U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5236

Prepared in cooperation with the Washington County Water Conservancy District

Numerical Simulation of Groundwater Movement 

and Managed Aquifer Recharge from Sand Hollow 

Reservoir, Hurricane Bench area, Washington 

County, Utah
Cover photograph: View of the North Dam of Sand Hollow Reservoir and Sand Hollow 

Basin. Photo by Tom M. Marston, January 2012.
Numerical Simulation of Groundwater 

Movement and Managed Aquifer Recharge 

from Sand Hollow Reservoir, Hurricane 

Bench area, Washington County, Utah

By Thomas M. Marston and Victor M. Heilweil

Prepared in cooperation with the Washington County Water Conservancy District

Scientific Investigations Report 2012–5236

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Marston, T.M., and Heilweil, V.M., 2012, Numerical simulation of groundwater movement and managed aquifer 

recharge from Sand Hollow Reservoir, Hurricane Bench area, Washington County, Utah: U.S. Geological Survey 

Scientific Investigations Report 2012–5236, 34 p.
iii

Contents

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

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

Hydrogeologic Setting ................................................................................................................................. 3

Aquifer Properties ............................................................................................................................... 3

Navajo Sandstone and Overlying Soils................................................................................... 3

Kayenta Formation ..................................................................................................................... 7

Groundwater Conditions and Movement ........................................................................................ 7

Groundwater Budget .......................................................................................................................... 9

Recharge...................................................................................................................................... 9

Discharge................................................................................................................................... 12

Numerical Simulation of Groundwater Flow ......................................................................................... 13

Model Construction .......................................................................................................................... 14

Spatial and Temporal Discretization ..................................................................................... 14

Aquifer Characteristics ........................................................................................................... 14

Boundary Conditions ............................................................................................................... 18

No-Flow Boundaries ...................................................................................................... 18

Recharge Boundaries .................................................................................................... 19

Discharge Boundaries ................................................................................................... 19

Model Calibration .............................................................................................................................. 19

Sensitivity Analysis .................................................................................................................. 20

Composite Scaled Sensitivities ..................................................................................... 20

Parameter Correlation Coefficient ............................................................................... 21

Influence Statistics ......................................................................................................... 21

Nonlinear Regression .............................................................................................................. 21

Parameter Adjustment and Final Values .............................................................................. 21

Hydraulic Conductivity and Anisotropy ....................................................................... 21

Specific Yield and Specific Storage ............................................................................. 22

Natural Recharge as Infiltration of Precipitation .............................................. 22

Reservoir Recharge ............................................................................................... 22

Drain and River Discharge .................................................................................... 22

Model Accuracy ................................................................................................................................ 24

Reservoir Recharge, Drain Discharge, and River Discharge ........................................... 24

Simulated Water Levels .......................................................................................................... 25

Environmental Tracers ............................................................................................................ 25

Model Projections ............................................................................................................................. 25

Model Limitations .............................................................................................................................. 28

Summary ...................................................................................................................................................... 32

References Cited ........................................................................................................................................ 33

Figures

1. Map showing the Hurricane Bench study area, Washington County, Utah .......................... 2

2. Location map of bedrock fractures and faults in the Hurricane Bench area, Utah.............. 4
iv

Tables

1. Range and mean values for porosity and hydraulic conductivity from soils and 

Navajo Sandstone core samples collected near the North Dam of Sand Hollow 

Reservoir, Utah............................................................................................................................. 5

2. Specific-capacity values for Washington County Water Conservancy District, Utah, 

production wells .......................................................................................................................... 5

3. Calibrated hydraulic properties for parameters within the model domain for the 

Hurricane Bench area, Utah.................................................................................................... 18

3. Map showing location of monitoring wells, production wells, irrigation wells, 

and drains in the Hurricane Bench area, Utah....................................................................... 6

4. Map showing potentiometric surface of the Navajo aquifer prior to completion 

of Sand Hollow Reservoir, Hurricane Bench area, Utah ...................................................... 8

5. Graph showing observed water levels from 1971 to 2009 at the (C-42-14)12dbb-1 

monitoring well in the Hurricane Bench area, Utah.............................................................. 9

6. Map showing potentiometric surface of the Navajo aquifer near Sand Hollow 

Reservoir, Hurricane Bench area, Utah, August 2009 ......................................................... 10

7. Graph showing estimated and simulated groundwater recharge from Sand Hollow 

Reservoir, Hurricane Bench area, Utah................................................................................. 11

8. Graph showing estimated withdrawals from wells in the Hurricane Bench area, 

Utah, 1975−2009 ......................................................................................................................... 12

9. Graph showing measured pumping and simulated discharge from the West Dam, 

West Dam Spring, and North Dam drains, Hurricane Bench area, Utah......................... 13

10. Map showing model grid of the groundwater system and zones of low, medium, 

and high recharge from precipitation in the Hurricane Bench area, Utah...................... 15

11. Generalized cross sections along column 19 and row 51 of the groundwater-flow 

model for the Hurricane Bench area, Utah ........................................................................... 16

12. Map showing hydraulic-conductivity zones of the Navajo Sandstone in the 

Hurricane Bench area, Utah.................................................................................................... 17

13. Graph showing composite scaled sensitivities for model parameters in the 

groundwater-flow model for the Hurricane Bench area, Utah.......................................... 20

14. Map showing bed conductance zones of the Reservoir Package representing 

vertical hydraulic conductivity beneath Sand Hollow Reservoir, Hurricane 

Bench area, Utah....................................................................................................................... 23

15. Graph showing temporally varying reservoir-bed hydraulic conductivity parameters that 

define a portion of the total recharge simulated in flat low-lying areas in Sand Hollow 

Reservoir, Hurricane Bench area, Utah................................................................................. 24

16. Graphs showing observed and simulated water levels at monitoring wells in Sand 

Hollow, Hurricane Bench area, Utah...................................................................................... 26

17. Map showing difference between observed and simulated water levels measured 

in December 2009 for monitoring wells around Sand Hollow Reservoir, 

Hurricane Bench area, Utah.................................................................................................... 29

18. Map showing simulated groundwater-flow paths from Sand Hollow Reservoir, 

Hurricane Bench area, Utah.................................................................................................... 30

19. Map showing projected travel time of managed aquifer recharge moving through 

the Navajo Sandstone in Sand Hollow, Hurricane Bench area, Utah .............................. 31
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

acre-foot per month (acre-ft/mo) 1,233 cubic meter per month (m3/mo)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Specific capacity

gallon per minute per foot 

[(gal/min)/ft]

0.2070 liter per second per meter [(L/s)/m]

Hydraulic conductivity

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

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d) 

Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88).

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

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

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot squared 

per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L). 
vi

Abbreviations

CFCs chlorofluorocarbons

Cl:Br chloride-to-bromide ratio

CSS composite scaled sensitivity

USGS U.S. Geological Survey

WCWCD Washington County Water Conservancy District
Numerical Simulation of Groundwater Movement and 

Managed Aquifer Recharge from Sand Hollow Reservoir, 

Hurricane Bench area, Washington County, Utah 

By Thomas M. Marston and Victor M. Heilweil

was simulated as well withdrawals, shallow drains at the base 

of reservoir dams, and seepage to the Virgin River. During 

calibration, variables were adjusted within probable ranges to 

minimize differences among model-simulated and observed 

water levels, groundwater travel times, drain discharges, and 

monthly estimated reservoir recharge. In general, the model 

adequately simulated water levels, and most simulated water 

levels were within 10 ft of measured water levels. Simulated 

arrival times of environmental tracers were within the range 

of observed arrivals at nearby monitoring wells from 2003 to 

2008. The total simulated drain discharge from 2002 to 2009 

was 9,500 acre-ft. The total simulated recharge from Sand 

Hollow Reservoir between 2002 and 2009 was approximately 

94,000 acre-ft. Predictive modeling revealed an average travel 

time of 800 years from 2009 for water recharged in Sand 

Hollow Reservoir to reach the Virgin River, with the earliest 

arrival occuring in 500 years.

Introduction

The Hurricane Bench area of Washington County, Utah, is 

an elevated bench, approximately 6 miles (mi) wide and 11 mi 

long, extending south from the Virgin River near the town of 

Hurricane, Utah (fig. 1). The area is bounded by the Hurricane 

Fault on the east side and by the erosional extent of the Jurassic Navajo Sandstone and Kayenta Formation on the west and 

south sides. The Navajo Sandstone is as much as 2,000 feet 

(ft) thick beneath Hurricane Bench. The local groundwater 

system is in the Navajo Sandstone and Kayenta Formation, 

and water movement is from south to north. The aquifer is 

unconfined, and natural recharge to the groundwater system 

as infiltration of precipitation occurs in higher elevation areas 

to the south and in areas where the Navajo Sandstone is thinly 

covered or exposed. Discharge from the groundwater system 

occurs as natural seepage to the Virgin River and as withdrawals from municipal and irrigation wells. A small part of the 

Navajo Sandstone in the northeastern part of the Hurricane 

Bench study area is covered by surface-flood basalts.

Sand Hollow Reservoir is located in Sand Hollow basin 

within the Hurricane Bench area, about 10 mi east of St. 

George, Utah (fig. 1). The reservoir is an off-channel facility operated by the Washington County Water Conservancy 

Abstract

The Hurricane Bench area of Washington County, Utah, 

is a 70 square-mile area extending south from the Virgin 

River and encompassing Sand Hollow basin. Sand Hollow 

Reservoir, located on Hurricane Bench, was completed in 

March 2002 and is operated primarily as a managed aquifer 

recharge project by the Washington County Water Conservancy District. The reservoir is situated on a thick sequence of 

the Navajo Sandstone and Kayenta Formation. Total recharge 

to the underlying Navajo aquifer from the reservoir was about 

86,000 acre-feet from 2002 to 2009. Natural recharge as 

infiltration of precipitation was approximately 16,800 acrefeet for the same period. Discharge occurs as seepage to the 

Virgin River, municipal and irrigation well withdrawals, and 

seepage to drains at the base of reservoir dams. Within the 

Hurricane Bench area, unconfined groundwater-flow conditions generally exist throughout the Navajo Sandstone. Navajo 

Sandstone hydraulic-conductivity values from regional aquifer 

testing range from 0.8 to 32 feet per day. The large variability 

in hydraulic conductivity is attributed to bedrock fractures that 

trend north-northeast across the study area.

A numerical groundwater-flow model was developed to 

simulate groundwater movement in the Hurricane Bench area 

and to simulate the movement of managed aquifer recharge 

from Sand Hollow Reservoir through the groundwater system. 

The model was calibrated to combined steady- and transientstate conditions. The steady-state portion of the simulation 

was developed and calibrated by using hydrologic data that 

represented average conditions for 1975. The transient-state 

portion of the simulation was developed and calibrated by 

using hydrologic data collected from 1976 to 2009. Areally, 

the model grid was 98 rows by 76 columns with a variable 

cell size ranging from about 1.5 to 25 acres. Smaller cells 

were used to represent the reservoir to accurately simulate 

the reservoir bathymetry and nearby monitoring wells; larger 

cells were used in the northern and southern portions of the 

model where water-level data were limited. Vertically, the 

aquifer system was divided into 10 layers, which incorporated 

the Navajo Sandstone and Kayenta Formation. The model 

simulated recharge to the groundwater system as natural infiltration of precipitation and as infiltration of managed aquifer 

recharge from Sand Hollow Reservoir. Groundwater discharge 
2 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 1. Hurricane Bench study area, Washington County, Utah.

River

Virgin

River

Virgin

Leeds

Anderson

Junction

Hurricane

La Verkin

EXPLANATION

Pumping well

Sand Hollow watershed

Hurricane Bench study area

Fault—Bar and ball on downthrown side

(C-40-13)28dcb-2

Hurricane Fault

Sandstone

Mountain

(C-42-14)12dbb-2

Hurricane Fault

113°30' 113°25' 113°20'

37°15'

37°05'

37°10'

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

Sand

Mountain

Sand

Hollow

Resort

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

15

Hurricane Cliffs

15

15 Study area

70

U T A H

40

80 Salt Lake

City

St George 5 miles

to southwest

Sand

Hollow

Reservoir
Hydrogeologic Setting 3

District (WCWCD) using water diverted from the Virgin 

River. The reservoir is operated for surface-water storage 

and managed recharge to the underlying aquifer system. The 

reservoir began filling in early 2002 and reached full pool in 

2005 at an altitude of 3,060 ft. From 2005 to 2009, annual 

reservoir stage fluctuation below full pool was no greater than 

20 ft (Heilweil and Marston, 2011). The site of the reservoir is 

underlain by the Navajo Sandstone, which is partially covered 

by thin soils and eolian sands. Sand Hollow basin is located 

on a hinge line of a north-northeast trending syncline that 

forms a structural basin known as the Sand Mountain syncline 

(Hurlow, 1998).

The Hurricane Bench area has been the subject of interdisciplinary, cooperative investigations of groundwater hydrology 

and geochemistry since 1999. Previous reports document prereservoir groundwater conditions in Hurricane Bench prior to 

March 2002 (Heilweil and others, 2000); pre-reservoir vadosezone and groundwater studies in Sand Hollow (Heilweil and 

Solomon, 2004; Heilweil and others, 2006; Heilweil and 

others, 2007; Heilweil and McKinney, 2007), pond and trench 

infiltration studies adjacent to the reservoir (Heilweil and 

others, 2004; Heilweil and Watt, 2011), and pre- and post-reservoir groundwater conditions, water budgets, and estimates 

of groundwater recharge from the reservoir from March 2002 

through December 2009 (Heilweil and others, 2005; Heilweil 

and Susong, 2007; Heilweil and others, 2009a; Heilweil and 

Marston, 2011). These reports also contain monitoring-well 

and production-well completion information, water-quality 

data, and precipitation data.

The purpose of this report is to describe the groundwater 

hydrology of the Hurricane Bench area and to present the 

construction, calibration, and projected results of a numerical simulation of the groundwater system in the Hurricane 

Bench area, including recharge from Sand Hollow Reservoir. 

A model was developed to estimate hydraulic properties of the 

Navajo Sandstone and Kayenta Formation, to test the conceptual understanding of the effects of managed aquifer recharge 

that commenced at Sand Hollow Reservoir in March 2002, 

and to project future movement of this recharge through the 

aquifer system. The flow-system concepts in the simulation are 

defined by previous referenced studies conducted in the Hurricane Bench area. This study is a cooperative effort between 

the WCWCD and the U.S. Geological Survey (USGS).

Hydrogeologic Setting

The Hurricane Bench area is underlain primarily by Navajo 

Sandstone that is either exposed at the surface or covered by a 

veneer of soil or surface-flood basalts (Hurlow, 1998; Heilweil 

and others, 2000, pl. 1). The Kayenta Formation underlies the 

Navajo Sandstone and is considered part of the groundwater 

system.

Although the total stratigraphic thickness of the Navajo 

Sandstone in this region is more than 2,000 ft, much of this 

has been eroded within the study area, and the sandstone 

pinches out to the west and south of Sand Hollow Reservoir. 

The combined thickness of the Navajo Sandstone and Kayenta 

Formation is estimated to be about 2,900 ft in the vicinity of 

Sand Hollow Reservoir, with 850 ft attributed to the Kayenta 

Formation (Hurlow, 1998). Stratigraphic units in the Hurricane 

Bench area west of the Hurricane Cliffs have been displaced 

downward by more than 2,000 ft as a result of vertical movement along the Hurricane Fault, which is located near the 

eastern boundary of the Hurricane Bench study area (fig. 1).

The Navajo Sandstone is characterized as well-sorted, 

fine-grained quartzose sandstone loosely cemented with 

calcite (Cordova, 1978). Predominant cross-bedding features 

reflect its eolian depositional environment (Hurlow, 1998). 

Because the Navajo Sandstone in the study area is only loosely 

cemented and well sorted, it has relatively high porosity and 

permeability. The Kayenta Formation underlies the Navajo 

Sandstone and contains some sandstone layers with similar 

permeability to the Navajo Sandstone separated by less permeable layers of siltstone. The interbedded siltstone layers of the 

Kayenta Formation likely inhibit vertical movement of water. 

The Jurassic Moenave Formation, consisting primarily of siltstone, underlies the Kayenta Formation and is considered less 

permeable than either the Navajo Sandstone or the Kayenta 

Formation.

In response to observations made by the WCWCD in 

2004 regarding the large range of production rates in wells 

completed in the Navajo Sandstone (100 to 2,000 gallons per 

minute (gal/min)), a study was conducted to produce detailed 

fracture maps of the Navajo Sandstone in the area near Sand 

Hollow Reservoir (P.D. Rowley, Geologic Mapping, Inc., 

unpub. data, 2004). The results of this study showed many 

fracture zones associated with small faults that trend northnortheast in the east-central portion of the study area (fig. 2). 

Two of these fracture zones were documented in a trench 

excavated parallel to the North Dam of Sand Hollow Reservoir 

(Heilweil and Solomon, 2004). Another zone of fracturing was 

located on the northeast side of the reservoir. A previous study 

by Hurlow (1998) investigated fracture density for a larger 

region of the Navajo Sandstone that included the Hurricane 

Bench. Results of this study indicated that the area between 

Sandstone Mountain and the Virgin River, just north of the 

Hurricane Bench area (fig. 1), is highly fractured, with fracture 

density decreasing to the south toward Sand Mountain.

Aquifer Properties

Navajo Sandstone and Overlying Soils

Near the North Dam of Sand Hollow Reservoir, 14 soil 

samples, 4 shallow weathered sandstone core samples, and 

13 deeper non-weathered sandstone core samples were collected and analyzed for hydraulic conductivity and porosity 

(Heilweil and others, 2004; table 1). The porosity of 14 local 

soil samples collected near the North Dam of Sand Hollow 

Reservoir ranged from 29 to 45 percent (Heilweil and others, 

2004). These soils commonly overlie the Navajo Sandstone in 

low-lying areas of the study area. Laboratory measurements of 

saturated hydraulic conductivity of these soil samples ranged 
4 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 2. Bedrock fractures and faults in the Hurricane Bench area, Utah.

270

7 15

292

37

330 297

60

00

277

22

67

15

60

315

45

7

60

337

113°22'30" 113°17'30"

37°12'30"

37°07'30"

113°27'30"

37°02'30"

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

0 1 2 3 MILES

0 1 2 3 KILOMETERS

EXPLANATION

Hurricane Bench study area

Spatial extent of mapping within the Hurricane 

Bench study area (P.D. Rowley, unpub. data, 2004)

Fault mapping (P.D. Rowley, unpub. data, 2004)

Trace of fault (certain and concealed)

Trace of inferred fault

Fracture measurement sites (modified from 

Hurlow, 1998)

Fracture measurement site with attitude of 

principal set(s), in degrees from North

High fracture density

Medium fracture density

Low fracture density

22

67

West Dam drain

West Dam Spring drain

North Dam drain

Sand

Hollow

Reservoir

Sandstone

Mountain

Sand

Mountain
Hydrogeologic Setting 5

from 0.01 to 0.2 foot per day (ft/d). The porosity of four shallow (about 3.2-feet depth), weathered sandstone core samples 

collected at the same site ranged from 20 to 26 percent, and 

the saturated hydraulic conductivity ranged from 0.33 to 

0.76 ft/d. The porosity of 13 deeper (6.5 to 62-foot depth), 

non-weathered sandstone core samples from the same trench 

ranged from 20 to 27 percent, and the saturated hydraulic 

conductivity ranged from 0.03 to 1.4 ft/d (Heilweil and others, 

2004). Cordova (1978, table 3) reported an effective porosity 

of about 17 percent from laboratory analysis of 12 sandstone 

samples from selected outcrops in and around the study area. 

The average effective porosity from the two samples collected 

within the study area was 13.5 percent.

An aquifer test conducted at Winding Rivers Corporation well (C-42-14)12dbb-2 in the Hurricane Bench area, 

approximately 5 mi southwest of Hurricane (fig. 1), yielded a 

saturated horizontal hydraulic conductivity of 0.8 to 2.2 ft/d, 

on the basis of an assumed aquifer thickness of 1,350 ft and 

500 ft, respectively, in the calculation (Heilweil and others, 

2000). Transmissivity and storage-coefficient values from this 

test were 1,075 feet squared per day (ft2/d) and 0.002, respectively. Results of an aquifer test conducted at the WCWCD 

well (C-40-13)28dcb-2 in Anderson Junction, just north of 

the study area, indicated that fracture-related anisotropy can 

strongly influence directional permeability within the Navajo 

Sandstone. The hydraulic conductivity from this test ranged 

from 1.3 to 32 ft/d (Heilweil and others, 2000). The lower permeability is characteristic of unfractured sandstone, whereas 

higher permeability is characteristic of preferential flow along 

fractures.

Many of the higher production-rate wells operated by 

WCWCD are located in or near fracture zones. The specific 

capacity calculated for 13 WCWCD production wells ranged 

from 0.4 to 6.1 gallons per minute per foot ((gal/min)/ft) 

(table 2). Specific capacity of a well is indicative of transmissivity. Generally, wells with large values of specific capacity 

are located near mapped fracture zones (fig. 2 and 3, wells 1, 

21, and 23), and wells with small values of specific capacity 

are in areas where no fractures are mapped (fig. 2 and 3, wells 

3 and 17). There are also wells that are located near fracture 

zones (fig. 2 and 3, wells 2, 8, 9, 18, 19, 20, and 22) that have 

low to intermediate values of specific capacity.

No aquifer testing has been done to determine vertical 

hydraulic conductivity of the Navajo Sandstone within the 

study area. Horizontal and vertical hydraulic-conductivity 

values determined from laboratory analyses of Navajo Sandstone samples within the Upper Colorado River Basin were 

compiled by Weigel (1987, table 5). The averages for vertical and horizontal hydraulic conductivity of 24 samples were 

about 0.8 ft/d and 1.1 ft/d, respectively. The ratio of vertical 

to horizontal laboratory-determined hydraulic-conductivity 

values for the 24 pairs of samples ranged from 0.13 to 2.7, 

averaging about 0.4. It is possible, however, that these discrete 

core samples do not accurately represent the bulk vertical 

hydraulic conductivity or vertical-to-horizontal anisotropy 

ratios for the Navajo Sandstone within the study area. The 

lowest vertical hydraulic conductivity in a layered sedimentary 

formation controls the overall vertical hydraulic conductivity 

of that formation. Therefore, it is likely that in some parts of 

the Navajo Sandstone, the vertical movement of groundwater 

could be more restricted than is indicated by the average of the 

laboratory-determined values. Lower overall vertical hydraulic-conductivity values and vertical-to-horizontal hydraulicconductivity ratios can result from thin, low-permeability horizontal layers that consist of fine-grained interdunal deposits 

Table 1. Range and mean values for porosity and hydraulic conductivity from soils and Navajo Sandstone core samples collected near the North Dam of Sand 

Hollow Reservoir, Utah.

[ft, feet; ft/d, feet per day; =

~, approximately equal to]

Sample type

Depth of sample, 

in ft below land 

surface

Number of samples Range of porosity,

in percent

Arithmetic mean of 

porosity, in percent

Range of saturated 

hydraulic conductivity, in ft/d

Geometric mean of 

saturated hydraulic 

conductivity, in ft/d

Soil Land surface 14 29–45 39 0.01–0.20 0.07

Weathered sandstone≅ 3.2 4 20–26 22 0.33–0.76 0.56

Sandstone 6.5–62 13 20–27 24 0.03–1.4 0.33

Table 2. Specific-capacity values for Washington County Water 

Conservancy District, Utah, production wells.

[WCWCD, Washington County Water Conservancy District; (gal/min)/ft, 

gallons per minute per foot]

WCWCD Well Number Specific Capacity, in (gal/min)/ft

Well 1 5.20

Well 2 1.27

Well 3 1.20

Well 8 2.01

Well 9 2.27

Well 17 0.40

Well 18 2.52

Well 19 2.20

Well 20 3.23

Well 21 6.13

Well 22 1.55

Well 23 4.18
6 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 3. Monitoring wells, production wells, irrigation wells, and drains in the Hurricane Bench area, Utah.

113°22'30" 113°17'30"

37°12'30"

37°07'30"

113°27'30"

37°02'30"

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

0 1 2 3 MILES

0 1 2 3 KILOMETERS

EXPLANATION

Hurricane Bench study area

Monitoring well

Washington County Water Conservancy 

District production well

Irrigation well

West Dam drain

West Dam Spring drain

North Dam drain

Well 17

Well 4 Well 3

Well 2

Well 22

Well 21

Well 8

Well 9

Well 20

Well 19

Well 23 Well 1

WD 9

WD 8

WD 7

WD 5

WD 4

WD 6

WD 3

WD 1

WD 20

WD 19 WD 18

WD 17

WD 18

WD RJ

WD 16

WD 15

WD 14

WD 13

WD 12

WD 11

WD 10

(C-42-13)6bac-1

(C-42-13)18bcb-1

(C-42-14)25adc-2

(C-42-14)12dbb-1

Sand

Hollow 

Reservoir 

WD 2
Hydrogeologic Setting 7

or have greater cementation than within the zone sampled for 

laboratory analyses. Conversely, vertical fracturing would 

increase the vertical hydraulic conductivity and vertical-tohorizontal hydraulic-conductivity ratios for the aquifer above 

the laboratory ratios.

Kayenta Formation 

No aquifer testing has been done in or near the study area 

to determine the horizontal hydraulic conductivity, vertical 

hydraulic conductivity, or storage properties of the Kayenta 

Formation. A study conducted by Cordova and others (1972, 

table 11) reported a horizontal hydraulic-conductivity value 

of 1 ft/d on the basis of specific-capacity data from a well in 

St. George screened in the Kayenta Formation. The storage 

coefficient estimated from the specific-capacity data was 0.006 

(Cordova and others, 1972, table 11). Estimated horizontal 

hydraulic conductivity from slug tests in the Kayenta Formation near Sheep Springs, about 2 mi northwest of St. George, 

ranged from 0.1 to 0.6 ft/d (Jensen and others, 1997).

Horizontal and vertical hydraulic-conductivity values 

have been determined from laboratory analysis of Kayenta 

Formation samples within the Upper Colorado River Basin, 

Utah, and Colorado (Weigel, 1987, table 5). The average 

horizontal hydraulic-conductivity value of 12 core samples 

was about 0.5 ft/d and ranged from 8.2 x 10-4 to 1.4 ft/d. The 

vertical hydraulic-conductivity values of two samples were 

8.2 x 10-4 ft/d and 0.5 ft/d. The ratios of vertical-to-horizontal 

hydraulic conductivity for these two samples were 0.36 and 

1.0, respectively. The large range in values could be caused 

by the alternating siltstone, silty mudstone, and sandstone layers within the formation. As with laboratory analyses of core 

samples from the Navajo Sandstone, it is possible that these 

discrete vertical samples do not accurately represent the bulk 

hydraulic conductivity or vertical-to-horizontal anisotropy 

ratios for the Kayenta Formation. Also, hydraulic properties of 

the Kayenta could vary regionally between the Upper Colorado River Basin and the central Virgin River basin.

Groundwater Conditions and Movement

Groundwater in the Navajo Sandstone and Kayenta Formation generally is unconfined in the Hurricane Bench area. Prior 

to filling of the reservoir in 2002, water levels were higher 

south of the reservoir site and lower north of the reservoir site. 

Groundwater generally moved northward from Sand Mountain, which has the highest estimated rates of recharge from 

precipitation in Sand Hollow basin (Heilweil and McKinney, 

2007). Horizontal hydraulic gradients, calculated by dividing 

the difference in water-level altitudes between two points by 

the distance separating these locations, indicated an average 

hydraulic gradient of about 0.006 ft/ft (30 ft/mi) during 2001 

in the vicinity of the reservoir site (fig. 4). The groundwater 

was intercepted primarily by irrigation wells located farther 

north of the reservoir site, although water levels measured in 

1996 and 1997 in the northern part of Hurricane Bench indicated that some groundwater moved toward the Virgin River 

(Heilweil and others, 2000). Water-level altitudes indicated 

that groundwater does not flow from the higher-altitude Pine 

Valley Mountains to the north, under the Virgin River and the 

Hurricane Bench area. The substantial offset of the Navajo 

Sandstone and Kayenta Formation, along with fine-grained 

fault gouge associated with the Hurricane Fault, likely restricts 

groundwater flow across the fault. The Hurricane Fault juxtaposes the Navajo Sandstone with the Permian Queantoweap 

Sandstone and Pakoon Formation, and the Pennsylvanian 

Callville Limestone (Hurlow, 1998). The hydrologic properties 

of these units are locally unknown.

One-time measurements of groundwater levels were 

made at monitoring well (C-42-13)18bcb-1 in 1958, monitoring well (C-42-14)25adc-2 in 1959, and monitoring 

well (C-42-13)6bac-1 in 1975 (fig. 3). Water levels were 

measured annually from 1971 to 2009 at monitoring well 

(C-42-14)12dbb-1 (fig. 5; Burden and others, 2010). Water 

levels were measured monthly at monitoring wells WD 1 

through WD 5 and WD RJ from 1995 through 2009; water 

levels were measured monthly at WD 6 through WD 14 from 

mid-2001 through 2009; and water levels were measured 

monthly at WD 15 through WD 20 during the latter half of 

2009 (fig. 3). Prior to 2002, water-level fluctuations in Sand 

Hollow generally were less than 10 ft, except at monitoring 

wells WD 1 and WD 2, which showed decreased water levels 

caused by groundwater withdrawals from nearby Well 9 during 2000 and 2001. Historical water levels from 1958 to 1975 

at well (C-42-14)25ada-1 indicated a depth to water directly 

beneath the reservoir site of about 100 ft and depth to water in 

wells located north of the reservoir from 30 to 100 ft. Prior to 

2002, depth to the water table south, east, and west of the reservoir site ranged from about 50 to 150 ft below land surface. 

The substantial vadose-zone thickness represented a large 

volume available to store managed aquifer recharge.

Filling of Sand Hollow Reservoir began in March 2002. 

The reservoir stage rose from about 2,980 ft above sea level 

in March 2002 to a maximum of about 3,060 ft in May 2006, 

when the reservoir was first filled to capacity. The reservoir 

stage receded to about 3,040 ft in December 2007 and then 

fluctuated between about 3,040 and 3,060 ft during 2008 and 

2009. The topographically lowest part of the reservoir bottom, adjacent to the North Dam, was inundated with surface 

water in 2002 and 2003. As the reservoir continued to fill 

from 2004 through 2006, the areal extent of surface water 

increased toward the south in a line roughly perpendicular to 

the North Dam. The monitoring wells nearest the northern side 

of the reservoir, therefore, were the first to show water-level 

responses and hydraulic connection with the reservoir. Water 

levels in wells WD 1, 2, 6, and 9 rose rapidly beginning in 

the spring of 2002. Water levels in wells WD 3 and WD 11, 

located farther south along the western side of the reservoir, 

began to rise rapidly in November 2002 and January 2003, 

respectively. Water levels in wells WD 10 and WD 12, located 

on the eastern side of the reservoir, and in wells WD 7 and 

WD 8, located along the southern side of the reservoir, began 

rising in the latter half of 2003. During 2004−09, water levels 
8 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

3000

3000

3000

3000

3200

3200

3200

3300

Sand

Hollow

Reservoir

ater flow groundw Direction of

113°23' 113°22'30''

37°07'30"

37°06'

EXPLANATION

0

2,950

Base from U.S. Geological Survey digital data, 1:100,000, 1980

Universal Transverse Mercator projection, Zone 12 Vertical datum is NAVD 88

0.5 1 MILE

0 0.5 1 KILOMETER

Potentiometric contour—Shows altitude in feet above sea 

level at which water level would have stood in tightly 

cased wells, various dates. Dashed where approximately 

located. Potentiometric-contour interval 5 feet. 

Well—Number is identification and water-level altitude, in 

feet

Measured between June and September, 2001

Measured in 2000

Measured in 1999

2,940

2,955

2,960

2,965

2,930

2,925

2,905

2,900

2,895

2,920

2,910

2,915

2,935

2,945

2,950

WD RJ 2,896

WD4 2,902

WD13 2,953

Well 4 2,939

WD10 2,941

Well 2 2,932 Measured in 2002

2,955

WD8 2,968

2,949

WD5 2,923

WD3 2,931

WD14 2,939

Well 17 2,940 2,943

WD1 2,913

2,928

2,940

Well 12 2,930

2,936

Well 9 2,915

WD11 2,940

WD9 2,924

2,911

2,910

Well 1 2,928

WD2 2,907

WD6 2,910

WD13 2,953

Modified from Heilweil and others, 2005

Figure 4. Potentiometric surface of the Navajo aquifer prior to completion of Sand Hollow Reservoir, Hurricane Bench area, Utah.
Hydrogeologic Setting 9

in monitoring wells closer to the reservoir (WD 3, 7, 8, 9, 

10, 11, and 12) generally fluctuated in response to changes in 

reservoir elevation. Water levels in the monitoring wells near 

the North Dam (WD 1, 2, and 6), however, rose sharply during 

the winters of 2003−04, 2004−05, 2005−06, and 2008−09 in 

association with the temporary cessation of pumping at nearby 

production wells 8 and 9. Water levels in wells farther from 

the reservoir (WD 4, 5, RJ, 13, and 14) generally showed a 

more subdued rise in response to recharge beneath the reservoir. Water levels have been measured only since May 2009 

in wells WD 15 through WD 20. Compared to pre-reservoir 

conditions, water levels in monitoring wells in the vicinity 

of Sand Hollow Reservoir increased by as much as 120 ft by 

August 2009. Depth to water in these wells ranged from less 

than 5 ft below land surface adjacent to the reservoir to about 

90 ft below land surface about 1 mi north of the reservoir. In 

2009, the regional gradient between wells WD 9 (3,041 ft altitude) and WD RJ (2,909 ft altitude) was 0.021 ft/ft (105 ft/mi), 

indicating more than a three-fold increase from pre-reservoir 

conditions. Potentiometric-contour values indicated that in 

2009 groundwater was moving away from Sand Hollow Reservoir in all directions (fig. 6).

Groundwater Budget

Recharge

The only groundwater recharge in the Hurricane Bench 

area prior to the completion of Sand Hollow Reservoir was 

natural infiltration of precipitation. Since its completion in 

2002, seepage from the reservoir into the underlying groundwater system has become the primary form of recharge. 

Recharge rates in the study area were derived by Heilweil 

and others (2007) by using a least-squares linear regression 

between three surficial parameters (soil coarseness, topographic slope, and downgradient distance from outcrop) and 

the percentage of precipitation that becomes net infiltration 

based on environmental tracer data from excavations and boreholes at Sand Hollow Reservoir (Heilweil and others, 2006). 

Estimated recharge from precipitation for the Hurricane Bench 

study area is 2,100 acre-feet per year (acre-ft/yr) and averages 

0.8 inches per year (in/yr), which is approximately 10 percent 

of precipitation. The highest recharge rates—greater than 2 

in/yr—are found in coarser soils in areas downgradient from 

sandstone outcrops at higher altitude, which receive more 

Figure 5. Observed water levels from 1971 to 2009 at the (C-42-14)12dbb-1 monitoring well in the Hurricane Bench area, Utah. Water-level altitude, in feet above sea level

2,850

2,855

2,860

2,865

2,870

2,875

2,880

2,885

2,890

January

1971

January

1974

January

1977

January

1980

January

1983

January

1986

January

1989

January

1992

January

1995

January

1998

January

2001

January

2004

January

2007

January

2010

Well (C-42-14)12dbb-1
10 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 6. Potentiometric surface of the Navajo aquifer near Sand Hollow Reservoir, Hurricane Bench area, Utah, August 2009.

0 0.5 1 1.5 MILES

0 0.5 1 1.5 KILOMETERS

Base from State of Utah 25-cm aerial imagery, 2006

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

EXPLANATION

113°23'45" 113°22'30" 113°21'15"

37°8'45"

37°7'30"

37°6'15"

WD 10 3,041—Well name, followed by 

measured water-level altitude, in feet 

above sea level, August 2009. Dot

indicates nested wells

Potentiometric contour—Shows altitude, 

in feet above sea level at which water 

level would have stood in tightly cased 

wells, August 2009. Contour interval 5 feet

Arrow showing direction of groundwater 

movement 

3,040

3,000

2,990

3,010

2,980

3,020

3,040

2,910

2,920

2,930

2,940

2,950

2,960

2,970

3,020

3,030

Sand

Hollow

Reservoir

WD 4 2,927

WD RJ 2,909

WD 17 2,907

WD 18 2,907

WD 19 2,909

WD 20 2,906

WD 1 2,970

WD 2 2,958

WD 6 2,955

WD 5 2,974

WD 3 3,020

WD 9 3,041

WD 15 2,985

WD 16 2,986

WD 11 3,009

WD 12 3,024

WD 8 3,040

WD 10 3,041

WD 13 3,011

WD 7 3,030

WD 14 2,976

Modified from Heilweil and Marston, 2011
Hydrogeologic Setting 11

precipitation. Moderate recharge rates of 10−50 millimeters 

per year (mm/yr) are found in upland areas with coarser soils, 

away from sandstone outcrops. The lowest recharge rates 

(less than 10 mm/yr) are characteristic of lower elevations 

beneath finer-grained soils not receiving runoff from sandstone 

outcrops. There are high-recharge areas south of Sand Hollow Reservoir by outcrops near Sand Mountain (fig. 1). The 

remaining upland area south of the reservoir, where outcrops 

of sandstone are less prevalent, has moderate recharge rates. 

The areas north and east of the reservoir have a mixture of low 

and moderate recharge rates.

A previous study by Heilweil and others (2000) indicated 

that groundwater recharge associated with applied irrigation water occurs east and northeast of the reservoir in the 

Hurricane Bench area; an irrigation-induced recharge of 

1,050 acre-ft/yr was estimated for this area. A later study by 

Heilweil and McKinney (2007), however, found that the same 

area likely receives little to no groundwater recharge because 

of the fine-grained sediment that is common to that area. The 

area northeast of the reservoir is also the location of Bench 

Lake, which was the source of clay material used in constructing the cores of both the North and West Dams of Sand Hollow Reservoir. The presence of large amounts of fine-grained 

material in the area typifies a low infiltration rate. 

Reservoir recharge from March 2002 through December 2009 was calculated with a water-budget equation that 

accounts for surface-water inflows to and outflows from Sand 

Hollow Reservoir, precipitation directly on the reservoir, 

evaporation from the reservoir, and changes in surface-water 

storage based on stage/bathymetry relations (Heilweil and 

Marston, 2011). Estimated monthly recharge rates beneath the 

reservoir ranged from about 0.001 to 0.43 ft/d between March 

2002 and December 2009. Monthly reservoir recharge ranged 

from about 3,500 acre-ft to less than 100 acre-ft resulting in a 

total annual recharge range from about 5,000 acre feet (acre-ft) 

in 2008 to about 18,000 acre-ft in 2005 (fig. 7). Estimates of 

monthly recharge have an associated uncertainty of 6 to 14 

percent.

Previous studies by Heilweil and others (2004), Heilweil 

and others (2009b), and Heilweil and Marston (2011) have 

investigated several flow-limiting processes that are likely 

inhibiting recharge through the reservoir bed sediments and 

underlying aquifer beneath Sand Hollow Reservoir. These 

include declining hydraulic gradients, the accumulation of silt 

and biofilms, and gas clogging. Although there is significant 

month-to-month variability in recharge rates, the trends in 

monthly recharge rates have been lumped into three periods: 

Period 1, March through June 2002, which had very high initial rates and then a steep decline as the vadose zone became 

saturated and the reservoir came into direct hydraulic connection with the aquifer; Period 2, mid-2002 through mid-2007, 

when recharge rates fluctuated but generally declined gradually; and Period 3, mid-2007 through 2009, when recharge 

rates were relatively constant (Heilweil and Marston, 2011, 

fig. 10).

Figure 7. Estimated and simulated groundwater recharge from Sand Hollow Reservoir, Hurricane Bench area, Utah.

January

2006

January

2008

January

2007

January

2003

January

2002

January

2004

January

2005

January

2009

January

2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

Reservoir recharge, in acre-feet per month

Simulated reservoir recharge

Calculated reservoir recharge

Reservoir stage

2,985

3,005

3,025

3,045

3,065

3,085

Reservoir stage altitude, in feet above sea level
12 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Discharge

The two predominant types of groundwater discharge 

from the Hurricane Bench area prior to the completion of 

Sand Hollow Reservoir were seepage to the Virgin River 

and well withdrawals. Natural groundwater discharge to the 

Virgin River takes place between the town of La Verkin and 

the erosional extent of the Navajo Sandstone approximately 

3.5 mi to the west. A previously published seepage study 

(Herbert, 1995) indicated a gain of about 7.2 cubic feet per 

second (ft3/s; 5,200 acre-ft/yr) along this reach of the Virgin 

River from the Navajo Sandstone on both the north (Sandstone Mountain) and the south (Hurricane Bench) sides of the 

river. On the basis of estimates of recharge from precipitation (Heilweil and McKinney, 2007), it is assumed that 3 ft3

/s 

(2,100 acre-ft/yr) or a little less than half of the estimated 

7.2 ft3/s of discharge to the Virgin River came from the Hurricane Bench prior to irrigation pumping in the mid-1970s. 

This includes the flow from one small spring (Berry Spring) 

with a reported discharge of 50 acre-ft/yr in 1968 (Wilkowske 

and others, 1998). There was no reported spring discharge on 

the west side of the Hurricane Bench study area prior to the 

filling of Sand Hollow Reservoir. Although seepage to the 

Virgin River along the north part of the Hurricane Bench study 

area could increase in the future as a result of rising groundwater levels associated with reservoir recharge, increased 

seepage and spring discharge along the west side is unlikely 

because of the lower-permeability rocks (Moenave and Chinle 

Formations) that have a nearly vertical dip associated with the 

Virgin River anticline (Heilweil and others, 2000, plate 1).

Prior to the mid-1970s, very few irrigation wells existed in 

the Hurricane Bench area; most water-rights claims were filed 

from the mid-1970s to the mid-1990s. Records of irrigationwell withdrawals are sparse, but an inventory of irrigationwell discharge was completed during the 1995 growing 

season. Well withdrawals from irrigation wells for years other 

than 1995 were estimated by using land-use data from Landsat 

imagery of the irrigated area north of Sand Hollow Reservoir 

during the months of June, July, or August for available years 

between 1975 and 2009. The values of annual estimated irrigation withdrawals were determined by multiplying the inventoried 1995 irrigation discharge with a ratio of irrigated-land 

coverage for each year between 1975 and 2009 to the irrigated-land coverage in 1995. All of the irrigation for this area 

is supplied by groundwater withdrawals; there are no surfacewater diversions. The uncertainty of estimation of withdrawals by using land use is likely greater than 20 percent. 

Well withdrawals for irrigation in the Hurricane Bench area 

increased rapidly from 1976 to 1980 and have been highly 

variable, peaking in the mid-1990s at about 2,700 acre-ft/yr 

(fig. 8). With the exception of two municipal wells supplying Sky Ranch, near the Hurricane Cliffs southeast of Sand 

Hollow, all of the pumping in the Hurricane Bench area for 

irrigation prior to filling of the reservoir in 2002 took place 

north of the reservoir. Well withdrawals for municipal use by 

the WCWCD began during January 2004; the pumping was 

Figure 8. Estimated withdrawals from wells in the Hurricane Bench area, Utah, 1975−2009. 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

2000200120022003200420052006200720082009

0

500

1,000

1,500

2,000

2,500

3,000

3500

Cumulative annual pumping well discharge, in acre-feet per year

Irrigation withdrawals

Municipal withdrawals
Numerical Simulation of Groundwater Flow 13

seasonal through 2005 and then remained relatively constant 

from 2006 through 2009. Total pumping from wells 1, 2, 8, 

9, 17, and 21 generally ranged from 150 to 275 acre-feet per 

month (acre-ft/mo) during 2006−09 (Heilweil and Marston, 

2011, fig. 3). Reported withdrawal values from the WCWCD 

have an associated uncertainty of at least 5 percent.

After reservoir completion, seepage to drains adjacent to 

Sand Hollow Reservoir became an additional type of discharge. Shallow groundwater has been discharging into drains 

parallel to the North and West Dams since 2003. The North 

Dam drain and the West Dam Spring drain were constructed in 

20-ft-deep trenches in the Navajo Sandstone that extend parallel to each dam 3,000 ft and 6,000 ft in length, respectively. 

The second drain parallel to the West Dam, referred to as the 

West Dam drain, is a 10-ft-deep trench 1,500 ft in length farther downgradient of the West Dam Spring drain (fig. 3). Each 

of these drains operates as a French drain with a pump located 

at the lowest altitude in each of the trenches. Pumping by the 

WCWCD at these drains is based on saturation conditions and 

is not continuous. The sporadic nature of this pumping was 

demonstrated during 2008, when pumping rates varied from 

about 50 to more than 500 acre-ft/mo (fig. 9). Recorded drain 

pumping generally has an associated uncertainty of at least 

5 percent.

Numerical Simulation of 

Groundwater Flow

A numerical model was developed to represent the groundwater system in the Hurricane Bench area. The primary objective of this model was to simulate managed aquifer recharge 

from Sand Hollow Reservoir and the subsequent movement of 

groundwater through the Hurricane Bench area. The numerical 

simulation represents initial steady-state hydrologic conditions 

in 1975 prior to (1) a period of increased irrigation-well withdrawals, which peaked in 1995, and (2) recharge from Sand 

Hollow Reservoir beginning in 2002. Data available from 

1975 to 2009 include monitoring-well water levels and irrigation-well withdrawals. Additional data available from 2002 

to 2009 include reservoir recharge, reservoir stage, pumping 

from drains parallel to the dams, municipal-well withdrawals, and tracer-based groundwater travel times. The “Model 

Construction” section of this report discusses the details of 

discretization, boundary conditions, and model parameters. 

The “Model Calibration” section discusses how the model 

was adjusted to match observed data. The “Model Accuracy” 

section discusses how adequately the model simulated the 

groundwater system. The “Model Projections” section (1) 

Figure 9. Measured pumping and simulated discharge from the West Dam, West Dam Spring, and North Dam drains, Hurricane Bench 

area, Utah.

January

2006

January

2008

January

2007

January

2003

January

2004

January

2005

January

2009

January

2010

0

100

200

300

400

500

600

Drain discharge, in acre-feet per month

Simulated drain discharge

Reported drain pumping
14 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

discusses simulated travel times for water originating at Sand 

Hollow Reservoir that ultimately will discharge to the Virgin 

River, (2) evaluates the change in groundwater storage, and (3) 

evaluates the long-term discharge to the Virgin River.

The groundwater-flow model was constructed with a 

modified version of MODFLOW-2005 that allows for temporal discretization of reservoir bed conductance and thickness in the Reservoir Package (Fenske and others, 1996). 

MODFLOW-2005 is the most recent version of the threedimensional, finite-difference, groundwater-flow model 

known as MODFLOW (Harbaugh, 2005). Sensitivity analysis 

and parameter estimation used to guide the construction and 

calibration of the model utilized UCODE_2005 (Poeter and 

others, 2005), which incorporates sensitivity analysis, parameter estimation, and uncertainty evaluation.

Model Construction

The model described in this report uses parameters and 

zones (Harbaugh, 2005) to define much of the input data. 

A parameter is a single value, which is given a name and 

determines the value of a variable in the finite-difference 

groundwater-flow equation at one or more model cells. Where 

parameters are used, the data value for a cell is either the 

parameter value or the calculated product of the parameter 

value and a cell multiplier, which could apply to many cells 

and can be described by using zones. Sensitivity analysis was 

used to refine model construction, parameter estimation, and 

model calibration.

Construction of the groundwater-flow model was accomplished by horizontally and vertically discretizing the hydraulic properties of the groundwater system, establishing model 

boundaries that depict conceptual hydrologic boundaries, and 

assigning model parameters to all stresses and aquifer properties except reservoir stage. Because managed aquifer recharge 

from Sand Hollow Reservoir is the dominant stress, model 

construction focused on being able to adequately simulate 

reservoir recharge and numerous water levels in proximity to 

the reservoir.

Spatial and Temporal Discretization

Areally, the model is discretized into a grid of rectangular cells and each cell has homogenous properties. Active 

cells, which delineate the lateral boundaries of the simulated 

groundwater system, generally correspond to the lateral extent 

of the Navajo Sandstone and Kayenta Formation in the Hurricane Bench area (fig. 10). The rectangular model grid contains 

98 rows and 76 columns. Cell size is variable, and active cells 

range in size from about 1.5 to 25 acres (about 0.002 to 0.04 

square mile (mi2)). Areas of small cell size represent the area 

beneath and surrounding Sand Hollow Reservoir. This more 

accurately simulates the reservoir bathymetry for the stageto-volume relation used by MODFLOW to determine if the 

reservoir is covering a cell and to provide finer discretization 

for the relatively large number of observation wells that are 

closely spaced near the reservoir. The model grid is oriented 

so that columns of cells are generally parallel to the dominant 

fracture direction in Sand Hollow, which is approximately 

40 degrees from true north (Heilweil and Solomon, 2004).

Vertically, the model is divided into 10 layers. The seven 

upper layers equally divide the Navajo Sandstone, with thicknesses up to about 300 ft in the northern part of the study area. 

The bottom three layers represent the Kayenta Formation and 

have a uniform thickness of about 280 ft (fig. 11). Because 

the Navajo Sandstone is more eroded in the southern part 

of the active domain, layers in the Navajo Sandstone tend 

to thin from north to south. Because the Moenave Formation, consisting primarily of siltstone, underlies the Kayenta 

Formation, the bottom of the Kayenta is considered a no-flow 

boundary. The altitude of the bottom of the Navajo Sandstone 

was obtained from a structure-contour map of the Navajo 

Sandstone that included the Hurricane Bench area (Hurlow, 

1998, plate 5A). The altitude of the bottom of the Kayenta 

Formation was set equal to 850 ft lower than the bottom of the 

Navajo Sandstone (Hurlow, 1998; Heilweil and others, 2000). 

MODFLOW-2005 requires that layers be assigned as confined 

or convertible. Convertible layers automatically change from 

unconfined to confined conditions if the layer becomes fully 

saturated. A confined layer is one in which transmissivity is 

constant and specific storage instead of specific yield is used. 

In the Hurricane Bench area, full saturation of the uppermost 

part of the Navajo Sandstone results in standing water at the 

land surface rather than confined conditions. To prevent the 

model from simulating unrealistic confined conditions at land 

surface, the top of layer 1 was set to 200 ft above land-surface 

altitude.

Temporally, the model was discretized into 26 annual 

stress periods from 1975 through 2000 and one 14 month 

stress period for 2001 and January and February 2002 because 

water levels and withdrawals were measured or estimated 

annually prior to the construction of the reservoir. Agricultural 

irrigation began in the late-1970s in the Hurricane Bench area. 

Because effects of nearby irrigation withdrawals can be seen 

in the long-term monitoring well (C-42-14)12dbb-1 after 1977 

(fig. 5), the mid-1970s were considered to represent steadystate flow conditions prior to groundwater development for 

agriculture on Hurricane Bench and to represent the first and 

only steady-state stress period in the model (fig. 5). Ninetyfour monthly stress periods were used from March 2002 

through December 2009, corresponding to initial filling and 

subsequent recharge from Sand Hollow Reservoir. Monthly 

hydrologic data for this period include water-level data from 

nearby monitoring wells, reservoir stage, calculated reservoir 

recharge, municipal-well withdrawals, and pumpage from 

drains (Heilweil and Marston, 2011).

Aquifer Characteristics

Unconfined conditions in the Navajo Sandstone are 

represented by the uppermost partially saturated cell for each 

location within the model. Fully saturated cells that lie beneath 

partially saturated cells represent portions of the Navajo Sandstone and Kayenta Formation that are simulated as confined. 

The hydraulic properties that control simulated water levels 
Numerical Simulation of Groundwater Flow 15

Figure 10. Model grid of the groundwater system and zones of low, medium, and high recharge from precipitation in the Hurricane 

Bench area, Utah. 

113°22'30" 113°17'30"

37°12'30"

37°07'30"

113°27'30"

37°02'30"

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

0 1 2 3 MILES

0 1 2 3 KILOMETERS

EXPLANATION

Zones defining natural recharge parameters 

and discharge for Hurricane Bench

rch 1

rch 2

rch 3

River cell

Drain cell

Well cell

Hurricane Bench study area and model domain

Line of cross section

A B

A'

B'

A A'

Sand

Hollow

Reservoir

Row

Column
16 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 11. Generalized cross sections along column 19 and row 51 of the groundwater-flow 

model for the Hurricane Bench area, Utah. 

are hydraulic conductivity and storage. Under unconfined 

conditions, storage is defined as specific yield; under confined 

conditions, it is defined as specific storage. Initial specified 

values for these parameters were within the ranges of hydraulic properties listed in table 3. During calibration, hydraulic 

conductivity of the Navajo Sandstone was delineated by 

zones (fig. 12) to allow more variability in the model domain 

to match areas of fractures and provide better calibration. 

The MODFLOW-2005 parameters for horizontal hydraulic 

conductivity for both the Navajo and Kayenta (hk1 through 

hk6 and kayentahk), horizontal anisotropy for the Navajo 

and Kayenta (hani1 through hani6 and hani_kynta), vertical 

hydraulic conductivity for the Kayenta (kayentavk), vertical 

anisotropy for the Navajo (vka1), specific yield for the Navajo 

(sy1), and specific storage for both the Navajo and Kayenta 

(ss1) are the actual values; no multiplier array was used to 

modify them.

Virgin River Sand Hollow Reservoir

Navajo

Sandstone

Kayenta

Formation

10

20 30

40 50

60

70 80

90

Row Number

EXPLANATION

A’ A

Horizontal hydraulic conductivity, 

in feet per day

2.39

2.09

1.47

Sand Hollow Reservoir 0.50

Navajo

Sandstone

Kayenta

Formation

10 20 30 40 50

60

Column Number

B

B’

5,000

4,000

3,000

2,000

1,000

0

3,000

2,000

1,000

0

Thickness, in feet Thickness, in feet

A B

A'

B'

Sand

Hollow

Reservoir
Numerical Simulation of Groundwater Flow 17

Figure 12. Hydraulic-conductivity zones of the Navajo Sandstone in the Hurricane Bench area, Utah.

113°22'30" 113°17'30"

37°12'30"

37°07'30"

113°27'30"

37°02'30"

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

0 1 2 3 MILES

0 1 2 3 KILOMETERS

EXPLANATION

Zones defining aquifer property parameters for 

the Navajo Sandstone

hk1 and hani1

hk2 and hani2

hk3 and hani3

hk4 and hani4

hk5 and hani5

hk6 and hani6

Hurricane Bench study area and model domain

Sand Hollow Reservoir
18 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Table 3. Calibrated hydraulic properties for parameters within the model domain for the Hurricane Bench area, Utah.

[ft/day, feet per day; ft3/ft3, cubic foot of water per cubic foot of matrix; —, not applicable]

Parameter Description Published ranges of values Calibrated value

hk1 Horizontal hydraulic conductivity (ft/day) 0.33 to 32 0.73

hani1 Horizontal anisotropy (unitless) 0.45 to 43.8 2.02

hk2 Horizontal hydraulic conductivity (ft/day) 0.33 to 32 0.68

hani2 Horizontal anisotropy (unitless) 0.49 to 47.1 3.52

hk3 Horizontal hydraulic conductivity (ft/day) 0.33 to 32 0.65

hani3 Horizontal anisotropy (unitless) 0.51 to 49.2 3.22

hk4 Horizontal hydraulic conductivity (ft/day) 0.33 to 32 0.73

hani4 Horizontal anisotropy (unitless) 0.45 to 43.8 2.00

hk5 Horizontal hydraulic conductivity (ft/day) 0.33 to 32 35.2

hani5 Horizontal anisotropy (unitless) 0.01 to 0.91 1.07

hk6 Horizontal hydraulic conductivity (ft/day) 0.33 to 32 1.06

hani6 Horizontal anisotropy (unitless) 0.09 to 8.8 3.65

vka1 Vertical anisotropy (unitless) 0.13 to 2.7 0.80

sy1 Specific yield (ft3/ft3) 0.10 to 0.17 0.12

ss1 Specific storage (1/ft) 7.0x10-7 to 2.5x10-6 4.0x10-6

kayentavk Vertical hydraulic conductivity (ft/day) 8.2x10-4 to 0.5 0.01

kayentahk Horizontal hydraulic conductivity (ft/day) 8.2x10-4 to 6 0.5

rch1 Recharge multiplier (unitless) — 1

rch2 Recharge multiplier (unitless) — 1

rch3 Recharge multiplier (unitless) — 1

resk1 Reservoir-bed vertical hydraulic conductivity (ft/day) Unknown 0.6

resk2 Reservoir-bed vertical hydraulic conductivity (ft/day) Unknown 0.009

resk3 Reservoir-bed vertical hydraulic conductivity (ft/day) Unknown 0.005

hi_K Reservoir-bed vertical hydraulic conductivity (ft/day) Unknown 0.09

rivbed Riverbed hydraulic conductivity (ft/day) Unknown 30.0

Boundary Conditions

The boundary conditions assigned in the groundwater-flow 

model mathematically represent the conceptual understanding of the boundaries in the actual groundwater system. These 

boundaries include no-flow boundaries, specified-flux boundaries, and head-dependent flux boundaries. These boundaries 

define the physical limits of the model and simulate recharge 

to and discharge from the groundwater system. No-flow 

boundaries are considered impermeable, and no flow is simulated across them. Specified-flux boundaries allow a specified 

rate of water through the boundary and are used to simulate 

some sources of recharge and discharge in the model. Headdependent flux boundaries simulate flow across the boundary 

proportional to the difference in heads across the boundary and 

are used to simulate some sources of recharge and discharge in 

the model.

No-Flow Boundaries

The entire outside border of the model domain is simulated 

as a no-flow boundary (fig. 10). The no-flow boundary on the 

east side of the model domain represents the mapped extent 

of the Navajo Sandstone and Kayenta Formation near the 

Hurricane Fault. The boundary on the southeastern, south, and 

western sides of the model domain is simulated as a no-flow 

boundary, representing the erosional extent of these geologic 

units and where no significant seepage occurs. Likewise, the 

northern side of the model domain is simulated as a no-flow 

boundary except in layer 1, where groundwater can discharge 

to the Virgin River. 
Numerical Simulation of Groundwater Flow 19

Recharge Boundaries

Recharge from infiltration of precipitation was simulated 

as a specified-flux boundary with the Recharge Package (Harbaugh, 2005) and was applied to the topmost active layer. In 

this model, a base rate and multiplier array obtained from infiltration rates calculated in a study by Heilweil and McKinney 

(2007) were used to simulate natural recharge as a specified 

flux for the study area. MODFLOW-2005 allows the value of 

recharge flux to be defined as one or more parameters. Three 

recharge parameters and associated zones were defined as follows: rch1 corresponds to the zone of intermediate recharge, 

rch2 corresponds to the zone of low recharge, and rch3 corresponds to the zone of high recharge (fig. 10). Recharge from 

precipitation was simulated as the product of the recharge 

parameter (base rate) and the natural recharge multiplier.

Recharge from Sand Hollow Reservoir was simulated as 

a head-dependent flux boundary with a modified version of 

the Reservoir Package (Fenske and others, 1996). The modified version allows for reservoir-bed thickness and hydraulic 

conductivity to vary temporally. This was needed because calibration to reservoir recharge, drain discharge, and water levels 

near the reservoir could not be achieved with non-varying 

reservoir bed hydraulic conductivity. The Reservoir Package 

computed flow from the reservoir to the groundwater system 

as a function of the simulated water level in the uppermost 

active cell, the water-level altitude of the reservoir (stage), 

and the reservoir-bed conductance. The Reservoir Package 

assumed that water moves from the reservoir to the aquifer 

under saturated conditions and water exchange between the 

reservoir and the groundwater system is instantaneous. The 

Reservoir Package was only active for cells within the specified area of potential inundation for Sand Hollow Reservoir 

and only when the reservoir stage exceeded reservoir-bed 

altitude. Fine spatial discretization (a small model-cell size 

relative to the size of the reservoir) was required to accurately 

represent the stage-to-active reservoir cell relation.

Data requirements for the Reservoir Package include reservoir stage, altitude of the bottom of the reservoir, thickness 

of the reservoir bed sediment, and hydraulic conductivity of 

the reservoir bed sediment. Reservoir stage for Sand Hollow 

Reservoir has been recorded daily since the reservoir began 

filling in March 2002 (Washington County Water Conservancy 

District, written commun., April 4, 2011). Detailed topographic mapping of the Sand Hollow Reservoir site conducted 

by RB&G Engineering, Inc. (1994) prior to construction was 

used to establish the bottom altitude of the reservoir. The 

contour interval for reservoir bathymetry is 10 ft. Existing 

pre-reservoir sediment thickness was assumed to be 5 ft for 

the entire reservoir on the basis of measurements made during 

excavation of the North and West Dams (Heilweil and others, 

2007).

Discharge Boundaries 

Simulated discharge from the groundwater system was to 

the Virgin River, the North Dam and West Dam drains, and to 

pumping wells. Discharge to the Virgin River was simulated 

as a head-dependent boundary with the River Package (Harbaugh, 2005). The River Package computes flow from the 

groundwater system as a function of groundwater levels in 

cells that coincide with the river boundary, the water level in 

the river (stage), and the riverbed conductance. The riverbed 

conductance is defined as a parameter in this model and represents the product of the hydraulic conductivity of the riverbed 

and the seepage area of the river cell. Because the river is 

known to gain groundwater in this area (Herbert, 1995) the 

river stage was set equal to the river bottom in the model. This 

prevents leakage from the river and minimizes the effects of 

local flow paths near the river.

Shallow groundwater discharge to drains at the foot of 

the North and West Dams of Sand Hollow Reservoir was 

simulated with the head-dependent Drain Package (Harbaugh, 

2005). The Drain Package computes flow from the groundwater system as a function of groundwater levels in cells that 

contain the drain boundary, the altitude of the base of the 

drain, and the drain conductance. The drain conductance is 

defined as a parameter in this model. The three shallow drains 

at the base of the reservoir (West Dam Spring drain, West 

Dam drain, North Dam drain) are pumped sporadically, which 

induces a stress on the aquifer similar to a shallow well.

Discharge from both irrigation and municipal wells (fig. 8) 

was simulated as specified flux using the Well Package (Harbaugh, 2005). Two well parameters were defined in the model: 

one for irrigation wells and the other for municipal wells.

Model Calibration

The objective of the numerical groundwater-flow model 

calibration was to obtain a simulation that reasonably represents groundwater recharge, movement, discharge, and 

measured water levels in order to predict the movement 

of recharge from Sand Hollow Reservoir and to estimate 

the additional storage capacity of the underlying Navajo 

Sandstone and the long-term discharge to the Virgin River. 

Model calibration was accomplished by minimizing the sum 

of squared errors between simulated and observed data in 

UCODE-2005 (Poeter and others, 2005). During calibration, 

the numerical results were constrained by observed water 

levels, drain discharge, river discharge, reservoir recharge, 

and the groundwater travel times to monitoring wells from 

environmental tracer analyses. Observations utilized in the 

Observation Package in MODFLOW (Harbaugh, 2005) and 

UCODE (Poeter and others, 2005) for calibration are monitoring-well water levels, estimated monthly reservoir recharge, 

pumpage from drains (Heilweil and Marston, 2011), and 

discharge to the Virgin River in the steady-state stress period 

(Herbert, 1995). Environmental tracer arrival times were used 

for comparison to evaluate model performance (Heilweil and 

Marston, 2011). Calibration relied on both trial and error comparison and formal parameter estimation by regression using 

UCODE-2005 (Poeter and others, 2005).

Initially, all model parameters (table 3), except areal 

recharge and well parameters, were allowed to vary during 

regression. Recharge from precipitation was not allowed 
20 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

to vary because it had been defined by previous studies 

(Heilweil and McKinney, 2007). The well parameter for the 

WCWCD wells was not allowed to vary because withdrawal 

data supplied by the WCWCD for municipal wells were more 

accurate (within 5 percent) than other stresses in the model. 

The well parameter for the irrigation wells was never varied 

because the estimated withdrawals produced a good match 

with recorded long-term monitoring-well water levels in well 

(C-42-14)12dbb-1.

Sensitivity Analysis

Sensitivity analysis was used to evaluate the information 

provided by the observations for the estimation of all defined 

parameters, to eliminate insensitive parameters from the 

regression, and to determine areas where parameters could be 

further divided or combined. Sensitivity analysis for parameters in the model was carried out using UCODE-2005 (Poeter 

and others, 2005). For the Hurricane Bench area model, 31 

parameters representing hydraulic properties, recharge, and 

well withdrawals were used; 14 of these were estimated by 

regression at some point during the calibration process, and 

6 other parameters were adjusted by trial and error. Eleven 

parameters were assigned and not varied during calibration.

Composite Scaled Sensitivities

The sensitivity of observations to parameters was used to 

aid model calibration. Composite scaled sensitivity (CSS) can 

be used to evaluate whether available observations provide 

adequate information to estimate each parameter (Hill and 

others, 2000, p. 96) and can provide an overall view of the 

average amount that simulated values change in response to a 

1-percent change in the parameter (Hill and Tiedeman, 2007, 

p. 50). The relative size of CSS values was used to assess the 

need for additional parameters and zones. Relatively large 

CSS values indicate that observations contain enough information to represent that aspect of the system in more detail, 

thereby justifying the use of additional parameters such as 

more hydraulic conductivity or recharge zones. A relatively 

small CSS value (about two orders of magnitude less than 

the largest CSS value) indicates that the observations provide 

insufficient information with which to estimate the parameter (Hill and Tiedeman, 2007, p. 50). After initial sensitivity 

analysis, parameters with small CSS values generally were 

assigned a fixed value or joined with a parameter with a similar value. The CSS values indicate that observations provide 

more information about natural recharge, hydraulic conductivity, and specific yield than about other model parameters, such 

as specific storage and riverbed conductance (fig. 13).

Figure 13. Composite scaled sensitivities for model parameters in the groundwater-flow model for the Hurricane Bench area, Utah. 

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

Composite scaled sensitivity

hk1

hani1

hk2

hani2

hk3

hani3

hk4

hani4

hk5

hani5

hk6

hani6

vka1

sy1ss1

kayentavk

kayentahk

rch1rch2rch3

resk1resk2resk3

rivbed

hani_kynta

hi_K

ndd

wdd

wdsd

wcwcd

n_wells

Parameters

Aquifer properties

Natural recharge

Reservoir recharge

Drain, well, and river discharge

Adjustable parameter, background 

shows aquifer properties (blue), 

or reservoir recharge (red)
Numerical Simulation of Groundwater Flow 21

Sensitivity analysis and regression were based on weighted 

residuals between simulated and observed values. Because 

water-level and drawdown observations were more accurate 

than recharge and discharge observations, they had larger 

weights and, typically, larger weighted residuals. The weighting and the fact that water-level and drawdown observations were much more numerous and widely distributed than 

recharge and discharge observations in the numerical model 

caused them to have more influence on the parameters. As 

a result, some parameters that control recharge from the 

reservoir and discharge to the drains were adjusted outside of 

regression to more closely match those observations.

Parameter Correlation Coefficient

Parameter correlation coefficients were used to evaluate 

whether parameters can be estimated uniquely by regression 

(Hill and Tiedeman, 2007, p. 53). A correlation coefficient 

with an absolute value close to 1.00 indicates that the two 

parameters involved likely cannot be estimated uniquely. Generally, absolute values greater than 0.95 are cause for concern, 

but values as small as 0.85 are reported in UCODE output 

because these lower correlations can affect the uncertainty of 

parameter estimates. Where parameter correlation was high, 

the value of the correlated parameter with the smallest CSS 

was assigned a value and not adjusted during regression. 

One set of parameters (hk5 and hani5) in the model yielded 

a parameter correlation coefficient near 1.0; therefore, during regression, hani5 was set to 1.07 and not adjusted. This 

value, based on trial and error, was less than the other hani 

values (2.0–3.52) because of the likely greater permeability 

along both 40- and 280-degree fracture orientations mapped 

by Hurlow (1998) just north of the study area near Sandstone 

Mountain (fig. 2).

Influence Statistics 

The RESIDUAL_ANALYSIS program (Poeter and others, 

2005) was used to calculate additional statistics useful for 

finding observation errors and model construction errors, and 

for highlighting changes in model construction that can lead to 

more realistic values of model parameters and better calibration. Two metrics of these are the Cook’s D and DFBETAS 

statistics. The Cook’s D statistic identified observations that, 

if omitted, would cause the greatest changes in estimated 

parameter values. The DFBETAS statistic identified observations that were influential in the estimation of each parameter 

(Poeter and others, 2005, p. 181). If nonlinear regression led to 

unreasonable parameter values in the Hurricane Bench model, 

or if regression statistics indicated that a parameter change 

improved one part of the model but made the fit worse in other 

areas, these statistics were used to make model changes.

Nonlinear Regression

Nonlinear regression was used to find parameter values 

that produce simulations that best fit the observations by 

adjusting parameter values to minimize the least-squares 

objective function by using weighted residuals. The fit 

between model simulated values and associated observations 

was quantified by using a weighted least-squares objective 

function (Hill and Tiedeman, 2007, p. 27−28). The weighting 

used in this objective function was based on observation errors 

presented in the “Groundwater Budget” section of this report 

and assumes that errors in the observations are uncorrelated.

Parameter Adjustment and Final Values 

During model calibration, values of horizontal hydraulic 

conductivity and horizontal anisotropy of the Navajo Sandstone, and values of reservoir bed conductance, were changed 

by modifying both the distribution of parameters (assigning and adjusting spatial zones) and by changing parameter 

values. Vertical anisotropy of the Navajo Sandstone, specific 

yield, specific storage, and vertical hydraulic conductivity 

of the Kayenta Formation were modified only by changing 

parameter values; these parameters were not assigned spatial zones. Parameters for reservoir bed conductance were 

divided and refined even though the CSS values were not high 

(fig. 13). This achieved a better match between simulated 

water levels and observed water levels in some locations. Values of other parameters were not changed during calibration 

but are discussed in the following sections.

Hydraulic Conductivity and Anisotropy

Sensitivity analysis indicated that the observations provided enough information to represent the horizontal hydraulic 

conductivity of the Navajo Sandstone by using at least six 

zones (fig. 12) and 12 parameters (table 2), rather than one 

homogenous value. In addition to hydraulic conductivity, 

each zone of the Navajo Sandstone includes a parameter for 

horizontal anisotropy to simulate enhanced hydraulic conductivity along the dominant north-northeast trending fractures in 

the study area. Both the spatial distribution of the zones and 

the values of the horizontal hydraulic conductivity parameters 

were adjusted during model calibration to cause simulated 

water levels to more closely match observed water levels. The 

final adjusted horizontal anisotropy ratios ranged from 1.07 

to 3.65, resulting in hydraulic-conductivity values of 1.47 to 

38 ft/d in the N 40 E direction (table 3). The adjusted vertical 

anisotropy (vka1) final value of 0.8 is within the known range 

of 0.13 to 2.7 (Heilweil and others, 2000) but implies that the 

Navajo Sandstone is more permeable in the vertical direction 

than in the horizontal direction (NW-SE) in the model. The 

final hydraulic property estimates from regression all lie near 

or within the previously published ranges of values. Horizontal hydraulic conductivity and anisotropy of the Navajo 

Sandstone in zone 1 of the model (hk1, hani1) are also very 

sensitive parameters because of the large size of this zone and 

its abundance of observations. Typically, this area would be 

split on the basis of its high degree of sensitivity, but it was 

left intact because it contains a large area where fractures have 

not been mapped because of soil cover.
22 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

The northwestern portion of the model domain near the 

Virgin River has a simulated hydraulic conductivity that is 

more than an order of magnitude greater than the surrounding 

hydraulic conductivities. This condition is plausible because 

of the proximity of the area to Sandstone Mountain, an area 

known to be highly fractured and known to have hydraulicconductivity values of 32 ft/d on the basis of the Anderson 

Junction aquifer test.

Horizontal hydraulic conductivity for the Kayenta Formation was held constant at 0.5 ft/d. This value was not adjusted 

because the associated parameter was insensitive to model 

observations. The value of this fixed parameter was based on 

previous work presented in the “Aquifer Properties” section of 

this report. The final calibrated vertical hydraulic-conductivity 

parameter for the Kayenta Formation was 0.01 ft/d; values of 

simulated reservoir recharge were sensitive to this parameter 

based on trial and error adjustments. In this study, flux observations had a much greater uncertainty than head observations. 

As a result, UCODE indicated that this parameter was insensitive (fig. 14) because there was less weight assigned to flux 

observations than to head observations.

Specific Yield and Specific Storage

One specific yield parameter (sy1) was assigned for the 

Navajo Sandstone; this value applies to the uppermost saturated cell in the model at any given location. Under unconfined conditions that exist in the Navajo Sandstone in the Hurricane Bench area, specific yield was considered to be equal 

to effective porosity. One specific storage parameter (ss1) was 

defined, which represents fully saturated conditions in either 

the Navajo Sandstone or the Kayenta Formation. The value of 

both parameters was adjusted during model calibration. The 

final value of 0.12 for sy1 was determined by regression and is 

within the range of reported effective porosity values of 0.10 

to 0.17 for the Hurricane Bench area (table 3). The final value 

of 4.0 x 10-6 per foot for ss1 was assigned by trial and error 

because model observations did not provide enough information to estimate it (fig. 13). The assigned value was slightly 

greater than known values, which range from 7.0 x 10-7 to 

2.5 x 10-6 per foot (Heilweil and others, 2000).

Natural Recharge as Infiltration of Precipitation

The model was very sensitive to the natural recharge 

parameters (rch1, rch2, rch3) prior to the completion of Sand 

Hollow Reservoir in 2002, but became less sensitive to these 

parameters when reservoir recharge dominated the system. 

Three parameters were assigned to simulate natural recharge 

as infiltration of precipitation. Each recharge parameter (rch1, 

rch2, rch3) was a multiplier value for an associated zoned area 

(fig. 10) representing intermediate, low, or high infiltration 

rates, respectively. Simulated water-level observations were 

very sensitive to rch1 and rch3 (fig. 13). None of the three 

parameters were adjusted, however, because of the relatively 

large amount of information available about infiltration in the 

basin (Heilweil and McKinney, 2007). All other parameter 

values determined in this model calibration were estimated on 

the assumption that estimated recharge from precipitation is 

correct. The resulting average natural recharge rates for each 

of the fixed parameter zones (rch1, rch2, rch3) were 20, 8, and 

31 mm/yr, respectively. The annual total simulated natural 

recharge to the Hurricane Bench study area was approximately 

2,100 acre-ft.

Reservoir Recharge

Initially, one parameter representative of the hydraulic 

conductivity of the reservoir bed was assigned to the spatial extent of the reservoir with a value equal to the average 

horizontal hydraulic conductivity of the Navajo Sandstone. 

Comparison of calculated reservoir recharge rates from 2002 

to 2009 to simulated recharge from the reservoir by using one 

zone and one conductivity through time showed that simulated 

recharge was approximately 30 percent more than the estimated recharge of 86,000 acre-ft, however. To help improve 

the match between model-simulated and calculated recharge 

from Sand Hollow Reservoir, the reservoir bed conductance 

parameter was split into four parameters (resk1, resk2, resk3, 

and hi_K) and assigned to two different zones within the 

boundary of Sand Hollow Reservoir (fig. 14). These parameters were defined even though the CSS values were not high 

(fig. 13) because better matches were achieved with calculated 

reservoir recharge, and between simulated water levels and 

observed water levels in some locations. The reservoir-bed 

hydraulic-conductivity parameters (resk1, resk2, and resk3) 

that simulate clogging processes varied temporally, and all 

corresponded to a zone beneath the deepest part of the reservoir. The parameter hi_K did not vary temporally and corresponded to a higher permeability zone on the edge of the reservoir (fig. 14). This zone occupied the approximate perimeter 

of the reservoir and represented areas in the reservoir that were 

less likely to be subject to limiting processes, such as siltation 

either by their relatively steep grades and exposed bedrock or 

by the late date at which reservoir filling caused saturation. 

The zone that occupied the low flat-lying areas of the reservoir (fig. 14) was defined by three parameters. Reservoir-bed 

conductivity in this zone was defined as a time-variable value, 

where time-dependent conductivity values were linearly interpolated in three segments defined by three parameter values 

(resk1, resk2, and resk3) representing March 2002, August 

2002, and July 2007, respectively. The first segment, between 

March 2002 and August 2002, represented the initial high rate 

of recharge; the middle segment, between August 2002 and 

July 2006, represented a period of generally decreasing rates 

of recharge; and the last segment, from July 2006 to December 2009, represented a period of generally static calculated 

recharge rates (fig. 15).

Drain and River Discharge

Simulated discharge from the shallow drains was compared 

to reported pumping from the drains during model calibration. 

Three parameters (ndd, wdd, and wdsd) corresponding to the 

North Dam Drain, West Dam Drain, and combined West Dam 

and West Dam Spring Drains, respectively, were assigned a 
Numerical Simulation of Groundwater Flow 23

Figure 14. Bed conductance zones of the Reservoir Package representing vertical hydraulic conductivity beneath Sand Hollow 

Reservoir, Hurricane Bench area, Utah. 

0 0.5 1 1.5 MILES

0 0.5 1 1.5 KILOMETERS

EXPLANATION

113°23'45" 113°22'30" 113°21'15"

37°8'45"

37°7'30"

37°6'15"

Reservoir-bed conductance zones 

and parameters for Sand Hollow 

Reservoir

resk1, resk2, resk3

hi_K

Sand

Hollow

Reservoir

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983
24 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

conductance multiplier value equal to the horizontal hydraulic conductivity of the Navajo Sandstone in proximity to the 

drain, which was approximately 0.6 to 0.7 ft/d. Observations 

were not sensitive to these parameters (fig. 13).

One parameter representing riverbed hydraulic conductivity (rivbed) was assigned a value of 30 ft/d for all stretches of 

the Virgin River. Simulated water levels at observation wells 

were not sensitive to the riverbed parameter, and this large 

value allowed groundwater to easily leave the system at this 

boundary.

Model Accuracy

Because of the large amount of water-level, recharge, 

and discharge data associated with Sand Hollow Reservoir, 

the model provided a relatively accurate estimate of hydraulic properties in the vicinity of the reservoir. Because of the 

paucity of both water-level measurements and aquifer stresses 

away from the reservoir, hydraulic properties and fluxes 

simulated by the model in other parts of the Hurricane Bench 

area had a greater degree of uncertainty, such that other combinations of recharge, discharge, and aquifer properties could 

yield similar or improved results. Observations provided little 

information for some model parameters, such as rivbed (fig. 

13); as a consequence, it is difficult to assess how well these 

parameters were estimated in the simulation.

Reservoir Recharge, Drain Discharge, and 

River Discharge

The simulated total reservoir recharge between 2002 and 

2009 was approximately 94,000 acre-ft, which is within 10 

percent of the calculated value of 86,000 acre-ft (fig. 7). Temporal trends closely matched calculated reservoir recharge, 

except during spring 2002 and spring 2003. These periods 

corresponded to large pumping events during the early filling 

of the reservoir, and the mismatches were likely the result of 

not simulating vadose zone saturation, which is a limitation of 

the Reservoir Package in MODFLOW.

The simulated total drain discharge from 2003 to 2009 was 

approximately 9,500 acre-ft—about 30 percent less than the 

reported amount of 14,200 acre-ft. Simulating drain pumping 

as head-dependent discharge through a hydraulic conductivity contrast (drain conductance) likely underestimated the 

discharge because it did not simulate the effects of dewatering the adjacent saturated flow system due to pumping. As a 

result of the vertical discretization in the model, however, the 

Well Package could not be used to simulate this drain pumping 

because wells were simulated as single node discharge points 

in the center of a cell. The thickness of layer 1 near Sand Hollow was approximately 200 ft; thus, the point of stress for a 

well would have been 80 ft below the base of the drains when 

using the Well Package. At this greater depth, the stress would 

have resulted in an inaccurate groundwater-flow gradient 

Figure 15. Temporally varying reservoir-bed hydraulic conductivity parameters that define a portion of the total recharge simulated in 

flat low-lying areas in Sand Hollow Reservoir, Hurricane Bench area, Utah.

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Total rechargerate,in feetperday

Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06 Jan-07 Jan-08 Jan-09 Jan-10

Total groundwater-recharge rate

Temporally-variable hydraulic conductivity

Time-variable hydraulic conductivity, in feet per day

resk1

resk2

resk3

0

0.1

0.2

0.3

0.4

0.5

0.6
Numerical Simulation of Groundwater Flow 25

in proximity to the reservoir, adversely affecting simulated 

reservoir recharge. Though trends in simulated drain discharge 

generally approximated those of the reported drain pumping 

(fig. 9), there were large differences in late 2006 and from 

mid-2007 through 2009. These periods coincided with operation of the pumps in the West Dam Spring drain, which were 

turned on intermittently. Differences between simulated and 

reported drain discharge during 2004 and 2005 were much less 

because the North Dam Drain was the only drain in operation 

until July 2005 and was pumped on a more constant basis.

Final model output yielded an approximate gain of 2,160 

acre-ft to the Virgin River during 1975 (steady-state conditions) for the model domain, with 1,740 acre-ft in 1995, and 

1,880 acre-ft in 2009. The year 1995 represented the lowest 

discharge to the Virgin River during the simulation because 

1995 corresponded to the greatest stress to the aquifer as a 

result of well withdrawals from 1975 to 2009. Discharge to the 

Virgin River had not recovered to pre-reservoir steady state 

amounts by 2009. The simulated discharge to the Virgin River 

was within the range of half of the estimated 5,220 acre-ft/

yr discharge to the river for reaches of the river measured by 

Herbert (1995) within the study area.

Simulated Water Levels

Simulated water levels were generally within 10 ft of measured water levels at most observation sites (fig. 16) throughout the period of simulation. Larger differences between 

simulated and observed water levels of up to approximately 

15 ft were generally found at observation sites located more 

than 1 mi north of the reservoir near wells WD RJ and WD 17 

through WD 20 (fig. 17). Except for WD RJ, these monitoring 

wells had a much shorter period of record compared to WD 

1-14 that limited a comparison of trends in the simulated and 

observed hydrographs, which are not presented in this report 

but are available in Heilweil and Marston (2011). An observation well located even farther north, (C-42-14)12dbb-1, had 

similar simulated versus observed water-level differences 

of 0 to 15 ft for the period of record, but the shape of the 

simulated hydrograph compared to the observed hydrograph 

showed an adequate match (fig. 16). The overall combined fit 

of the model to both the water-level and reservoir recharge 

observations indicated that the simulated hydraulic properties 

and boundary conditions were well constrained and provided 

a reasonable representation of the groundwater system. The 

simulated increase in water levels in the aquifer resulting 

from reservoir recharge corresponded to a simulated increase 

in groundwater storage of about 70,000 acre-ft from 2002 to 

2009. This value is less than the simulated reservoir recharge 

of 94,000 acre-ft for the same period. The difference is attributed primarily to discharge from wells and drains.

Environmental Tracers

To determine if the model adequately simulated groundwater-flow directions and travel times, MODPATH (Pollock, 

1994) was used to compare simulated travel times from the 

reservoir to the monitoring wells to travel times estimated on 

the basis of the observed arrival of environmental tracers in 

those monitoring wells. The chemical and isotopic characteristics of reservoir recharge water, including the chloride-to-bromide ratio (Cl:Br), major-ion chemistry, chlorofluorocarbons 

(CFCs), tritium (3H), and field parameters (total dissolved-gas 

pressure, dissolved oxygen, and specific conductance), were 

used to track groundwater movement in the aquifer. The concentration of each of these constituents in reservoir recharge is 

significantly different than in the native groundwater, making 

them useful tracers of the movement of reservoir water into 

the surrounding aquifer. The arrival of recharge from the reservoir has been documented in three monitoring wells (WD 6, 

WD 9, and WD 11) near the reservoir (Heilweil and Marston, 

2011). In general, these tracers indicated that reservoir water 

arrived at the nearby wells in the following years: arrival at 

WD 9 between 2003 and 2006, arrival at WD 11 between 2005 

and 2006, and arrival at WD 6 between 2005 and 2009.

MODPATH predicted the simulated arrival time for 

reservoir water at WD 9 between 2003 and 2006, at WD 11 

between 2004 and 2008, and no arrival at WD 6 by 2009 

(fig. 18). Simulated arrival times for WD 9 and WD 11 

matched observed arrival times adequately. In contrast to 

the environmental tracer data, MODPATH particles did not 

arrive at WD 6 because of the pumping influence of well 8, 

which is located between the reservoir and WD 6. Observed 

tracer arrivals in WD 6 were likely the result of more rapid 

groundwater movement in fractures than was simulated by 

the groundwater model, which assumes flow is through a 

homogenous, porous media. Although the fractured areas in 

Hurricane Bench were simulated as zones of greater hydraulic conductivity, they were simulated as porous media rather 

than fracture flow. Both well 8 and WD 6 lie within a zone of 

mapped fractures (Heilweil and others, 2000; P.D. Rowley, 

Geologic Mapping, Inc., unpub. data, 2004). Open fractures 

observed by Heilweil and Solomon (2004) could allow for 

greater localized groundwater velocities not explicitly simulated by MODFLOW.

Model Projections

The groundwater-flow model was used to predict the future 

movement of reservoir water through the groundwater system 

toward the Virgin River. The model also was used to estimate 

the additional groundwater storage if the reservoir continued to operate until a new steady state groundwater system 

was achieved. The projection assumed the same distribution 

of groundwater withdrawal from wells as was recorded in 

September 2009, which was approximately equal to the March 

2006 through December 2009 average. During this stress 

period, most of the pumping took place at well 8 and well 9, 

with lesser withdrawals at well 2 and well 21. In the future 

projection, the reservoir stage was specified as constant at the 

December 2009 value of 3,044 ft, or about 16 ft below full 

stage. The effects of withdrawals from other wells or different 

reservoir stage conditions were not projected. The projection 

(beginning in January 2010) simulated these conditions for 
26 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 16. Observed and simulated water levels at monitoring wells in Sand Hollow, Hurricane Bench area, Utah.

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

April

2001

April

2003

April

2005

April

2007

April

2009

April

2011

April

2001

April

2003

April

2005

April

2007

April

2009

April

2011

WD1 WD2

WD3 WD4

WD5 WDRJ

Observed

Simulated

Water-level altitude, in feet above sea level

See figure 3 for location of monitoring wells
Numerical Simulation of Groundwater Flow 27

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

April

2001

April

2003

April

2005

April

2007

April

2009

April

2011

April

2001

April

2003

April

2005

April

2007

April

2009

April

2011

Observed

Simulated

WD6

WD10 WD11

WD8 WD9

WD7

Water-level altitude, in feet above sea level

See figure 3 for location of monitoring wells

Figure 16. Observed and simulated water levels at monitoring wells in Sand Hollow, Hurricane Bench area, Utah.—Continued
28 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 16. Observed and simulated water levels at monitoring wells in Sand Hollow, Hurricane Bench area, Utah.—Continued

April

2001

April

2003

April

2005

April

2007

April

2009

April

2011

April

2001

April

2003

April

2005

April

2007

April

2009

April

2010

Observed

Simulated

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

3,060

2,840

2,860

2,880

2,900

2,920

2,940

2,960

2,980

3,000

3,020

3,040

WD14

WD12 WD13

December

1973

December

1983

December

1993

December

2003

December

2013

(C-42-13)18bcb-1 Simulated

(C-42-13)18bcb-1 Observed

(C-42-14)12dbb-1 Simulated

(C-42-14)12dbb-1 Observed

(C-42-13)6bac Simulated

(C-42-13)6bac Observed

(C-42-14)25adc-2 Simulated

(C-42-14)25adc-2 Observed

Water-level altitude, in feet above sea level

See figure 3 for location of monitoring wells

one 1,000-year stress period by using ten 100-year time steps. 

To estimate the travel time of reservoir water to the Virgin 

River, MODPATH (Pollock, 1994) was used to track particles 

with starting locations at the top of layer 1 underneath Sand 

Hollow Reservoir. The effective porosity was assumed to 

equal the calibrated specific yield of 0.12. The earliest arrival 

times at the Virgin River were after approximately 500 years, 

with an average arrival time for most particles of 800 years 

(fig. 19). Some of the particles that had starting locations 

beneath the central portions of the reservoir had longer travel 

times and flow paths that extended into the deeper parts of the 

Navajo Sandstone and the Kayenta Formation. Approximately 

50 percent of the particles had arrival times in excess of 1,000 

years. Steady-state conditions, where simulated groundwater 

recharge and discharge are approximately equal and there is 

no change in storage, were attained approximately 300 years 

after December 2009. The predicted additional groundwaterstorage capacity associated with reservoir recharge in the Hurricane Bench area after December 2009 was approximately 

325,000 acre-ft.

Model Limitations

The numerical model is a simplified representation of the 

groundwater system and includes limitations regarding the 

simulation of natural recharge, uncertainty in hydraulic conductivity in areas far from the reservoir, assumptions regarding 

discharge to the Virgin River, the use of an equivalent porous 

media model to simulate preferential flow in a fractured 

dual-permeability aquifer system, the assumption of uniform 

storage properties, not including the possibility of upwelling 

higher-salinity groundwater as a form of recharge, and the 

simulation of withdrawals from shallow drains using the headdependent Drain package.

In this model, natural recharge as infiltration of precipitation was set to values obtained from Heilweil and McKinney 

(2007) rather than adjusting it during parameter estimation. 

Also, average long-term recharge rates were assigned rather 

than accounting for seasonal and annual variability.
Numerical Simulation of Groundwater Flow 29

Figure 17. Difference between observed and simulated water levels measured in December 2009 for monitoring wells around Sand 

Hollow Reservoir, Hurricane Bench area, Utah.

EXPLANATION

Monitoring well head residual—In feet, as a 

difference between measured water levels in 

December 2009 and model-computed head

Hurricane Bench study area

Note: Negative residuals (green number) indicate 

simulated heads greater than observed heads

Sand

Hollow 

Reservoir 

113°23'45" 113°21'15"

37°8'45"

37°7'30"

37°6'15"

0 0.5 1 MILE

0 0.5 1 KILOMETER

WD 18

-15.15

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

(C-42-14)12dbb-1

-12.68

WD 20

-11.66

WD 19

-9.24

WD RJ

-13.03

WD 18

-15.15

WD 17

-14.67

WD 4

-9.83

WD 6

-3.10 WD 2

0.29

WD 1

0.51

WD 5

3.88

WD 16

4.97

WD 15

3.64

WD 9

12.98

WD 3

9.87

WD 14

1.33 WD 11

-5.27

WD 7

6.79

WD 8

2.04

WD 12

12.99

WD 10

10.85

WD 13

3.36
30 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

Figure 18. Simulated groundwater-flow paths from Sand Hollow Reservoir, Hurricane Bench area, Utah.

WD 9

WD 8

WD 7

WD 6

WD 5

WD 4

WD 3

WD 2

WD 1

WD 20

WD 18 WD 19

WD 17

WD 16

WD 15

WD 12

WD 11

WD 10

WD RJ

(C-42-13)18bcb-1

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

EXPLANATION

113°23' 113°22'

37°08'

37°07'

37°06'

Sand

Hollow

Reservoir

Particle tracking travel time, by year

2002

2003

2004

2005

2006

2007

2008

2009

Monitoring well and number

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

WD 18
Numerical Simulation of Groundwater Flow 31

Figure 19. Projected travel time of managed aquifer recharge moving through the Navajo Sandstone in Sand Hollow, Hurricane Bench 

area, Utah.

113°22'30"

37°07'30"

113°20'

37°10'

Imagery from U.S. Department of Agriculture, 2011

Universal Transverse Mercator projection, Zone 12

North American Datum of 1983

0 0.5 1 1.5 MILES

0 0.5 1 1.5 KILOMETERS

EXPLANATION

Particle tracking travel time to 

sink or endpoint, in years since 

2009

0 to 24

Greater than 24 to 55

Greater than 55 to 94

Greater than 94 to 143

Greater than 143 to 199

Greater than 199 to 286

Greater than 286 to 448

Greater than 448 to 727

Greater than 727 to 911

Greater than 911 to 1,000

Sand 

Hollow

Reservoir
32 Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir

The model simulated a greater rise in water levels farther 

from the reservoir, such as in the area of well WD RJ, than 

was observed. Hydraulic-conductivity values assigned to this 

part of the model could have been too low. The calibrated 

permeability closer to the reservoir was a better approximation where the aquifer had undergone more stress and more 

observations for calibration were available.

Another source of uncertainty in the model is the representation of groundwater discharge to the Virgin River. Discharge 

was simulated to the river from the Navajo Sandstone south 

of the river. Measured groundwater discharge to the river 

included flow from north of the river, which was not simulated 

in the model. The amount of this groundwater discharge, from 

the southern part of the Navajo Sandstone cannot be quantified 

but only estimated as less than the 7.2 ft3/s measured in 1995. 

Water-level data near the river and in the northeast portions of 

the model grid would improve the simulation of groundwater 

to surface-water interaction along the entire reach of the Virgin 

River within the model domain.

The aquifer was assumed to be a homogenous porous 

media, and the model did not explicitly simulate flow through 

specific fractures. The current model only approximated 

fracture flow through bulk anisotropy. This could explain the 

later-than-observed arrival times of environmental tracers at 

well WD 6. Simulating flow along individual fracture zones 

was not within the scope of this study.

One value for specific yield was used for the entire simulated Navajo Sandstone. Estimates of additional groundwater 

storage capacity based on future model predictions assumed a 

constant value for specific yield. Spatial and temporal variability in specific yield could increase or decrease this value, but 

no data were available to evaluate spatial variability.

A study by Heilweil and others (2000) indicated that 

upwelling groundwater from underlying formations contributes up to 1,100 acre-ft/yr to the Navajo Sandstone and 

Kayenta Formation for a 6-mi2 area located west of Hurricane. 

This estimate was based only on a solute mass-balance calculation, rather than direct measurement of this potential source 

of recharge. Also, this upwelling of higher salinity groundwater is located in the vicinity where large withdrawals were 

made in the 1990s and could have been a short-term transient 

response to pumping. Because of its relatively small amount 

and the uncertainty associated with its long-term occurrence, it 

was not considered in this study’s groundwater budget. Further 

investigation of this form of recharge would be necessary 

before inclusion in this groundwater-flow model.

Another limitation of the model is the simulation of 

withdrawals from shallow drains by using the Drain Package. Use of the Well Package to simulate this discharge would 

have required much finer vertical discretization of the shallow 

part of the aquifer system than was practical for this regional 

groundwater-flow model.

Summary

The purpose of this report was to present the construction, 

calibration, and projected results of a numerical simulation of 

groundwater movement in the Hurricane Bench area, including managed aquifer recharge from Sand Hollow Reservoir. 

This study incorporated data presented in previous reports 

that documented pre-reservoir vadose-zone and groundwater 

conditions in Sand Hollow and the Hurricane Bench area prior 

to March 2002; pond and trench infiltration studies adjacent 

to the reservoir; and post-reservoir groundwater conditions, 

water budgets, and estimates of groundwater recharge from 

the reservoir from March 2002 through December 2009.

Natural recharge as infiltration of precipitation was 

approximately 2,100 acre-ft/yr from 2002 to 2009 (totalling about 16,800 acre-ft). Discharge included seepage to 

the Virgin River, municipal- and irrigation-well withdrawals, and drains at the base of reservoir dams. Seepage from 

the Hurricane Bench to the Virgin River was estimated to be 

about 2,100 acre-ft/yr under natural conditions. Total well 

withdrawals from 1975 to 2009 ranged from 130 to about 

3,000 acre-ft/yr. Managed aquifer recharge from Sand Hollow 

Reservoir to the underlying Navajo aquifer was calculated 

to be about 86,000 acre-ft from 2002 to 2009. Groundwater 

levels in monitoring wells closest to the reservoir generally 

rose between 2002 and 2006 and then fluctuated with reservoir 

altitude and nearby pumping from production wells. Water 

levels in monitoring wells farther from the reservoir were still 

rising through 2009.

The objective of the numerical groundwater-flow model 

was to obtain a simulation that reasonably represented groundwater recharge, movement, discharge, and measured water 

levels in the Hurricane Bench area. The simulated recharge 

from Sand Hollow Reservoir between 2002 and 2009 was 

approximately 94,000 acre-ft, which was within 10 percent of 

the calculated value. The simulated total drain discharge from 

2003 to 2009 was approximately 9,500 acre-ft. This value 

was 4,700 acre-ft less than reported drain pumpage because 

passive drains were simulated by using a seepage face that did 

not simulate the spatially larger effects of dewatering due to 

pumping. A head-dependent groundwater discharge of about 

1,740 acre-ft to the Virgin River was simulated during 1995, 

increasing to about 1,880 acre-ft in 2009. Simulated arrival 

times of environmental tracers at monitoring wells WD 9 

and WD 11 were within the ranges of observed arrival times 

from 2003 to 2008. In general, the groundwater-flow model 

adequately simulated water levels, and most simulated water 

levels were within 10 ft of the measured water levels. The 

overall fit of the model to water levels, estimated reservoir 

recharge, and measured discharge indicated that the simulated 

hydraulic properties and boundary conditions provided a reasonable representation of the groundwater-flow system.

The simulated change in water levels caused by managed 

aquifer recharge at Sand Hollow Reservoir corresponded to 

an increase in groundwater storage of about 70,000 acre-ft 

between 2002 and 2009. The model predicted an estimated 
References Cited 33

325,000 acre-ft of total additional groundwater storage for the 

Hurricane Bench area when steady state is eventually reached 

(approximately 300 years from 2009), assuming constant reservoir operating conditions and well withdrawals, as observed 

in late 2009. The simulated earliest arrival time of reservoir 

water to the Virgin River was 500 years from 2009, with an 

average arrival time of 800 years from 2009.

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Marston and Heilweil—Numerical Simulation of Groundwater Movement and Managed Aquifer Recharge from Sand Hollow Reservoir, Hurricane Bench 

area, Washington County, Utah—Scientific Investigations Report 2012–5236

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