respectively, and mean observed values are 8.50 ft 

and 113.6 ft3/s, respectively; the difference in flow is less than 

2 percent. The simulated upstream stage and flow at South 

Mills are 8.49 ft and 111.7 ft3/s, and mean observed values 

are 8.49 ft and 131.2 ft3/s. The simulated upstream lake level 

at the spillway from Lake Drummond to the Feeder Ditch is 

15.95 ft., the same as the mean observed lake level of 15.95 ft. 

Simulated flow at the spillway from Lake Drummond to the 

Feeder Ditch is 110.5 ft3/s, about 2 percent less than observed 

flow 112.7 ft3/s. As described previously, the flows at the 

structures managed by the USACE are very sensitive to SWR1 

weir elevation settings, which in the model were manually set 

to achieve the close match between observed and simulated 

values.

As judged by the high group error value (table 15), and 

a fraction of simulated inundated cells (58.1%) that is greater 

than observed inundated cells (4.7% for SAR/InSAR and 

2.5% for DSWE), simulated inundation does not appear to 

be a good fit with observed inundation (fig. 15). However, 

visual comparisons indicate that simulated inundation exhibits spatial patterns similar to the DSWE and SAR observed 

inundations (fig. 15). Simulated and observed images all show 

areas of inundation to the southwest of Lake Drummond, 
48 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

which corresponds to observed ponding in the 2011 Burn 

Area. Simulated wet areas generally correspond well to wet 

areas reported by Refuge staff and include the burned area 

south of Lake Drummond, the Pasquotank River flood plain, 

the foot of the western scarp, and the northwestern Swamp. 

The large mean inundation error is caused partly by the model 

cell size of 500 ft, which is larger than the observed inundation raster sizes of 98 ft (30 m) for DSWE and 75 ft (23 m) 

for SAR/InSAR. To have an error of zero for a single cell 

would require that the approximately 45 SAR/InSAR raster 

values or 26 DSWE raster values within each model cell be 

0 or 1, matching the simulated 0 or 1 inundation value. This is 

unlikely to happen in areas subject to inundation, except during very wet or dry periods. 

Water-budget mass balance errors were -0.4 percent for 

the groundwater budget and 0.0 percent for the surface-water 

budget. The relatively low errors indicate that the model has 

adequate numerical stability.

Sensitivity

Sensitivity describes how much simulated water levels and flows change in response to a change in parameter 

value. If simulated flows and water levels change a lot when 

a parameter X is changed, then sensitivity to parameter X is 

high. A sensitivity analysis of the 17 calibrated parameters in 

the model (table 12) was performed by using PEST during 

model calibration. Composite normalized parameter sensitivities, which describe the information content of observations relative to a specific model parameter (Doherty, 2010), 

were calculated (eq. 8). Comparison of composite parameter 

sensitivities shows the relative contribution of each parameter or parameter group to the objective function during the 

automated parameter estimation process. Different observation 

data, model structure, or parameterization results in different sensitivities. So the best use of the composite normalized 

parameter sensitivities is to gauge relative sensitivities of 

parameters within the same model. The composite sensitivity 

of parameter, i, is calculated as (eq. 8)

S

J w J

m

i

T

ii

2

1

2   (8)

where

J is the Jacobian (sensitivity) matrix,

T is the matrix transpose operation,

w is the diagonal matrix of observation weights, 

and

m is the number of observations with nonzero 

weights.

The most sensitive parameter in the model, by far, is 

etratemax, the maximum evapotranspiration rate, used by 

MODFLOW to calculate the simulated ET rate for each cell. 

The sensitivity of etratemax is an order of magnitude greater 

than all other parameters because it is the primary control on 

vertical outflow rates from the aquifer. The next most sensitive 

parameters are the Manning’s roughness coefficients, which 

control the simulated loss of head for flow in the ditches. 

All three of the Manning’s roughness coefficient parameters 

in the model (manngtypical, manngcanal, manngpasquo) 

have calibrated values of 0.05, which is the maximum value 

(PARUBND) assigned during the PEST calibration. The value 

of 0.05 is at the high range of acceptable values and corresponds to natural channels in poor condition. Most ditches in 

the study are straight with dense vegetation along the edges 

and frequent downed trees and brush in the channels. A higher 

maximum limit of 0.1 was tried in an additional PEST run, 

which pushed manngtypical and manngpasquo from 0.05 

to 0.1 and yielded a marginally better calibration fit. But it 

was decided that a value of 0.1 was unreasonably high for 

the ditches in the Swamp, based on reported values for the 

Manning’s roughness coefficient (Chow, 1959). Therefore the 

Manning’s coefficient values in the model should be considered as assigned to upper limits, given understanding of the 

system, and based on the automated calibration results, rather 

than assigned as part of the optimization regression.

Simulated Hydrology and Water 

Management

Results from the model simulations provide new understanding of swamp hydrology and options for water management. The calibrated baseline model, which simulates average 

spring conditions, provides results that improve our understanding of water balances, levels, and flow directions. Additional model runs, or scenarios (table 16, scenario 1), were 

developed to simulate hypothetical wet and dry conditions 

and different WCS management strategies. These alternative 

climatic conditions and water-management strategies were 

simulated by changing boundary conditions of the calibrated 

model. Climatic conditions were assigned (tables 2 and 10) on 

the basis of averaged observations of precipitation and temperature during spring 2011, when dry conditions prevailed, 

and spring 2015, when wet conditions prevailed. Water-management actions were simulated by adjusting SWR1 Process 

weir levels in the model input files. 

Additional WCSs proposed for construction by the Refuge were simulated in Scenario 4 (table 16). Scenarios 5 and 

6 simulate flooding of the Swamp by raising elevations of all 

existing active and proposed WCSs to the elevation of nearby 

land-surface elevations. Scenarios 7 and 8 simulate draining 

of the Swamp by removing all existing active and proposed 

WCS.
Simulated Hydrology and Water Management 49

Table 16. Description of hydrologic simulation scenarios for the 

three-dimensional model of the Great Dismal Swamp, Virginia and 

North Carolina.

[WCS, water control structure]

Scenario Climatic 

conditions

Weir 

level 

settings

Water control 

structures 

included

1 Baseline Average Low Those active 

in 2015

2 Wet Wet Low Those active 

in 2015

3 Dry Dry High Those active 

in 2015

4 Future WCS 

added

Average Low Proposed 

+ active 2015

5 Flood the 

swamp wet

Wet Very high Proposed 

+ active 2015

6 Flood the 

swamp dry

Dry Very high Proposed 

+ active 2015

7 Drain the 

swamp wet

Wet Very low Proposed 

+ active 2015

8 Drain the 

swamp dry

Dry Very low Proposed 

+ active 2015

Average Spring Hydrologic Conditions

The baseline scenario (Scenario 1) simulates average 

spring hydrologic conditions from 2005 through 2015. A water 

budget was calculated for the simulated combined (groundwater and surface water) hydrologic system (table 17). Precipitation is the primary source of water to the study area, representing about 900 ft3

/s of the approximately 974 ft3/s inflow or 

92 percent, whereas surface-water and groundwater inflows 

from the south and west make up the remainder. Surface-water 

inflows account for 48 ft3/s or 5 percent of simulated inflows, 

whereas groundwater inflows account for 26 ft3/s (3 percent).

Total simulated outflow from the study area is 976.7 ft3/s, 

consisting of ET equal to 541.7 ft3/s (55%), surface outflow of 395.5 ft3

/s (41%), and groundwater discharge of 

39.5 ft3/s (4%) (table 17). These outflow values are exclusive of exchanges within the Swamp between the aquifer 

and ditches. Simulated spring ET has a rate of 30.4 in/yr, 

obtained by dividing the volumetric ET rate of 541.7 ft3/s by 

the active model area. This calibrated ET rate is less than the 

estimated annual average rate of 36 in/yr reported by Heath 

(1975) for the Albemarle-Pamlico region and less than the 

MODIS average ET rate for spring months during 2005–14 of 

50.7 in/yr, which includes the southern part of the study area, 

but the calibrated ET rate is greater than the average value of 

27.6 in/yr for the study area estimated by Sanford and Selnick 

(2013). ET rates during the calibration period April through 

June would be expected to increase from relatively low rates 

in early April when most vegetation has little leaf surface area 

to maximum rates in May and June when plant growth reaches 

maximum rates for the year. Simulated ET rates agree with 

those presented by Heath (1975) and Sanford and Selnick 

(2013) for the study area. Some confidence in the simulated 

value comes from noting that the value is derived from the 

water balance enforced by the model. The MODIS estimated 

ET rate is likely too high, especially considering that it is 

equal, apparently by coincidence, to observed mean precipitation at the one meteorological station (USC00448837) in the 

study area. 

As expected for a shallow unconfined aquifer system in 

a saturated landscape, simulated groundwater levels (fig. 25) 

show spatial patterns similar to those seen in land-surface 

elevations (fig. 2). The highest groundwater levels are found 

along the western scarp boundary, whereas the lowest are in 

the northwest, northeast, and southeast corners where tidal 

creeks are represented by specified head boundaries. 

Groundwater levels are generally higher away from 

ditches and lower close to ditches, indicating that groundwater 

generally discharges to ditches. This is further confirmed by 

groundwater budget results for groundwater/surface-water 

exchange (table 17) showing that the groundwater discharge 

rate to the ditches (455.3 ft3/s) is greater than the rate of 

recharge to the aquifer from ditches (106.1 ft3/s). Simulated 

vertical head gradients have small magnitudes across the study 

area, generally from -0.0001 to +0.00001 (fig. 26). Positive 

Table 17. Simulated water budget for the calibrated baseline 

three-dimensional hydrologic model, Scenario 1, of the Great 

Dismal Swamp, Virginia and North Carolina.

[ET, evapotranspiration; Head dep bounds, head dependent boundaries; SWR, 

surface-water routing; --, no data]

Hydrologic component Description

In Out

(cubic feet 

per second)

Wells Groundwater inflow 

from scarp

26.4 --

Lateral flow Surface-water inflow 

from scarp

47.6 --

Recharge Recharging precipitation 899.9 --

ET Evapotranspiration -- 541.7

Head dep bounds Groundwater exchange 

with tidal rivers

-- 39.5

Boundary flow Surface-water outflow to 

tidal rivers

-- 395.5

TOTAL 973.8 976.7

SWR Leakage1 Groundwater/surfacewater exchange106.1 455.3

1

Internal exchanges between surface water and groundwater are not 

included as part of the overall water balance of the hydrologic system. “In” 

is surface inflow to the groundwater system, while “Out” is groundwater 

discharge to the surface-water system.
50 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

0 2.5 5 KILOMETERS

0 2.5 5 MILES

Base from U.S. Geological Survey 

National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36°45'57.6" N and 

37°57'57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State 

Plane Virginia South

Simulated groundwater 

 level, in feet

1.000 to 3.000

3.001 to 5.000

5.001 to 7.000

7.001 to 9.000

9.001 to 11.000

11.001 to 13.000

13.001 to 15.000

15.001 to 16.000

16.001 to 17.000

17.001 to 18.000

18.001 to 19.000

19.001 to 20.000

20.001 to 21.000

21.001 to 22.000

22.001 to 26.000

> 26.000

Dry cell

Inactive cell

EXPLANATION

Active model extent

Major road

Lake Drummond 

 boundary

58

17

158E

158

17 32N

-76°30' -76°20'

36°40'

36°30'

VIRGINIA

NORTH

CAROLINA

LAKE

DRUMMOND

58

17

158E

158

17 32N

-76°30' -76°20'

36°40'

36°30'

VIRGINIA

NORTH

CAROLINA

LAKE

DRUMMOND

58

17

158E

158

17 32N

-76°30' -76°20'

36°40'

36°30'

VIRGINIA

NORTH

CAROLINA

LAKE

DRUMMOND

58

17

158E

158

17 32N

-76°30' -76°20'

36°40'

36°30'

VIRGINIA

NORTH

CAROLINA

A B

C D

LAKE

DRUMMOND

Figure 25. Simulated groundwater levels in the Great Dismal Swamp, Virginia and North Carolina, and dry cells in A, Layer 1, B, Layer 

2, C, Layer 3, and D, Layer 4.
Simulated Hydrology and Water Management 51

£

£

¬

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

58

32

58

13

17

64

32

32N

158E

17

13

LAKE 

DRUMMAND

158

EXPLANATION

Vertical gradient 

 (dimensionless)

> 0.000010

0.000010

-0.000010

< -0.000010

0.000000

Dry cell

Lake cell

Model extent

Active model 

 extent

Major road

Figure 26. Simulated vertical head gradients between the water table and Layer 4 in the three-dimensional model of Great Dismal 

Swamp, Virginia and North Carolina. Positive values indicate upward groundwater flow; negative values indicate downward flow.
52 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

vertical gradients indicate upward groundwater flow, and 

negative values indicate downward flow. The small magnitude 

of the vertical head gradients is a result of low topographic 

gradients (figs. 2 and 4) and an absence of groundwater withdrawals or other aquifer stresses across most of the study area. 

Vertical gradients are upward at ditches, with the exception 

of a few locations, again indicating groundwater discharge to 

most ditches. The few locations where surface water recharges 

the aquifer are shown as red model cells containing a ditch or 

SWR1 reach (fig. 26). These locations include South Ditch, 

the eastern portion of Lateral Ditch, Sherrill Ditch, Cross 

Canal Ditch just west of Weyerhauser Ditch, and East Ditch 

near Juniper Ditch. 

On the basis of simulated groundwater and surface-water 

levels and of simulated groundwater flow paths, subbasins 

draining to points of major discharge are delineated for baseline average spring conditions (Scenario 1; fig. 27). Under 

different climatic conditions or weir level settings, some of 

these drainage subbasin boundaries can be expected to change 

(not shown). Flow leaving Lake Drummond moves east to the 

intersection of the Feeder Ditch and the Dismal Swamp Canal, 

and at that point flow divides and moves north to Deep Creek 

and south to South Mills. Although not simulated in this study 

because it was outside the study’s scope, changing lock weir 

levels at Deep Creek and South Mills can be expected to affect 

the part of Lake Drummond outflow going to each end of the 

Dismal Swamp Canal.

Simulated groundwater flow paths (fig. 28) indicate 

groundwater generally discharges to the nearest ditch, which is 

in agreement with the expectation of little underflow beneath 

ditches. In just a few locations, such as at West Ditch at the 

foot of the scarp (fig. 29), simulated groundwater passes 

under a ditch without discharging upward, then continues on 

to discharge at a more distant ditch, but this is an exception 

to the general pattern. The pattern of most simulated groundwater discharging to the nearest ditch is likely affected by the 

assigned thickness of the model layers. Ditches penetrate the 

entire thickness of model Layers 2 and 3 and a part of model 

Layer 4. Adding a model layer below Layer 4 or assigning 

a greater thickness to model Layer 4 would likely result in 

more simulated groundwater passing under ditches without 

discharging.

Simulated inundation (fig. 25A) indicates areas of the 

Swamp that are relatively wetter or drier on the land surface. 

Because Layer 1 holds water stored on the land surface, any 

Layer 1 cell that is not dry indicates land-surface inundation; 

that is, the water table is above land surface. Simulated wet 

areas generally correspond to wet areas reported by Refuge 

staff, including the Burn Area south of Lake Drummond, the 

foot of the western scarp, and the northwestern Swamp. In 

the hummocky terrain of the Swamp, complete inundation of 

the land surface is rare, except in Burn Area where there has 

been substantial peat loss. Typically, inundation in the Swamp 

manifests as standing water in low elevation hollows, whereas 

slightly higher elevation hummocks remain above water. 

Because the modeled land-surface elevation represents the low 

end of the lidar derived elevations in a model cell, inundation estimates likely overpredict the extent of standing water 

during average spring conditions. If the model is converted to 

be transient in the future, it could be used to simulate seasonal 

inundation conditions. 

Depth of the water table below land surface is an important factor for plant growth and ground fire risk and is simulated by the model (fig. 30). Under average spring conditions 

(model Scenario 1), simulated depths to the water table range 

from more than 10 ft along the western scarp to less than zero 

in inundated areas. Simulated depths to groundwater help to 

illustrate where existing groundwater conditions match those 

described in the literature for undrained peatlands. The early 

spring (April) is typically when seasonal water levels in the 

Swamp (Day and others, 1988) and nearby forested peatlands 

in North Carolina are highest (Heath, 1975; Richardson, 

1991). In undrained pocosin peatlands similar to those in the 

Swamp, spring water-level highs are near 0 ft below land surface, and late summer and fall lows approach 2 ft below land 

surface (Richardson, 2003; Wang and others, 2015). Areas of 

the Swamp that are inundated or have depths to groundwater near 0 ft (fig. 30) likely approximate where the effects of 

drainage on spring water levels are less pronounced. Depth to 

groundwater approaching 2 ft during spring is indicative of 

drained conditions and corresponds to portions of the Refuge 

with a high density of ditches (fig. 30). Measures of vegetation response to water-table depth typically take into account 

the time-varying nature of water-table depth; for example, see 

USACE Environmental Laboratory (1987). Future work could 

include simulation of time varying water levels and percent 

time inundated if the model were updated to be transient.

Wet and Dry Simulation Results

Hydrologic conditions under hypothetical wet and dry 

climatic conditions are simulated by making changes to 

boundary conditions: recharge, ET, lateral inflows, and weir 

settings (tables 1, 2, and 10). For simulated wet conditions 

(Scenario 2), weirs were left at the same elevations as the 

average conditions in baseline Scenario 1, whereas for dry 

conditions (Scenario 3), weir elevations were raised to reflect 

the typical Refuge management actions of adding stop logs 

(boards) to weirs to raise water levels and prevent the Swamp 

from drying out. 

As expected, groundwater levels are higher under wet 

conditions (Scenario 2, fig. 31B) than during average climatic 

conditions (Scenario 1, fig. 31A), with greater groundwaterlevel increases adjacent to ditches, to the north and west of 

Lake Drummond, and along the Pasquotank River (fig. 32A). 

Under wet conditions 7,654 model cells, or 43,928 acres, that 

are unsaturated under average conditions contain water owing 

to higher simulated water-table elevations. Cells that are least 

partially saturated under average conditions have an average 

water-table elevation increase of 0.48 ft under wet conditions. 

Simulated flows at major WCSs increase by an average factor 

of 2.0, which can be seen in the comparison of simulated flows 
Simulated Hydrology and Water Management 53

Dismal Swamp Canal

Pasquotank

River

Feeder Ditch

Deep

Creek

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

South

Mills

58

32

58

13

17

64

32

32N

158E

17

13

LAKE 

DRUMMAND

158

SWR 

 (ditch, stream,

 river or Lake

 Drummond)

Subbasin drainage

 area boundary

Active model extent

Major road

Stream or river

General flow

 direciton 

EXPLANATION

Figure 27. Subbasins and generalized flow directions that are based on simulated baseline hydrologic conditions, model Scenario 1, 

in the three-dimensional model of Great Dismal Swamp, Virginia and North Carolina. (SWR, surface-water reach simulated with the 

MODFLOW- SWR1 Process)
54 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

Dismal Swamp Canal

Pasquotank

River

Feeder Ditch

Deep

Creek

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

South

Mills

58

32

58

13

17

64

32

32N

158E

17

13

LAKE 

DRUMMAND

158

SWR 

 (ditch, stream,

 river or Lake

 Drummond)

Subbasin drainage

 area boundary

Active model extent

Major road

Stream or river

Simulated flow path

HFB boundary

EXPLANATION

Figure 28. Simulated flow paths under baseline conditions, model Scenario 1, in the three-dimensional model of Great Dismal Swamp, 

Virginia and North Carolina. (SWR, surface-water reach simulated with the MODFLOW- SWR1 Process; HFB, Hydraulic Flow Boundary)
Simulated Hydrology and Water Management 55

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

West Ditch

Interior Ditch

Lateral Ditch

South Ditch

Corapeake Ditch

SWR 

 (ditch, stream,

 river or Lake

 Drummond)

Subbasin drainage

 area boundary

Active model extent

Simulated flow path

HFB boundary

EXPLANATION

36°

34'

36°

32’

-76°34' -76°32' -76°30'

VIRGINIA

NORTH 

CAROLINA

0 0.2 0.4 MILES

0 0.2 0.4 KILOMETERS

VIRGINIA

-76°30' -76°20'

36°

40'

36°

30'

Chesapeake

Suffolk

NORTH

CAROLINA

Figure 29. Simulated flow paths at the foot of the scarp near the intersection of West Ditch and South Ditch, Great Dismal Swamp, 

Virginia and North Carolina, under baseline conditions, model Scenario 1, of the three-dimensional model. (SWR, surface-water reach 

simulated with the MODFLOW-SWR1 Process; HFB, Hydraulic Flow Boundary)
56 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

58

32

58

13

17

64

32

32N

158E

17

13

LAKE 

DRUMMAND

158

EXPLANATION

Simulated depth to

 water, in feet below

 land surface 

< 0.0

0.0

2.0

> 2.0

Inactive cell

Lake cell

Model extent

Active model 

 extent

Major road

Figure 30. Simulated depth to water table under baseline conditions, model Scenario 1, of the three-dimensional model of Great 

Dismal Swamp, Virginia and North Carolina.
Simulated Hydrology and Water Management 57 0 2.5 5 KILOMETERS 0 2.5 5 MILES EXPLANATION Simulated groundwater level, in feet 17.001 to 18.000 18.001 to 19.000 19.001 to 20.000 20.001 to 21.000 21.001 to 22.000 22.001 to 26.000 > 26.0 Dry cell Inactive cell 1.000 to 3.000 3.001 to 5.000 5.001 to 7.000 7.001 to 9.000 9.001 to 11.000 11.001 to 13.000 13.001 to 15.000 15.001 to 16.000 16.001 to 17.000 Active model extent Major road Lake Drummond boundary

£

£

£

£

£

£

¬

£

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'36°30'

64

32

58

17

158 E

158

32

13

58

17

32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-secondLambert Conformal Conic projectionStandard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W North American Datum of 1983 State Plane Virginia South

City of

Suffolk City of

Chesapeake

A

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°40'36°30'

64

32

58

17

158E

158

32

13

58

17

32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-secondLambert Conformal Conic projectionStandard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

B

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°40'36°30'

64

32

58

17

158E

158

32

13

58

17 32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-secondLambert Conformal Conic projectionStandard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W North American Datum of 1983 State Plane Virginia South

City of

Suffolk City of Chesapeake

C

Figure 31. Simulated groundwater levels in model Layer 4 of the three-dimensional model of Great Dismal Swamp, Virginia and North Carolina, under A, baseline (model Scenario 1), B, wet (model Scenario 2), and C, dry (model Scenario 3) climatic conditions.
58 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

0 2.5 5 KILOMETERS

0 2.5 5 MILES

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EXPLANATION

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Change in simulated 

 groundwater level in Layer 4, 

 in feet, under wet conditions 

 (scenario 2), relative to 

 baseline conditions 

 (scenario 1)

Active model extent

Major road

Ditch

Lake Drummond boundary

Proposed WCS

Existing WCS

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

1.01 to 2.00

2.01 to 3.00

>= 3.01

EXPLANATION

!(

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Change in simulated 

 groundwater level in Layer 4, 

 in feet, under dry conditions 

 (scenario 3), relative to 

 baseline conditions 

 (scenario 1)

Active model extent

Major road

Ditch

Lake Drummond boundary

Proposed WCS

Existing WCS

<= -3.00

-2.99 to -2.00

-1.99 to -1.50

-1.49 to -1.00

-0.99 to -0.50

-0.49 to -0.20

-0.19 to -0.10

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'

36°

30'

64

32

58

17

158E

158

32

13

58

17

32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

B

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'

36°

30'

64

32

58

17

158 E

158

32

13

58

17 32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk City of

Chesapeake

A

Figure 32. Change in simulated groundwater levels relative to baseline conditions for A, wet conditions (model Scenario 2 levels minus 

Scenario 1 levels) and B, dry conditions (model Scenario 3 levels minus Scenario 1 levels) from the three-dimensional model of Great 

Dismal Swamp, Virginia and North Carolina. (WCS, water control structure)
Simulated Hydrology and Water Management 59

for Scenarios 2 and 1 (table 18). The WCS at the intersection 

of Bull Boulevard Ditch and Insurance Ditch has a simulated 

flow increase from 0 to 6.9 cubic feet per second.

Simulated groundwater levels are lower under dry 

climatic conditions (Scenario 3, fig. 31C) than under average 

conditions (Scenario 1, fig. 31A), with the greatest groundwater-level decreases seen adjacent to ditches, to the north and 

west of Lake Drummond, and along the Pasquotank River 

(fig. 31B). Simulated groundwater levels are lower under dry 

conditions, despite higher weir levels being assigned at many 

WCSs to simulate the Refuge practice of adding stop logs to 

raise water levels during dry conditions. Under dry climatic 

conditions, 9,799 model cells, or 56,238 acres, which are at 

least partially saturated under average conditions, become 

unsaturated as a result of falling groundwater levels. Model 

cells that remain saturated have a simulated mean groundwater 

level decrease of 0.44 ft. In a comparison of Scenarios 3 and 1, 

simulated flow decreased by an average of 56 percent at major 

WCSs (table 18). 

Water Management Simulation Results

The Refuge has expanded its ability to manage water 

levels in recent years (2010–16) by installing additional WCSs 

with adjustable weirs. An important goal for the hydrologic 

simulation model is to serve as a tool to improve the Refuge 

staff’s understanding of how different weir level settings in the 

Refuge affect water levels and flows and to help guide Refuge 

management decisions about installation of additional WCSs. 

To understand how changing weir levels and adding WCSs 

could affect Swamp hydrology, five model scenarios were 

Table 18. Simulated flow rates at major water control structures in Great Dismal Swamp, Virginia and North Carolina, using a threedimensional model.

[All flows are measured in cubic feet per second. WCS, water control structure; Blvd, boulevard; --, no data]

Description of Flow

WCS 

Number1

Flow rates at WCS under different scenarios

1 

Baseline

2 

Wet

3 

Dry

4 

Future 

WCS 

added

5 

Flood 

the 

swamp, 

wet

6 

Flood the 

swamp, 

dry

7 

Drain 

the 

swamp, 

wet

8 

Drain 

the 

swamp, 

dry

Canal discharge at South Mills Lock 70 111.7 216.0 50.4 89.1 190.9 40.8 213.0 53.5 

Canal discharge at Deep Creek Lock 1 111.3 209.0 53.3 93.7 214.5 46.2 213.1 56.9 

Portsmouth Ditch discharge to Big Entry Ditch 6 24.2 49.5 11.2 34.4 60.4 17.6 25.0 6.2 

Jericho Ditch discharge to Shingle Creek 4 20.4 38.6 8.6 24.5 51.0 10.1 48.3 7.6 

Big Entry Ditch discharge towards Canal 8 1.0 1.9 0.5 1.2 4.8 0.7 1.9 0.5 

Weir 1 discharge to Martha Washington Ditch 47 39.9 61.7 12.4 24.2 -- -- 88.0 24.3 

Lake Drummond discharge to Feeder Canal 28 110.5 216.3 60.4 86.3 168.1 45.2 201.0 61.4 

Bull Blvd Ditch discharge to Insurance Ditch 62 -- 6.9 -- -- -- -- 2.9 0.0 

County Line Ditch discharge towards the Pasquotank River 65 29.3 82.0 7.8 46.8 14.1 -- 132.9 23.5 

Newland Dike Ditch discharge to 158 Ditch 72 1.4 2.3 1.0 1.4 2.2 1.0 2.3 1.0 

1

WCS number shown in figure 12.

simulated (Scenarios 4–8) with various WCS weir settings 

under wet and dry climatic conditions (table 16). 

Simulation of Proposed Water Control Structures 

New WCSs proposed for construction in the Swamp were 

added to the model in Scenario 4 (table 1, figs. 12 and 33). 

Eight new WCSs were added, and eight existing WCS were 

modified in the model with new weir elevations corresponding 

to proposed changes. The locations and weir levels for the new 

and modified WCSs were provided for the study by Refuge 

staff. Higher weir levels and additional WCSs were added 

in Scenario 4; otherwise, the model was unchanged from the 

average baseline conditions (Scenario 1). 

The changes to WCSs cause simulated water levels and 

flows to change relative to the baseline conditions (Scenario 

4 – Scenario 1) (fig. 33). Surface-water levels increase in most 

ditches in the north with the exception of East Ditch, and 

adjacent groundwater levels also increase (fig. 33). Raising 

groundwater water levels in the northeastern part of the study 

area is a management goal for the Refuge, and simulation 

results indicate the proposed WCSs will achieve that goal. 

The simulated groundwater budget also changes relative to the 

baseline with more discharge to ditches (+0.6 percent), more 

recharge to groundwater from ditches (+7.2 percent), more 

water lost to ET (+0.2 percent), and less groundwater discharge to tidal stream boundaries (-0.6 percent). 

Simulated surface-water flows under the proposed new 

WCSs (Scenario 4) and assigned higher weir levels both 

increase and decrease relative to the baseline (Scenario 1). 

More surface-water flow exits the Swamp from Portsmouth 
60 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

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Washington

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Weyerhaeuser

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Middle

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Northeast

Interior

Lateral

Jericho

Lynn

Feeder

County Line

Bull Boulevard

Insurance

LAKE

DRUMMOND

36°40'

36°30'

-76°30' -76°20'

0 1 2 MILES Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

VIRGINIA

NORTH

CAROLINA

City of

Suffolk

City of

Chesapeake

0 1 2 KILOMETERS 

58

32

58

13

17

64

32

32N

158E

17

13

158

Change in simulated

 groundwater level in Layer 4,

 in feet, caused by adding 

 proposed WCS (scenario 4), 

 relative to baseline 

 conditions (scenario 1)

-2.99 to -2.00

-1.99 to -1.50

-1.49 to -1.00

-0.99 to -0.50

-0.49 to -0.20

-0.19 to -0.10

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

1.01 to 2.00

2.01 to 3.00

>= 3.01

!(

!

Model extent

Active model 

 extent

Major road

Ditch

Lake Drummond 

 boundary

Proposed WCS

Existing WCS

EXPLANATION

Figure 33. Changes in simulated groundwater levels caused by the addition of proposed water control structures and raising of weir 

levels at existing water control structures (model Scenario 4 levels minus Scenario 1 levels) from the three-dimensional model of Great 

Dismal Swamp, Virginia and North Carolina. (WCS, water control structure)
Simulated Hydrology and Water Management 61

Ditch (+49.5 percent) and Jericho Ditch (+44.1 percent), 

whereas less flow exits the Dismal Swamp Canal at South 

Mills (-12.6 percent) and Deep Creek (-10.4 percent) 

(table 18).

Flood the Swamp Wet/Dry

In model Scenarios 5 and 6, weir levels are raised to their 

highest possible elevations at all WCSs, including proposed 

structures. Such a hypothetical management action would be 

carried out in practice by adding stop logs and closing screw 

gates, generally raising ditch water levels as high as possible. These management actions are not expected to occur 

in practice, and the model scenario was simulated to evaluate 

the maximum limit of WCS effects on hydrologic conditions 

in the Swamp. Raising weirs to their highest possible levels 

is simulated under wet conditions (Scenario 5) and under dry 

conditions (Scenario 6).

Under wet conditions, extreme maximum weir levels 

(Scenario 5) increase simulated spring surface-water levels and groundwater levels relative to standard weir levels 

(Scenario 2). Groundwater levels in active cells rise by an 

average of 0.28 ft, and the land surface is inundated in an 

additional 3,103 cells, equivalent to 17,809 acres (fig. 34A). 

Under dry conditions, extreme maximum weir levels (Scenario 6) increase simulated spring surface-water levels and 

groundwater levels relative to those for standard weir levels 

(Scenario 3). Groundwater levels in active cells rise by an 

average of 0.19 ft, and the land surface is inundated in an additional 5,526 cells, equivalent to 31,715 acres (fig. 34B). Under 

wet and dry conditions (Scenarios 5 and 6), flows mostly 

increase slightly relative to those for standard weir levels 

(Scenarios 2 and 3) at WCSs north of Lake Drummond and 

decrease slightly at WCSs from Lake Drummond to the south 

(table 18). This is likely caused by increased weir levels on 

Portsmouth, East, and Jericho Ditches (WCS 18, 19, 20, and 

21 in fig. 12) that retain more water in the north and reduce 

flows south toward Lake Drummond. 

Drain the Swamp Wet/Dry

Weir levels are lowered to their lowest possible elevations at all WCSs, including proposed structures, in model 

Scenarios 7 and 8. Such a hypothetical management action 

would be carried out in practice by removing all stop logs and 

opening all screw gates, generally lowering ditch water levels 

as much as possible. These management actions are unlikely 

to occur in practice, and the model scenario was included primarily to evaluate what might occur if the WCSs are managed 

to actively drain the Swamp during either wet or dry climatic 

conditions. Lowering weirs to their lowest possible levels 

under wet conditions is simulated in Scenario 7 and under dry 

conditions in Scenario 8.

Under wet conditions, extreme minimum weir levels 

(Scenario 7) decrease simulated spring surface-water levels 

and groundwater levels relative to those for standard weir 

levels (Scenario 2). Groundwater levels in active cells fall by 

an average of 0.17 ft, and water levels drop below land surface 

(surface no longer inundated) in 2,177 cells, or 12,494 acres, 

compared to standard weir levels (fig. 35A). Under dry conditions, extreme minimum weir levels (Scenario 8) decrease 

simulated surface-water levels and groundwater levels relative 

to those for standard weir levels (Scenario 3). Groundwater 

levels in active cells fall by an average of 0.22 ft, and water 

levels drop below land surface (surface no longer inundated) 

in 1,072 cells, or 6,152 acres (fig. 35B). 

Imposing extreme low weir levels (Scenarios 7 and 8) 

increases and decreases flows (table 18) at major WCSs 

relative to flows for standard weir levels (Scenarios 2 and 3). 

The largest flow changes under wet conditions are decreases 

in Portsmouth Ditch discharge to Big Entry Ditch (WCS 6), 

decreases in Lake Drummond spillway discharge to the Feeder 

Ditch (WCS 28), and increases in Weir 1 and County Line 

Ditch discharges to the Pasquotank River (WCSs 47 and 65). 

These changes are caused by the interplay of various factors, 

including altered ET fluxes caused by changes in water-table 

depths, reduced base levels at the downstream ends of ditches, 

and changes in directions and rates of flow in ditches and in 

the aquifer. The distance to which groundwater levels change 

in response to changes in ditch water levels depends on the 

distance from the ditch, the magnitude of water-level changes 

in the ditch, and the presence of other nearby ditches. 

Model Limitations

Users need to be aware that the hydrologic model has 

limitations and provides an imperfect representation of Swamp 

hydrology. Model limitations are caused by spatial and temporal discretization, boundaries, and inaccuracies associated 

with parameterization and assignment of parameter values. 

The model does not simulate hydrology outside the horizontal 

boundaries of the active model area (fig. 2) or to depths below 

land surface of about 25 ft in most areas. Although groundwater flow rates below the 25-ft depth are expected to be very 

small relative to flow above the 25-ft depth, as discussed in the 

“Groundwater Levels and Flow Patterns” section, the chosen 

depth of the active model does affect simulations of deeper 

flow, such as groundwater flowing under ditches, and ignores 

any possible interactions with the deeper regional groundwater-flow system. Because the model cells are 500 ft on a side, 

the model cannot simulate variability in water levels or flows 

at resolutions less than 500 ft. 

Land-surface elevation assignments in the model affect 

simulated groundwater and surface-water levels, which can 

introduce simulation errors. Assignment of a single landsurface elevation to each 500-ft model cell contributes to these 

errors because the actual land-surface elevation has variability 

within each model cell. Assigned land-surface elevation values 

in the model represent lower land-surface elevations, such as 

depressions between trees, rather than the higher land-surface 

elevations, such as hummocks surrounding trees. Bias can 
62 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

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EXPLANATION

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!

Change in simulated 

 groundwater level in Layer 4, 

 in feet, caused by raising 

 WCS weirs to their highest 

 levels under wet conditions 

 (scenario 5), relative to 

 baseline wet conditions 

 (model scenario 2)

Active model extent

Major road

Ditch

Lake Drummond boundary

Proposed WCS

Existing WCS

EXPLANATION

!(

!

Change in simulated 

 groundwater level in Layer 4, 

 in feet, caused by raising 

 WCS weirs to their highest 

 levels under dry conditions 

 (scenario 6), relative to 

 baseline dry conditions 

 (model scenario 3)

Active model extent

Major road

Ditch

Lake Drummond boundary

Proposed WCS

Existing WCS

-1.49 to -1.00

-0.99 to -0.50

-0.49 to -0.20

-0.19 to -0.10

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

1.01 to 2.00

2.01 to 3.00

>= 3.01

-2.99 to -2.00

-1.99 to -1.50

-1.49 to -1.00

-0.99 to -0.50

-0.49 to -0.20

-0.19 to -0.10

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

1.01 to 2.00

2.01 to 3.00

>= 3.01

0 2.5 5 KILOMETERS

0 2.5 5 MILES

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'

36°

30'

64

32

58

17

158E

158

32

13

58

17

32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

B

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'

36°

30'

64

32

58

17

158 E

158

32

13

58

17 32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk City of

Chesapeake

A

Figure 34. Changes in simulated groundwater levels caused by raising weir levels to their highest possible elevations at existing and 

proposed water control structures under A, wet conditions (model Scenario 5 levels minus Scenario 2 levels) and B, dry conditions 

(model Scenario 6 levels minus Scenario 3 levels) in the three-dimensional model of Great Dismal Swamp, Virginia and North Carolina. 

(WCS, water control structure)
Simulated Hydrology and Water Management 63

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Camp

Middle

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Northeast

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Corapeake

West

South

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East

Portsmouth

Hudnell

Big Entry

Washington

Williamson

Camp

Middle

Rosemary

Northeast

Jericho

Lynn

Corapeake

West

South

Cross Canal

RailroadWeyerhaeuser

Kim Saunders

Interior

Lateral

Feeder

County Line

Bull Boulevard

Insurance

EXPLANATION

!(

!

Change in simulated 

 groundwater level in Layer 4, 

 in feet, caused by dropping 

 WCS weirs to their lowest 

 levels under wet conditions 

 (scenario 7), relative to 

 baseline wet conditions 

 (model scenario 2)

Active model extent

Major road

Ditch

Lake Drummond boundary

Proposed WCS

Existing WCS

EXPLANATION

!(

!

Change in simulated 

 groundwater level in Layer 4, 

 in feet, caused by dropping 

 WCS weirs to their lowest 

 levels under dry conditions 

 (scenario 8), relative to 

 baseline dry conditions 

 (model scenario 3)

Active model extent

Major road

Ditch

Lake Drummond boundary

Proposed WCS

Existing WCS

<= -3.00

-2.99 to -2.00

-1.99 to -1.50

-1.49 to -1.00

-0.99 to -0.50

-0.49 to -0.20

-0.19 to -0.10

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

1.01 to 2.00

2.01 to 3.00

>= 3.01

<= -3.00

-2.99 to -2.00

-1.99 to -1.50

-1.49 to -1.00

-0.99 to -0.50

-0.49 to -0.20

-0.19 to -0.10

-0.09 to 0.10

0.11 to 0.20

0.21 to 0.50

0.51 to 1.00

1.01 to 2.00

2.01 to 3.00

>= 3.01

0 2.5 5 KILOMETERS

0 2.5 5 MILES

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'

36°

30'

64

32

58

17

158E

158

32

13

58

17

32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk

City of

Chesapeake

B

LAKE

DRUMMOND

VIRGINIA

NORTH

CAROLINA

-76°30' -76°20'

36°

40'

36°

30'

64

32

58

17

158 E

158

32

13

58

17 32N

Base from U.S. Geological Survey National Elevation Data 1/3 arc-second

Lambert Conformal Conic projection

Standard Parallels 36° 45' 57.6" N and 37° 57' 57.6" N; central meridian 78° 30' W 

North American Datum of 1983 State Plane Virginia South

City of

Suffolk City of

Chesapeake

A

Figure 35. Changes in simulated groundwater levels caused by lowering weir levels to their lowest possible elevations at existing 

and proposed water control structures under A, wet conditions (model Scenario 7 levels minus Scenario 2 levels) and B, dry conditions 

(model Scenario 8 levels minus Scenario 3 levels) of the three-dimensional model of Great Dismal Swamp, Virginia and North Carolina. 

(WCS, water control structure)
64 Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.

be expected when comparing simulated depth to water with 

actual depth to water, unless observations are made at low 

points and not on the top of hummocks. Depth to the water 

table is important in the model because it affects simulated ET.

Because the model is steady-state rather than transient, 

it has the limitation of being unable to simulate hydrologic 

changes over time. For example, seasonal fluctuations in 

water-table elevations or flows resulting from a storm cannot 

be simulated with the steady-state model. Actual hydrologic 

conditions in the Swamp are constantly changing as climatic 

conditions change, and the system adjusts to these changes. 

The simulations of hypothetical wet and dry conditions 

performed in this study do not represent actual conditions 

in spring 2011 or 2015, respectively, but rather hypothetical dry and wet climatic conditions imposed for an indefinite 

length of time. The model is designed so that, in the future, it 

can be readily updated to be capable of simulating transient 

conditions.

Assigning recharge equal to 100 percent of observed 

precipitation and then relying on the EVT package to remove 

water is appropriate for the setting of the study area but can 

introduce error, namely overassignment of net recharge, if 

the water table is deeper than 8.86 ft and therefore below the 

extinction depth and unavailable to the ET boundary condition. Results for all eight of the model scenarios, presented in 

the “Simulated Hydrology and Water Management” section, 

indicate that from 0.9 percent to 1.3 percent of active model 

cells have water-table depths greater than 8.86 ft. Nearly all of 

those cells are located along the western boundary where landsurface elevations are higher along the scarp than elsewhere. 

For the scenarios studied here, the potential for over-assignment of net recharge does not outweigh the benefits of the 

approach used to assign recharge and ET. However, the user 

needs to be aware of the approach when simulating conditions 

where the water table is more than 8.86 feet below the land 

surface over much of the study area and may want to consider 

using alternate assignment of recharge and ET. 

Summary and Conclusions

In a study conducted by the U.S. Geological Survey, in 

cooperation with the U.S. Fish and Wildlife Service (USFWS), 

a three-dimensional numerical model based on MODFLOWNWT and the Surface Water Routing Process was developed 

to simulate groundwater and surface-water in the Great Dismal 

Swamp (Swamp) of Virginia and North Carolina. Development of the numerical model has led to improved understanding of hydrology in the Great Dismal Swamp and provides 

a tool to assist USFWS National Wildlife staff who manage 

water in the Swamp. The model simulates historical average 

spring hydrologic conditions for the years 2005 through 2015. 

Calibration results indicate that the model provides a reasonable representation of average spring hydrologic conditions in 

the study area. Simulated heads and flows are most sensitive 

to model parameters controlling evapotranspiration rates and 

to Manning’s roughness coefficients, which control frictional 

head loss in ditches. 

Simulation results show that hydrologic input to the 

Swamp is dominated (92%) by precipitation. More than 

one-half of simulated outflow from the Swamp is through 

evapotranspiration, and the remainder is mostly surface-water 

discharge. Ditches capture most groundwater flow, although 

that simulated result likely depends on the assigned layer 

thicknesses in the model. 

Simulations of water-management scenarios can be used 

by Refuge staff to inform their water-management decisions. Raising weir levels can be used to suppress fires in the 

Swamp; model results indicate that raising all weirs from 

their standard levels to their highest levels under dry conditions will inundate an additional 31,709 acres, equivalent to 

21 percent of the active model area. Raising weir levels during 

wet conditions provides only a small amount of water retention capacity in the context of preventing flooding during large 

rainfall events. Model results indicate that 0.28 feet (ft) of 

water can be stored by raising all weirs, including proposed 

water control structures (WCSs), to their highest levels. This is 

considered a high-end estimate because simulation results are 

for steady-state conditions, whereas actual conditions require 

some time (perhaps weeks or months) for groundwater in 

interior areas of the Swamp to fully respond to changes in weir 

levels.

The three-dimensional model can be used to simulate 

additional water management scenarios besides those studied 

and presented in this report. The model could also be quite 

useful for analyzing time varying hydrologic conditions if 

future work was done to make the model transient. For example, the fraction of time that the land surface was inundated 

could be simulated and used to inform decisions about forest 

management within the Swamp.
References Cited 65

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For additional information, contact:

Director, Virgina Water Science Center

U.S. Geological Survey

1730 East Parham Road,

Richmond, VA 23228

or visit our website at:

http://va.water.usgs.gov/

Publishing support provided by West 

Trenton Publishing Service Center
Eggleston and others—Hydrologic Conditions and Simulation of Groundwater and Surface Water in the Great Dismal Swamp of Va. and N.C.—Scientific Investigations Report 2018–5056

ISSN 2328-0328 (online)

ISSN 2328-031X (print)

https://doi.org/10.3133/sir20185056 Printed on recycled paper