U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5262

Prepared in cooperation with the Carson Water Subconservancy District

Assessing Potential Effects of Changes in Water Use With 

a Numerical Groundwater-Flow Model of Carson Valley, 

Douglas County, Nevada, and Alpine County, California
Cover photograph: Simulated rate and direction of groundwater flow in Carson Valley.
Assessing Potential Effects of Changes 

in Water Use With a Numerical 

Groundwater-Flow Model of Carson 

Valley, Douglas County, Nevada, and 

Alpine County, California

By Richard M. Yager, Douglas K. Maurer, and C.J. Mayers

Prepared in cooperation with the Carson Water Subconservancy District

Scientific Investigations Report 2012–5262

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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Suggested citation:

Yager, R.M., Maurer, D.K., and Mayers, C.J., 2012, Assessing potential effects of changes in water use with a numerical groundwater-flow model of Carson Valley, Douglas County, Nevada, and Alpine County, California: U.S. Geological 

Survey Scientific Investigations Report 2012–5262, 73 p.
iii

Contents

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

Introduction.....................................................................................................................................................2

Previous studies....................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Geographic Setting .......................................................................................................................................4

Geologic Setting.............................................................................................................................................6

Hydrologic Setting ......................................................................................................................................10

Streamflow and Mountain Runoff ...................................................................................................10

Irrigation...............................................................................................................................................13

Groundwater Pumpage......................................................................................................................13

Transmissivity .....................................................................................................................................14

Storage .................................................................................................................................................16

Groundwater Flow .......................................................................................................................................21

Exchange of Groundwater and Surface Water ............................................................................21

Groundwater Levels ...........................................................................................................................21

Water Budget ......................................................................................................................................26

Groundwater Budget..........................................................................................................................26

Groundwater-Flow Model ..........................................................................................................................29

Model Design.......................................................................................................................................29

Discretization..............................................................................................................................29

Boundaries..................................................................................................................................29

Inflows ................................................................................................................................29

Outflows..............................................................................................................................33

Streamflow Routing..........................................................................................................34

Hydraulic Properties .................................................................................................................36

Hydraulic Conductivity.....................................................................................................36

Storage ...............................................................................................................................37

Model Calibration................................................................................................................................37

Observations...............................................................................................................................37

Model Fit......................................................................................................................................39

Simulated groundwater flow ............................................................................................................48

Parameters...........................................................................................................................................48

Water Budgets ....................................................................................................................................52

Model uncertainty...............................................................................................................................54

Model Application........................................................................................................................................56

Base Scenario.....................................................................................................................................57

Scenario 1 ............................................................................................................................................59

Scenarios 2A and 2B..........................................................................................................................59

Scenario 3 ............................................................................................................................................61

Prediction Uncertainty.......................................................................................................................66

Summary .......................................................................................................................................................67

Hydrogeology.......................................................................................................................................67
iv

Groundwater-Flow Model .................................................................................................................67

Model Application...............................................................................................................................69

References Cited..........................................................................................................................................70

Appendix 1. Modifications to MODFLOW ................................................................................................75

Appendix 2. Modifications to PCGN Solver.............................................................................................83

Figures

1. Map showing location of the Carson River basin and the Carson Valley study area ............. 3

2. Map showing vegetation and land use in and near Carson Valley, 2005 .................................. 5

3. Graph showing annual precipitation at Minden, Nevada, for period of record 

(1930–2005) and average annual precipitation for 1971–2000; Palmer Drought 

Severity Index for western Nevada, 1930–2005 .......................................................................... 6

4. Map showing locations of towns and approximate depth to water (spring 2006) in 

Carson Valley .................................................................................................................................... 7

5. Map showing geologic units, depth to bedrock, faults, and locations of aquifer tests; 

selected wells likely penetrating Tertiary sediments; and east-west hydrogeologic 

cross section .................................................................................................................................... 8

6. Hydrogeologic cross-section A-A’ and conceptual model of groundwater flow paths 

in Carson Valley ................................................................................................................................ 9

7. Map showing location of U.S. Geological Survey and Federal Water Master gages, 

perennial and ephemeral watersheds, areas where effluent has been applied for 

irrigation, and irrigation distribution system in Carson Valley ............................................... 11

8. Graph showing groundwater pumpage for irrigation, municipal, domestic and other 

uses in Carson Valley, 1946–2005 .................................................................................................13

9. Graph showing relation between transmissivity estimates obtained from specific 

capacity and aquifer-test data ......................................................................................................14

10. Map showing location of wells with specific-capacity data from drillers’ logs .....................15

11. Graph showing distribution of transmissivity in wells with screen lengths greater 

than 99 feet in Quaternary and Tertiary sediments....................................................................16

12. Map showing location of wells with screened lengths greater than 99 feet and the 

kriged distribution of log transmissivity ...................................................................................... 17

13. Map showing geologic units and the estimated distribution of specific yield in basinfill sediments of Carson Valley from Dillingham (1980) ............................................................ 20

14. Map showing water-table altitude in Carson Valley, spring 1998 ............................................ 22

15. Graph showing water-level fluctuations,1981–2006, in wells on the western alluvial 

fans; shallow wells on the valley floor; deep flowing wells on the valley floor; deep 

irrigation wells near the valley floor; wells on the eastern side of the valley; and 

wells with water levels affected by changes in water use .................................................... 23

16. Map showing components of the mean annual water budget for Carson Valley, water 

years 1990–2005 .............................................................................................................................. 27

17. Diagram showing inflow and outflow of water from agricultural use; domestic use ........... 28

18. Map showing model domain and distribution of Quaternary and Tertiary sediments in 

Carson Valley groundwater model and location of hypothesized Hot Springs Fault ......... 30

19. Map showing assumed lateral and vertical distribution of Quaternary and Tertiary 

sediments..........................................................................................................................................31
v

20. Map showing distribution of mountain-front recharge, effluent application, and 

evapotranspiration in Carson Valley groundwater model ....................................................... 32

21. Map showing location of municipal, domestic, and irrigation supply wells in Carson 

Valley groundwater model ............................................................................................................ 34

22. Graph showing residual plots for simulated absolute heads in Carson Valley 

groundwater model: relation between simulated and observed values; relation 

between simulated values and weighted residuals ................................................................ 39

23. Map showing simulated water table in the summer quarter of water year 2005, 

locations and rates of surface-water exchange, and spatial distribution of 

absolute-head residuals ............................................................................................................... 42

24. Graphs showing residual plots for simulated water-level changes in the Carson 

Valley groundwater model: relation between simulated and observed values; 

relation between simulated values and weighted residuals .................................................. 43

25. Map showing simulated and observed water-level hydrographs for selected wells 

in the Carson Valley ....................................................................................................................... 44

26. Graphs showing residual plots for simulated flows in Carson Valley groundwater model: 

relation between simulated and observed values; relation between simulated values and 

weighted residuals ......................................................................................................................... 45

27. Graphs showing simulated and observed flow hydrographs for streamflow gaging 

stations in the Carson Valley: Carson River near Carson City; Carson River near 

Genoa; West Fork Carson River at Dressler Lane; Brockliss Slough at Ruhenstroth 

dam; and Brockliss Slough at Scossa box ................................................................................. 46

28. Graph showing simulated and observed loss in flow in the Carson River, water 

years 1971–2005 .............................................................................................................................. 47

29. Simulated water table along section A-A’ in the summer quarter of water year 2005.......... 48

30. Image showing simulated rate and direction of groundwater flow in Carson Valley............ 49

31. Maps showing simulated rate of recharge from streamflow diverted to irrigated 

fields in Carson Valley in water year 2005: spring quarter; summer quarter........................ 50

32. Graph showing composite-scaled sensitivities of optimal parameters in Carson 

Valley groundwater model ............................................................................................................ 51

33. Graph showing groundwater budgets simulated for water years 1971–2005......................... 52

34. Graphs showing cumulative water budgets simulated for water years 1971–2005: 

inflow; outflow................................................................................................................................. 53

35. Graphs showing projected Palmer drought severity index; range in pumping 

specified in 55-year predictive simulations................................................................................ 57

36. Map showing water-table decline from 2010 through 2060 simulated by the base 

scenario using the Carson Valley groundwater model ............................................................ 58

37. Graph showing annual groundwater budgets, 2006–60 simulated by base scenario .......... 59

38. Map showing additional groundwater withdrawals included in scenario 1, additional 

water-table decline from 2010 through 2060 simulated by scenario 1 compared to 

the base scenario, and simulated water-level hydrographs at selected wells for the 

base scenario and scenario 1 ...................................................................................................... 60

39. Graphs showing annual groundwater budgets from 2051 through 2060 simulated by 

base scenario and scenario 1; annual loss in flow in the Carson River, 2051–60 

predicted by base scenario and scenario 1............................................................................... 61

40. Map showing additional groundwater withdrawals included in scenario 2A, 

additional water-table decline 2010–60 simulated by scenario 2A compared to 

the base scenario, and simulated water-level hydrographs at selected wells for 

the base scenario and scenario 2A ............................................................................................ 62
vi

41. Map showing additional groundwater withdrawals included in scenario 2B, 

additional water-table decline 2010–60 simulated by scenario 2B, and simulated 

water-level hydrographs at selected wells for base scenario and scenario 2B ................ 63

42. Graphs showing annual groundwater budgets, 2051–60 simulated by base scenario 

and scenario 2A; annual loss in flow in the Carson River, 2051–60 predicted by 

base scenario and scenario 2A ................................................................................................... 64

43. Graphs showing annual groundwater budgets, 2051–60 simulated by base scenario 

and scenario 2B; annual loss in flow in the Carson River 2051–60 predicted by base 

scenario and scenario 2B ............................................................................................................. 64

44. Map showing additional groundwater withdrawals included in scenario 3, 

agricultural land removed from irrigation, additional water-table decline 2010–60 

simulated by scenario 3, and simulated water-level hydrographs at selected 

wells for base scenario and scenario 3 ..................................................................................... 65

45. Graphs showing annual groundwater budgets, 2051–60, simulated by base scenario 

and scenario 3; annual loss in flow in the Carson River 2051–60 predicted by base 

scenario and scenario 3 ................................................................................................................ 66

Tables

1. USGS and FWM gaging stations on the Carson River and selected tributaries and 

diversion ditches ............................................................................................................................ 12

2. Summary of aquifer tests in Carson Valley .................................................................................. 18

3. Mean annual water budget for Carson Valley for water years 1990 through 2005 ............... 27

4. Components of the groundwater budget for Carson Valley....................................................... 28

5. Groups of stream segments for which streambed hydraulic conductivity was specified ... 35

6. Groups of observations included in model calibration: heads ;flows ...................................... 38

7. Parameter values in Carson Valley groundwater model specified or estimated 

through nonlinear regression....................................................................................................... 40

8. Simulated mean-annual groundwater budget for Carson Valley from water year 

1971 through 2005 .......................................................................................................................... 52

9. Simulated annual water budgets for Carson Valley in the 1992 and 1997 water years......... 54

10. Future water-use scenarios represented in 55-year predictive simulations .......................... 56
vii

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in) 2.54 centimeter (cm)

inch (in) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

)

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

million gallons (Mgal) 3,785 cubic meter (m3

)

cubic foot (ft3

) 28.32 cubic decimeter (dm3) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

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

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3/s)

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

/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

foot per second (ft/s) 0.3048 meter per second (m/s)

foot per minute (ft/min) 0.3048 meter per minute (m/min)

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

foot per year (ft/yr) 0.3048 meter per year (m/yr)

cubic foot per second (ft3

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

cubic foot per second per square

mile [(ft3

/s)/mi2]

0.01093 cubic meter per second per square

kilometer [(m3/s)/km2]

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

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

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

mile per hour (mi/h) 1.609 kilometer per hour (km/h) 

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) 

Application rate

pounds per acre per year 

[(lb/acre)/yr]

1.121 kilograms per hectare per year

[(kg/ha)/yr]

Leakance

foot per day per foot [(ft/d)/ft] 1 meter per day per meter

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter

[(mm/yr)/m]

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

°C = (°F-32)/1.8
viii

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

(NAVD 88).

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

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.
ix

Acknowledgments

The authors thank Mimi Moss of Douglas County Community Development, Planning 

Division, Ron Roman and Carl Ruschmeyer of Douglas County Public Works, and Edwin 

James of the Carson Water Subconservancy District for providing information and assistance 

in developing the predictive simulations. Rob Anderson of RO Anderson Engineering, Michael 

Bennett of Lumos and Associates, Inc., Dale Bugenig of EcoLogic, Mark Gonzales of the 

Gardnerville Water Co., Carl Rushmeyer of Douglas County Public Works, Bruce Scott of 

Resource Concepts, Inc., and Bob Spellberg of the Gardnerville Ranchos General Improvement 

District who provided aquifer-test data that were used in this report. Donald Callahan, Julian 

Larrouy, Gary Stone, and Dave Wathen of the Federal Water Master’s Office provided data and 

information on irrigation diversions and historical irrigation practices. Scott Clark, Tony Eng, 

and Steve Walmsley of the Nevada Department of Water Resources provided data and information on the distribution of groundwater pumping in Carson Valley and areas irrigated with 

perennial streams of the Carson Range. 
x
Assessing Potential Effects of Changes in Water Use With 

a Numerical Groundwater-Flow Model of Carson Valley, 

Douglas County, Nevada, and Alpine County, California

By Richard M. Yager, Douglas K. Maurer and C.J. Mayers

Abstract

Rapid growth and development within Carson Valley in 

Douglas County, Nevada, and Alpine County, California, has 

caused concern over the continued availability of groundwater, and whether the increased municipal demand could either 

impact the availability of water or result in decreased flow 

in the Carson River. Annual pumpage of groundwater has 

increased from less than 10,000 acre feet per year (acre-ft/yr) 

in the 1970s to about 31,000 acre-ft/yr in 2004, with most of 

the water used in agriculture. Municipal use of groundwater 

totaled about 10,000 acre-feet in 2000. In comparison, average 

streamflow entering the valley from 1940 to 2006 was 344,100 

acre-ft/yr, while average flow exiting the valley was 297,400 

acre-ft/yr. 

Carson Valley is underlain by semi-consolidated Tertiary 

sediments that are exposed on the eastern side and dip 

westward. Quaternary fluvial and alluvial deposits overlie 

the Tertiary sediments in the center and western side of the 

valley. The hydrology of Carson Valley is dominated by 

the Carson River, which supplies irrigation water for about 

39,000 acres of farmland and maintains the water table less 

than 5 feet (ft) beneath much of the valley floor. Perennial and 

ephemeral watersheds drain the Carson Range and the Pine 

Nut Mountains, and mountain-front recharge to the groundwater system from these watersheds is estimated to average 

36,000 acre-ft/yr. 

Groundwater in Carson Valley flows toward the Carson 

River and north toward the outlet of the Carson Valley. An 

upward hydraulic gradient exists over much of the valley, and 

artesian wells flow at land surface in some areas. Water levels 

declined as much as 15 ft since 1980 in some areas on the 

eastern side of the valley. Median estimated transmissivities 

of Quaternary alluvial-fan and fluvial sediments, and Tertiary 

sediments are 316; 3,120; and 110 feet squared per day (ft2/d), 

respectively, with larger transmissivity values in the central 

part of the valley and smaller values near the valley margins. 

A groundwater-flow model of Quaternary and Tertiary 

sediments in Carson Valley was developed using MODFLOW 

and calibrated to simulate historical conditions from water 

years 1971 through 2005. The 35-year transient simulation 

represented quarterly changes in precipitation, streamflow, 

pumping and irrigation. Inflows to the groundwater system 

simulated in the model include mountain-front recharge from 

watersheds in the Carson Range and Pine Nut Mountains, valley recharge from precipitation and land application of wastewater, agricultural recharge from irrigation, and septic-tank 

discharge. Outflows from the groundwater system simulated 

in the model include evapotranspiration from the water table 

and groundwater withdrawals for municipal, domestic, irrigation and other water supplies. The exchange of water between 

groundwater, the Carson River, and the irrigation system was 

represented with a version of the Streamflow Routing (SFR) 

package that was modified to apply diversions from the irrigation network to irrigated areas as recharge.

The groundwater-flow model was calibrated through 

nonlinear regression with UCODE to measured water levels 

and streamflow to estimate values of hydraulic conductivity, 

recharge and streambed hydraulic-conductivity that were represented by 18 optimized parameters. The aquifer system was 

simulated as confined to facilitate numerical convergence, and 

the hydraulic conductivity of the top active model layers that 

intersect the water table was multiplied by a factor to account 

for partial saturation. Storage values representative of specific 

yield were specified in parts of model layers where unconfined 

conditions are assumed to occur. The median transmissivity 

(T) values (11,000 and 800 ft2/d for the fluvial and alluvial-fan 

sediments, respectively) are both within the third quartile of T

values estimated from specific-capacity data, but T values for 

Tertiary sediments are larger than the third quartile estimated 

from specific-capacity data. The estimated vertical anisotropy 

for the Quaternary fluvial sediments (9,000) is comparable to 

the value estimated for a previous model of Carson Valley. The 

estimated total volume of mountain-front recharge is equivalent to a previous estimate from the Precipitation-Runoff Modeling System (PRMS) watershed models, but less recharge is 

estimated for the Carson Range and more recharge is estimated for the Pine Nut Mountains than the previous estimate.

Simulated flow paths indicate that groundwater flows faster 

through the center of Carson Valley and slower through the 

lower hydraulic-conductivity Tertiary sediments to the east. 

Shallow flow in the center of the valley is towards drainage 
2 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

channels, but deeper flow is generally directed toward the 

basin outlet to the north. The aquifer system is in a dynamic 

equilibrium with large inflows from storage in dry years and 

large outflows to storage in wet years. Pumping has historically been less than 10 percent of outflows from the groundwater system, and agricultural recharge has been less than 10 

percent of inflows to the groundwater system. Three principal 

sources of uncertainty that affect model results are: (1) the 

hydraulic characteristics of the Tertiary sediments on the eastern side of the basin, (2) the composition of sediments beneath 

the alluvial fans and (3) the extent of the confining unit represented within fluvial sediments in the center of the basin. 

The groundwater-flow model was used in five 55-year 

predictive simulations to evaluate the long-term effects of 

different water-use scenarios on water-budget components, 

groundwater levels, and streamflow in the Carson River. The 

predictive simulations represented water years 2006 through 

2060 using quarterly stress periods with boundary conditions 

that varied cyclically to represent the transition from wet to 

dry conditions observed from water years 1995 through 2004. 

The five scenarios included a base scenario with 2005 pumping rates held constant throughout the simulation period and 

four other scenarios using: (1) pumping rates increased by 70 

percent, including an additional 1,340 domestic wells, (2A) 

pumping rates more than doubled with municipal pumping 

increased by a factor of four over the base scenario, (2B) 

pumping rates of 2A with 2,040 fewer domestic wells, and (3) 

pumping rates of 2A with 3,700 acres removed from irrigation. The 55-year predictive simulations indicate that increasing groundwater withdrawals under the scenarios considered 

would result in as much as 40 ft and 60 ft of water-table 

decline on the west and east sides of Carson Valley, respectively. The water table in the central part of the valley would 

remain essentially unchanged, but water-level declines of as 

much as 30 ft are predicted for the deeper, confined aquifer. The increased withdrawals would reduce the volume of 

groundwater storage and decrease the mean downstream flow 

in the Carson River by as much as 16,500 acre-ft/yr. If, in 

addition, 3,700 acres were removed from irrigation, the reduction in mean downstream flow in the Carson River would be 

only 6,500 acre-ft/yr. The actual amount of flow reduction is 

uncertain because of potential changes in irrigation practices 

that may not be accounted for in the model. The projections of 

the predictive simulations are sensitive to rates of mountainfront recharge specified for the Carson Range and the Pine Nut 

Mountains. The model provides a tool that can be used to aid 

water managers and planners in making informed decisions. 

A prudent management approach would include continued 

monitoring of water levels on both the east and west sides of 

Carson Valley to either verify the predictions of the groundwater-flow model or to provide additional data for recalibration 

of the model if the predictions prove inaccurate.

Introduction

Rapid population growth and suburban development in 

Carson Valley, west-central Nevada (fig. 1), is causing concern over the continued availability of water resources in the 

future. The increased demand for groundwater for municipal 

use within the valley could impact the availability of water. 

In addition, the increased municipal demand for groundwater 

could also reduce discharge to the Carson River and, in turn, 

impact downstream water users that depend on sustained river 

flow. A numerical groundwater-flow model that simulates 

the exchange of water between the aquifer and surface water 

in Carson Valley was developed to evaluate the impacts of a 

range of future groundwater management alternatives. The 

model provides a tool that can be used to aid water managers 

and planners in making informed decisions. 

The potential effects of groundwater withdrawals on flow 

in the Carson River have previously been estimated using a 

groundwater-flow model developed for Carson Valley in the 

early 1980s (Maurer, 1986; Prudic and Wood, 1995). Prior 

model simulations indicated that flow in the Carson River 

was the source of 70 to 80 percent of groundwater pumped in 

Carson Valley (Maurer, 1986, p. 105; Prudic and Wood, 1995, 

p. 28). These model simulations also indicated that the reduction of irrigated lands resulted in increased flow in the Carson 

River that could offset the effects of pumping (Maurer, 1986, 

p. 90). These results were considered preliminary, however, 

because the volume of pumping during the model-calibration 

period (1981–83) was less than 5 percent of the total water 

budget, the period of model calibration was short, and the grid 

cells used in the model were relatively large (1 square mile; 

Maurer, 1986, p. 82). 

Previous studies

In February 2003, the U.S. Geological Survey (USGS) 

began a study in cooperation with Douglas County, Nevada, 

and the Carson Water Subconservancy District to improve 

estimates of water-budget components in Carson Valley in 

order to address concerns over continued residential growth. 

As part of the study, five reports have been published. The 

first two developed improved estimates of the distribution of 

precipitation in Carson Valley (Maurer and Halford, 2004) and 

flow in perennial streams tributary to the Carson Valley (Maurer and others, 2004). The third report provided estimates of 

(1) evapotranspiration (ET) from various types of vegetation 

and land use, (2) recharge from precipitation on the northern 

and eastern parts of Carson Valley, and (3) the location and 

rates of streamflow losses and gains from streams and irrigation ditches on the valley floor (Maurer and others, 2006). The 

fourth report presented estimates of water-budget components 

for Carson Valley and presented preliminary estimates of the 

potential effects of changes in water use (Maurer and Berger, 

2007). The fifth report used the Precipitation-Runoff Modeling System (PRMS; Leavesly, 1983) watershed models of the 
Introduction 3

Figure 1. Location of the Carson River basin and the Carson Valley study area.

DOUGLAS

COUNTY

ALPINE

COUNTY

WASHOE

COUNTY

PERSHING

COUNTY

CHURCHILL

COUNTY

MINERAL

COUNTY

LYON

COUNTY

STOREY

COUNTY

Carson

Lake

Stillwater Point

Reservoir

Lahontan

Reservoir

Pyramid

Lake

Carson

Sink

Truckee

Canal

Truckee

River

Carson

River

Walker

Walker

River

River

West

Walker River

East

East

West

Fork

Fork

Humboldt

River

120° 119° 118°

40°

39°

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Universal Transverse Mercator projection, zone 11

North American Datum of 1983 NEVADA CALIFORNIA

Carson

City

Dayton

Dayton

Valley

Eagle

Valley

NEVADA

CALIFORNIA

Reno

Minden

Genoa

Gardnerville

Markleeville

Woodfords

Fernley

Hazen

Fallon

Bradys Hot

Springs

Lahontan

Valley

Churchill

Valley

Silver

Springs Lahontan

Dam

Carson Valley

study area

Carson Regna

Pine Nut Mountains

EXPLANATION

Boundary of Carson River basin

Boundary of Hydrographic Areas

Gaging station

0 10 20 30 MILES

20100 30 KILOMETERS

Lake

Tahoe

Los

Angeles

Las Vegas

CARSON

RIVER

BASIN

San

Francisco

Reno

CALIFORNIA NEVADA

Sierra

Nevada
4 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Carson Range and Pine Nut Mountains to estimate groundwater recharge and ephemeral streamflow from the mountains 

surrounding the valley (Jeton and Maurer, 2007).

The final phase of work included a refinement of the 

groundwater-flow model for Carson Valley. Previous reports 

had cited the need for a more detailed groundwater-flow model 

to better estimate the discharge of water through evapotranspiration by native plants and irrigated crops, and the volumes of 

water exchanged between the groundwater and surface water 

systems in Carson Valley (Maurer and Berger, 2007, p. 44, 

55, 56). The model developed for the study described herein 

provides estimates of the groundwater-budget components for 

Carson Valley and was used to evaluate the potential effects of 

changes in water use on groundwater levels in Carson Valley 

and on flow in the Carson River exiting Carson Valley.

Purpose and Scope

The purpose of this report is to describe the development, 

calibration, and application of the groundwater-flow model for 

Carson Valley. The report presents a discussion of the hydrogeologic setting of Carson Valley, including a description of 

the geology, estimates of streamflow, groundwater withdrawals and aquifer properties, and a description of the groundwater-flow system. The report also presents results of five 

predictive simulations for water years1 2006–60 made with the 

calibrated model to estimate the potential effects of changes in 

water use. The predictive simulations included a base simulation in which water use remained unchanged from 2005 

conditions; two simulations that increased municipal pumping 

from 9,000 acre-feet per year (acre-ft/yr) to 22,000 and 33,000 

acre-ft/yr, respectively; a fourth simulation in which the higher 

municipal pumping rate was concentrated in several municipal 

wells rather than dispersed through many domestic wells; and 

a final simulation in which the higher municipal-pumping rate 

was combined with a removal of about 3,700 acres of land 

from irrigation. 

Geographic Setting 

Carson Valley is mainly in Douglas County, south of 

Carson City, Nevada’s capital (fig. 1). The southern end of the 

valley extends about 3 miles (mi) into Alpine County, California. The floor of the valley is oval-shaped, about 20-mi long 

and 8-mi wide, and slopes from about 5,000 feet (ft) above sea 

level at the southern end to about 4,600 ft at the northern end. 

On the western side of Carson Valley the Carson Range of the 

Sierra Nevada rises abruptly from the valley floor with peaks 

ranging from 9,000 to 11,000 ft in altitude; on the eastern side, 

the Pine Nut Mountains rise more gradually to peaks ranging 

from 8,000 to 9,000 ft. 

1 A water year extends from October through the following September and 

is designated in the calendar year in which it ends.

For purposes of this study, the boundary of Carson Valley was delineated as a 396-square mile (mi2; 253,570-acre) 

subarea within the Carson Valley Hydrographic Area, which 

extends to the headwaters of the East and West Forks of the 

Carson River (fig. 1). The model domain was selected to 

include only unconsolidated and semi-consolidated sediments 

that transmit most of the groundwater in the aquifer system 

that underlies the Carson Valley. The headwaters of the West 

and East Forks of the Carson River upstream of gaging stations are outside the model domain. Bedrock underlies the 

points where the West and East Forks of the Carson River 

cross the domain boundary, however, restricting groundwater 

inflow from areas to the south. 

On the western side of the valley, sagebrush and bitterbrush cover steep alluvial fans, and manzanita and ponderosa 

pine trees cover the slopes of the Carson Range (fig. 2). 

Alluvial fans and foothills of the Pine Nut Mountains on the 

eastern side of the valley are covered with sagebrush and rabbitbrush, but pinyon and juniper trees occur at higher altitudes. 

The valley floor corresponds to the area covered with pasture 

grasses and crop lands (mainly alfalfa), and native phreatophytes (plants whose roots tap the water table). Greasewood, 

rabbitbrush, and big sage occupy the northern end of the 

valley floor, and riparian willows and cottonwood trees line 

the Carson River and many irrigation ditches. A land-use map 

by Maurer and Berger (2007, p. 7) was used as the basis for 

the specification of evapotranspiration rates (the consumptive 

use of water by pasture grasses, crops, and phreatophytes) 

and rates of groundwater recharge and discharge from various 

types of land use in the model developed for this study. 

Carson Valley lies in the rain shadow of the Sierra Nevada, 

and annual precipitation decreases greatly eastward from the 

crest of the mountains to the valley floor. Annual precipitation at the town of Minden (fig. 3) averages 8.4 inches (in.) 

for the period 1971–2000 (National Oceanic and Atmospheric 

Administration, 2002, p. 12). In contrast, the crest of the Carson Range receives about 40 inches per year (in/yr) , and the 

crest of the Pine Nut Mountains receives from 15 to 18 in/yr 

(Maurer and Halford, 2004, p. 35). From 1984 to 1992 and 

from 1999 to 2004, conditions were drier than normal with 

annual precipitation considerably less than average (fig. 3A). 

The Palmer Drought Severity Index (PDSI, National Oceanic 

and Atmospheric Administration, 2006), based on long-term 

weather conditions, provides an indication of regional meteorological wet and dry periods (fig. 3B). The PDSI indicates dry 

conditions have dominated western Nevada since about 1999.

Douglas County has grown from a population of about 

28,000 in 1990 to 46,000 in 2006, an increase of 66 percent 

(Economic Research Service, 2007). The major towns in 

Carson Valley are Minden, Gardnerville, and Genoa; however, 

much of the increased growth has taken place in outlying 

subdivisions. The major subdivisions include the Gardnerville 

Ranchos and Ruhenstroth south of Gardnerville, Fish Spring 

Flat east of Gardnerville, and Johnson Lane and Indian Hills 

north of Minden (fig. 4). In addition, residential development 

has spread along the eastern and western margins of the valley 
Geographic Setting 5

Figure 2. Vegetation and land use in and near Carson Valley, 2005.

LYON COUNTY

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

Clear Creek

Dangberg

Reservoir

Buckeye

Creek

Creek

Pine Nut

Creek

Indian

East

Fork

West

Fork

Carson

River

50

88

207

395

89

395

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Vegetation and land-use imagery from the Carson Valley Conservation District 

(BAE SYSTEMS Advanced Technologies, Inc. 

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

119° 52' 30" 119° 45' 119° 37' 30"

39°00'

38°52'30"

CARSON RANGE

Gardnerville

Minden

Genoa

Muller Lane

Johnson Lane

Mottsville Lane

Centerville Lane

Gardnerville

Ranchos

Jacks

Valley Indian

Hills

Hot Springs

Mountain EXPLANATION

VEGETATION

Wetland

Riparian

Sagebrush and bitterbrush

Rabbitbrush and greasewood

Pasture grasses

Alfalfa or other crops

Trees or other mountain-block 

vegetation

LAND USE

Residential or commercial

Open water

Sprinkler-irrigated areas

Carson Valley study area 

boundary

CARSON CITY BOUNDARY

DOUGLAS COUNTY
6 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

floor, and into scattered locations throughout the valley floor 

on land that historically has been agricultural. 

Geologic Setting

Geologic units that were delineated in Carson Valley by 

Maurer and Berger (2007, p. 9) based on the map of Stewart and Carlson (1978) were modified for the design of the 

groundwater-flow model in this study (fig. 5). Granitic, metamorphic, and volcanic rocks of Jurassic to Tertiary age that 

form the mountain blocks surrounding the valley floor were 

grouped into a single consolidated rock unit. These consolidated rocks are overlain by Tertiary sediments and Quaternary 

alluvial fans and fluvial sediments. Maurer and Berger (2007, 

p. 9) delineated Quaternary gravel deposits capping the Indian 

Hills in the northwestern part of the valley and Quaternary 

eolian sand deposits in the northeastern part of the valley. 

In this study the relatively thin Quaternary gravel deposits 

capping the Indian Hills (Stewart, 1999, p. 2) were grouped 

with Tertiary sediments. The Quaternary eolian sand deposits are also likely thin veneers in the northeastern part of the 

valley and so were grouped with the underlying alluvial-fan 

deposits.

Basin and range faulting during the Tertiary Period from 

7 to 10 million years ago produced the present topography 

in Carson Valley (Muntean, 2001, p. 9). Faulting uplifted 

the Carson Range and the Pine Nut Mountains and downdropped consolidated rock beneath the floor of Carson Valley. 

Volcanic rocks and sediments deposited contemporaneous 

with this faulting have since become semi-consolidated. 

These semi-consolidated Tertiary sediments are exposed 

mainly on the eastern side of the valley, but dip towards the 

west and probably underlie the entire valley (figs. 5, 6). The 

Tertiary sediments vary in degree of compaction and lithology 

(Pease, 1980, p. 14), varying from fine-grained and tuffaceous 

siltstone with isolated lenses of sandstone and conglomerate 

to sandstone and conglomerate (Muntean, 2001, p. 18–31). 

Figure 3. A, Annual precipitation at Minden, Nevada, for period of record (1930–2005) and average annual precipitation for 1971–2000; 

B, Palmer Drought Severity Index for western Nevada, 1930–2005.

Annual precipitation at Minden, Nevada, 1930–2005

Palmer drought severity index

A

B Annual precipitation, in inches Palmer drought severity index

-8

-6

-4

-2

0

2

4

6

8

10

0

2

4

6

8

10

12

14

16

18

20

1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Average annual precipitation

8.4 in. (1971–2000)

Area of severe to extreme wetness

Area of severe to extreme drought
Geologic Setting 7

Figure 4. Locations of towns and approximate depth to water (spring 2006) in Carson Valley. 

LYON COUNTY

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

Clear

Creek

Dangberg

Reservoir

Buckeye

Creek

Creek

Pine

Nut

Creek

Indian

East

Fork

West

Fork

Carson

River

89

395

88

395

50

207

50

88

208

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

119° 52' 30" 119° 30' 

38°45'

39°07'30"

CARSON RANGE

Gardnerville

Minden

Genoa

Gardnerville

Ranchos

Jacks

Valley Indian

Hills

Hot Springs

Mountain

EXPLANATION

Ruhenstroth

50

Fish

Spring

Flat

Snowshoe Thompson Ditches

Diamond

Valley

PINE NUT MOUNTAINS

Carson City

Douglas County

 Airport

Line of equal depth to water, 

approximately located. 

Interval, in feet, is variable. 

Datum is land surface 

modified for spring 2006 

from Maurer and Peltz 

(1994)

Carson Valley study area 

boundary

Long-term streamflow 

gaging station on Carson 

River 

CARSON CITY BOUNDARY

DOUGLAS COUNTY

Johnson Lane

Muller Lane

Mottsville Lane

Centerville

Lane

5

5

50

200

150

100

50

400

100

20

20

5

20

20

20

200

5

5

200

200

200

50

100

150

100

20

200

50

50

100

50 100

50

50

150

200

200

50

100

20

20

150

100

200

100

200
8 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

0

1000

1500

500 2000

200

1200

2500

750

3000

100

3500

4000

0

200

200

2000

0

200

200

200

200

2500

200

500

100

0

500

0

0

500

200

0

2500

0

1000

0

1200

0

0

0

0

0

1000

1000

200

0

0

1500

0

1500

200

0

0

500

200

0

200

1200

Dangberg

Reservoir

River

East

Fork

West

Fork

Carson

Buckeye

Creek

Creek

Pine

Nut

89

395

88

395

207

50

88

208

50

Johnson Lane

5

1

2

3

8

4

9

7

6

11

18

28

27

35

37

36

30

12

29

33

32

31

34

26 23

25 22

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

119°48' 119°27'

39°10'

38°50'

A A'

CARSON RANGE

PINE NUT MOUNTAINS

Carson

City

CARSON CITY BOUNDARY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

Walleys

Hot Springs

408

695

1270

10

14

15 16

17

19 20

21

24

A’A

Quaternary

200

EXPLANATION

Tertiary

Tertiary to

Triassic 408

10

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated bedrock

Accommodation zone (Muntean, 2001)

Quaternary fault

Depth to bedrock in feet below land surface. Contour 

interval 500 feet, unless noted

Location of hydrogeologic cross section

Carson Valley study area boundary

Hypothesized Hot Springs Mountain fault

Well with depth to top of Tertiary sediments, bedrock, 

or bottom of well, in feet

Well with aquifer test, number is well number, table 2

Base geology from Stewart and Carlson (1978) at 1:250,000 scale.

Modified from geologic mapping by Stewart (1999) at 1:100,000 scale; by 

Armin and John (1983), Stewart and Noble (1979) at 1:62,500 scale; and 

by dePolo and others (2000), Garside and Rigby (1998), and Muntean 

(2001) at 1:24,000 scale. Faults from U.S. Geological Survey (2003). 

Dressler

Bench

Figure 5. Geologic units, depth to bedrock, faults, and locations of aquifer tests; selected wells likely penetrating Tertiary sediments; 

and east-west hydrogeologic cross section. 
Geologic Setting 9

Muntean (p. 35) divided the Tertiary sediments into subsections on the basis of age and provided a detailed description of 

the stratigraphy of the Tertiary sediments in each subsection. 

The aggregate thickness of the Tertiary sediments is estimated 

to exceed 3,000 ft (Muntean, pl. 5). 

Throughout the Quaternary Period (within the past 2 

million years), unconsolidated sediments, herein referred to 

as basin fill, have been deposited by the Carson River and 

tributary streams from the Carson Range and the Pine Nut 

Mountains. The Quaternary sediments deposited by tributary 

streams form coalescing alluvial fans that consist of coarseto fine-grained, poorly sorted mixtures of sand, gravel, silt, 

and clay at the base of the mountains (fig. 5). The Quaternary 

deposits of the Carson River are fluvial sediments that consist 

of well-sorted sand and gravel, interbedded with fine-grained 

silt and clay from over-bank flood deposits. These fluvial sediments underlie most of the valley floor. The alluvial-fan and 

fluvial sediments likely interfinger beneath the margins of the 

valley, but detailed information is not available on the distribution of these sediments with depth. The Dressler Bench, 

mapped as an alluvial-fan deposit at the southern end of the 

valley between the East and West Forks of the Carson River, 

is actually a river terrace consisting of boulders, cobbles, and 

gravel (Dillingham, 1980, p. 18; dePolo and others, 2000).

The mountains bounding Carson Valley are west-tilted 

structural blocks (Stewart, 1980, p. 113), and Carson Valley overlies the down-dropped western part of the Pine Nut 

Mountain block (Moore, 1969, p. 18). Continued westward 

tilting of the blocks is indicated by recent faulting along the 

base of the Carson Range (Pease, 1980, p. 15) and by displacement of the Carson River to the extreme western side of 

the valley (Moore, 1969, p. 18). A steep, well-defined normal 

fault has created a 5,000-ft escarpment along the eastern side 

of the Carson Range, but a wider zone of faulting consisting 

of numerous faults is present on the eastern side of the valley 

(fig. 5). Muntean (2001) describes an accommodation zone 

on the eastern side of the valley where the vertical offset of 

faults north and south of the zone decreases toward the zone. 

A continuous section of tilted Tertiary sedimentary deposits is 

present within the accommodation zone. The zone is thought 

to be related to the location of maximum displacement on the 

fault bounding the western side of Carson Valley, directly to 

the west across the valley (Muntean, 2001, p. 80). 

A gravity survey by Maurer (1984) estimated the depth to 

bedrock underlying the Quaternary and Tertiary sediments in 

Carson Valley. The gravity data indicate a deep basin beneath 

the western part of the valley, as much as 5,000 ft below land 

surface about 1.5 mi southeast of Wallys Hot Springs (fig. 5). 

The basin rapidly decreases in depth eastward from about 

3,500 ft below land surface near US 395 to 1,500 ft below land 

surface about 2 mi east of the highway. The rapid change in 

depth extends from the western side of Hot Springs Mountain 

on the north through a ridge on the bedrock surface 1 mi east 

of Minden (fig. 5). The steep eastern side of the bedrock valley 

corresponds to the western extent of Quaternary faults and 

suggests the presence of a major buried fault. This hypothesized fault is informally called the Hot Springs Mountain 

fault in this report. Bedrock is relatively shallow east of the 

Hot Springs Mountain fault; depth to bedrock ranges from less 

than 200 ft east of Johnson Lane to as much as 1,000 ft north 

of Fish Spring Flat. Further east, structural basins beneath 

the northeast and southeast sides of the valley have estimated 

depths as great as 2,500 ft and 1,000 ft below land surface, 

respectively.

Sediments that overlie bedrock on the western side of the 

valley consist of both Quaternary and Tertiary sediments. 

Tertiary sediments constitute the entire thickness of sediments 

overlying bedrock on the eastern side of Carson Valley, with 

the exception of Fish Spring Flat (fig. 6). The geometry of 

geologic units within the transition from Tertiary sediments 

Figure 6. Hydrogeologic cross-section A-A’ and conceptual model of groundwater flow paths in Carson Valley. 

0

-1,000

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

Altitude, in feet above sea level

0 5 10 15 20

Distance, in miles

Vertical exaggeration X5

Pine Nut

Mountains

Carson

Range

A’

A Perennial and

ephemeral 

Infiltration of streamflow

ephemeral 

streamflow

Tertiary sediments

Tertiary 

sediments

Alluvial fan

sediments

Fluvial

sediments Confining 

layers

Pumping

ET

GW-SW

exchange

Hypothesized

Hot Springs Mountain fault

Consolidated 

bedrock

Groundwater

 inflow

Groundwater

 inflow

US 395

well log

Carson

River

?

?

Upward 

gradient

Fish Spring Flat

two miles south of section
10 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

exposed on the eastern side of the valley to Quaternary sediments that cover the western side of the valley has not been 

well characterized. The Tertiary sediments probably underlie the alluvial fans on the eastern side of the valley and are 

assumed to underlie basin-fill sediments to the west. The 

Tertiary sediments could have either been down-dropped 

sequentially to the west by a wide zone of faults on the eastern 

side of the valley, or offset within a narrow zone along the 

hypothesized Hot Springs Mountain fault. 

The semi-consolidated nature of the Tertiary sediments 

makes it difficult to differentiate these sediments from unconsolidated Quaternary sediments based on descriptions in most 

drillers’ logs. During drilling the semi-consolidated sediments 

are likely broken up and disaggregated, resulting in an appearance similar to that of unconsolidated sediments. Drillers’ logs 

from wells located in exposures of Tertiary sediments often 

indicate layers of clay exceeding 100 ft in thickness; the drillers’ descriptions included the adjectives “cemented” for sand 

and gravel layers and “hard” for clay layers. Descriptions of 

“cemented” sand or gravel or “hard” clay have also been noted 

for sediments at shallow depths where Tertiary sediments are 

likely hundreds of feet deep, however, suggesting that these 

descriptive terms from drillers’ logs are not always a reliable 

means for identifying Tertiary sediments. It is also difficult to 

determine the thickness of clay that could reliably be used to 

indicate the presence of Tertiary sediments. For these reasons 

the depth to Tertiary sediments beneath Quaternary sediments 

and Fish Spring Flat is uncertain.

A few drillers’ logs provide descriptions from which the 

presence or absence of Tertiary sediments can be inferred. 

On the eastern side of the valley floor, one log describes a 

“tight and unproductive formation” from about 408 to 700 feet 

below land surface that probably represents the top of the 

Tertiary sediments (fig. 5). On the western side of the valley 

drillers’ logs suggest that semi-consolidated Tertiary sediments 

have not been penetrated. One log for a well near the bedrock 

ridge south of Gardnerville indicates granite was encountered 

at a depth of 695 ft below land surface, but another log for a 

1,270-ft deep well near Walleys Hot Springs indicates only 

unconsolidated basin-fill sediments were encountered (fig. 5). 

Neither log contained descriptions that could be interpreted as 

clearly identifying Tertiary sediments. 

Information from the drillers’ logs was used to estimate 

the geometry of geologic units in the zone of transition from 

Tertiary sediments on the east to basin-fill sediments on the 

west. It was assumed that east of the Hot Springs Mountain 

fault, the top of the Tertiary sediments dips westward at about 

2.5 degrees from the westernmost exposure to a depth of 408 

ft at the well where the log indicates the top of Tertiary sediments had been penetrated (fig. 6). It was also assumed that 

the top of the Tertiary sediments is offset downward about 800 

ft along the inferred Hot Spring Mountain fault, which is consistent with the log south of Gardnerville where granite was 

encountered at 695 ft and no Tertiary sediments were detected 

(fig. 5).

Hydrologic Setting 

The hydrology of Carson Valley is dominated by the 

Carson River. The East and West Forks of the Carson River 

enter the valley from the south and flow northward to join 

near Genoa (fig. 4). The combined flow continues northward 

and exits the valley southeast of Carson City. Streamflow is 

diverted across the valley floor through a network of unlined 

canals, ditches, and sloughs that form a complex system for 

flood irrigation of about 39,000 acres of mainly alfalfa and 

native pasture grasses (fig. 2; Maurer and Berger, 2007, p. 44). 

Infiltration of surface water through channels of the Carson 

River and the irrigation system and beneath irrigated fields 

maintains a shallow water table beneath much of the western 

side of the valley floor where depth to ground water is less 

than 5 ft (fig. 4). 

Streamflow and Mountain Runoff 

Streamflows in the East and West Forks of the Carson River and in the Carson River where it exits the valley near Carson City have been measured with overlapping 

periods of record from 1940 to 2006 (gages 3, 1, and 9, 

respectively; fig. 7). Average annual flow for the period was 

76,200 acre-ft/yr at the West Fork Carson River near Woodfords, California; 267,900 acre-ft/yr at the East Fork Carson 

River near Gardnerville; and 297,400 acre-ft/yr at the Carson 

River near Carson City (table 1). Irrigation diversions from the 

river have been recorded by the Federal Water Master (FWM) 

since 1984 at 12 locations on major ditches (fig. 7 and table 1). 

Mountain runoff is an additional source of streamflow into 

Carson Valley. Twelve perennial streams tributary to the Carson River drain the Carson Range within the study area (fig. 7; 

Maurer and others, 2004). Clear Creek also drains the Carson 

Range and joins the Carson River in the northernmost part 

of the study area. The combined annual flow of the perennial 

streams including Clear Creek averaged about 25,000 acre-ft 

for water years 1990–2002 (Maurer and others, 2004, p. 14). 

Flow from the 12 perennial streams (fig. 7) is used for irrigation on land adjacent to the Carson Range during the spring 

and summer quarters of each water year (termed the irrigation season herein). Three perennial streams drain the southern and eastern parts of Carson Valley. Indian Creek drains 

uplands adjacent to the Carson Range south of the study area 

and discharged an average annual flow of about 5,000 acre-ft 

for water years 1990–2002 (Maurer and Berger, 2007, p. 22). 

Buckeye and Pine Nut Creeks drain the Pine Nut Mountains, 

but streamflow only reaches the valley floor and the Carson 

River during spring runoff in extremely wet years or large 

floods. The combined, average annual flow for these two 

streams was about 1,400 acre-ft for water years 1990–2002 

(Maurer and others, 2004, p. 14).

Previous reports have estimated daily volumes of mountain 

runoff. Daily volumes of runoff for the 12 perennial streams 

draining the Carson Range and Clear Creek were computed 
Hydrologic Setting 11

Figure 7. Location of U.S. Geological Survey and Federal Water Master gages, perennial and ephemeral watersheds, areas where 

effluent has been applied for irrigation, and irrigation distribution system in Carson Valley. 

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

Dangberg

Reservoir Carson River

East

Fork

West

Fork

Buckeye

Creek

Creek

Pine

Nut

Clear

Creek

Creek

Indian

Muller Lane

89

395

88

395

207

50

88

208

50

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

119°48' 119°27'

39°10'

38°50'

CARSON RANGE

PINE NUT MOUNTAINS

CARSON CITY COUNTY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

9

FWM12

EXPLANATION

 

Area of effluent application and storage (Alvarez and Seiler, 2004)

Water-rights irrigation area

U.S. Geological Survey gage and number, table 1

Federal Water Master (FWM) gage and number, table 1

Stream reach, from Maurer and Berger (2007)

Losing reach

Gaining reach

Boundary of perennial watershed

Boundary of ephemeral watershed

Carson Valley study area boundary

FWM12

8

9

6

7

10

FWM5

FWM9

FWM7

FWM8

FWM4 FWM6

FWM3

FWM2

FWM1

FWM10

FWM11

3

1

4

5

2
12 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

or estimated for the period 1990–2002 by Maurer and others 

(2004) using gage data and miscellaneous streamflow measurements. The estimated daily volumes of runoff provided 

information for the calibration of watershed models developed 

by Jeton and Maurer (2007) using the PRMS. The watershed 

models were based on climate data consisting of daily values 

of precipitation, minimum and maximum air temperature, and 

on land-cover and soils maps, and topography. PRMS model 

output for each watershed consisted of daily values for runoff, 

evapotranspiration, and groundwater recharge to basin-fill 

sediments in Carson Valley. Parameters from the perennial 

watershed models were used to estimate mountain runoff from 

the intervening ephemeral watersheds for the period 1990–

2002 (fig. 7; Jeton and Maurer, 2007, p. 45). 

Mountain-front recharge refers to groundwater recharge 

from mountain watersheds to the Quaternary basin-fill sediments in Carson Valley, together with infiltration of streamflow near the base of the mountains. Maurer and Berger (2007, 

p. 36) concluded that runoff from ephemeral watersheds in the 

Carson Range recharges the Quaternary sediments, whereas 

runoff from perennial watersheds does not. Runoff from 

perennial watersheds either flows to the Carson River during 

winter and spring or is diverted to irrigated lands during the 

irrigation season. The conclusion that ephemeral runoff infiltrates to Quaternary sediments is based on the lack of active 

channels across alluvial fans adjacent to the Carson Range. 

The conclusion that runoff from perennial watersheds does 

not infiltrate to Quaternary sediments is based on streamflow 

Table 1. USGS and FWM gaging stations on the Carson River and selected tributaries and diversion ditches. 

[Flow data from gages shown in bold were used in model calibration] 

Gage name Site number, 

fig. 7

Decimal 

latitude 1

Decimal

longitude 1 Station ID 2 Period of record(water years)

Mean annual 

streamflow 

for period of record 

(acre-feet)

West Fork Carson River at Woodfords, 

CA

1 38.76974869 119.8327026 10310000 1938–2006 76,200 3

Fredericksburg Canyon Creek near 

Fredericksburg, CA

2 38.8272222 119.7988899 10310300 1989–2000 3,200

East Fork Carson River near 

Gardnerville, NV

3 38.84598923 119.7022018 10309000 1940–2006 267,900

Indian Creek above mouth near 

Gardnerville, NV 

4 38.8791667 119.7011111 10309035 1995–1998 8,500

Pine Nut Creek near Gardnerville, NV 5 38.8594444 119.5672222 10309050 1981–1997 920

Buckeye Creek near Minden, NV 6 38.98305556 119.5730556 10309070 1981–1997 650

Daggett Creek near Genoa, NV 7 38.96527888 119.8486111 10310400 1988–2006 1,400

Carson River near Genoa 8 38.99777985 119.8225021 10310407 2001–2006 260,600

Carson River near Carson City, NV 9 39.10834122 119.7110977 10311000 1940–2006 297,400

Clear Creek near Carson City, NV 10 39.11333333 119.7972222 10310500 1989–2006 4,100

Snowshoe Thompson Ditch nos. 1 and 2 FWM1 38.77027893 119.8281021 C76A 1984–2006 6,200

Fredericksburg Ditch FWM2 38.81084061 119.7755966 C78 1984–2006 7,600

West Fork at Dressler Lane FWM3 38.86833954 119.7600021 C79 1984–2006 29,000

Brockliss Slough at Ruhenstroth Dam FWM4 38.88972855 119.7789001 C80 1984–2006 26,900

Brockliss Slough at Scossa Box FWM5 38.93389130 119.8060989 C81 1984–2006 22,800

Allerman Canal FWM6 38.89332962 119.6996994 C82 1984–2006 25,300

Virginia Ditch FWM7 38.91500854 119.7149963 C83 1984–2006 11,800

Rocky Slough FWM8 38.91249847 119.7164001 C84 1984–2006 17,200

Edna Slough FWM9 38.91500092 119.7363968 C85 1984–2006 5,400

Cottonwood Slough FWM10 38.92361832 119.7305984 C86 1984–2006 6,100

Henningson Slough FWM11 38.72277908 119.7324982 C88 1984–2006 5,100

Heyburn Ditch FWM12 38.98139954 119.7600021 C89 1984–2006 6,700

1 Datum NAD 1983. 

2 Eight digit ID indicates U.S. Geological Survey gage, others are Federal Water Master gages. 

3 Mean annual runoff for water years 1940–2006 for comparison with East Fork Carson River near Gardnerville, Nevada, and Carson River near Carson City, Nevada. 
Hydrologic Setting 13

measurements of perennial streams where they cross the 

alluvial fans that indicate that little flow is lost to infiltration. 

The lack of infiltration could result from a shallow water table 

that is established near perennial stream channels. Runoff 

from both perennial and ephemeral watersheds in the Pine Nut 

Mountains is assumed to recharge the Tertiary sediments.

Irrigation

Since about 1980, irrigation of land in Carson Valley has 

been guided by the Alpine Decree (U.S. District Court, 1980). 

The Alpine Decree adjudicates the surface-water rights for 

the Carson River and is based largely on historical practices 

in place since the late 1800s (U.S. District Court, 1980, p. 5). 

Under the Decree, the distribution of surface water is based 

on the prior appropriation doctrine, which holds that the first 

person to have put the water to beneficial use has a higher priority, or right, to the water than subsequent users. The Decree 

lists the year that water was first put to beneficial use for about 

800 individual claims, as well as the location and total acreage 

irrigated and the source from which the water is diverted. The 

FWM uses the Alpine Decree to manage the irrigation system 

and distribute water across the valley. Land irrigated by runoff 

from the 12 perennial streams draining the Carson Range, 

referred to as water-rights areas herein (fig. 7), is not included 

in the Alpine Decree.

Other sources of irrigation water include groundwater and 

wastewater effluent. Groundwater is pumped mainly from 

Quaternary sediments and generally discharged to ditches in 

the irrigation system and applied by flood irrigation. However, 

some sprinkler irrigation systems have been in operation since 

the mid-1980s southeast of the Gardnerville Ranchos and 

since the late 1990s on the eastern side of the valley (fig. 7). 

Wastewater effluent imported from the Lake Tahoe basin and 

generated within Carson Valley is also stored and used for 

irrigation (fig. 7). Maurer and Berger (2007, p. 41) estimated 

effluent applied for irrigation averaged about 9,500 acre-ft 

annually during 1990–2005. 

Water applied for irrigation either returns to the irrigation system through surface runoff (return flow, also referred 

to as tail water), is lost through evapotranspiration (ET), or 

infiltrates to the water table. Return flow from irrigated fields 

in Carson Valley in the 1970s was estimated to be 30 to 50 

percent of the applied water (Guitjens and Mahannah, 1972; 

Guitjens and others, 1976; and Guitjens and others, 1978). 

Since the 1980s laser-leveling of agricultural fields has probably improved irrigation efficiency and reduced losses to 

return flow to 20 to 30 percent of the applied water (Arlen 

Neil, Vada Hubbard, Natural Resources Conservation Service, 

oral commun., 2006). 

Groundwater Pumpage

Groundwater pumpage for domestic, municipal, and 

industrial use in Carson Valley has increased greatly since 

1971 when it was about 600 acre-ft/yr. Pumpage for irrigation ranged from about 3,000 acre-ft/yr in wet years to 10,000 

acre-ft/yr in dry years in the early 1970s (Glancy and Katzer, 

1976, p. 59). Total annual pumpage had increased to as much 

as 20,000 acre-ft/yr by the late 1990s (fig. 8), and total pumpage increased to 31,000 acre-ft/yr in 2004, a relatively dry 

year (Maurer and Berger, 2007, p. 42). Agricultural irrigation 

has historically been the greatest use of pumped water, with 

lesser volumes pumped during wet years when sufficient 

surface water was available for irrigation, and greater volumes 

pumped during dry years. Pumpage for irrigation decreased 

from 67 percent of the total volume pumped in 1970 to 29 percent in 2005. This reduction in pumpage was primarily offset 

by an increase in pumping for municipal use, which increased 

to about 10,000 acre-ft annually by 2000. Domestic pumpage 

accounted for about 16 percent of the total volume of water 

pumped in 2005. Historical pumping rates were estimated 

from inventories conducted by the Nevada State Engineer’s 

Office for water years 1987–2005 (Scott Clark, Tony Eng, 

Nevada Division of Water Resources (NDWR), written and 

oral commun., 2006, 2007), inventories made by the USGS for 

water years 1981–87 (Maurer, 1986, p. 62–63; Berger, 1987, 

p. 14; Berger, 1990, p. 9), and estimates made by Maurer and 

others (2009, p. 40) for water years 1946–81. 

Figure 8. Groundwater pumpage for irrigation, municipal, 

domestic and other uses in Carson Valley, 1946–2005. Annual pumpage, in acre-feet

35,000

30,000

25,000

20,000

15,000

10,000

5,000

0

1940 1950 1960 1970 1980 1990 2000 2010

Total

Irrigation

Municipal

Domestic

Other
14 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Transmissivity 

The transmissivity of Tertiary and Quaternary sediments 

of Carson Valley was initially estimated from aquifer tests of 

37 wells in Carson Valley (table 2 and fig. 5). The aquifer test 

data were obtained from files at the USGS office in Carson 

City, Nevada, Douglas County, and water purveyors in Carson 

Valley. Most of the data were collected from municipal-supply 

wells from single-well tests conducted using step-drawdown 

and constant-rate pumping; seven of the tests used multiple 

observation wells. Twenty-two of the aquifer tests were analyzed using the spreadsheet program of Halford and Kuniansky (2002) based on the Cooper-Jacob, straight-line method 

(Cooper and Jacob, 1946) to analyze constant rate tests, and a 

method developed by Lee (1982) to analyze step-drawdown 

tests. Transmissivity estimates (table 2) are averages of two 

estimates for wells that have both constant-rate and stepdrawdown tests. Most of the remaining tests were analyzed by 

ECO:LOGIC (2000–05, various references listed in table 2, on 

file at the USGS office in Carson City) using a program called 

Well Hydraulics Interpretation Package (WHIP) developed 

by Hydro-Geo Chem, Inc. (1988). Selected aquifer-test data 

analyzed by ECO:LOGIC using the WHIP program were 

reanalyzed using the program of Halford and Kuniansky 

(2002). The resulting estimates of transmissivity and storage 

were almost identical. 

Estimates of transmissivity ranged from 20 to 31,000 feet 

squared per day (ft2/d; table 2). Most aquifer tests were conducted in wells screened in Quaternary sediments, although 

two wells are screened entirely in Tertiary sediments. The 

tops and bottoms of the known screened intervals of the wells 

tested ranged from 40 to 305 ft and from 86 to 650 ft below 

land surface, respectively, with the exception of well 29. Well 

29 is screened from 100 to 1,100 ft below land surface and 

penetrates the entire thickness of Tertiary sediments on the 

eastern side of Carson Valley (fig. 5). The largest values of 

transmissivity (more than 15,000 ft2/d) were estimated for 

wells screened in Quaternary basin-fill sediments within about 

0.5 mi of the Carson River and at well 21 near the eastern 

side of the valley floor. Drillers’ logs indicate these wells 

penetrate well-sorted gravel and sand deposited by the Carson 

River. Much lower values of transmissivity (160 to 800 ft2/d) 

were estimated for the two wells screened entirely in Tertiary 

sediments. 

The specific capacity (ratio of well yield to drawdown) 

of pumped wells was also used to estimate transmissivity in 

areas where aquifer-test data were not available (Lohman, 

1972, p. 52). A relation was developed between the transmissivity estimated from the aquifer tests and the specific capacity for the 37 wells for which both transmissivity and specific 

capacity data were available (fig. 9). The relation indicates 

that specific capacity is proportional to transmissivity and 

that a linear model can explain 84 percent of the variability of 

the data. This relation was used to estimate transmissivity for 

about 1,100 wells with specific-capacity data in Carson Valley 

(fig. 10). The screened length of the 1,100 wells ranges from 

4 to 540 ft, with 114 wells having screened lengths greater 

than 99 ft. No significant relation was apparent between the 

screened length and the estimated transmissivity for the 1,100 

wells.

The 114 wells with screened lengths greater than 99 ft 

were divided into three groups according to the type of sediment screened by each well: (1) Quaternary alluvial fan, 

(2) Quaternary fluvial, and (3) Tertiary sediments (fig. 11). 

The median transmissivity computed for each group was 

316; 3,120; and 110 ft2/d, respectively. An analysis of variance indicates that transmissivity is significantly larger in 

wells screened in Quaternary fluvial sediments, but there is no 

significant difference in transmissivity between wells screened 

in Quaternary alluvial fan and Tertiary sediments. A similar 

analysis was conducted by dividing all 1,100 wells into the 

same three groups, but no significant differences were found, 

probably because there are a large number of wells with 

relatively short screens (less than 20 ft) that do not accurately 

reflect the overall transmissivity of each sediment type. Transmissivity, in square feet per day

Specific Capacity, in gallons per minute per foot

0.1 1.0 10 100

100,000

1,000

10

y = 217.24x1.1242

R2= 0.8445

Figure 9. Relation between transmissivity estimates obtained 

from specific capacity and aquifer-test data.
Hydrologic Setting 15

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

Creek

Indian

Carson River

East

Fork

West

Fork

Buckeye

Creek

Creek

Pine

Nut

89

395

88

395

207

50

88

208

50

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

119°48' 119°27'

39°10'

38°50'

CARSON RANGE

PINE NUT MOUNTAINS

CARSON CITY COUNTY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

EXPLANATION

Carson Valley study area boundary

Location of wells with specific-capacity information

Quaternary fluvial sediments

Quaternary alluvial-fan sediments

Tertiary sediments

Dangberg

Reservoir

Figure 10. Location of wells with specific-capacity data from drillers’ logs. 
16 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

A kriged interpolation of the log-transformed values 

of transmissivity from the 114 wells was computed with 

an assumed Guassian variogram with a variance (sill) of 

1.0 (ft2/d)2, a nugget of 0.45 (ft2/d)2 and a range of 11,000 ft. 

The distribution of transmissivity indicates larger values in the 

central part of the valley underlying the East and West Forks 

of the Carson River, with smaller values along the valley margins (fig. 12). This pattern is similar to that mapped by Prudic 

and Herman (1996) for the Paradise Valley in Humboldt 

County, Nevada.

Storage

Estimates of the distribution of specific yield in Carson 

Valley were made by Dillingham (1980, p. 23) using descriptions of aquifer materials in 250 drillers’ logs (fig. 13). The 

resulting map shows areas of specific yield greater than 20 

percent approximately parallel to the East Fork Carson River 

west of the Minden/Gardnerville area, near the center of the 

valley, and in small areas on the eastern side of the valley and 

near the state line on the southwestern side of the valley. Estimated specific yield is about 15 percent throughout most of 

the valley, but decreases to 10 percent around the margins of 

the valley and beneath the Carson River in the northern part of 

the valley. Morris and Johnson (1967) reported ranges of specific yield for different types of rocks and sediments based on 

laboratory determinations of samples collected throughout the 

United States from 1948 to 1960 and analyzed at the USGS 

Denver laboratory. Ranges in values determined for water-laid 

sediments, typical of those in Carson Valley, were from 17 to 

20 percent for silt and clay, from 28 to 45 percent for sand, 

and from 31 to 44 percent for gravel (Morris and Johnson, 

1967, p. 22–24). These ranges suggest the values of specific 

yield estimated by Dillingham (1980) on the basis of lithology 

could be underestimated. The mapped values shown in figure 

13, therefore, are assumed to qualitatively depict the distribution of specific yield in the Carson Valley. Aquifer tests listed 

in table 2 also include estimates of storage coefficient values 

ranging from 0.1, representing unconfined conditions, to 

0.0002, representing confined conditions, but values estimated 

from single-well tests are considered approximations only.

44

56

14

Log Transmissivity, in feet squared per day

-1

0

1

2

3

4

5

Geologic Unit

Alluvium Fluvial

Quaternary sediments Tertiary

sediments

Percentile

90

75

50

25

10

14

EXPLANATION

Number of samples

Third quartile

Outlier less than 10th percentile or 

greater than 90th percentile

Value from model calibration

Figure 11. Distribution of transmissivity in wells with screen 

lengths greater than 99 feet in Quaternary and Tertiary sediments. 
Hydrologic Setting 17

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

Dangberg

Reservoir

River

East

Fork

West

Fork

Carson

Buckeye

Creek

Creek

Pine

Nut

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

119°48' 119°27'

39°10'

38°50'

CARSON RANGE

PINE NUT MOUNTAINS

CARSON CITY BOUNDARY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

Walleys

Hot Springs

Base geology from Stewart and Carlson (1978) at 1:250,000 scale.

Modified from geologic mapping by Stewart (1999) at 1:100,000 scale; by 

Armin and John (1983), Stewart and Noble (1979) at 1:62,500 scale; and 

by dePolo and others (2000), Garside and Rigby (1998), and Muntean 

(2001) at 1:24,000 scale. 

Model

boundary

Carson

City

EXPLANATION

Log Transmissivity of Quaternary 

sediments, in square feet per day

4 to 4.5

3.5 to 4

3 to 3.5

Tertiary sediments

Consolidated bedrock

Stream or ditch

Carson Valley study area boundary

Well with specific-capacity data and screen length greater 

than 99 feet

2.5 to 3

2 to 2.5

1.5 to 2

Figure 12. Location of wells with screened lengths greater than 99 feet and the kriged distribution of log transmissivity. 
18 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley Well number (fig. 5) Screened interval (feet below land surface) Sediment type in screened interval Analyzed by Date of test Type of test Pump rate 1 (gallons per minute) Test duration 2 (hours) Specific capacity 3 (gallons per minute per foot) Transmissivity 4 (feet squared per day) Storage coefficient (dimensionless) 5

1 110–330 Alluvial fan EcoLogic (2003b, p. 14) 07/16/03 Single well, step and constant 80–247 245

1

12 5.6 1,400 0.013

2 134–335 Alluvial fan EcoLogic (2005, p.19) 04/18/05 Multi-well, step and constant 316–546 307

1

24 4.5 1,200 0.0007

3 120–280 Alluvial fan EcoLogic (2002a, p. 15) 11/06/01 Multi-well, step and constant 82–128 100

2

48 1.9 500 0.0017

4 100–370 Fluvial USGS 04/04/96 Single well, step 700–2,000 3 77.5 20,000 Not estimated5 102–492 Alluvial fan EcoLogic (2002b, p. 2) 01/18/02 Single well, step and constant 2,000–2,500 2,250 2448 13.1 2,300 0.05

6 55–103 Alluvial fan USGS 6 09/12/96 Single well, step and constant 150–450 375

3

24 5.5 2,300 Not estimated

7 55–103 Alluvial fan USGS 6 09/16/96 Single well, step and constant 150–450 200

36 7.5 1,900 Not estimated

8 106–436 Alluvial fan EcoLogic (2000, p. 3) 04/07/98 Multi-well, constant 1,200 8 20.8 8,700 0.025

9 80–374 Alluvial fan USGS 6 09/23/96 Single well, step 250–900 12 7.1 1,000 Not estimated

10 60–300 Alluvial fan USGS 6 10/13/96 Single well, step and constant 250–1,100 800

3

24 14.4 2,300 Not estimated

11 150–300? Alluvial fan EcoLogic (2004b, p.7) 06/01/04 Multi-well, step and constant 300–950 950

1

12 14.9 2,400 0.0005

12 100–400 Fluvial USGS 6 01/13/70 Single well, constant 2,000 48 35.9 8,300 Not estimated

13 ?–270 Fluvial EcoLogic (2001b, p. 4) 09/17/01 Single well, step 20–24 1 0.3 20 0.0008

14 105–325 Fluvial EcoLogic (2003a, p.13) 02/06/03 Multi-well, constant, several periods 1,037 10 days 17.6 4,700 0.001

15 105–125 Fluvial USGS 6 09/21/06 Single well, constant 5 4 1.5 640 Not estimated

16 305–325 Fluvial USGS 6 09/21/06 Single well, constant 5 4 0.3 47 Not estimated

17 100–282 Alluvial fan EcoLogic (2001c, p. 4) 10/29/01 Multi-well, step 96–200 1 24.9 4,300 0.001

18 71–86 Tertiary Kleinfelder (1992, p. 9) 11/15/91 Multi-well, constant 20 72 0.9 800 0.0005

19 200–360 Fluvial USGS 6 01/17/86 Single well, constant 2,500 24 38.3 31,000 Not estimated

20 175–498 Fluvial USGS 6 06/11/85 Single well, step 500–1,100 24 18.2 2,000 Not estimated

21 110–510 Fluvial USGS 6 11/17/94 Single well, step and constant 900–1,800 1,800

1

24 37.0 23,000 Not estimated

22 40–290 Fluvial USGS 6 03/05/93 Single well, step and constant 625–800 750

2

12 27.7 5,000 Not estimated

Table 2. Summary of aquifer tests in Carson Valley. 
Hydrologic Setting 19

Well 

number 

(fig. 5)

Screened 

interval

(feet below 

land surface)

Sediment 

type in 

screened 

interval

Analyzed by Date of test Type of test Pump rate 1 (gallons per minute) Test duration 2 (hours) Specific capacity 3 (gallons per minute per foot) Transmissivity 4 (feet squared per day) Storage coefficient (dimensionless) 5

23 95–295 Fluvial USGS 6 12/11/03 Single well, constant 1,400 24 24.8 18,000 Not estimated

24 125–305 Fluvial USGS 6 12/05/03 Single well, constant 1,000 28 12.9 7,700 Not estimated

25 100–300 Fluvial USGS 6 02/11/95 Single well, constant 1,800 16 24.6 19,000 Not estimated

26 140–400 Fluvial USGS 6 07/03/89 Single well, constant 1,350 24 10.1 3,000 Not estimated

27 155–430 Alluvial fan EcoLogic (2001a, p. 3) 06/11/01 Single well, step 240–600 1 4.5 700 0.1

28 160–440 Alluvial fan EcoLogic (2001a, p. 6) 06/11/01 Single well, step 120–306 1 24.8 6,300 0.0002

29 100–1,100 Tertiary USGS 6 11/17/05 Single well, constant 139 5 1.2 160 Not estimated

30 90–420 Alluvial fan Tertiary? USGS 6 12/14/63 Single well, step 460–1,550 10 7.2 2,000 Not estimated

31 270–650 Alluvial fan USGS 6 04/28/04 Single well, constant 1,800 48 13.6 3,300 Not estimated32 260–500 Fluvial USGS 6 08/14/97 Single well, step and constant 1,000–1,800 1200 223 8.7 1,200 Not estimated33 230–490 Alluvial fan USGS 6 01/27/01 Single well, constant 445 48 7.9 3,300 Not estimated34 240–390 Alluvial fan USGS 6 05/26/05 Single well, step and constant 508–1,000 1000 348 8.3 5,900 Not estimated35 100–220 Alluvial fan EcoLogic (2005?) 05/18/05 Single well, step and constant 38 24 1.8 380 0.000336 95–285

Alluvial fan 

decomposed 

granite

EcoLogic (2004a, 

p. 8) 06/26/05 Single well, constant 150 48 2.8 780 0.0004

37 55–425

Alluvial fan 

decomposed 

granite

EcoLogic (2004a, 

p. 8) 06/26/05 Single well, constant 200 48 5.0 1,300 0.0025

Table 2. Summary of Aquifer tests in Carson Valley.—Continued

1 Upper number range in rates for step test, lower number rate for constant test. 2 Upper number duration for each step, lower number duration for constant test. 3 Calculated from constant-rate test or from final step if only step test available. 4 Average of step and constant tests where both available. 5 Values estimated from single-well tests are considered approximations only. 6 Analyzed using spreadsheet program of Halford and Kuniansky (2002), rounded to two significant figures. 
20 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 13. Geologic units and the estimated distribution of specific yield in basin-fill sediments of Carson Valley from Dillingham (1980).

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

Dangberg

Reservoir

River

East

Fork

West

Fork

Carson

Buckeye

Creek

Creek

Pine

Nut

West

Fork

East

Fork

89

395

88

395

207

50

88

208

50

Johnson Lane

15

10

10

20

20

10

10

20

20

20

5

15

5

10

15

5

5

Quaternary

Tertiary

Tertiary to

Triassic

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated rocks

Carson Valley study area boundary

Area of equal specific yield, in 

percent (from Dillingham, 1980)

10

EXPLANATION

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

119°48' 119°27'

39°10'

38°50'

CARSON RANGE

PINE NUT MOUNTAINS

Carson

City

CARSON CITY BOUNDARY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

Walleys

Hot Springs
Groundwater Flow 21

Groundwater Flow

Previous investigations show that groundwater flows from 

the west and east towards the Carson River and then northward (fig. 14; Berger and Medina, 1999). Along the northsouth axis of the valley, hydraulic gradients range from about 

100 feet per mile (ft/mi) in the southwestern part of the valley 

to about 5 ft/mi in the northern part of the valley. The hydraulic gradient is eastward at about 100 ft/mi beneath alluvial 

fans on the western side of the valley, whereas gradients are 

westward and range from 20 to 100 ft/mi on the eastern side of 

the valley (Maurer, 1986, p. 18). Water-level data for Tertiary sediments exposed on the eastern side of the valley are 

sparse, but indicate a gradient toward the west. Water levels 

near Indian Hills in the northwestern part of the valley, not 

included on the map developed by Berger and Medina (1999) 

but available in the NWIS database, indicate a gradient from 

Jacks Valley toward Carson Valley through Tertiary sediments 

beneath Indian Hills.

The bedrock surrounding and underlying Carson Valley is relatively impermeable to groundwater flow, although 

many wells drilled in these rocks produce sufficient water for 

domestic use. In the semi-consolidated Tertiary sediments, 

lenses of sand and gravel are the main water-bearing units and 

probably transmit most groundwater flow. The Quaternary 

sediments that form alluvial fans surrounding the valley and 

fluvial deposits that underlie the flood plain of the Carson 

River are the principal aquifers in Carson Valley (Maurer, 

1986, p. 17).

An upward hydraulic gradient exists throughout most of 

the valley (Maurer, 1986, p. 17) and artesian flow (indicating 

confined conditions) occurs in some wells screened at depths 

greater than 200 to 300 ft below land surface. Dillingham 

(1980, p. 40), however, concluded from inspection of drillers’ 

logs that a continuous confining unit is not present throughout 

the valley. Confined conditions likely result from discontinuous clay beds that are 30 to 40 ft thick and occur at depths of 

200 to 300 ft. Confined conditions and artesian flow also occur 

at shallower depths (less than 100 ft) on the western side of the 

valley. At these locations, the distal portions of coarse-grained 

alluvial fans have probably been buried by fine-grained floodplain deposits of the Carson River, creating confined conditions (Maurer, 1986, p. 17).

Exchange of Groundwater and Surface Water 

In the center of the valley where the water table is shallow, groundwater is hydraulically connected to the network 

of streams and irrigation ditches. Maurer and Berger (2007, 

p. 51) compared streambed temperatures and groundwater 

levels with stream stage and concluded that the greatest losses 

from the Carson River and the irrigation system (ditches 

and canals) to groundwater occur mainly in the southeastern 

part of the valley, although losses also occur in other parts 

of the valley during the irrigation season and in wet years. 

In contrast, both the Carson River and the irrigation system 

appeared to gain flow from groundwater discharge on the 

western side of the valley and north of Muller Lane (fig. 7). 

The direction of the hydraulic gradient between the water 

table and stream changes between wet and dry years, and 

during the irrigation season, resulting in a complex pattern of 

flow. Maurer and Berger (2007, p 24) calculated the difference 

between mean daily surface-water inflow to and outflow from 

Carson Valley for the period 1990–2002 using the measured 

flow of the Carson River and estimates of mean-daily streamflow in tributaries. The calculations indicated that surfacewater inflow to the basin was greater than outflow from the 

basin during the irrigation season (when the evapotranspiration rate was high), and that streamflow losses to groundwater and consumptive use during the period averaged about 

89,000 acre-ft/yr. Conversely, surface-water outflow from the 

basin was greater than inflow to the basin during the winter 

and spring (when the evapotranspiration rate was low) when 

streamflow gains totaled about 16,000 acre-ft. A net annual 

loss of streamflow in Carson Valley of 73,000 acre-ft/yr was, 

therefore, computed for the period 1990–2002.

Groundwater Levels

Groundwater levels have been measured by the USGS 

in about 70 wells in Carson Valley since December 2004 

(selected wells are shown on fig. 14). Measurements were first 

made in many of these wells in the early 1980s for a study 

by Maurer (1986) and additional measurements have been 

made at various times through 2004. Water levels in alluvialfan sediments on the western side of Carson Valley showed 

seasonal fluctuations of about 5 to 35 ft during the early 1980s 

(fig. 15A) and fluctuations of as much as 40 ft in response to 

the transition from wet periods to dry periods (1985–92, wells 

1 and 2). Shallow wells on the valley floor indicate smaller 

magnitude seasonal and long-term fluctuations ranging from 

about 2 to 8 ft, although data for dry years in the late 1980s 

and wet years in the mid 1990s are not available for many 

wells (fig. 15B). Although groundwater levels fluctuate in 

response to both seasonal and annual variations in recharge, 

water levels among multiple wells do not display a long-term 

trend over the period of record, indicating that the aquifer is in 

a state of dynamic equilibrium. 

Water levels in artesian wells can fluctuate from about 5 to 

20 ft above land surface during winter months to about 10 to 

20 ft below land surface during summer months when pumping for irrigation occurs (fig. 15C). No obvious trend in water 

levels is apparent in these wells from 1981 to 2005. Water levels in deep irrigation wells (fig. 15D) appear to have declined 

5 to 10 ft by 2005 since the mid-1990s, possibly in response 

to dry conditions. The largest declines in water level have 

occurred on the eastern side of Carson Valley where areas 

of increased development are supplied largely by individual 

domestic wells (fig. 15E). In the Johnson Lane area (well 5), 

water levels declined about 10 ft from 1981 to 2005, while 
22 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

Dangberg

Reservoir

Carson

River

East Fork

West Fork

Buckeye Creek

Pine Nut Creek

Buckeye

Effluent

Reservoir

395

88

50

4,650

4,700

4,800

4,750

4,800

5,090

4,880

Ditch or slough

Well and number—See figure 15 for wells screened in,

Quaternary fluvial sediments

Quaternary alluvial-fan sediments

Tertiary sediments

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Universal Transverse Mercator projection, zone 11

North American Datum of 1983

21 3 4 5 6 MILES

21 3 4 5

0

0 6 KILOMETERS

119°50' 119°40'

39°0'

38°50'

CARSON RANGE

CARSON CITY COUNTY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

Reservoir, not used after 1997

Reservoir, used from 1993 to 2005

Reservoir, continued use

Carson Valley study area boundary

Water-table altitude—Spring 1998, in feet above sea 

level, interval 10 feet (from Berger and Medina, 1999)

10

EXPLANATION

17

8

19

Johnson Lane

Ruhenstroth

4

8

1

2

3

7

6

5

22

18

9

14

21

13 16 15

11

12

10

20

19

17

Fish

Spring

Flat

Allerman #1

Allerman #4

Allerman #2

Figure 14. Water-table altitude in Carson Valley, spring 1998. 
Groundwater Flow 23

Figure 15. Water-level fluctuations,1981–2006, in A, wells on the western alluvial fans; B, shallow wells on the valley floor; C, deep 

flowing wells on the valley floor; D, deep irrigation wells near the valley floor; E, wells on the eastern side of the valley; and F, wells with 

water levels affected by changes in water use.

0

20

40

60

Depth to water, in feet below land surface Depth to water, in feet below land surface

0

20

40

60

0

20

40

60

0

20

40

60

1981 1985 1990 1995 2000 2005

Well 22

Well 12

Well 2

Well 1

1981 1985 1990 1995 2000 2005

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

0

2

4

6

8

10

Well 13

Well 21

Well 15

Well 11

A Wells on the western 

alluvial fan

B Shallow wells on the 

valley floor

 Location of wells shown on fig. 14
24 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 15. Water-level fluctuations,1981–2006, in A, wells on the western alluvial fans; B, shallow wells on the valley floor; C, deep 

flowing wells on the valley floor; D, deep irrigation wells near the valley floor; E, wells on the eastern side of the valley; and F, wells with 

water levels affected by changes in water use.—Continued Depth to water, in feet below land surface Depth to water, in feet below land surface

C Deep flowing wells 

on the valley floor

D Deep irrigation wells 

near the valley 

floor

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

Well 14

Well 10

Well 9

Well 3

1981 1985 1990 1995 2000 2005

Well 20

Well 18

Well 16

Well 4

-10

0

0

0

0

10

20

30

40

50

60

-10

10

20

30

40

50

60

-10

10

20

30

40

50

60

-10

10

20

30

40

50

60

1981 1985 1990 1995 2000 2005

 Location of wells shown on fig. 14
Groundwater Flow 25

Figure 15. Water-level fluctuations,1981–2006, in A, wells on the western alluvial fans; B, shallow wells on the valley floor; C, deep 

flowing wells on the valley floor; D, deep irrigation wells near the valley floor; E, wells on the eastern side of the valley; and F, wells with 

water levels affected by changes in water use.—Continued Depth to water, in feet below land surface Depth to water, in feet below land surface

E Wells on the eastern 

side of the valley

F Wells with water levels 

affected by changes 

in water use

Well 5

Well 17

Well 19

0

20

40

60

0

20

40

60

120

140

160

180

1981 1985 1990 1995 2000 2005

40

60

80

100

120

140

40

60

80

100

120

140

40

60

80

100

120

140

1981 1985 1990 1995 2000 2005

Well 8

Well 6

Well 7

 Location of wells shown on fig. 14

water levels in Fish Spring Flat (well 17) and the Ruhenstroth 

subdivision (well 19) declined more than 15 ft. These areas 

are far from irrigated lands, so recharge is limited solely to 

groundwater inflow from the Pine Nut Mountains. 

Water levels in wells on the eastern side of Carson Valley 

also reflect changes in the operation of the irrigation system. 

Water levels within 1 mi of the Allerman # 4 reservoir (fig. 14) 

declined about 20 ft by 2005 since the use of reservoir was 

discontinued in 1997 (fig. 15F, wells 6 and 8). The declines 

likely resulted from the cessation of infiltration from the 

reservoir that maintained water levels in the surrounding area. 

Conversely, water levels in well 7 near the mouth of Buckeye 

Creek (fig. 15F) rose about 20 ft by 2005, likely as a result of 

infiltration losses from the Buckeye effluent reservoir (fig. 14) 

that was constructed in the 1990s 1 mi to the northeast. 
26 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Water Budget

Inflows and outflows to Carson Valley can be summarized 

in a water budget using the following equation:

Precipvf + Streamin + MFrch + Eff + GWin = 

ETvf + Streamout +Pumpnet (1)

where 

Precipvf is precipitation on the valley floor [L3/T];

Streamin is flow in the tributaries to the Carson River 

entering the valley (including perennial 

flow from the Carson Range) [L3/T];

MFrch is mountain-front recharge (including 

perennial flow from the Pine Nut 

Mountains and ephemeral flow from 

both the Carson Range and the Pine Nut 

Mountains) [L3/T];

Eff is wastewater effluent imported for irrigation 

from outside Carson Valley [L3/T];

GWin is groundwater inflow from Eagle Valley 

[L3/T],

ETvf is evapotranspiration from the vegetation on 

the valley floor [L3/T];

Streamout is flow in the Carson River exiting the valley 

[L3/T]; and

Pumpnet is net pumping from groundwater, defined as 

total pumping minus wastewater irrigation 

and septic discharge [L3/T].

Components of the annual water budget for Carson Valley 

have been estimated by previous studies (Maurer and Berger, 

1997; Maurer and others, 2004; Maurer and Halford, 2004; 

Maurer and others, 2004; Maurer and Berger, 2007; Jeton and 

Maurer, 2007). Annual inflows and outflows listed in table 3 

and shown in figure 16 are estimates compiled for a 15-year 

(yr) period (1990–2005) when precipitation was typically 

below average (fig. 3). The largest components of the budget 

are inflow of the Carson River (East Fork and West Fork) to 

the valley (329,000 acre-ft or 74 percent) and outflow of the 

Carson River from the valley (278,000 acre-ft or 62.6 percent). 

The net loss in downstream flow in the Carson River within 

Carson Valley is 51,000 acre-ft. Inflows from precipitation on 

the valley floor and imported wastewater from the Lake Tahoe 

basin account for 10.8 percent of the budget, whereas mountain-front recharge from the Carson Range and the Pine Nut 

Mountains provide 8.1 percent. Evapotranspiration from phreatophytes and irrigated fields on the valley floor accounts for 

32.9 percent of outflow, whereas 4.6 percent of the outflow is 

discharged through consumptive use of pumped groundwater.

Groundwater Budget

Inflows and outflows to groundwater in Carson Valley 

can be summarized in a water budget using the following 

equation:

MFrch + Valleyrch + Agrch + Septic + Streaminfil + GWin = 

ETwt +Pump + Streamdisch (2)

where 

Valleyrch is valley recharge from precipitation on 

the valley floor (Precipvf) and recharge 

from fields irrigated with wastewater and 

perennial flow from the Carson Range 

[L3/T];

Agrch is agricultural recharge from irrigated fields, 

computed as applied irrigation water minus 

losses through return flows and crop ET 

[L3/T];

Septic is septic-tank discharge [L3/T];

Streaminfil is infiltration of streamflow from the Carson 

River and its tributaries and the irrigation 

system to groundwater [L3/T];

ETwt is evapotranspiration from the water table on 

the valley floor [L3/T];

Pump is total pumping of groundwater [L3/T]; and

Streamdisch is groundwater discharge to the Carson River, 

its tributaries, and the 

irrigation system [L3/T].

Components of the groundwater budget for the Carson 

Valley (table 4) were either specified in or computed by the 

groundwater-flow model. Precipitation becomes groundwater 

recharge only in vegetated areas on the valley floor where the 

water table is shallow and during months when precipitation 

exceeds evapotranspiration. A portion of irrigation water is 

assumed lost through return flow to irrigation ditches and then 

further reduced by subtracting crop evapotranspiration losses 

(fig. 17A). The remaining irrigation water (if any) is assumed 

to infiltrate to the water table beneath the irrigated fields. 

Mountain-front recharge from watersheds in the Carson Range 

and Pine Nut Mountains was estimated from PRMS simulation results discussed previously and then adjusted during 

model calibration. Valley recharge is specified in the model 

and defined separately from agricultural recharge, which is 

computed based on volumes of streamflow diverted for irrigation. Infiltration of streamflow to groundwater was computed 

in model simulations, whereas infiltration from septic tanks 

(fig. 17B) was specified as a constant rate of 21.5 cubic feet 

per day (ft3/d; 0.18 acre-ft/yr) per household based on the 

volume of effluent treated per hook-up reported in northern 

Douglas County (Cathe Poole, Douglas County Utilities, 

written commun., 2006; Frank Johnson, Minden-Gardnerville 

Sanitation District, oral commun., 2006). Inflow from the 

Eagle Valley was computed in model simulations.
Groundwater Flow 27

Table 3. Mean annual water budget for Carson Valley for water years 1990 through 2005. 

[ac-ft/yr; acre-feet per year] 

Inflow Symbol Volume 

(1,000 ac-ft/yr) Percent Outflow Symbol Volume 

(1,000 ac-ft/yr) Percent

Precipitation on valley 

floor Precipvf 38 8.6 Evapotranspiration from valley floor Etvf 146 32.9

Streamflow Streamin Carson River Streamout 278 62.6

East Fork Carson River 256 57.6 Net pumping 1 Pumpnet 20.3 4.6

West Fork Carson River 73 16.4

Carson Range 30 6.8

Mountain-front recharge MFrch

Carson Range 20 4.5

Pine Nut Mountains 16 3.6

Imported effluent Eff 9.8 2.2

Inflow from Eagle Valley GWin 1.5 0.3

Total 444.3 100 444.3 100

1 Total pumping minus wastewater irrigation and septic discharge.

Figure 16. Components of the mean annual water budget for Carson Valley, water years 1990–2005.

Eagle Valley

inflow

1,500

Carson River 278,000

Evapotranspiration

from valley floor146,000

Mountain-front

recharge

20,000

Net pumping

20,300

Precipitation

on valley floor

38,000 Perennial

streams

Carson 30,000

Range West Fork Carson River

73,000

Imported 

effluent

9,800

East Fork Carson River

256,000

Mountain-front 

recharge

16,000

Pine

Nut

Mountains

EXPLANATION

9,800 Flow, in acre-feet per year

Inflow

Outflow

Carson Valley study area boundary

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Shaded-relief from 30-meter Digital Elevation Model

Sun illumination from northwest at 30 degrees above horizon

Universal Transverse Mercator projection, zone 11
28 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Table 4. Components of the groundwater budget for Carson Valley. 

Inflow Symbol Model value Outflow Symbol Model value

Mountain-front recharge 1 MFrch Specified Evapotranspiration from 

water table Etwt computed

Valley recharge Valleyrch Specified Pumping Pump specified

Agricultural recharge Agrch Computed Streams and ditches Streamdisch computed

Infiltration

Septic tanks Septic Specified

Streams and ditches Streaminfil Computed

Inflow from Eagle Valley GWin Computed

1 Adjusted by a factor in model calibration.

A Agricultural use

B Domestic use

Evapotranspiration from lawn

Evapotranspiration from crops

Pumping from groundwater

Water application

Infiltration

to water table

Infiltration

to water table

Water table

Well

Septic tank

Delivery ditch

Return

ditch

Return flow

Figure 17. Inflow and outflow of water from A, agricultural use; B, domestic use. 
Groundwater-Flow Model 29

Discharge of groundwater in Carson Valley is assumed to 

occur through evapotranspiration in areas where phreatophytes 

grow and the depth to water is 35 ft or less, or in irrigated 

areas where the depth to water is 10 ft or less. The volume of 

water discharged through evapotranspiration and into streams 

and ditches was computed in model simulations. The volume of water pumped for domestic, municipal, agricultural, 

and other uses was specified in model simulations. There 

were 3,642 households in Carson Valley in 2005, and each 

household was assumed to pump 134 ft3/d (1.12 acre-ft/yr) 

for domestic use. In addition, there were 12 agricultural and 

municipal wells that each pumped more than 50,000 ft3/d 

(419 acre-ft/yr) in 2005.

Groundwater-Flow Model

Groundwater flow through Quaternary and Tertiary 

sediments in Carson Valley was simulated using MODFLOW-2005 (Harbaugh, 2005), a three-dimensional, numerical finite-difference groundwater-flow model. The exchange of 

water among groundwater, the Carson River, and the irrigation system was represented with a version of the Streamflow 

Routing (SFR2) package (Niswonger and Prudic, 2005) that 

was modified to apply diversions from the irrigation network 

to irrigated areas as recharge. A 35-yr transient simulation was 

designed and calibrated to represent historical conditions for 

water years 1971–2005, a period of increasing development 

and groundwater pumping. A steady-state period representing 

conditions in 1970 provided initial conditions for the transient simulation. The 35-yr transient simulation represented 

quarterly changes in precipitation, streamflow, pumping, and 

irrigation. Model results were used to quantify certain components of the groundwater budget (table 4) and the exchange 

between surface water and groundwater, and to depict the 

rate and direction of groundwater flow in Carson Valley. The 

calibrated model was then applied in 55-yr transient simulations (2006–60) to predict changes in groundwater levels and 

discharges that would result from alternative future water-use 

scenarios.

Model Design

The model domain covers an area of 242.8 mi2 that 

includes the portion of the study area underlain by Quaternary and Tertiary sediments. The model domain is bounded 

to the west by the Carson Range and to the east by the Pine 

Nut Mountains (fig. 18). The southern boundary is located 

near where the East and West Forks of the Carson River 

emerge from the bedrock uplands and onto the valley floor, 

whereas the northern boundary coincides with the watershed 

divide upstream of the gage on the Carson River near Carson 

City NV. The model represents groundwater flow through 

the unconsolidated sediments with a saturated thickness that 

ranges from 20 to 5,000 ft. The model also represents the 

exchange of groundwater and surface water in the Carson 

River and its tributaries, and includes an approximate representation of the complex irrigation system in Carson Valley.

Discretization

The model domain is discretized using a regularly spaced 

grid with cells that measure 550 ft on each side. The model 

grid was oriented along the north-south axis of the valley 

approximately parallel with the general direction of surfaceand groundwater flow in the valley, and likely parallel with 

relict, buried stream channels of the Carson River. The cell 

size was selected to be small enough to represent the exchange 

of water between groundwater and surface water, yet large 

enough for efficient numerical computation. The 35-yr simulation is divided into 140 quarterly (3-month) periods, each 

containing 6 time steps that ranged from 12 to 19 days.

The top surface of the model represents land-surface elevation computed from 3-ft (ground sample distance) LiDAR 

data on the valley floor (BAE SYSTEMS Advanced Technologies, Inc., 2004) and a 30-ft digital elevation model in higher 

terrain. The bottom model boundary represents the bedrock 

surface mapped by gravity surveys (fig. 5). The unconsolidated sediments are divided into 10 model layers; the extents 

and thicknesses of model layers were chosen to conform to the 

assumed distribution of Quaternary and Tertiary sediments in 

Carson Valley (fig. 19). The top three model layers represent 

fluvial sediments in the center of the valley and alluvial-fan 

sediments to the east and west (fig. 18). The lateral extent of 

model layer 1 is limited to the relatively flat portion of the 

valley. Model layers 4 through 10 primarily represent Tertiary sediments that are exposed on the eastern side of Carson 

Valley and dip beneath the Quaternary sediments to the west 

(fig. 19). The Tertiary sediments are assumed to be displaced 

(down-dropped) 800 ft west of the hypothesized Hot Springs 

Mountain fault, so that layers 4 through 7 represent fluvial 

sediments west of the fault (fig. 18). The average thickness of 

model layers increases from 80 ft at the top of the model to 

almost 900 ft at the bottom of the model. 

Boundaries

All the lateral and bottom boundaries of the model are 

assumed to be no-flow (impervious to flow) with the exception 

of part of the northern boundary that is treated as head-dependent flow (fig. 18). This boundary represents underflow (GWin) 

through basin-fill sediments from the Eagle Valley (fig. 1) and 

was assigned hydraulic-head altitudes based on water levels 

measured in 2006 in seven wells that straddle the boundary.

Inflows

Valley recharge derived from precipitation on the valley 

floor (Precipvf) and the application of irrigation water (Agrch) is 

represented as specified flow (recharge) in the top model layer. 

Precipitation is assumed to occur throughout the areas covered 
30 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 18. Model domain and distribution of Quaternary and Tertiary sediments in Carson Valley groundwater model and location of 

hypothesized Hot Springs Fault.

NEVADA

CALIFORNIA

DOUGLAS COUNTY

ALPINE COUNTY

LYON COUNTY

Dangberg

Reservoir

River

East

Fork

West

Fork

Carson

Buckeye

Creek

Creek

Pine

Nut

West

Fork

East

Fork

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

0 2 4 6 8 MILES

0 2 4 6 8 KILOMETERS

119°48' 119°27'

39°10'

38°50'

CARSON RANGE

PINE NUT MOUNTAINS

Carson

City

CARSON CITY BOUNDARY

DOUGLAS COUNTY

Gardnerville

Minden

Genoa

A A’

Model

boundary

Carson River 

near Carson

City

Carson River

near Genoa

FWM5

FWM4

FWM3

Fish

Spring

Flat

Quaternary

Tertiary

Tertiary to

Triassic

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated rocks

Carson Valley study area boundary

Hypothesized fault

Stream segment in streamflow routing

Line of section

Head-dependent flow

Streamflow gage

EXPLANATION

A’A
Groundwater-Flow Model 31

by phreatophytes or irrigated fields (fig. 20) and is specified 

quarterly during the 35-yr simulation period using the distribution of mean annual precipitation estimated by Maurer and 

Halford (2004, p. 35) for water years 1990–2005. Quarterly 

precipitation in each water year of the 35-yr period is based on 

measured daily precipitation at Minden, Nevada.

 Irrigation water is applied in irrigated fields using water 

diverted from the 12 perennial streams that drain the Carson 

Range, application of effluent wastewater, and diversions 

from the Carson River and the adjoining irrigation system 

(discussed further in the section describing streamflow routing). The volume of irrigation water is reduced by 30 percent 

to account for return flow that is assumed to remain in the 

irrigation system. The resulting volume of applied water is 

then reduced by factors of 70 and 80 percent to account for 

crop ET losses in the spring and summer quarters, respectively. The remaining volume is then allocated as specified 

flow (recharge) equally among the model cells that represent 

the irrigated parcel.

Figure 19. Assumed lateral and vertical distribution of Quaternary and Tertiary sediments. Labels associated with geologic units refer 

to parameters listed in table 7.

A Plan view

Qal-NW

Qfl

Qal-E

TerN

Teraz

TerS

TerE

TerNW

Qal-SE

Qal-W

A A’

Hypothesized fault 0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

B Oblique view (not to scale)

A’

A

Qfl

Qal-NW Qal-E

TerN

Teraz

TerE

Terdeep

Hypothesized fault Verrtical exaggeration x 5

TerNW
32 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 20. Distribution of mountain-front recharge, effluent application, and evapotranspiration in Carson Valley groundwater model.

Dangberg

Reservoir

River

Carson

Buckeye Creek

Creek

Pine

Nut

West

Fork

East

Fork

Gardnerville

Minden

Jack’s Valley Model boundary

Fish

Spring

Flat

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated bedrock

Stream segment in 

streamflow routing

Carson Valley study 

area boundary

EXPLANATION

Evapotranspiration

Vegetation classes

Riparian

Wetlands

Phreatophytes

Irrigated fields

Mountain-front recharge

Pine Nut Mountains

Northeast

Southeast

Carson Range

Northwest

West

Effluent application

Quaternary

Tertiary and older
Groundwater-Flow Model 33

Irrigation water is applied to water-rights areas near the 

foot of the Carson Range (fig. 7) in the irrigation season based 

on runoff from the 12 perennial streams that drain the Carson 

Range computed by the PRMS watershed models discussed 

previously. Recharge from effluent wastewater is represented 

in the irrigation season based on the reported annual volume permitted by the Nevada Department of Environmental 

Protection and reports from effluent suppliers and end users 

(Hal Bird, South Tahoe Public Utility District, written commun., 2005, Kelvin Ikehara; Douglas County Sewer Improvement District No. 1, written commun., 2010; Frank Johnson, 

Minden Gardnerville Sanitation District, written commun., 

2010; Jim Usher, Bently Agrodynamics Ranch Manager, oral 

commun., 2010). 

Mountain-front recharge (MFrch) is represented through 

specified flows (simulated as injection wells) to cells in the 

layer below the top active layer. The cells are located along 

the foot of the Carson Range on the western side of the valley 

and along stream channels draining the Pine Nut Mountains on 

the eastern side of the valley (fig. 20). Recharge volume was 

based on daily values estimated by PRMS watershed models discussed previously for the period 1978–2005. Annual 

recharge volumes for the period 1970–77 were calculated from 

the measured precipitation at Minden and a relation between 

recharge and precipitation derived for the period 1978–2005. 

The PRMS recharge values were adjusted in model calibration 

in four areas: the western alluvial fans and Jacks Valley along 

the Carson Range (west and northwest areas), and the northern 

and southern zones along the Pine Nut Mountains (northeast 

and southeast areas).

Infiltration of septic-tank discharges (Septic) were represented through specified flows (simulated as injection wells) 

in the top active model layer. The number of recharge cells 

specified to represent septic-tank discharge increased from 

about 450 in 1970 to almost 4,200 in 2005. The recharge 

cells were located from parcels mapped in a GIS database by 

Douglas County (Dawn Patterson, Douglas County MultiAgency Geographic Information Center [MAGIC], written 

commun., 2006). Infiltration of streamflow from the Carson 

River and its tributaries and the irrigation system to groundwater (Streaminfil) is discussed further in the section describing 

streamflow routing.

Outflows

Evapotranspiration (ET) from groundwater is represented 

as a head-dependent boundary in the top model layer for 

four vegetation classes: irrigated pasture grasses and alfalfa, 

native phreatophytes (mainly rabbit brush and greasewood), 

wetlands, and riparian willows. The rate of ET loss per unit 

surface area QET was computed in transient simulations using 

a depth-dependent function:

 ( ( )) max

ET ET

ET

ET Q H LS d d = −− (3)

where 

ETmax is maximum ET rate [L/T],

dET is extinction depth below which no ET occurs 

[L],

H is hydraulic head [L], and

LS is land surface altitude [L].

The maximum ET rates (ETmax) are specified quarterly for 

irrigated crops, phreatophytes, and wetlands based on monthly 

rates reported by Maurer and others (2006, p. 22). ET rates for 

riparian willows are based on monthly rates estimated for willows in the Walker River basin about 40 mi east of Carson Valley (Allander and others, 2009, p. 35). The annual ETmax rates 

are 3.0 ft/yr for irrigated alfalfa and pasture grasses, 1.9 ft/yr 

for native phreatophytes, 4.4 ft/yr for wetlands, and 3.8 ft/yr 

for riparian willows. The extinction depth for phreatophytes, 

wetlands, and riparian willows is specified as 30 ft, whereas 

the extinction depth for irrigated alfalfa and pasture grasses is 

specified as 10 ft.

Pumping of groundwater for municipal, domestic, irrigation, and other water supplies is represented by specified flow 

boundaries (discharge wells). The number of wells increases 

from about 520 in 1970 to almost 4,000 in 2005. About 90 

percent of the wells supply water for domestic use. Altitudes 

of well screens were used to assign the wells to model layers, 

but this information is available for less than 10 percent of the 

wells, mainly those that are used for municipal or irrigation 

water supply. Depths for other remaining wells were assigned 

150 ft below land surface for domestic wells and 300 ft below 

land surface for municipal, industrial, and irrigation wells. 

The infiltration of water from conveyance losses associated 

with municipal supplies is assumed to be negligible in model 

simulations. Water pumped for irrigation at a fish hatchery is 

discharged to the irrigation system and is simulated through 

streamflow routing described in the following section.

Estimates of annual pumpage for water years 1970–80 

(Maurer and others, 2009, p. 34–39) are based on the number 

of wells in use each year for municipal, domestic, irrigation, 

and other supplies. Estimates of annual pumpage for water 

years 1981–86 (Maurer, 1986, p. 62–63; Berger, 1987, p. 14; 

Berger, 1990, p. 9) are based on reports from water purveyors 

for municipal supply and power usage/pump efficiency data 

for irrigation supply. Estimates of annual pumping for water 

years 1987–2005 are based on NDWR pumping inventories 

as described by Clark (2006). Annual pumpage volumes are 

apportioned quarterly based on reported monthly metered 

pumping for municipal supply and power usage for irrigation supply. Quarterly pumping volumes (as a percentage of 

annual volume) for municipal supplies in each water year are 

13, 7, 31, and 49 percent for the fall, winter, spring and summer quarters, respectively. The quarterly volume of domestic 

pumping is assumed to follow the same pattern. Quarterly 

pumping volumes for irrigation supplies for the same four 

quarters are 7, 0, 9, and 84 percent.
34 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

The locations of the principal municipal supply wells 

were determined from Global Positioning Satellite (GPS) 

data collected by NDWR, Douglas County, and water purveyors (fig. 21). The locations of irrigation supply wells were 

determined from the township, range, section, and quarter 

location on water-right permits listed in NDWR reports, and 

then adjusted using historical maps on file at the USGS office 

in Carson City, Nevada. Domestic wells were located using 

mapped parcels supplied by Douglas County (Dawn Patterson, 

Douglas County MAGIC, written commun., 2006). The parcel 

map also provides the dates when domestic wells and septic 

tanks were installed.

Streamflow Routing

The exchange of water between groundwater, the Carson 

River, and the adjoining irrigation system was represented 

with the Streamflow Routing (SFR2) package (Niswonger and 

Prudic, 2005). This head-dependent boundary allows groundwater discharge where stream stage is higher than the water 

table (losing reaches), stream infiltration to groundwater where 

stream stage is lower than the water table (gaining reaches) 

and the diversion of water for irrigation. The volume of 

water exchanged between the groundwater and surface-water 

systems is calculated on a cell by cell basis as the difference 

in head between stream stage and the water table multiplied 

by the streambed conductance. The SFR2 package accounts 

Figure 21. Location of municipal, domestic, and irrigation supply wells in Carson Valley groundwater model.

East

Fork

West

Fork

Buckeye Creek

Creek

Pine

Nut

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated bedrock

Stream segment in 

streamflow routing

EXPLANATION

Quaternary

Tertiary and older

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

Model boundary

Jack’s Valley

Minden

Pumped wells, rate in acre-feet per year

100 to 1,000

10 to 100

1 to 10

Less than 1

Agricultural Domestic Municipal

and other

Fish

Spring

Flat
Groundwater-Flow Model 35

for water that enters and flows through the irrigation system 

using Manning’s equation to compute the stream depth at each 

location within the network as a function of flow and channel 

geometry (Niswonger and Prudic, 2005). The SFR2 package 

was modified for the groundwater model of Carson Valley to 

apply diversions from the irrigation system to irrigated areas 

as recharge, as discussed in the appendix 1. The volume of the 

diverted water is reduced by two factors that account for return 

flow and crop ET losses (as was done for irrigation water 

derived from water-rights areas and wastewater); the remainder is specified as recharge to groundwater.

The SFR2 boundary represents the Carson River and its 

tributaries, and 25 canals, ditches, and sloughs within the 

irrigation system as 432 stream segments that comprise 2,656 

reaches (model cells; fig. 18). The model also represents 520 

diversions for irrigation. An 8-point channel with a V-shaped 

bottom is defined for each stream reach; this configuration 

provides a smooth transition of stream depths from high to 

low flow and facilitates numerical convergence. The SFR2 

boundary requires definition of a conductance term C:

K LW str C

l = (4)

where

Kstr is hydraulic conductivity of the streambed [L/T],

L is length of stream reach [L],

W is width of stream reach [L], and

l is thickness of streambed [L].

The quantity of flow exchanged between groundwater and 

streamflow in a model cell is the stream conductance times the 

difference between the groundwater level and stream stage.

The widths of the East and West Forks and the main stem 

of the Carson River range from 30 to 140 ft, whereas widths 

of channels in the irrigation system range from 17 to 60 ft, 

as determined from the LiDAR data. Streambed thickness 

was assigned an arbitrary value of 1 ft. The channel bottom elevations were assumed to be 6 ft and 3 ft below land 

surface for natural and manmade channels, respectively. The 

stream segments were divided into 12 groups on the basis of 

location within the valley and the type of underlying sediment (table 5). The Kstr values for six groups were estimated 

through model calibration. The Kstr values for the remaining 

six groups were computed from these estimated values based 

on location and stream width.

Inflows from the East and West Forks of the Carson River 

and Clear Creek are specified for each period in the 35-year 

simulation based on flow measured at USGS gages. Because 

of the short period of record for available streamflow data, 

inflow from Indian Creek is specified as 2 percent of the flow 

in the East Fork Carson River. Inflows to Pine Nut Creek and 

Buckeye Creek for the 35-yr period were from gaged data 

for water years 1981–97 and from runoff computed using the 

PRMS watershed models described earlier for the remaining years. Although the West Fork Carson River gage is 

located 2 mi upstream of the model boundary, the exchange 

of water between the channel and groundwater is assumed 

Table 5. Groups of stream segments for which streambed 

hydraulic conductivity was specified. 

[Values were estimated for groups that are shaded]

Hydraulic-conductivity group Stream segment

Carson River

Main stem Carson River downstream of Genoa

East Fork Carson River East Fork

West Fork Carson River West Fork

Creeks

Buckeye Creek

Pine Nut Creek

Clear Creek

Irrigation ditches

Upland

Center

Rocky Slough

Edna Wilslef Ditch

North

Heyburn Ditch

East

Allerman Canal

Virginia Ditch

South

Dressler Ditch

Company Ditch

Fredericksburg Ditch

Falke and Tillman Ditch

Jones Company Ditch

Thran Ditch

Lowland

Center

Heise Company Ditch

Home Slough

Henningston Slough

Saint Louis Straight Ditch

North

Middle Ditch

Rosser Ditch

Williams Slough

East

Cottonwood Slough

Martin Slough

Poleline Ditch

West

Brockliss Slough

Fredericksburg Ditch

Johnson Slough

Park and Bull Slough
36 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

to be negligible between the gage and the model boundary. 

Although diversions from the channel do occur along this 

reach, most of the diverted water is assumed to return to the 

channel upstream of the model boundary (Maurer and Berger, 

2007, p. 21). 

The volume of streamflow diverted from the East and West 

Forks of the Carson River to nine ditches during each quarter 

of the year are approximate and determined from monthly 

flows recorded by the FWM from 1984 to 2005. Diversions 

in water years prior to 1984 were estimated by comparing the 

mean annual flow measured in the East Fork and the West 

Fork of the Carson River with recorded ditch flows from 

1984 to 2005 to determine the appropriate monthly allocation. 

Streamflow diversions to 18 ditches without recorded flow are 

based on estimates of flow capacity reported by Unger (2006, 

p. 86, 139). The ditches are assumed to flow at 75 percent of 

capacity during the irrigation season until the flow in the East 

Fork Carson River falls below 200 cubic feet per second (ft3/s) 

and flow in the West Fork Carson River falls below 180 ft3/s. 

Once streamflow drops below these thresholds, diversions to 

ditches are regulated by the FWM and rotated periodically so 

that the ditches are assumed to flow at 50 percent of capacity 

(Hess and Taylor,1999, p. 16,17). Little or no flow enters the 

ditches during the non-growing season when irrigation is not 

applied. 

Irrigated parcels that receive diversions from the irrigation system are listed in the Alpine Decree and mapped in a 

GIS database prepared for the U.S. Fish and Wildlife Service 

(Andrew Stroud, History Mapping Services, digital dataset, 

August 20, 2008). Recharge is applied to model cells that 

correspond to the irrigated parcels during the irrigation season 

of each water year. The volume of water diverted for irrigation is reduced by 30 percent to account for return flow that 

is assumed to remain in the irrigation system. The resulting 

volume of applied water is then reduced by factors of 70 and 

80 percent to account for crop ET losses in the spring and 

summer quarters, respectively. The remaining volume is then 

allocated as recharge equally among the model cells that represent the irrigated parcel.

The volume of water diverted for each parcel is based on 

the water right granted by the Alpine Decree, but the volume 

is limited by the available flow within the source segment of 

the irrigation system. The SFR2 package routes streamflow 

through the irrigation system from upstream segments to 

downstream segments. The supplied diversion of irrigation 

water, therefore, reduces the remaining flow in the irrigation system. If the flow in the source segment is less than the 

water right, then the available flow in the source segment is 

diverted and the segment is dewatered. Flow diversions were 

specified as a percentage of flow rather than a volume of flow 

at seven stream junctions to ensure that flow was available in 

downstream segments. Diversions from the irrigation system to irrigated parcels are computed in order by the priority 

of each water right, which is based on its age. Some water 

rights were transferred during the 35-yr simulation period as 

development removed agricultural land from irrigation. The 

water-rights database maintained by the Nevada Division of 

Water Resources (http://water.nv.gov/waterrights) was used to 

track the largest transfers and to adjust allocations of diverted 

water in the simulation. In some cases, however, the history of 

water-right transfers is difficult to determine precisely, so the 

simulation represents an approximation of the actual changes 

in water use over the period. 

Hydraulic Properties

Hydraulic Conductivity

Horizontal hydraulic-conductivity values (Kxy) for Quaternary sediments were computed from the kriged distribution 

of log transmissivity presented earlier (fig. 12), assuming a 

saturated thickness that was equivalent to the median depth of 

wells included in the kriged interpolation (200 ft). The Quaternary sediments were divided into five zones that included the 

four alluvial-fan deposits (Qal-NW, Qal-W, Qal-E, Qal-SE) as well 

as the fluvial sediments (Qfl; fig. 19). Factors were estimated 

through model calibration for each of the five zones to adjust 

the magnitude of horizontal hydraulic-conductivity values 

within each zone. Vertical hydraulic-conductivity values (Kz) 

were computed using three vertical-anisotropy values (Kxy/Kz) 

estimated through model calibration for the eastern and western alluvial fans and the fluvial sediments. Horizontal anisotropy (Kx/Ky) was also specified for the western alluvial fan to 

represent preferred flow paths in the direction of deposition 

from the front of the Carson Range eastward to Carson Valley. 

The western alluvial fan is composed of several coalescing 

fan deposits. Groundwater flows mainly through transmissive 

layers within each fan deposit from the mountain front toward 

the valley, and there is probably little hydraulic connection 

between adjacent fans. A horizontal-anisotropy value of 5 was 

specified to increase simulated flow from west to east and 

limit flow from higher elevations in the south towards lower 

elevations in the north. 

Horizontal hydraulic-conductivity values for Tertiary sediments were estimated through model calibration for five zones 

delineated on the basis of sections described by Muntean 

(2001). Tertiary sediments in the eastern part of Carson Valley 

were divided into four zones: the accommodation zone (Teraz) 

and areas to the north, and to the south and east (TerN, TerS, 

and TerE; fig. 19). An additional zone was defined for Tertiary 

sediments in the northwest near Jacks Valley (TerNW). The 

Kxy values were estimated for the northern, northwestern, and 

southern zones, while the Kxy values for accommodation and 

eastern zones were assumed to be twice and one-half, respectively, the northern Kxy value on the basis of lithology. A sixth 

zone was delineated for buried sediments in the deeper parts 

of the basin beneath the Quaternary sediments (Terdeep) and 

assigned a Kxy value of 1 foot per day (ft/d). Two verticalanisotropy values were estimated through model calibration 

for the Tertiary sediments. One Kxy/Kz value was estimated for 

the accommodation and southern zones, and a second Kxy/Kz

value was estimated for the remaining zones.
Groundwater-Flow Model 37

The aquifer system was assumed to be confined to 

facilitate the numerical convergence of the solution of the 

groundwater-flow equation. This assumption implies that the 

transmissivity of model layers that intersect the simulated 

water table is fixed during the transient simulation and is not 

a function of simulated head. The top model boundary was 

specified as the land surface, so the hydraulic conductivity was 

multiplied by a factor (Satfactor) to account for partial saturation of model layers that intersect the water table:

sim layer

factor layer

− bottom Sat thick = head (5)

where 

headsim is the head computed by a prior simulation, 

bottomlayer is the elevation of the bottom of the model 

layer, and 

thicklayer is the layer thickness. 

Satfactor was set to 0.1 x thicklayer in areas where the simulated head was below the bottom of the model layer.

This modification was necessary because MODFLOW uses 

the full layer thickness to compute transmissivity for confined 

model layers. The simulated head is a function of Satfactor, so 

an iterative procedure was used to update Satfactor values. This 

entailed repetitively running the calibrated model with updated 

Satfactor values to compute a new head distribution, and then 

recalculating Satfactor until the change in model error was less 

than 0.2 percent. The assumption of confined conditions was 

later shown to have had little effect on model results, as discussed in the later section on “Model Uncertainty.”

In many geologic settings, hydraulic conductivity is known 

to decrease systematically with depth. A power function 

similar to that applied to the Death Valley area in Nevada and 

California by Belcher (2004) is used to calculate the decrease 

in horizontal hydraulic-conductivity below a threshold depth 

D:

Kdepth = Kxy 10–λd (6)

where 

Kdepth is the horizontal hydraulic-conductivity [L/T] 

at depth d [L] below D, and 

λ is a decay factor [L-1]. 

The threshold depth D and depth-decay factor λ specified for Quaternary sediments were 500 ft and 5 x 10-4 ft-1, 

respectively, whereas the corresponding values for the Tertiary 

sediments were specified as 200 ft and 8 x 10-4 ft-1. As a result, 

hydraulic-conductivity values of Quaternary sediments were 

constant to greater depths than those of Tertiary sediments 

and they decreased less rapidly with depth than did values of 

Tertiary sediments.

Storage

Specific storage (Ss) and specific yield (Sy) values were 

specified for both Quaternary and Tertiary sediments. 

Although the aquifer system was treated as confined in the 

transient simulation, storage values representative of specific 

yield were specified in parts of model layers where unconfined 

conditions are assumed to occur. For example, Sy values were 

assigned to layer 1, which represented fluvial sediments, and 

to parts of layer 4 where Tertiary sediments are exposed at 

land surface. Storage properties in the rest of layer 4 where 

Tertiary sediments are overlain by Quaternary sediments were 

assigned Ss values. The same Ss value (2 x 10-6 ft-1) was specified for both Quaternary and Tertiary sediments based on work 

by Halford and Plume (2011). Four Sy values were specified, 

with one value for Tertiary sediments and three values for the 

eastern and western alluvial fans and the fluvial sediments. 

The Sy values for fluvial sediments were adjusted by multiplying the specified value by a normalized distribution of the 

Sy values mapped by Dillingham (1980) that were presented 

earlier (fig. 13).

Model Calibration

The groundwater-flow model was calibrated by adjusting values of hydraulic conductivity, recharge, and streambed 

hydraulic-conductivity to minimize the difference between 

simulated groundwater levels and streamflows with those 

observed in wells and at gaging stations. Optimum parameter 

values were estimated through a nonlinear regression with 

UCODE 2005 (Poeter and others, 2005) that utilized parallel 

processing with a 28-processor cluster to minimize computation time. Computation of the exchange of water between the 

irrigation system and groundwater at the SFR2 boundary made 

the groundwater-flow problem highly nonlinear, a condition 

that was aggravated by the ten-fold variation in recharge from 

wet years to dry years. As a result, a numerical solution was 

difficult to obtain and necessitated a modification to the PCGN 

solver (Naff and Banta, 2008) described in appendix 2. Even 

with this modification a relatively large convergence criterion 

for hydraulic head (0.01 ft) was required to obtain a solution. The accuracy of the solution was checked by comparing volumetric flow budgets and volumes of stream leakage 

computed by the PCGN solver with those computed with a 

smaller convergence criterion (0.001 ft) by the NWT solver 

(Niswonger and others, 2011). Both results were quite similar. 

Although the NWT solver provided a more accurate numerical 

solution, the computation time was ten times longer than with 

the PCGN solver, which precluded use of the NWT solver in 

model calibration.

Observations

The nonlinear regression compared the simulated results 

with 5,686 water levels measured in 237 wells and flow at 

five streamflow-gaging stations. Water-level measurements 
38 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

were obtained from the USGS National Water Information 

System (NWIS) database for well data. This database included 

information on the screened interval and the well depth. The 

measurement depth in the model was assumed to be the well 

depth if the screened interval was not noted. Forty wells are 

shallow (less than 20 ft deep), whereas the mean well depth 

for the remaining wells is about 200 ft with an average screen 

length of 70 ft. 

The wells were divided into four groups based on the 

period of record for each well (table 6A). Water levels in 

group 1 wells were generally measured from 1981 through 

2005, whereas group 2 wells have a shorter period of record 

(12 years), with most measurements made after 1990. Group 

3 wells have generally been measured for only 2 years since 

2000. The remaining wells were assigned to group 4 and have 

mostly been measured once, although several wells were 

measured for 3 to 4 years in the 1980s. Different weights were 

specified for absolute head observations (the first measured 

water level) than for water-level-change observations, which 

were more numerous. Higher weights were assigned to absolute head observations where multiple measurements were 

made (group 1 wells), and lower weights were assigned to 

wells with only single or a few observations of absolute head 

(group 4 wells). Table 6A lists the assumed standard deviation 

σ in measurement error for each type of observation by group. 

The weight w for each observation is equal to 1/σ 2.

Streamflow measurements obtained from the USGS NWIS 

database and the FWM database were compared; simulated 

flows were computed by the SFR2 package. Average, quarterly 

flow-observations used in model calibration were computed 

from mean daily flows measured at two USGS gaging stations 

and monthly flows measured at three FWM gaging stations. 

The period of record for the USGS Carson River gaging station near Carson City spanned the entire simulation period 

(1970–2005), whereas measured flow was available at the 

USGS Carson River gaging station near Genoa only from 

2002 through 2005 (table 6B). Flow measurements for the 

FWM gaging stations were available from 1984–2005. All 

flow observations were assigned weights based on a coefficient of variation in measurement error of 10 percent. The 

resulting weights in flow observations resulted in weighted 

residuals (observed minus simulated values) that were comparable in magnitude with weighted residuals in heads, so that 

both observation groups influenced the regression equally. 

Quarterly flow observations for six other FWM gaging stations (Allerman Canal-FWM6, Heyburn Ditch-FWM12, 

Virginia Ditch-FWM7, Cottonwood Slough-FWM10, Edna 

Slough-FWM9 and Rocky Slough-FWM8, fig. 7) were used to 

specify flow diversions from the irrigation system at the SFR2 

boundary. Hydrographs of measured flows at these sites were 

compared with simulated flows to verify that the simulation 

adequately represented the historical period, but these data 

were not included in the nonlinear regression.

Table 6. Groups of observations included in model calibration A, heads and B, flows. 

A. Heads Standard deviation, in feet

Group Wells Median period of record

Median number 

of 

measurements

Absolute 

head

Water-level 

change

1 27 24 68 1 0.45

2 23 12 34 1.4 0.58

3 26 2 11 2 0.71

4 161 — 1 3.2 1

B. Flows

Gaging station

Source Number 1 Name Period of record

Coefficient 

of variation, 

percent

USGS

8 Carson River near Genoa 1/2002–9/2005 10

9 Carson River near Carson City 1/1971–9/2005 10

FWM

FWM3 West Fork Carson River at Dressler Lane 4/1984–9/2005 10

FWM4 Brockliss Slough at Ruhenstroth dam 4/1984–9/2005 10

FWM5 Brockliss Slough at Scossa box 4/1984–9/2005 10

1 Number given in table 1.
Groundwater-Flow Model 39

Model Fit

Residual plots for heads and flows (observed minus simulated values) indicate that the model simulates the groundwater system reasonably well. Residual plots for absolute 

head show little bias in model error with the mean weighted 

residual near zero (fig. 22). The standard error (SE) of the 

weighted residuals for absolute head is 16.6 ft, which is 

slightly larger than the SE for water-level change and flow and 

indicates greater influence in the regression (table 7). The SE 

in the unweighted absolute head is 39.2 ft or 3.6 percent of the 

1,090-ft measured range. Nearly all the simulated heads are 

within 50 ft of the measured value. The largest errors in absolute head occur along the basin margins; heads are generally 

under predicted within the western alluvial fans that border the 

Carson Range and over predicted in eastern alluvial fans that 

border the Tertiary sediments (fig. 23). Heads are also under 

predicted along Pine Nut Creek west of Fish Spring Flat. 

A 

B Observed head, in feet

4,600

4,700

4,800

4,900

5,000

5,100

5,200

4,600 4,700 4,800 4,900 5,000 5,100 5,200

4,600 4,700 4,800 4,900 5,000 5,100 5,200

-100

-75

-50

-25

0

25

50

75

100

Weighted residual, in feet

Simulated heads, in feet

Group 1

Group 2

Group 3

Group 4

Under predicted

Over predicted

Figure 22. Residual plots for simulated absolute heads in 

Carson Valley groundwater model: A, relation between simulated 

and observed values; B, relation between simulated values and 

weighted residuals.
40 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

[Optimal values are shaded. ft/d, feet per day; ft, feet; in/yr, inches per year; ft-1, inverse feet]

Parameter Abbreviation 1 Value

Approximate 

95-percent individual 

confidence interval

Coefficient 

of variation, 

percent

Horizontal hydraulic-conductivity, Kxy:

Quaternary sediments, factor 2

fluvial sediments Qfl 4.7 5.7–4.4 2.3

western alluvial fan Qal-W 3.7 4.1–3.3 4.4

northwestern alluvial fan Qal-NW 6.8 7.5–6.2 2.5

eastern alluvial fan Qal-E 9.9 10.8–9.2 1.8

southeastern allluvial fan Qal-SE 0.6

Tertiary sediments, ft/d

northern sediments TerN 5.8 6.1–5.5 1.6

northwestern sediments TerNW 3 3.3–2.8 4.2

southern sediments TerS 6.9 7.3–6.5 1.5

accomodation zone Teraz 11.6

eastern sediments TerE 2.9

buried sediments Terdeep 1

Vertical anisotropy, Kxy/Kz, dimensionless:

Quaternary sediments

fluvial sediments 8,500 9,700–7,400 0.8

western alluvial fans 1.4

eastern alluvial fans 2.6

Tertiary sediments

northern sediments 770 1,000–580 2.2

northwestern sediments 770

southern sediments 1.2

accomodation zone 1.2

eastern sediments 770

buried sediments 770

Storage:

Specific yield, Sy, dimensionless:

Quaternary sediments

fluvial sediments 0.32

western alluvial fans 0.2

eastern alluvial fans 0.1

Tertiary sediments 0.1

Specific storage, Ss, ft-1 3: 2. x 10-6

Table 7. Parameter values in Carson Valley groundwater model specified or estimated through nonlinear regression.
Groundwater-Flow Model 41

[Optimal values are shaded. ft/d, feet per day; ft, feet; in/yr, inches per year; ft-1, inverse feet]

Parameter Abbreviation 1 Value

Approximate 

95-percent individual 

confidence interval

Coefficient 

of variation, 

percent

Recharge, Rch, percent 4:

Quaternary sediments

west Carson Range 43 47–40 4.1

northwest Carson Range 5 100 110–90 3.5

Tertiary sediments

northeast Pine Nut Mountains 200 6 210–190 1.5

southeast Pine Nut Mountains 62 67–57 4.1

Streambed hydraulic conductivity, Kstr, ft/d:

Carson River

main stem 7.3 11–4.7 11

East Fork 6.3 11–3.5 17

West Fork 2.1

Creeks 8 9.9–6.5 5.3

Irrigation ditches

Upland

center 0.9

north 0.2 0.3–0.2 7

east 0.3

south 0.5

Lowland

center 15.4 24–10 8.2

north 21.6

east 15.4

Standard error of weighted residuals, ft

Absolute water level 16.6

Water-level change 10.3

Flow 6.7

Table 7. Parameter values in Carson Valley groundwater model specified or estimated through nonlinear regression.—Continued

1 See figure 19. 

2 Factor multiplied by interpolation. 

3 Quaternary and Tertiary sediments. 

4 Percent of value calculated by PRMS watershed models. 

5 Jack’s Valley. 

6 Value at assumed upper constraint. 
42 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Carson River

Nut

Pine

Creek

Buckeye Creek

West Fork

East Fork

A’A

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated bedrock

Line of section

Line of equal simulated 

head, in feet-Contour 

interval 100 feet, 

unless noted

EXPLANATION

Quaternary

Tertiary and older

A A’

Model boundary

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

144

121

140

Surface-water exchange; Log value in cubic 

feet per day (+ is infiltration to aquifer; – is 

discharge to stream)

5 to 6

4 to 5

3 to 4

2 to 3

–2 to 2

–2 to –3

–3 to –4

–4 to –5

–5 to –6

Residual in head, in feet (observed minus simulated)

Greater than 100

50 to 100

25 to 50

0 to 25

–25 to 0

–50 to –25

–100 to –50

5500

4700

5000

4800

5000

6000

5500

5500

4750

4650

Fish

Spring

Flat

Gardnerville

Ranchos

Ruhenstroth

Johnson Lane

Figure 23. Simulated water table in the summer quarter of water year 2005, locations and rates of surface-water exchange, and spatial 

distribution of absolute-head residuals.
Groundwater-Flow Model 43

Figure 24. Residual plots for simulated water-level changes 

in the Carson Valley groundwater model: A, relation between 

simulated and observed values; B, relation between simulated 

values and weighted residuals.

A 

B Observed change in head, in feet Weighted residual, in feet

Simulated change in heads, in feet

Group 1

Group 2

Group 3

Group 4

−50

−40

−30

−20

−10

10

20

30

40

50

0

−50 −40 −30 −20 −10 0 10 20 30 40 50

Under predicted

Over predicted

−100

−75

−50

−25

25

0

50

75

100

−50 −40 −30 −20 −10 0 10 20 30 40 50

Residual plots for water-level change show little bias for 

most of the 5,449 measurements with the mean weighted 

residual near zero (fig. 24). The SE of the weighted residuals for water-level change is 10.3 ft, and the absolute error is 

5.7 ft or 8.3 percent of the 68-ft measured range. The largest 

residuals are for three wells located along the western alluvial 

fans and in Jacks Valley where observed changes are under 

predicted (for example, well 306, fig. 25), and for one well in 

Fish Spring Flat (well 121, fig. 23) where changes are over 

predicted. Water levels on the eastern side of the valley have 

declined more than 15 ft near Fish Spring Flat (wells 144 and 

150) and the Ruhenstroth subdivision (well 55) and more than 

10 ft near Gardnerville Ranchos (well 83). These observed 

declines are well simulated, although the absolute head in 

some areas is either over or under predicted (fig. 25). Waterlevel changes in response to transitions from wet to dry periods are observed and simulated along the western side of the 

valley (wells 137 and 306). Water-level changes in response 

to pumping in deep wells in the center of the valley are also 

well represented in the model simulation (wells 125 and 175). 

An additional 35-yr transient simulation was conducted that 

represented natural conditions by omitting groundwater pumping and streamflow diversions. No water-level declines were 

simulated for the Johnson Lane, Ruhenstroth, and Gardnerville 

areas, indicating that declines in these areas are the result of 

pumping. Water-level decline simulated near Fish Spring Flat 

was about one half that simulated by the calibration simulation, however, indicating that part of the decline in this area 

has been response to drier conditions since 1987.
44 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 25. Simulated and observed water-level hydrographs for selected wells in the Carson Valley.

Carson River

Pine

Creek

Nut

Buckeye Creek

West Fork

East Fork

Fish

Spring

Flat

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated bedrock

EXPLANATION

Quaternary

Tertiary and older

Model boundary

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

167/175

273

306

144

150

55

83

15

113

124/125

137

Gardnerville

Ranchos

Ruhenstroth

Johnson Lane

Shallow

Observed Simulated

Deep

167

175

124

125

Shallow

Observed Simulated

Deep

Shallow

Deep

Observed Simulated

1975 1980 1985 1990 1995 2000 2005 5,060

5,160

5,080

5,100

5,120

5,140

1975 1980 1985 1990 1995 2000 2005

273

4,640

4,740

4,660

4,680

4,700

4,720

1975 1980 1985 1990 1995 2000 2005

175

167

4,640

4,740

4,660

4,680

4,700

4,720

1975 1980 1985 1990 1995 2000 2005

137

4,700

4,800

4,720

4,740

4,760

4,780

1975 1980 1985 1990 1995 2000 2005

113

4,680

4,780

4,700

4,720

4,740

4,760

1975 1980 1985 1990 1995 2000 2005

15

4,720

4,820

4,740

4,760

4,780

4,800

1975 1980 1985 1990 1995 2000 2005

125

124

4,680

4,780

4,700

4,720

4,740

4,760

1975 1980 1985 1990 1995 2000 2005

83

4,680

4,780

4,700

4,720

4,740

4,760

1975 1980 1985 1990 1995 2000 2005

55

4,820

4,920

4,840

4,860

4,880

4,900

1975 1980 1985 1990 1995 2000 2005

144

5,060

5,160

5,080

5,100

5,120

5,140

1975 1980 1985 1990 1995 2000 2005

Water level, in feet

above sea level

150

4,780

4,880

4,800

4,820

4,840

4,860

306

Water level

144 Well and number

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level
Groundwater-Flow Model 45

A 

B Weighted residual, in feet

5

6

7

8

9

–50

–25

0

25

50

5 6 7 8 9

5 6 7 8 9

Log observed flows, in cubic feet per day

Log simulated flows, in cubic feet per day

Carson River near Carson City

Carson River at Genoa

West Fork Carson River at Dressler

Brockliss Slough at Ruhenstroth

Brockliss Slough at Scossa

Under predicted

Over predicted

Figure 26. Residual plots for simulated flows in Carson Valley 

groundwater model: A, relation between simulated and observed values; 

B, relation between simulated values and weighted residuals.

Residual plots for flows show little bias for measurements at the five gaging stations, although the 

lowest flows in the Carson River are over predicted 

(fig. 26A).The SE of the weighted residuals for flows 

is 6.7 ft, and weighted residuals are largest for the 

smallest flows (fig. 26B). Most observed flows at the 

USGS gages on the Carson River are well simulated, 

as are flows at the FWM gages on drainage ditches, 

although low flows in the Carson River are generally 

over predicted (fig. 27).

The greatest discrepancy between the observed 

and simulated flows in the Carson River likely results 

from the fact that storm runoff within the basin is 

not accounted for in the groundwater-flow model. 

For example, a rain on snow event at the beginning 

of water year 1997 caused flooding and sharply 

increased the observed outflow at the Carson River 

gage near Carson City (Thomas and Williams, 1997). 

The significance of storm runoff generated within 

Carson Valley is apparent in plots of simulated 

and observed changes in flow in the Carson River 

(fig. 28). As noted earlier in the report, the annual 

loss in downstream flow in the Carson River within 

Carson Valley was about 51,000 acre-ft/yr for the 

period 1990–2005. The simulated loss in flow 

matches observed losses reasonably well, although 

the simulated losses generally under predicted those 

observed since 1987. Large gains in flow were 

observed in water years 1983 and 1997, but losses 

were predicted by the model. If storm runoff were 

subtracted from the observed flow in the Carson 

River, the simulated flow would better match the 

observed flow. Storm runoff within the basin is not 

a frequent occurrence, however, and the volume of 

runoff has not been estimated.
46 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 27. Simulated and observed flow hydrographs for streamflow gaging stations in the Carson Valley: A, Carson River near Carson 

City; B, Carson River near Genoa; C, West Fork Carson River at Dressler Lane; D, Brockliss Slough at Ruhenstroth dam; and E, Brockliss 

Slough at Scossa box.

C

A

B

Streamflow, 

in million cubic feet per day

Streamflow, 

in million cubic feet per day

Streamflow, 

in million cubic feet per day

0

20

40

60

80

0.1

1

10

100

1,000

0.1

1

10

100

1,000

Oct-70 Sep-75 Sep-80 Sep-85 Sep-90 Sep-95 Sep-00 Sep-05

Oct-70 Sep-75 Sep-80 Sep-85 Sep-90 Sep-95 Sep-00 Sep-05

Oct-70 Sep-75 Sep-80 Sep-85 Sep-90 Sep-95 Sep-00 Sep-05

Observed

Simulated

Observed

Simulated
Groundwater-Flow Model 47

D

E Streamflow, 

in million cubic feet per day

Streamflow, 

in million cubic feet per day

Oct-70 Sep-75 Sep-80 Sep-85 Sep-90 Sep-95 Sep-00 Sep-05

Oct-70 Sep-75 Sep-80 Sep-85 Sep-90 Sep-95 Sep-00 Sep-05

0

20

40

60

80

0

20

40

60

80

Observed

Simulated

Figure 27. Simulated and observed flow hydrographs for streamflow gaging stations in the Carson Valley: A, Carson River near Carson 

City; B, Carson River near Genoa; C, West Fork Carson River at Dressler Lane; D, Brockliss Slough at Ruhenstroth dam; and E, Brockliss 

Slough at Scossa box.—Continued

Figure 28. Simulated and observed loss in flow in the Carson River, water years 1971–2005. Change in flow, in thousands of acre-feet per year Loss in flow Gain in flow

100

75

50

50

25

25

0

Water year

1970 1975 1980 1985 1990 1995 2000 2005

Storm runoff

Observed

Simulated
48 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Simulated groundwater flow

The distribution of simulated heads indicates that groundwater flows westward through the Tertiary sediments toward 

the center of Carson Valley and then northward toward the 

outlet of the Carson River (fig. 23). The location of the simulated 4,700-ft contour in the center of the valley is similar to 

that of the 4,700-ft contour estimated from observed water levels in spring 1998 (fig. 14). A section view indicates the simulated hydraulic gradient is nearly flat at lower elevations in 

Carson Valley, but the gradient slopes upward through Tertiary 

sediments to the east (fig. 29). Simulated vertical gradients are 

upward near the center of Carson Valley, but are over predicted by as much as 20 ft (well 175, fig. 25). Simulated flow 

paths indicate that groundwater flows faster through the center 

of Carson Valley where the hydraulic conductivity of fluvial 

sediments is larger, and groundwater flows slower through 

the smaller hydraulic-conductivity Tertiary sediments to the 

east (fig. 30). The rate of flow in all areas declines with depth, 

reflecting the assumed decrease in hydraulic conductivity. The 

direction of flow also changes with depth in the center of the 

valley as a result of the large vertical anisotropy estimated for 

the fluvial sediments. Shallow flow in the center of the valley 

is towards drainage channels, while deeper flow is generally 

directed toward the basin outlet to the north. 

The distribution of streamflow losses and gains simulated 

throughout the valley in the summer quarter of 2005 (fig. 23) 

is in general agreement with that determined by Maurer and 

Berger (2007, p. 50) from data collected over several years. 

The greatest losses to infiltration are simulated in the southeastern part of the valley, and the greatest gains are simulated 

on the western and northern parts of the valley. The simulated 

distribution of recharge derived from irrigation fluctuated 

throughout the simulation in response to changes in streamflow available for diversion. For example, in the spring 

quarter of water year 2005 simulated irrigation was applied 

to about 30,000 acres and provided 55,000 acre-ft of recharge 

(fig. 31A). In contrast, streamflow available for diversion during the summer quarter of water year 2005 provided irrigation 

water for only about 24,000 acres, resulting in only 25,000 

acre-ft of recharge (fig. 31B).

Parameters

The Carson Valley groundwater model was formulated 

with a total of 41 parameters, 18 of which were optimized 

through nonlinear regression. The values of other parameters 

were either fixed or estimated through regressions that lowered 

model error but did not converge to an optimal set of parameter values. Coefficients of variation (CV) for the 13 optimized 

hydraulic-conductivity and recharge parameters range from 

0.8 to 4.4 percent (table 7), indicating that the regression is 

sensitive to the parameters and that the values are well estimated. The estimated values for the five streambed hydraulicconductivity parameters are less certain with CV values 

ranging from 7 to 17 percent. There is no significant correlation among the model parameters. The individual confidence 

intervals in table 7 are approximate and indicate qualitatively 

the uncertainty in estimated parameter values.

The sensitivity of the model to the estimated parameter 

values was assessed by comparing the composite-scaled 

sensitivities (CSS), which indicate the amount of information 

provided by all observations for each parameter (Hill, 1998). 

The CSS statistics indicate that the parameter sensitivities 

are highest for the hydraulic conductivity (K) and recharge 

(Rch) parameters associated with the Tertiary sediments in 

0

-1,000

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

Altitude, in feet above sea level

0 5 10 15 20

Distance, in miles

Vertical exaggeration X5

Pine Nut

Mountains

Carson

Range

A’

A

Tertiary

sediments

Tertiary 

sediments

Alluvial fan

sediments

Fluvial

sediments

Hypothesized

Hot Springs Mountain fault

Carson US 395

River

140

144

Well and number

Simulated water table

Consolidated 

bedrock

Figure 29. Simulated water table along section A-A’ in the summer quarter of water year 2005.
Groundwater-Flow Model 49

Figure 30. Simulated rate and direction of groundwater flow in Carson Valley.

Traveltime, in years

Johnson Lane area

Ditch or slough

Jacks Valley

Vertical exaggeration x 3Oblique view. Not to scale.

Ruhenstroth

subdivision

Carson River

1050

Groundwater flow paths,indicate three-dimensionaldirection and rate of flow.Length of line indicatesdistance traveled.
50 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 31. Simulated rate of recharge from streamflow diverted to irrigated fields in Carson 

Valley in water year 2005: A, spring quarter; B, summer quarter.

A

B

Model boundary

Geologic units

Fluvial deposits

Alluvial fan deposits

Tertiary sediments

Consolidated bedrock

EXPLANATION

Quaternary

Tertiary and older

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Recharge from streamflow 

diverted to irrigated fields, 

in cubic feet per day

Less than 2,000

2,000 to less than 4,000

4,000 to less than 6,000

6,000 to less than 8,000

8,000 to 10,000

Greater than 10,000

Stream segment in 

streamflow routing

the northern and southern parts of the 

basin, and they are lowest for the Tertiary vertical-anisotropy parameters 

and streambed hydraulic-conductivity 

parameters for the Carson River 

(fig. 32). Together the CV and CSS 

statistics indicate that changes to the 

optimized parameter values with CV 

values less than 10 percent would 

result in proportionally large increases 

in model error.

The Kxy distribution for Quaternary fluvial and alluvial fan sediments 

in the Carson Valley groundwater 

model is obtained by multiplying 

the optimized Kxy parameter values 

(table 7) by the kriged distribution of 

transmissivity (T) values derived from 

the specific-capacity data (fig. 12), 

and dividing by the median wellscreen length (200 ft). Model Kxy

values are compared with the T values 

estimated from specific-capacity data 

(fig. 11) by multiplying the geometric-mean Kxy value by 200 ft. The 

resulting mean T values (11,000 and 

800 ft2/d for the fluvial and alluvial 

fan sediments, respectively) are both 

within the third quartile of T values 

estimated from specific-capacity data. 

The mean T value for the fluvial sediments is slightly less than the maximum value (13,000 ft2/d) estimated 

for a previous model of Carson Valley 

by Maurer (1986). Optimized values 

of streambed hydraulic-conductivity 

Kstr are either less than or comparable 

to estimated Kxy values of the Quaternary sediments, with larger values 

estimated for the Carson River and 

irrigation channels in lowlands near 

the center of the valley and smaller 

values estimated for channels in the 

uplands.

The optimized Kxy values for 

Tertiary sediments (3.3 to 6.9 ft/d) 

are larger than expected and yield 

transmisivity values more than 700 

ft2/d, which are higher than the third 

quartile of T values estimated from 

specific-capacity data (fig. 11). In 

an alternative simulation the Kxy 

values for Tertiary sediments were 

all reduced to a more reasonable 

value of 2 ft2/d. The Rch values for 

Tertiary sediments were also reduced 
Groundwater-Flow Model 51

0

10

20

30

40

50

Composite-scaled sensitivity, 

dimensionless

Parameter

Unit or stream

Kxy Kxy:Kz Rch Kstr

Quaternary

Alluvial

fan

Tertiary Tertiary Carson

Range

Carson

River

Irrigation

ditch

Pine

Nut

Mtns

FluvialWestNorthwestEastNorthwestNorthSouthQuaternaryNorthSouthWestNorthwestNortheastSoutheastMain stemEast ForkEastern creeksUpland centerUpland northLowland center

Figure 32. Composite-scaled sensitivities of optimal parameters in Carson Valley groundwater model.

by comparable amounts to preserve the same Kxy: Rch ratios. 

Model error increased by 10 percent, but the poorer model fit 

was due solely to additional error in water-level change, while 

errors in absolute head and streamflows decreased slightly. 

The use of more reasonable Kxy values for Tertiary sediments 

could, therefore, be justified to produce a model that slightly 

better matches heads and streamflows at the expense of a 

poorer match to water-level fluctuations.

The estimated vertical anisotropy Kxy /Kz for the Quaternary fluvial sediments is large (9,000) and reflects the presence of confining layers within the sediments. Maurer (1986) 

estimated a comparable value of vertical anisotropy (10,000) 

in his previous model of Carson Valley. Observed water-level 

changes in artesian wells screened in deeper parts of the valley 

are not well simulated with smaller Kxy /Kz values. The spatial 

distribution of fluvial sediments was divided into two zones 

in an alternative regression to separate the deeper sediments 

in the center of the valley from the shallower sediments, but 

the estimated Kxy /Kz value for each zone was unchanged. The 

Kxy /Kz values estimated for the Quaternary alluvial sediments 

are small (1.4 to 2.6), but the regression was not sensitive to 

these values, which were not optimized. The Kxy /Kz value 

estimated for the Tertiary sediments in the accommodation and 

southern zones is also small (both 1.2) and was not optimized 

for the same reason. The Kxy /Kz value estimated for remaining 

Tertiary sediments is large (500), which is reasonable because 

low permeability fine-grained (clay) layers are present within 

these sediments. In contrast, Tertiary sediments in the southern zone and in the accommodation zone are coarser grained 

(Muntean, 2001), so vertical anisotropy is expected to be less 

in both these zones.

Volumes of mountain-front recharge (MFrch) estimated 

by the regression (table 7) differ from those estimated from 

PRMS watershed models by factors that range from 43 to 

200 percent. The mean estimate of mountain-front recharge 

derived from the regression (36,000 acre-ft/yr) is equivalent 

to the PRMS estimate, but about 70 percent of the mountainfront recharge simulated in the Carson Valley model enters 

along the Pine Nut Mountains. In contrast, the PRMS models 

estimate a nearly equal distribution of mountain-front recharge 

between the Carson Range and the Pine Nut Mountains. An 

alternate regression in which the PRMS recharge estimates 

were specified in the model simulation resulted in a 15-percent increase in model error, with errors in Carson River flow 

accounting for nearly all the increase.

The regression was sensitive to the specific-yield parameters, but it produced values that were unreasonably large 

(greater than 0.4), so these parameters were later omitted 

from the regression. The specific-yield values for Quaternary 

fluvial sediments were obtained by multiplying the normalized 

distribution of values presented previously (fig. 13) by the factor 0.3, which yielded a mean value of 0.21. Areas of waterlevel decline are limited within Carson Valley, so water-level 

measurements provide little information concerning the value 

of specific yield. Evidently, the regression-derived estimates 

acted to dampen seasonal fluctuations in simulated water levels, but the cause of the unreasonable estimates could not be 

linked to a single set of observations.
52 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Water Budgets

Annual groundwater budgets were computed for each 

water year in the 35-yr transient simulation (fig. 33); the 

mean annual budget for the period is shown on table 8. All 

the components of the budgets were computed by the model 

except valley recharge (Valleyrch), septic-tank discharge (Septic) and pumping (Pump) which were specified (table 4). The 

contributions of flow from Eagle Valley (GWin) and Septic are 

too small to appear on the time-series plots and are neglected 

on the mean annual budget in table 8. The length of the bars 

rises and falls on the time-series plots and indicates transitions 

from wet to dry periods. Stream infiltration (Streaminfil) and 

mountain-front recharge (MFrch) are the largest inflow components, and stream discharge (Streamdisch) and evapotranspiration (ETwt) are the largest outflow components. Pumping has 

historically been less than 10 percent of discharge from the 

groundwater system, and agricultural recharge (Agrch) has been 

less than 10 percent of recharge to the groundwater system. 

The time-series plots indicate that storage is an important part 

of the budget with larger inflows from storage in dry years 

and larger outflows to storage in wet years. As a result, the 

aquifer system is in a dynamic equilibrium and it is difficult 

to define a steady-state period from which to calculate an 

annual groundwater budget that is representative of average 

conditions.

The cumulative water budget for Carson Valley portrays 

the relative contributions of budget components for water 

years 1971–2005 (fig. 34). Steeply rising portions of the curve 

indicate wet periods while flatter portions indicate dry periods. 

This budget only includes inflows and outflows of water to 

the basin. Agricultural recharge and stream infiltration and 

discharge are not included in this water budget because these 

components of the groundwater budget are transfers of water 

within the basin. The cumulative plots indicate that streamflow 

is the dominant water-budget component and that evapotranspiration is the second largest component. Inflows from 

mountain-front and valley recharge and storage contribute 

nearly equal volumes to the water budget. Outflows to wells 

and storage are also equal in volume.

Figure 33. Groundwater budgets simulated for water years 1971–2005. Flow, in thousands of acre-feet per year

Water year

-300

-200

-100

0

100

200

300

400

-400

0

Inflow

Outflow

1971 1975 1980 1985 1990 1995 2000 2005

Inflow

Stream infiltration

Mountain-front recharge

Valley recharge

Agricultural recharge

Storage

Outflow

Stream discharge

Evapotranspiration

Wells

Storage

EXPLANATION

Table 8. Simulated mean-annual groundwater budget for Carson Valley from water year 1971 through 2005. 

[Volumes in thousands of acre-feet] 

Inflow Volume Percent Outflow Volume Percent

Stream infiltration 120.2 48.2 Stream discharge 125.9 50.5

Mountain-front recharge 36.4 14.6 Evapotranspiration from 

water table 70.8 28.4

Storage 44.1 17.7 Storage 38.0 15.2

Valley recharge 32.1 12.9 Net pumping1 14.6 5.9

Agricultural recharge 16.8 6.7

Total 249.6 100.0 Total 249.3 100.0

1 Pumping minus infiltration of septic discharge. 
Groundwater-Flow Model 53

Figure 34. Cumulative water budgets simulated for water years 1971–2005: A, inflow; B, outflow.

A Inflow

B Outflow

Water year

Flow, in millions of acre-feet

Water year

Flow, in millions of acre-feet

Cumulative outflow

0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

16

1970 1975 1980 1985 1990 1995 2000 2005

1970 1975 1980 1985 1990 1995 2000 2005

Streamflow

Evapotranspiration

Wells

Storage 

Streamflow

Mountain-front recharge

Valley recharge

Storage and storm runoff
54 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Simulated water budgets for Carson Valley for water years 

1992 and 1997 (table 9) were prepared for comparison with 

the independently estimated, mean-annual water budget presented in table 3 for the period 1990–2005. Water years 1992 

and 1997 were the driest and wettest years, respectively, during this period. The overall budget errors (about 2 percent) are 

reasonable considering potential errors in estimating the waterbudget components. The simulated water budgets include 

storage and, therefore, differ from the mean budget estimates 

presented in table 3. Flows of individual budget components 

and the total flow for these dry and wet years bracket the 

comparable values in mean budget and are consistent with 

earlier assessments. An assumed contribution of storm runoff 

was required to balance the budget in water year 1997 when 

flooding occurred.

Table 9. Simulated annual water budgets for Carson Valley in 

the 1992 and 1997 water years. 

[Shaded volumes are estimated by Carson Valley groundwater-flow model. 

Volumes in thousands of acre-feet. ET, evapotranspiration]

Inflow 1992 1997

Streamflow:

East Fork 127.0 438.3

West Fork 39.6 132.9

Carson Range 1 1.4 22.5

Valley recharge 2 20.6 58.5

Mountain-front recharge 4.7 97.1

Storm runoff 3 — 63

Storage 4 42.5

Total 235.8 812.3

Outflow 1992 1997

Stream flow 101.5 599.5

Evapotranspiration

Groundwater 5 63.0 77.3

Agricultural 6 41.1 63.3

Net pumping 7 22.1 18.1

Storage 4 65.7

Total 227.7 823.9

1 Winter runoff. 

2 Includes precipitation, water-rights applications and imported effluent. 

3 Assumed runoff within basin. 

4 Net storage. 

5 ET from water table for all vegetation types. 

6 ET from application of stream-flow diversions on irrigated fields. 

7 Pumping minus infiltration of septic discharge. 

Model uncertainty

There are three types of errors that cause heads and flows 

simulated by the groundwater model to differ from those that 

are observed. The first source of error and most difficult to 

quantify results from errors in the model design, which entails 

representing the aquifer system as a set of mathematical equations. This design error reflects both the approximation of 

complex physical processes by theoretical equations and the 

uncertainty regarding the actual geometry of the aquifer system, which can only be characterized at a limited number of 

locations through boreholes or surface exposures. The second 

source of error is related to the values of hydraulic properties 

(parameters) that must be specified in the mathematical model. 

These values can sometimes be estimated independently 

through field measurements (for example, hydraulic conductivity through aquifer tests), but often must be inferred indirectly (for example, specific yield through grain-size analysis 

or recharge through flow measurements). The parameter error 

reflects both the uncertainty in the estimated values and in the 

pattern of their distribution within the model. Parameter error 

can be quantified using statistical measures that are computed 

by nonlinear regression. The third source of error results from 

measurement of the observations themselves and is reflected 

by weights assigned to the observations in the regression. This 

error is commonly assumed to be much less than the other two 

types of error.

Both design error and parameter error can be reduced 

through model calibration by considering alternative model 

designs and by estimating parameter values through nonlinear 

regression, but ultimately a point is reached beyond which 

further reduction in error is difficult. Halford and Plume 

(2011) refer to irreducible error, which can only be lowered by 

obtaining additional information concerning uncertain model 

processes, geometry, and parameters. The irreducible error, 

therefore, reflects the current state of knowledge concerning 

the aquifer system. Design error in the Carson Valley model 

was assessed through several alternative models, which are 

discussed in the following paragraphs. None of the alternatives greatly reduced model error. Some alternatives increased 

model error only slightly (less than 15 percent) and served 

to indicate the range of possible parameter values that can 

be considered. Parameter error in the Carson Valley model is 

reflected by the CSS and CV statistics and by the approximate 

individual confidence intervals computed for the parameter 

estimates. Parameter error could be further assessed (and 

perhaps reduced) through calibration using regularization with 

pilot points, as described by Doherty (2003) and Halford and 

Plume (2011). This approach was beyond the scope of the 

current study due to limitations of time and computational 

capacity.

One source of uncertainty in the model design is the 

assumption of confined conditions for the unconfined aquifer 

system. As discussed previously, unconfined conditions were 

accounted for in model layers that intersect the water table 

during calibration using the factor Satfactor in the computation 
Groundwater-Flow Model 55

of transmissivity to account for partial saturation and by the 

specification of storage values representative of specific yield. 

Recently (May 2011), an additional numerical solver became 

available within MODFLOW that allowed the solution of an 

unconfined version of the Carson Valley model (Niswonger 

and others, 2011). The unconfined solution presently requires 

a ten-fold increase in computation time to complete the 35-yr 

transient simulation and would be difficult to use in model 

calibration, but the results are compared with those of the 

calibrated model to assess how the assumption of confined 

conditions affects the model results.

The heads and flows computed by the unconfined transient 

simulation are similar to those computed with the confined 

simulation. For the unconfined transient simulation, the 

standard errors of weighted residuals in absolute head, waterlevel change, and flow are 16.9, 7.5, and 7.7 ft, respectively, 

indicating a comparable match to observed heads, a better 

match to water-level change, and a slightly poorer match to 

observed flows than those obtained with the confined simulation (table 7). The distribution of residuals for the unconfined 

simulation is also similar to that of the confined simulation. 

The largest differences between the simulations are in heads 

computed in Tertiary sediments near the eastern margin of the 

basin where the unconfined simulation computes much higher 

values (more than 100 ft higher). Actual heads in these areas 

are unknown, however, and the difference in the simulated 

water table between the two simulations is small within 3 mi 

of well 144 (fig. 29). Water levels computed by the unconfined 

transient simulation are below the depths of domestic wells 

specified in some areas, primarily near Johnson Lane and 

within the Ruhenstroth subdivision. As a result, pumping was 

not simulated from these domestic wells, and the total volume 

pumped was reduced by about 2 percent. This reduction has 

little effect on the simulated groundwater budget, though. The 

depths of domestic wells in these areas are probably deeper 

than the depth specified in the model (150 ft). The close agreement between the confined and unconfined transient simulations indicates that the parameter values estimated through 

model calibration are probably not affected by the assumption 

of confined conditions.

One alternative model was calibrated through nonlinear 

regression in which the 800-ft displacement of Tertiary sediments along the hypothesized Hot Springs Mountain fault 

was represented by two 400-ft steps: an upper step beneath 

the eastern boundary of the Quaternary alluvial fan sediments 

and a lower step along the assumed location of the fault. In 

this configuration the fluvial sediments extend further eastward beneath the alluvial fan sediments than in the calibrated 

model. The standard errors of weighted residuals in absolute 

head, water-level change, and flow are 16.1, 10.4, and 6.9 ft, 

respectively, indicating a comparable match to the observed 

values as in the calibrated model. The spatial distribution 

of absolute head residuals differs from that of the calibrated 

model; however, simulated heads are 20 to 40 ft lower on 

the eastern side of the basin beneath the alluvial fan sediments and in the Tertiary sediments. The actual geometry of 

the hypothesized fault is unknown, but these results indicate 

that eastern areas where heads computed with the calibrated 

model are over predicted could be underlain by more permeable fluvial sediments that extend eastward from the center of 

Carson Valley.

Representation of evapotranspiration (ET) from groundwater is another source of uncertainty. The simulated ET rate is 

determined by four factors in the model: the maximum ET rate 

(ETmax), volume of return-flow loss, the irrigation efficiency, 

and the extinction depth dET below which ET from the water 

table does not occur. There is uncertainty associated with the 

estimates for each of these factors and different combinations 

of values could be specified to obtain a similar ET rate, so the 

set chosen for the calibrated model is nonunique. ET simulated 

by the calibrated model from irrigated land is about 3.7 ft/yr, 

23 percent higher than the rate estimated by Maurer and others 

(2006, p. 22) for irrigated alfalfa and pasture grasses in Carson 

Valley. Changes to these values would affect the simulated 

streamflow, but they would have little effect on the relatively 

flat water table in the center of the valley where the irrigated 

fields are located.

There are three principal aspects of the calibrated model in 

which uncertainty in the model design or parameter values are 

likely to affect important model results: (1) the mountain-front 

recharge and hydraulic properties of the Tertiary sediments on 

the eastern side of the basin, (2) the composition of sediments 

beneath the alluvial fans, and (3) the extent of the confining 

layer represented within fluvial sediments in the center of 

the basin. Very little information is available concerning the 

thickness and hydraulic conductivity of the Tertiary sediments 

and the position of the water table within them. This information is important because the hydraulic gradient that drives 

flow through these sediments is determined by these hydraulic 

properties and by the volume of mountain-front recharge that 

enters the sediments. It is not possible to calibrate these values 

without information concerning the position of the water table. 

Most of the recent and future development in Carson Valley 

has or will occur on the eastern side of the basin adjacent to 

areas underlain by Tertiary sediments. The rate of simulated 

flow through these sediments determines the amount of predicted water-level decline (drawdown) in response to expected 

increases in pumping. Although the total rate of flow through 

the Tertiary sediments simulated by the Carson Valley groundwater model is similar to that estimated by the previous PRMS 

watershed models, the proportion of mountain-front recharge 

from the Carson Range and the southeastern Pine Nut Mountains is less than that estimated by PRMS models. The uncertainty associated with the volume of mountain-front recharge, 

and in the hydraulic properties of the Tertiary sediments, limits 

the reliability of model predictions regarding drawdown along 

the western and southeastern sides of the valley.

The largest errors in absolute head in the Carson Valley 

model are in areas underlain by alluvial fans on the east and 

west sides of the valley where most recent development has 

occurred. Reducing model error in these areas is important to 

improving model predictions. For example, heads in alluvial 
56 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

fans along the Carson Range are under predicted and the error 

in absolute head could be reduced by either by decreasing the 

Kxy of the alluvial fan sediments or increasing mountain-front 

recharge. However, both of these changes result in increased 

error in flow. The recharge estimate for the Carson Range 

estimated by PRMS watershed models was more than twice 

the value estimated through calibration of the Carson Valley 

groundwater model. This discrepancy reflects the uncertainty 

in determining this value and indicates that the representation of alluvial fans in the Carson Valley groundwater model 

could be improved. Little information is available concerning 

the composition of sediments that underlie the alluvial fans or 

how the alluvial fan sediments transition to the fluvial sediments in the center of the valley. Additional geologic data 

in these areas would support an improved conceptual model 

of deposition in these areas that could be incorporated in the 

Carson Valley groundwater model to reduce error and reduce 

uncertainty in the estimation of mountain-front recharge.

Finally, a large degree of vertical anisotropy is required 

within the fluvial sediments to simulate water-level changes 

observed at artesian wells in the center of the valley, but the 

simulated vertical gradient in hydraulic head is over predicted. 

The actual confining layer is probably not as extensive as that 

represented, and there are likely areas where no confining 

layer exists. Additional head data from pairs of shallow and 

deep wells would provide information concerning the magnitude of the vertical gradient within the valley. Alternatively, 

additional calibration using regularization with pilot points 

could be used to better represent the spatial heterogeneity of 

vertical anisotropy of the fluvial sediments. This parameter 

limits the hydraulic connection between shallow and deep flow 

systems in the valley and is important in predicting the extent 

of drawdown that would result from increased pumping from 

deep wells in the valley.

Model Application

The Carson Valley groundwater model was used in a series 

of five 55-yr predictive simulations to evaluate the long-term 

effects of the expected increase in demand for groundwater on 

water-budget components, groundwater levels, and streamflow 

in the Carson River. Details of several hypothetical changes 

were developed in consultation with Douglas County personnel (Mimi Moss, Douglas County Planning; Carl Ruschmeyer 

and Ron Roman, Douglas County Public Works) and the 

Carson Water Subconservancy District (Ed James, General 

Manager). 

The predictive simulations represented water years 2006–

60 using 220 quarterly stress periods. The head distribution 

computed at the end of the calibration simulation was specified as the initial condition, and rates of inflow and outflow 

specified at model boundaries for the end of the calibration 

period in water year 2005 were held constant through water 

year 2010. The boundary conditions were varied for water 

years 2011–60 to represent the transition from wet to dry 

conditions that occurred from water year 1995 through 2004 

(fig. 35). This 10-yr cycle was repeated five times to complete 

the 55-yr simulation. An alternative approach could have used 

predictions of climate models to adjust inflow and outflow 

rates, but such an approach was beyond the scope of the present study.

Four water-use scenarios were compared with a base 

simulation in which the rates and locations of pumping and 

surface-water irrigation were based on values specified for 

water year 2005 and held constant through water year 2060. 

Minor changes in the distribution of 2005 pumpage were 

included in the base scenario to more accurately represent the 

actual conditions after 2005. Pumpage from Minden wells was 

increased to compensate for decreased pumpage at Douglas 

County municipal wells that had previously supplied the Indians Hills, the East Valley water-service area, and parts of the 

Ruhenstroth water-service area.

Rates of groundwater pumping for municipal and domestic 

use were increased and the volume of surface water diverted 

for irrigation was decreased in the four water-use scenarios. In 

scenario 1 the 2005 municipal pumping rates were increased 

by 250 percent from 8,800 to 22,200 acre-ft/yr to supply 

projected population growth (table 10). Additional pumping 

from 1,340 domestic wells was assumed to occur outside the 

Douglas County water-service areas from parcels that did not 

contain wells in 2005. Groundwater pumping for municipal 

use was increased by a factor of four over the base scenario 

to 32,900 acre-ft/yr in scenario 2A to supply the anticipated 

Table 10. Future water-use scenarios represented in 55-year predictive simulations. 

Pumping, 1,000 acre-feet per year

Scenario

Maximum 

irrigated area 1,000 

acres

Municipal Domestic Agricultural Other Total

Base 40.9 8.8 4.1 7 3.0 22.9

1 40.9 22.2 5.3 7 4.6 39.1

2A 40.9 32.9 5.3 7 5.0 50.2

2B 40.9 35.2 3 7 5.0 50.2

3 37.2 32.9 5.3 5.3 5.2 48.7
Model Application 57

Figure 35. A, Projected Palmer drought severity index; B, range in pumping specified in 55-year predictive simulations. 

A

B

Calibration period Prediction period

1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050 2060

1970 1980 1990 2000 2010 2020 2030 2040 2050 2060

Palmer drought severity index

Pumpage rate, in millions of cubic feet per day

-8

-6

-4

-2

0

2

4

6

8

10

0

2

4

6

8

10

12

14

16

Severe to extreme wetness Wet

period

Severe to extreme drought Dry period

Historical rates

Senarios:

Base (2005 rates)

1

2A

EXPLANATION

Calendar year

demand associated with the maximum “build-out” of Carson 

Valley. The same pumping rates were also applied in scenario 

2B, but in water year 2020 pumping was assumed to cease 

from 2040 domestic wells within Douglas County waterservice areas and pumping at 6 supply wells for the city of 

Minden was increased to represent a hypothetical reallocation to the municipal water supply. Scenario 3 combined the 

maximum pumping rates in scenario 2A with a removal of 

3,700 acres of irrigated land from the water-service areas. 

Recharge from application of treated effluent for irrigation was 

applied at the same rates and locations as simulated in 2005 

for scenarios 1-3. Although increased municipal pumping 

would likely generate greater volumes of treated effluent, it is 

uncertain how or where the effluent may be used in the future. 

The greatest increase in pumping is simulated from wells 

that supplied the town of Minden in 2005 because the Minden 

wells have the most available water rights and the produced 

water meets drinking-water standards. Some municipal wells 

at other locations in Carson Valley produce water that contains arsenic concentrations that do not meet drinking-water 

standards (Paul and others, 2010, p. 2). Increased municipal 

pumping was also simulated for municipal wells that supply 

Gardnerville and the Gardnerville Ranchos and wells owned 

by Douglas County.

Base Scenario

The 55-year predictive base scenario indicates that watertable declines will mainly occur along the east side of Carson Valley and near the Gardnerville Ranchos in the south 

(fig. 36). Declines of more than 40 ft are predicted north of 

Buckeye Creek in 2060, but most of the predicted declines 

range from 5 to 40 ft. Less than 5 ft of water-table decline is 

predicted for the central part of Carson Valley and the Johnson Lane area. Water-level declines are predicted to continue 

through 2060 on the east side of the valley. Model uncertainty 

precludes the prediction of water-table declines in the upland 

areas north of Buckeye Creek and east of Fish Spring Flat. 

The time-series of annual groundwater budgets indicates 

that the aquifer system quickly reaches a dynamic equilibrium 

in the 55-year predictive simulation and reflects the alternating 

cycles of wet and dry periods (fig. 37). The largest components of the budget are the exchange of groundwater with 

streamflow, as in the calibration simulation presented earlier. 

Outflow to evapotranspiration remains relatively constant 

from year-to-year, but inflows from mountain-front recharge 

and from storage increase sharply during wet and dry periods, respectively. Outflows to storage also increase during 
58 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 36. Water-table decline from 2010 through 2060 simulated by the base scenario using the Carson Valley groundwater model. 

Carson River

Pine Nut Creek

Model boundary

Study-area boundary

0 1 2 3 4 5 MILES

Buckeye Creek

West Fork

East Fork

10 2 3 4 5 KILOMETERS

Gardnerville

Minden

Johnson Lane

Fish

Spring

Flat

Ruhenstroth

Gardnerville

 Ranchos

Base from U.S. Geological Survey digital data, 1:100,000, 1988

Hillshade from 30-meter USGS National Elevation Dataset, 1999

Universal Transverse Mercator projection, zone 11, North American Datum of 1983

119° 52' 30" 119° 30' 

38°

45'

30"

39°

07'

30" EXPLANATION

Simulated water-table decline, 

in feet

5 to 10

10 to 20

20 to 40

More than 40

Uncertain
Model Application 59

wet periods, while outflow to wells remains a relatively small 

component of the groundwater budget throughout the 55-year 

period. The annual loss in downstream flow in the Carson 

River within Carson Valley is about 38,000 acre-ft. 

Scenario 1

Groundwater withdrawals for municipal and domestic 

use are 14,600 acre-ft/yr larger in scenario 1 than in the base 

scenario, as a result of increased pumping at existing municipal wells and the assumed installation of 1,340 new domestic 

wells. Additional water-level declines, below the levels predicted by the base scenario, are predicted in scenario 1 for the 

east side of Carson Valley and near the Gardnerville Ranchos 

(fig. 38). The additional declines range from 5 to 40 ft on the 

east side of the valley to more than 40 ft near Gardnerville 

Ranchos, resulting in total water-table declines of as much as 

50 ft below 2005 conditions in both areas. Additional waterlevel declines of 5 to 10 ft are also predicted along the western 

side of the valley, although these declines would be less if 

estimates of mountain-front recharge from PRMS watershed 

models were applied. Predicted water-level hydrographs 

indicate relatively rapid declines at well 144 in Fish Spring 

Flat and well 29 in Gardnerville Ranchos. Predicted water 

levels continue to decline through 2060 at the former location, 

but they stabilize at the latter location (fig. 38). More gradual 

declines are predicted at wells 55 and 150 on the east side of 

the valley near Ruhenstroth and Buckeye Creek, respectively. 

Water-table declines are not indicated by hydrographs predicted for wells in the central part of the valley, but seasonal 

drawdowns in wells 83 and 113 more than double as a result 

of the higher pumping rate at the Minden and Gardnerville 

wells. The higher pumping rate also produces a pronounced 

downward hydraulic gradient at wells 124 and 125 southwest 

of Minden, but the upward hydraulic gradient at wells 167 and 

175 to the northwest generally is preserved despite the higher 

pumping rate.

The groundwater budget for scenario 1 is similar to that 

of the base scenario, but the total flow through the aquifer 

is slightly larger. For example, in water year 2051 inflows 

from streams and storage increase by 1,900 and 2,200 acre-ft, 

respectively (fig. 39A). Outflows to streams and evapotranspiration in water year 2051 decrease by 11,700 and 3,300 acreft, respectively. The additional decrease in Carson River flow 

in scenario 1, compared to the base scenario, is 9,600 acre-ft 

per year, and accounts for about two-thirds of the increase in 

pumping (fig. 39B). This result is consistent with that from a 

previous groundwater-flow model of Carson Valley developed 

by Maurer (1986) that indicated 70 to 75 percent of pumped 

water was derived from a decrease in outflow of the Carson 

River.

Scenarios 2A and 2B

Groundwater withdrawals for municipal and domestic 

use are 25,300 acre-ft/yr more in scenario 2A than in the base 

scenario as a result of increased pumping at existing municipal wells. The installation of 1,340 new domestic wells is 

assumed, as in scenario 1. Total groundwater withdrawals are 

the same in scenario 2B as in scenario 2A, but in water year 

2020 pumping from 2,040 domestic wells ceases and pumping from 6 municipal wells increases in scenario 2B. Areas 

of additional water-level decline (below the levels predicted 

by the base scenario) are more extensive on the east side of 

the valley by 2060 in scenario 2A than in scenario 1, resulting 

in total water-table declines of 60 ft below 2005 conditions. 

Figure 37. Annual groundwater budgets, 2006–60 simulated by base scenario. Flow, in thousands of acre-feet per year

Water year

-300

-200

-100

0

100

200

300

400

-400

0

Inflow

Outflow

Inflow

Stream infiltration

Mountain-front recharge

Valley recharge

Agricultural recharge

Storage

Outflow

Stream discharge

Evapotranspiration

Wells

Storage

EXPLANATION

2006 2010 2020 2030 2040 2050 2060
60 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 38. Additional groundwater withdrawals included in scenario 1, additional water-table decline from 2010 through 2060 

simulated by scenario 1 compared to the base scenario, and simulated water-level hydrographs at selected wells for the base scenario 

and scenario 1.

Carson River

Pine Nut Creek

Buckeye Creek

West Fork

East Fork

EXPLANATION

Model boundary

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

167/175

273

144

150

55

83

113

124/125

29

Gardnerville

Minden

Johnson Lane

Fish

Spring

Flat

Ruhenstroth

Gardnerville

 Ranchos

Additional water-table decline (more 

than 5 feet), compared with base 

scenario, in feet

5 to 10

10 to 20

20 to 40

More than 40

Additional wells and wells with increased 

pumping, rate in acre-feet per year

More than 1,000

100 to 1,000

10 to 100

Less than 1

Domestic Municipal

and other

Simulated water level

Base

Scenario 1

Shallow

Deep

Shallow

Deep

Base Scenario 1

Base Scenario 1

2000 2010 2020 2030 2040 2050 2060 2000 2010 2020 2030 2040 2050 2060 4,640

4,740

4,660

4,680

4,700

4,720

150

4,760

4,860

4,780

4,800

4,820

4,840

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

167/175

4,650

4,750

4,670

4,690

4,710

4,730

113

4,650

4,750

4,670

4,690

4,710

4,730

124/125

4,650

4,750

4,670

4,690

4,710

4,730

83

4,640

4,740

4,660

4,680

4,700

4,720

29

4,700

4,800

4,720

4,740

4,760

4,780

55

4,820

4,920

4,840

4,860

4,880

4,900

Water level, in feet

above sea level

144

5,000

5,100

5,020

5,040

5,060

5,080

273

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060
Model Application 61

Additional water-level declines from 5 to 40 ft (below the 

levels predicted by the base scenario) are also predicted in 

scenario 2A on the west side of the valley (fig. 40). Additional 

water-level declines in scenario 2B are similar to those in scenario 2A, but not as extensive in areas where pumping from 

domestic wells ceases in 2020 (fig. 41).

Predicted water-level hydrographs in scenarios 2A and 

2B are similar and indicate as much as 10 ft of additional 

drawdown at wells 83 and 113 in the central part of the valley compared to the base scenario (figs. 40, 41). The upward 

hydraulic gradient at wells 167 and 175 northwest of Minden 

is also reduced. Total flow through the aquifer is larger in scenario 2A than in the base scenario. In water year 2051 inflows 

from streams and storage increase by 3,700 and 3,900 acre-ft, 

respectively, while outflows to streams and evapotranspiration decrease by 19,400 and 5,600 acre-ft, respectively (fig. 

42A). The decrease in mean-annual Carson River flow in 

scenario 2A is 16,300 acre-ft more than in the base scenario, 

and accounts for about 60 percent of the increase in pumping 

(fig. 42B). The annual groundwater budgets are nearly identical for scenarios 2A and 2B, but there is a slightly greater 

decrease in Carson River flow (16,500 acre-ft) in scenario 2B 

(figs. 43A, B). 

Scenario 3

Groundwater withdrawals for municipal and domestic use 

are the same in scenario 3 as in scenario 2A, but the maximum 

irrigated acreage is reduced about 13 percent by 3,700 acres in 

2010. The predicted distribution of water-table decline is similar to that predicted by scenario 2A, but slightly more decline 

is predicted south of Johnson Lane where irrigation ceases 

(figs. 40, 44). Diversions of streamflow for irrigation in scenario 3 average 10,100 acre-ft/yr less than in scenario 2A. In 

water year 2051 total flow through the aquifer is 5,100 acre-ft 

less in scenario 3 than in scenario 2A (fig. 45A). The predicted 

decrease in mean-annual Carson River flow in scenario 3 is 

only 6,500 acre-ft/yr less than in the base scenario (fig. 45B).

Figure 39. A, Annual groundwater budgets from 2051 through 

2060 simulated by base scenario and scenario 1; B, annual loss in 

flow in the Carson River, 2051–60 predicted by base scenario and 

scenario 1. Flow, in thousands of acre-feet per year

400

300

200

100

0

-100

-200

-300

-400

A

B

Base scenario (2005 pumping rates)

Scenario 1

Water year

Inflow

Stream infiltration

Mountain-front recharge

Valley recharge

Agricultural recharge

Storage

Outflow

Stream discharge

Evapotranspiration

Wells

Storage

EXPLANATION

Wet period Dry period

Wet period Dry period

2051 2055 2060

Loss in flow, in thousands of acre-feet per year

100

75

50

25

0

2051 2055 2060

Base scenario (2005 pumping rates)

Scenario 1
62 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 40. Additional groundwater withdrawals included in scenario 2A, additional water-table decline 2010–60 simulated by scenario 

2A compared to the base scenario, and simulated water-level hydrographs at selected wells for the base scenario and scenario 2A.

Carson River

Pine Nut Creek

Buckeye Creek

West Fork

East Fork

Model boundary

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

167/175

273

144

150

55

83

113

124/125

29

Gardnerville

Minden

Johnson Lane

Fish

Spring

Flat

Ruhenstroth

Gardnerville

 Ranchos

EXPLANATION

Additional water-table decline (more 

than 5 feet), compared with base 

scenario, in feet

5 to 10

10 to 20

20 to 40

More than 40

Additional wells and wells with increased 

pumping, rate in acre-feet per year

More than 1,000

100 to 1,000

10 to 100

Less than 1

Domestic Municipal

and other

Simulated water level

Base

Scenario 1

Shallow

Deep

Shallow

Deep

2000 2010 2020 2030 2040 2050 2060 2000 2010 2020 2030 2040 2050 2060 4,640

4,740

4,660

4,680

4,700

4,720

150

4,760

4,860

4,780

4,800

4,820

4,840

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

167/175

4,650

4,750

4,670

4,690

4,710

4,730

113

4,650

4,750

4,670

4,690

4,710

4,730

124/125

4,650

4,750

4,670

4,690

4,710

4,730

83

4,640

4,740

4,660

4,680

4,700

4,720

29

4,680

4,780

4,700

4,720

4,740

4,760

55

4,820

4,920

4,840

4,860

4,880

4,900

Water level, in feet

above sea level

144

5,000

5,100

5,020

5,040

5,060

5,080

273

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

WATER LEVEL, IN FEET

ABOVE SEA LEVEL

2000 2010 2020 2030 2040 2050 2060

Base Scenario 2A

2000 2010 2020 2030 2040 2050 2060

Base Scenario 2A
Model Application 63

Figure 41. Additional groundwater withdrawals included in scenario 2B, additional water-table decline 2010–60 simulated by scenario 

2B, and simulated water-level hydrographs at selected wells for base scenario and scenario 2B.

Carson River

Pine Nut Creek

Buckeye Creek

West Fork

East Fork

Simulated water level

Base

Scenario 2B

Domestic wells removed in 2020

Model boundary

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

167/175

273

144

150

55

83

113

124/125

29

Gardnerville

Minden

Johnson Lane

Fish

Spring

Flat

Ruhenstroth

Gardnerville

 Ranchos

EXPLANATION

Additional water-table decline (more 

than 5 feet), compared with base 

scenario, in feet

5 to 10

10 to 20

20 to 40

More than 40

Additional wells and wells with increased 

pumping, rate in acre-feet per year

More than 1,000

100 to 1,000

10 to 100

Less than 1

Domestic Municipal

and other

2000 2010 2020 2030 2040 2050 2060 2000 2010 2020 2030 2040 2050 2060 4,640

4,740

4,660

4,680

4,700

4,720

150

4,760

4,860

4,780

4,800

4,820

4,840

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

167/175

4,650

4,750

4,670

4,690

4,710

4,730

113

4,650

4,750

4,670

4,690

4,710

4,730

124/125

4,650

4,750

4,670

4,690

4,710

4,730

83

4,640

4,740

4,660

4,680

4,700

4,720

29

4,680

4,780

4,700

4,720

4,740

4,760

55

4,820

4,920

4,840

4,860

4,880

4,900

WAter level, in feet

above sea level

144

5,000

5,100

5,020

5,040

5,060

5,080

273

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

WAter level, in feet

above sea level

2000 2010 2020 2030 2040 2050 2060

Base Scenario 2B

2000 2010 2020 2030 2040 2050 2060

Base Scenario 2B

Shallow

Deep

Shallow

Deep
64 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 42. A, Annual groundwater budgets, 2051–60 simulated 

by base scenario and scenario 2A; B, annual loss in flow in the 

Carson River, 2051–60 predicted by base scenario and scenario 

2A. 

Figure 43. A, Annual groundwater budgets, 2051–60 simulated 

by base scenario and scenario 2B; B, annual loss in flow in the 

Carson River 2051–60 predicted by base scenario and scenario 2B. 

Flow, in thousands of acre-feet per year

400

300

200

100

0

-100

-200

-300

-400

A

B

Base scenario (2005 pumping rates)

Scenario 2A

Water year

Inflow

Stream infiltration

Mountain-front recharge

Valley recharge

Agricultural recharge

Storage

Outflow

Stream discharge

Evapotranspiration

Wells

Storage

EXPLANATION

Wet period Dry period

Wet period Dry period

2051 2055 2060

Loss in flow, in thousands of acre-feet per year

100

75

50

25

0

2051 2055 2060

Base scenario (2005 pumping rates)

Scenario 2A

Flow, in thousands of acre-feet per year

400

300

200

100

0

-100

-200

-300

-400

A

B

Base scenario (2005 pumping rates)

Scenario 2B

Water year

Inflow

Stream infiltration

Mountain-front recharge

Valley recharge

Agricultural recharge

Storage

Outflow

Stream discharge

Evapotranspiration

Wells

Storage

EXPLANATION

Wet period Dry period

Wet period Dry period

2051 2055 2060

Loss in flow, in thousands of acre-feet per year

100

75

50

25

0 2051 2055 2060

Base scenario (2005 pumping rates)

Scenario 2B
Model Application 65

Figure 44. Additional groundwater withdrawals included in scenario 3, agricultural land removed from irrigation, additional water-table 

decline 2010–60 simulated by scenario 3, and simulated water-level hydrographs at selected wells for base scenario and scenario 3.

Carson River

Pine Nut Creek

Buckeye Creek

West Fork

East Fork

Model boundary

0 1 2 3 4 5 MILES

10 2 3 4 5 KILOMETERS

167/175

273

144

150

55

83

113

124/125

29

Gardnerville

Minden

Johnson Lane

Fish

Spring

Flat

Ruhenstroth

Gardnerville

 Ranchos

EXPLANATION

Additional water-table decline (more than 5 feet), 

compared with base scenario, in feet

5 to 10

10 to 20

20 to 40

More than 40

Additional wells and wells with increased 

pumping, rate in acre-feet per year

More than 1,000

100 to 1,000

10 to 100

Less than 1

Domestic Municipal

and other

Agricultural land removed from irrigation

Simulated water level

Base scenario

Scenario 2B 

2000 2010 2020 2030 2040 2050 2060 2000 2010 2020 2030 2040 2050 2060 4,640

4,740

4,660

4,680

4,700

4,720

150

4,760

4,860

4,780

4,800

4,820

4,840

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

2000 2010 2020 2030 2040 2050 2060

167/175

4,650

4,750

4,670

4,690

4,710

4,730

113

4,650

4,750

4,670

4,690

4,710

4,730

124/125

4,650

4,750

4,670

4,690

4,710

4,730

83

4,640

4,740

4,660

4,680

4,700

4,720

29

4,680

4,780

4,700

4,720

4,740

4,760

55

4,820

4,920

4,840

4,860

4,880

4,900

WAter level, in feet

above sea level

144

5,000

5,100

5,020

5,040

5,060

5,080

273

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

Water level, in feet

above sea level

WAter level, in feet

above sea level

2000 2010 2020 2030 2040 2050 2060

Base Scenario 3

2000 2010 2020 2030 2040 2050 2060

Base Scenario 3

Shallow

Deep

Shallow

Deep
66 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Figure 45. A, Annual groundwater budgets, 2051–60, simulated 

by base scenario and scenario 3; B, annual loss in flow in the 

Carson River 2051–60 predicted by base scenario and scenario 3. Flow, in thousands of acre-feet per year

400

300

200

100

0

-100

-200

-300

-400

A

B

Base scenario (2005 pumping rates)

Scenario 3

Water year

Inflow

Stream infiltration

Mountain-front recharge

Valley recharge

Agricultural recharge

Storage

Outflow

Stream discharge

Evapotranspiration

Wells

Storage

EXPLANATION

Wet period Dry period

Wet period Dry period

2051 2055 2060

Loss in flow, in thousands of acre-feet per year

100

75

50

25

0 2051 2055 2060

Base scenario (2005 pumping rates)

Scenario 3

Prediction Uncertainty

The 55-yr predictive simulations indicate that continued 

withdrawals of groundwater at the rates estimated for 2005 

(base scenario) would result in about 5 to 40 ft of water-table 

declines on the eastern side of Carson Valley and near Gardnerville Ranchos by 2060. Increasing groundwater withdrawals to meet the maximum expected water-demand (scenario 

2A) would widen the extent of water-level declines on the east 

side of Carson Valley, ranging from 5 to 60 ft, and produce 

declines from 5 to 40 ft on the western side of the valley. 

However, the predicted declines on the western side of the valley would be less if values of mountain-front recharge derived 

from the PRMS watershed models were specified. The water 

table in the central part of the valley near the Carson River 

and the irrigation system would remain essentially unchanged 

under either withdrawal scenario, but heads in the deeper, confined part of the aquifer would decline as much as 30 ft. The 

increased groundwater withdrawals would be met primarily by 

decreased flow in the Carson River and would result in larger 

losses in streamflow than typically were observed under 2005 

conditions. However, there is uncertainty in the actual amount 

of decreased flow because of potential changes in future irrigation practices not accounted for in the model. For example, if 

greater amounts of treated effluent were used for irrigation of 

land previously irrigated from river diversions, the decrease 

in river flow would be less. Part of the additional groundwater 

demand would also be met by the loss of groundwater storage, 

as water levels are projected to continue to decline after 2060 

in both scenarios.

The projections of the predictive simulations are sensitive 

to rates of mountain-front recharge specified for the Carson 

Range and the Pine Nut Mountains. As discussed earlier, 

recharge rates previously estimated by PRMS watershed 

models were adjusted during calibration of the groundwater 

model; this adjustment resulted in a reduction of 12,000 acreft/yr of recharge from the Carson Range and little change in 

recharge from the Pine Nut Mountains. The areas of projected 

water-table decline on the west and east sides of Carson Valley 

are adjacent to these uplands, so uncertainty in the specified 

recharge rates results in uncertainty in the predicted declines. 

The projected area of water-table decline along the west side 

of the valley is confined to a narrow band between the Carson 

Range and the Carson River, but a much wider area of watertable decline is projected for the east side of Carson Valley. 

Little information is available on the east side of the valley 

that could constrain estimates of mountain-front recharge, 

such as the position of the water table or the hydraulic properties of the Tertiary sediments that underlie this area.

Mountain-front recharge rates in the predictive simulations are assumed to remain within the range of extreme rates 

observed in the past 35 years. The sensitivity of predicted 

water-table declines to rates of mountain-front recharge 

was assessed through an alternate scenario in which the 

same withdrawal rates were specified as in scenario 2A, but 

recharge rates were decreased by 50 percent along the Pine 
Summary 67

Nut Mountains. These changes resulted in water-table declines 

ranging from 5 to 60 ft on the east side of Carson Valley, and 

declines of more than 60 ft in areas east of Fish Spring Flat. 

For example, predicted declines by water year 2060 at wells 

150 and 144 were 21 and 56 ft greater than in scenario 2A 

(fig. 40). Although it is unlikely that average recharge rates 

would abruptly decrease by 50 percent, recharge rates could 

be reduced by either decreased precipitation or increased 

evapotranspiration in the future as a result of changing global 

weather patterns (Jeton and others, 1996; Dettinger and others, 2004). Additional predictive scenarios could be run using 

recharge rates based on projections of climate models that take 

into account these possible changes in recharge. A prudent 

management approach would include continued monitoring of 

water levels on both the east and west sides of Carson Valley 

to either verify the predictions of the groundwater flow model 

or to provide additional data for recalibration of the model if 

the predictions prove inaccurate.

Summary 

Rapid growth and development within Carson Valley in 

Douglas County, Nevada, and Alpine County, California, has 

caused concern over the continued availability of groundwater and whether the increased municipal demand could either 

impact the availability of water or result in decreased flow in 

the Carson River. A groundwater-flow model was developed 

in this study to provide estimates of the groundwater-budget 

components for Carson Valley and to evaluate the potential 

effects of changes in water use on groundwater levels in

Carson Valley and on flow in the Carson River downstream 

from Carson Valley.The model provides a tool that can be 

used to aid water managers and planners in making informed 

decisions. 

Hydrogeology

Carson Valley is underlain by semi-consolidated Tertiary 

sedimentary deposits as much as 3,000-ft thick that are overlain by Quaternary alluvial fans and fluvial sediments (basin 

fill) deposited by the Carson River and its tributaries. The 

Tertiary sediments are assumed to have been down-dropped 

about 800 feet along the hypothesized Hot Springs Mountain 

fault on the eastern side of the basin. The hydrology of Carson 

Valley is dominated by streamflow in the Carson River that is 

diverted across the valley floor through a network of unlined 

canals, ditches, and sloughs for flood irrigation of about 

39,000 acres and maintains a shallow water table less than 5 ft 

beneath much of the valley floor. Average streamflow entering 

the valley was 344,100 acre-ft/yr, while average flow exiting 

the was 297,400 acre-ft/yr. Total streamflow from perennial 

watersheds in the Carson Range is estimated as 30,000 acreft/yr, while mountain-front recharge is estimated as 20,000 

and 16,000 acre-ft/yr from west and east sides of the valley, 

respectively.Sources of irrigation water include the Carson 

River, runoff from perennial watersheds draining the Carson 

Range, groundwater pumped from Quaternary sediments, and 

wastewater effluent imported from the Lake Tahoe basin and 

generated within Carson Valley. Annual pumping of groundwater has increased from less than 10,000 acre-ft in the 1970s 

to about 31,000 acre-ft in 2004; municipal use of groundwater 

was about 10,000 acre-ft in 2000.

Median transmissivities computed from specific-capacity 

data for Quaternary alluvial fan, Quaternary fluvial, and 

Tertiary sediments are 316; 3,120; and 110 ft2/d, respectively, with larger transmissivity values in the central part of 

the valley underlying the East and West Forks of the Carson 

River and smaller values near the valley margins. Values of 

specific yield in Carson Valley are typically 15 percent, with 

values greater than 20 percent near the center of the valley, 

and values of 10 percent around the margins of the valley. 

Groundwater flows from the west and east toward the Carson 

River and then north toward the outlet of the Carson Valley. 

An upward hydraulic gradient exists over much of the valley, and artesian flow occurs in some areas, but a continuous 

confining unit is not thought to underlie the entire valley. The 

exchange of water between groundwater and streams changes 

from wet to dry years and during the irrigation season. Losses 

of flow from the Carson River and the irrigation system to 

groundwater occur mainly in the southeastern part of the valley, while discharge of groundwater to the river occurs on the 

north and west sides of the valley. Most of the aquifer is in a 

state of dynamic equilibrium, and groundwater levels fluctuate 

in response to both seasonal and annual variations in recharge 

and pumping, but water levels have declined 10 to 15 ft on the 

eastern side of the valley. Inflow and outflow to the Carson 

River account for more than two-thirds of the total budget, 

with a net loss of 51,000 acre-ft/yr from the river within Carson Valley. Pumped groundwater accounts for only 4.4 percent 

of the total budget. 

Groundwater-Flow Model

A model that simulates groundwater flow through Quaternary and Tertiary sediments in Carson Valley was designed 

and calibrated to historical conditions from water years 1970 

through 2005. The 35-yr transient simulation represented 

quarterly changes in precipitation, streamflow, pumping, and 

irrigation. The 242.8-mi2 model domain is discretized with 

a 550-ft grid and divided into 10 model layers that range in 

thickness from 80 to 900 ft. The top three model layers represent alluvial fan and fluvial sediments; the bottom seven layers 

primarily represent Tertiary sediments that are exposed on the 

eastern side of the valley and dip beneath the Quaternary sediments to the west. 

The top model boundary represents recharge derived from 

precipitation and irrigation, and evapotranspiration from vegetation. Recharge is represented in areas irrigated with water 

from the 12 perennial streams that drain the Carson Range, 
68 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

effluent wastewater, and from the Carson River and the adjoining irrigation system. Recharge is also applied to the model 

domain using wells to represent infiltration of septic-tank discharge and groundwater inflow from perennial and ephemeral 

watersheds in the Carson Range and Pine Nut Mountains. Discharge wells represent pumping of groundwater for municipal, 

domestic, irrigation, and other water supplies. The exchange 

of water between groundwater and the irrigation system was 

represented with a version of the Streamflow Routing (SFR2) 

package that was modified to apply diversions from the irrigation network to irrigated areas as recharge. 

The Quaternary sediments were divided into five zones 

representing the four alluvial fans and the fluvial sediments. 

The horizontal hydraulic-conductivity (Kxy) values for Tertiary sediments were estimated for five zones delineated on 

the basis of geology mapped by Muntean (2001). Vertical 

anisotropy values (Kxy/Kz) were estimated for the eastern and 

western alluvial fans, the fluvial sediments and three Tertiary 

zones. Horizontal anisotropy (Kx/Ky) was specified for the 

western alluvial fan to represent preferred flow paths in the 

direction of deposition from the front of the Carson Range 

eastward to Carson Valley. The aquifer system was assumed 

to be confined, but the hydraulic conductivity of the top active 

model layers that intersect the water table was multiplied by a 

factor to account for partial saturation. A power function was 

used to decrease the horizontal hydraulic-conductivity below 

threshold depths of 500 and 200 ft for the Quaternary and Tertiary sediments, respectively. A specific yield (Sy) value of 0.1 

was specified for Tertiary sediments; three Sy values ranging 

from 0.1 to 0.3 were specified for Quaternary sediments (the 

eastern and western alluvial fans and the fluvial sediments). 

Storage values representative of specific yield were specified in parts of model layers where unconfined conditions are 

assumed to occur.

The groundwater-flow model was calibrated with UCODE 

to estimate values of hydraulic conductivity, recharge, and 

streambed hydraulic-conductivity that minimized the difference between simulated groundwater levels and streamflows 

with 5,686 water levels measured in 237 wells and flow at 5 

streamflow-gaging stations. The model simulates the groundwater system reasonably well, and the standard error (SE) in 

absolute head is 37 ft or 3 percent of the 1,090-ft measured 

range, while the SE in water-level change is 5.8 ft or 9 percent 

of the 68-ft measured range. Observed water-level declines on 

the eastern side of the valley are well simulated, although the 

absolute head in some areas is either over-or underpredicted. 

The SE of the weighted residuals for flows is 6.7 ft.

A total of 41 parameters were specified in the groundwatermodel, 18 of which were optimized through nonlinear regression. Parameter sensitivities are highest for the hydraulic conductivity and recharge parameters associated with the Tertiary 

sediments in the northern part of the basin, and they are lowest 

for the Tertiary vertical-anisotropy parameters and streambed 

hydraulic-conductivity parameters for the Carson River. The 

mean transmissivity (T) values computed from optimized values of Kxy parameters (11,000 and 800 ft2/d for the fluvial and 

alluvial-fan sediments, respectively) are both within the third 

quartile of T values estimated from specific-capacity data. 

Optimized streambed hydraulic-conductivity values are larger 

for channels in lowlands near the center of the valley and 

smaller for channels in the uplands. The optimized Kxy values 

for Tertiary sediments (3.5 to 13 ft/d) yield transmisivity values that are larger than the third quartile of T values estimated 

from specific-capacity data. An alternate regression indicated 

that more reasonable Kxy values for Tertiary sediments (2 ft/d) 

would produce a model that slightly better matches heads and 

flows at the expense of a poorer match to water-level fluctuations. The estimated vertical anisotropy for the Quaternary 

fluvial sediments is large (9,900), but this value is comparable 

to the value estimated for the previous model of Carson Valley. Observed water-level changes in artesian wells are not 

well simulated with smaller values. Volumes of mountainfront recharge estimated by the regression are comparable to 

estimates from PRMS watershed models, but about 70 percent 

of the mountain-front recharge in the groundwater model 

enters along the Pine Nut Mountains, while the PRMS models 

estimated a nearly equal distribution of recharge between the 

Carson Range and the Pine Nut Mountains.

Simulated flow paths indicate that groundwater flows 

faster through the center of Carson Valley where the hydraulic conductivity of fluvial sediments is higher and slower 

through the lower hydraulic-conductivity Tertiary sediments 

to the east. Shallow flow in the center of the valley is towards 

drainage channels; deeper flow is generally directed toward 

the basin outlet to the north. Simulated groundwater budgets 

indicate that stream infiltration and mountain-front recharge 

are the largest inflow components, while stream discharge and 

evapotranspiration are the largest outflow components. Pumping has historically been less than 15 percent of the groundwater budget, while agricultural recharge has been less than 10 

percent. The aquifer system is in a dynamic equilibrium with 

large inflows from storage in dry years and large outflows to 

storage in wet years.

Uncertainty in simulation results primarily originates from 

errors in model design and parameter values that cause errors 

in simulated heads and flows and is a reflection of the current 

state of knowledge regarding the aquifer system. The uncertainty related to the assumption of confined conditions for an 

unconfined aquifer system was assessed by using a recently 

available (May 2011) numerical solver to simulate the aquifer 

system as unconfined. The model errors in the confined and 

unconfined simulations were comparable and indicate that 

the parameter values estimated through model calibration are 

largely unaffected by the assumption of confined conditions. 

There are three principal aspects of the calibrated model in 

which uncertainty is likely to affect important model results: 

(1) the hydraulic characteristics of the Tertiary sediments on 

the eastern side of the basin, (2) the composition of sediments 

beneath the alluvial fans, and (3) the extent of the confining 

layer represented within fluvial sediments in the center of 

the basin. Most of the development in Carson Valley has or 

will occur on the eastern side of the basin where very little 
Summary 69

information is available concerning the hydraulic properties 

of the Tertiary sediments or the position of the water table. 

The largest errors in absolute head are in areas underlain by 

alluvial fans on the east and west sides of the valley where 

little information is available concerning the composition of 

sediments that underlie the alluvial fans. Finally, the actual 

confining layer underlying the center of the valley is probably not as extensive as that represented. The confining-layer 

extent limits the hydraulic connection between shallow and 

deep flow systems and determines the extent of drawdown that 

would result from increased pumping at deep wells. Additional 

information in these three areas would improve the current 

state of knowledge concerning the aquifer system and serve to 

reduce model uncertainty.

Model Application

The Carson Valley groundwater model was used in a series 

of 55-yr predictive simulations to evaluate the long-term 

effects of the expected increasing demand for water on waterbudget components, groundwater levels, and streamflow in the 

Carson River. The predictive simulations represented water 

years 2006−60 using 220 quarterly stress periods. The initial 

heads were specified from the distribution computed at the end 

of the calibration simulation, and boundary conditions were 

varied cyclically to represent the transition from wet to dry 

conditions that occurred from water year 1995 through 2004. 

Four water-use scenarios were compared with a base simulation in which the rates and locations of pumping and surfacewater irrigation specified for water year 2005 were held constant through water year 2060. The four scenarios included: 

(1) total pumping rates increased by 70 percent, including 

an additional 1,340 domestic wells, (2A) total pumping rates 

more than doubled with municipal pumping increased by a 

factor of four, (2B) maximum pumping rates of 2A with 2,040 

fewer domestic wells, and (3) maximum pumping rates of 2A 

with 3,700 acres removed from irrigation.

Continued pumping at 2005 rates resulted in water-level 

declines of 5 to 40 ft on the east side of Carson Valley by 2060 

with less than 5 ft of decline in the central part of the valley. 

Increasing pumping by 16,200 acre-ft/yr in scenario 1 caused 

additional water-table declines (compared with the base scenario) of 5 to 10 ft on the east side of the valley and more than 

40 ft near Gardnerville Ranchos, resulting in as much as 50-ft 

declines below 2005 conditions in both areas. Steep declines 

in water levels are predicted for Fish Spring Flat and Gardnerville Ranchos that would continue through 2060, and the 

additional reduction in mean-annual Carson River flow (9,600 

acre-ft) accounts for two-thirds of the increase in pumping. 

Increasing pumping by 27,300 acre-ft/yr in scenario 2A relative to the base scenario caused more extensive water-table 

declines with as much as 60 ft of decline in Fish Spring Flat 

and Gardnerville Ranchos, and as much as 40 ft of decline on 

the west side of the valley. The additional reduction in meanannual Carson River flow in scenario 2A (16,300 acre-ft) 

over the base scenario accounts for 70 percent of the increase 

in pumping. There was little difference between simulation 

results for scenarios 2A and 2B, except for slight reduction in 

Carson River flow of 200 acre-ft/yr in scenario 2B. Reducing 

irrigated acreage by 13 percent in scenario 3 caused slightly 

more water-table decline south of Johnson Lane where irrigation ceases, but the Carson River flow was only 6,500 acre-ft 

less than in the base scenario.

The 55-yr predictive simulations indicate that increasing 

groundwater withdrawals to meet the maximum expected 

water-demand would result in 40 to 60 ft of water-table 

decline on the west and east sides of Carson Valley, respectively. The water table in the central part of the valley would 

remain essentially unchanged, but as much as 30 ft of head 

decline is expected in the confined deeper parts of the aquifer. The increased withdrawals would be offset primarily by 

decreased flow in the Carson River and by the loss of groundwater storage. There is uncertainty in the actual amount of 

decreased flow because of potential changes in irrigation 

practices not accounted for in the model such as an increased 

use of treated effluent for irrigation. The projections of the 

predictive simulations are sensitive to rates of mountain-front 

recharge specified for the Carson Range and the Pine Nut 

Mountains. For example, reducing the recharge rate by 50 percent resulted in 60 ft more decline in Fish Spring Flat than predicted by scenario 2A. Little information, such as the position 

of the water table or the hydraulic properties of the Tertiary 

sediments, is available on the east side of the valley where 

the most extensive areas of water-table decline are predicted. 

A prudent management approach would include continued 

monitoring of water levels on both the east and west sides of 

Carson Valley to either verify the predictions of the groundwater flow model, or to provide additional data for recalibration 

of the model if the predictions prove inaccurate.
70 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

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Carson Valley, Nevada- California: U.S. Geological Survey 

Water-Resources Investigations Report 84-4202, 20 p.

Maurer, D.K., 1986, Geohydrology and simulated response to 

ground-water pumpage in Carson Valley, a river-dominated 

basin in Douglas County, Nevada, and Alpine County, California: U.S. Geological Survey Water-Resources Investigations Report 86-4328, 109 p.
72 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Maurer, D.K., and Berger, D.L., 1997, Subsurface flow and 

water yield from watersheds tributary to Eagle Valley 

Hydrographic Area, west-central Nevada: U.S. Geological 

Survey Water-Resources Investigations Report 97-4191, 

56 p.

Maurer, D.K., and Berger, D.L., 2007, Water budgets and 

potential effects of water use changes for Carson Valley, Douglas County, Nevada, and Alpine County, California: U.S. Geological Survey Scientific Investigations 

Report 2006-5305, 63 p. Available at http://pubs.usgs.gov/

sir/2006/5305/index.html.

Maurer, D.K., Berger, D.L., Tumbusch, M.L., and Johnson, 

M.J., 2006, Rates of evapotranspiration, recharge from precipitation beneath selected areas of native vegetation, and 

streamflow gain and loss in Carson Valley, Douglas County, 

Nevada, and Alpine County, California: U.S. Geological 

Survey Scientific Investigations Report 2005-5288, 70 p.

Maurer, D.K., and Halford, K.J., 2004, Updated estimates of 

the distribution of average annual precipitation in Carson 

Valley, 1971-2000, Douglas County, Nevada, and Alpine 

County, California: Journal of the Nevada Water Resources 

Association, v. 1, no. 1, p. 20−39.

Maurer, D.K., Paul, A.P., Berger, D.L., and Mayers, C.J., 2009, 

Analysis of streamflow trends, ground-water and surfacewater interactions, and water quality in the upper Carson 

River Basin, Nevada and California: U.S. Geological Survey Scientific Investigations Report 2008-5238, 192 p.

Maurer, D.K., and Peltz, L.A., 1994, Potential for, and possible effects of, artificial recharge in Carson Valley, Douglas 

County, Nevada: U.S. Geological Survey Water-Resources 

Investigations Report 94-4126, 4 map sheets.

Maurer, D.K., Watkins, S.A., and Burrows, R.L., 2004, 

Updated computations and estimates of streamflows tributary to Carson Valley, Douglas County, Nevada, and Alpine 

County, California: U.S. Geological Survey Scientific 

Investigations Report 2004-5179, 29 p.

Moore, J.G., 1969, Geology and mineral deposits of Lyon, 

Douglas, and Ormsby Counties, Nevada: Nevada Bureau of 

Mines and Bulletin 75, 42 p.

Morris, D.A., and Johnson, A.I., 1967, Summary of hydrologic and physical properties of rock and soil materials, as 

analyzed by the hydrologic laboratory of the U.S. Geological Survey, 1948–60: U.S. Geological Survey Water-Supply 

Paper 1839-D, 42 p.

Muntean, T.W., 2001, Evolution and stratigraphy of the Neogene Sunrise Pass Formation of the Gardnerville sedimentary basin, Douglas County, Nevada: University of Nevada, 

Reno, unpublished Master’s thesis, 206 p.

Naff, R.L., and Banta, E.R., 2008, The U.S. Geological Survey 

modular ground-water model—PCGN: A preconditioned 

conjugate gradient solver with improved nonlinear control: 

U.S. Geological Survey Open-File Report 2008–1331, 35 p.

National Oceanic and Atmospheric Administration, 2002, 

Monthly station normals of temperature, precipitation, and 

heating and cooling degree days, 1971−2000, Climatology 

of the United States, No. 81, Nevada: National Climatic 

Data Center, Asheville, North Carolina, 26 p.

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National Climate Data Center, Palmer Drought Severity 

Index: National Oceanic and Atmospheric Administration, 

accessed March 7, 2006, at http://www.drought.noaa.gov. 

Note: This site will no longer be updated, but will remain 

archived. For the latest drought information, please visit the 

National Integrated Drought Information System.

Niswonger, R.G., Panday, Sorab, and Ibaraki, Motomu, 2011, 

MODFLOW-NWT, A Newton Formulation for MODFLOW-2005: U.S. Geological Survey Techniques and 

Methods 6–A37, 44 p.

Niswonger, R.G., and Prudic, D.E., 2005, Documentation 

of the Streamflow-Routing (SFR2) Package to include 

unsaturated flow beneath streams—A modification to SFR1 

(revised April 2010): U.S. Geological Survey Techniques 

and Methods, book 6, chap. A13, 50 p.

Paul, A.P., Maurer, D.E., Stollenwerk, K.G., and Welch, A.H., 

2010, In-situ arsenic remediation in Carson Valley, Douglas 

County, west-central Nevada: U.S. Geological Survey 

Scientific Investigations Report 2010-5161, 24 p. 

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Genoa Folio, Map 1Cg., scale 1:24,000.

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codes for universal sensitivity analysis, calibration and 

uncertainty evaluation: U.S. Geological Survey Techniques 

and Methods 6-A11, 283 p.

Prudic, D.E., and Herman, M.E., 1996, Ground-water flow 

and simulated effects of development in Paradise Valley, a 

basin tributary to the Humboldt River in Humboldt County, 

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103 p.

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ground-water pumping in Carson Valley, a river-dominated 

basin in Douglas County, Nevada, and Alpine County, California: U.S. Geological Survey Water-Resources Investigations Report 95-4174, 29 p.

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Mines and Geology Special Publications 4, 135 p.
Appendix 1. Modifications to MODFLOW 73

Stewart, J.H., 1999, Geologic map of the Carson City 30 x 60 

minute quadrangle, Nevada: Nevada Bureau of Mines and 

Geology, map 118, 1:100,000 scale, and accompanying text 

and references, 12 p. 

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Nevada: U.S. Geological Survey, prepared in cooperation 

with the Nevada Bureau of Mines and Geology, 1:500,000 

scale, 2 sheets.

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map of the Mount Siegel quadrangle, Nevada-California: 

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scale.

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1997 in the Carson River basin, California and Nevada: 

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agriculture and the environment in the upper Carson River 

basin: University of Nevada, Reno, unpublished Ph.D. dissertation, 264 p.

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tabulation and administrative provisions, United States vs. 

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no. D-183 BRT, 164 p.

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qfaults/.
74 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley
 USE GWFSFRMODULE,ONLY:RECHSAVE 75

Appendix 1. Modifications to MODFLOW

This appendix describes the changes made to the MODFLOW-2005 code to (1) enable simulation of recharge by 

applying diversions defined in the Streamflow Routing (SFR2) 

package directly to a defined group of model cells (diverted 

recharge), and (2) include an irrigation-efficiency term that 

decreases the volume of diverted water that is applied to 

account for losses through return flow and crop evapotranspiration. The changes required in the SFR2 input file are also 

described. These features are not available in the standard 

version of MODFLOW-2005 (Harbaugh, 2005, Version 1.8). 

At each diversion segment in SFR2, a specified flow [L3/T] is 

normally routed to one of the following: another segment, lake 

cells, or outside the model domain. The modification described 

herein also allows flow to be applied evenly over a group of 

model cells, assuming sufficient flow is available in the stream 

segment. The recharge rate is calculated as the total flow 

diversion, divided by the total cell area [L/T] (number of cells 

multiplied by the cell areas). In addition, this diverted recharge 

is subject to a user-defined efficiency factor that removes a 

portion of the flow from the model domain to represent evapotranspiration losses. Note all changes and additions are shown 

in bold below.

Main program: mf2005.f

Under section C7B-----READ AND PREPARE INFORMATION FOR STRESS PERIOD. READ USING PACKAGE READ 

AND PREPARE MODULES, the SFR unit number was added for use by the recharge package.

 IF(IUNIT(8).GT.0) CALL GWF2RCH7RP(IUNIT(8),IUNIT(44),IGRID)

Under section C7C2A---FORMULATE THE FINITE DIFFERENCE EQUATIONS.S, the RCH unit number was added for use 

by the SFR package.

 IF(IUNIT(44).GT.0) CALL GWF2SFR7FM(KKITER,KKPER,KKSTP,

 1 IUNIT(22),IUNIT(8),IGRID)

Under section C7C4----CALCULATE BUDGET TERMS. SAVE CELL-BY-CELL FLOW TERMS. add SFR and RCH unit 

numbers

CALL GWF2RCH7BD(KKSTP,KKPER,IUNIT(44),IGRID)

.

.

. 

 IF(IUNIT(44).GT.0) CALL GWF2SFR7BD(KKSTP,KKPER,IUNIT(15),

 1 IUNIT(22),IUNIT(46),IUNIT(55),NSOL,

 2 IUNIT(8),IGRID) !cjm (added IUNIT(8))

RCH Module: gwf2rch7.f

In specifications under SUBROUTINE GWF2RCH7RP define RECHSAVE array. This array is used to store the original recharge 

array data and account for the diverted recharge from SFR2 

 USE GLOBAL, ONLY:IOUT,NCOL,NROW,NLAY,IFREFM,DELR,DELC

 USE GWFRCHMODULE,ONLY:NRCHOP,NPRCH,IRCHPF,RECH,IRCH

 USE GWFSFRMODULE,ONLY:RECHSAVE
76 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Under section C4------MULTIPLY RECHARGE RATE BY CELL AREA TO GET VOLUMETRIC RATE, insert the bolded line 

below to initialize the RECHSAVE array with the RECH array values. 

 C4------MULTIPLY RECHARGE RATE BY CELL AREA TO GET VOLUMETRIC RATE.

 DO 50 IR=1,NROW

 DO 50 IC=1,NCOL

 RECH(IC,IR)=RECH(IC,IR)*DELR(IC)*DELC(IR)

 IF ( IUNITSFR.GT.0 ) RECHSAVE(IC,IR) = RECH(IC,IR) 

 50 CONTINUE

 END IF

In specifications under SUBROUTINE GWF2RCH7BD defined a number of new variables. 

 USE GLOBAL, ONLY:IOUT,NCOL,NROW,NLAY,IBOUND,BUFF, iout

 USE GWFBASMODULE,ONLY:MSUM,VBVL,VBNM,ICBCFL,DELT,PERTIM,TOTIM

 USE GWFRCHMODULE,ONLY:NRCHOP,IRCHCB,RECH,IRCH

 USE GWFSFRMODULE,ONLY:RECHSAVE

 C

 DOUBLE PRECISION RATIN,RATOUT,QQ, Q, rin2, zero

 CHARACTER*16 TEXT

 DATA TEXT /’ RECHARGE’/

 CHARACTER*16 txtsfrch 

 DATA txtsfrch/’ SFR RECHARGE’/

 INTEGER IUNITSFR

Under SUBROUTINE GWF2RCH7BD in section C2------CLEAR THE RATE ACCUMULATORS, initialize several variables to 

zero.

 C2------CLEAR THE RATE ACCUMULATORS.

 ZERO=0.0D0

 RATIN=ZERO

 RATOUT=ZERO

 rin2 = zero

 Q = zero

 QQ = zero

Under SUBROUTINE GWF2RCH7BD in section C5A-----IF CELL IS VARIABLE HEAD, THEN DO BUDGET FOR IT, 

calculate budget for variable head cell

 C5A-----IF CELL IS VARIABLE HEAD, THEN DO BUDGET FOR IT.

 IF(IBOUND(IC,IR,1).GT.0) THEN

 Q=RECH(IC,IR)

 QQ = Q (Note: QQ = Q is replaced by block of bolded code below)

 IF ( IUNITSFR.GT.0 ) THEN 

 QQ=RECHSAVE(IC,IR) 

 rin2 = rin2 + (Q-QQ)

 ELSE

 QQ = Q

 END IF
 END IF 77

Under SUBROUTINE GWF2RCH7BD in C6B-----IF CELL IS VARIABLE HEAD, THEN DO BUDGET FOR IT., calculate 

budget for variable head cell

 C6B-----IF CELL IS VARIABLE HEAD, THEN DO BUDGET FOR IT.

 IF(IL.EQ.0) GO TO 20

 IF(IBOUND(IC,IR,IL).GT.0) THEN

 Q=RECH(IC,IR)

 QQ = Q (Note: QQ = Q is replaced by block of bolded code below)

 IF ( IUNITSFR.GT.0 ) THEN

 QQ=RECHSAVE(IC,IR) 

 rin2 = rin2 + (Q-QQ)

 ELSE

 QQ = Q

 END IF

Under SUBROUTINE GWF2RCH7BD in C6D-----IF RECHARGE IS POSITIVE ADD TO RATIN, ELSE ADD IT TO 

RATOUT, calculate budget for variable head cell

 C6D-----IF RECHARGE IS POSITIVE ADD TO RATIN, ELSE ADD IT TO RATOUT.

 IF(QQ.GE.ZERO) THEN (Note: Q is replaced by QQ)

 RATIN=RATIN+QQ

 ELSE

 RATOUT=RATOUT-QQ

 END IF

 END IF

 20 CONTINUE

Under SUBROUTINE GWF2RCH7BD in C7C-----IF CELL IS VARIABLE HEAD, THEN DO BUDGET FOR IT, calculate 

budget for variable head cell

 C7C-----IF CELL IS VARIABLE HEAD, THEN DO BUDGET FOR IT.

 IF (IBOUND(IC,IR,IL).GT.0) THEN

 Q=RECH(IC,IR)

 QQ = Q (Note: QQ = Q is replaced by block of bolded code below)

 IF ( IUNITSFR.GT.0 )THEN

 QQ=RECHSAVE(IC,IR) 

 rin2 = rin2 + (Q-QQ)

 ELSE

 QQ = Q

 END IF

Under SUBROUTINE GWF2RCH7BD in C7E-----IF RECH IS POSITIVE ADD IT TO RATIN, ELSE ADD IT TO RATOUT, 

calculate budget for variable head cell

 C7E-----IF RECH IS POSITIVE ADD IT TO RATIN, ELSE ADD IT TO RATOUT.

 IF(QQ.GE.ZERO) THEN (Note: Q is replaced by QQ)

 RATIN=RATIN+QQ

 ELSE

 RATOUT=RATOUT-QQ

 END IF

 GO TO 29

 END IF
78 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

Insert code block below after C12-----INCREMENT BUDGET TERM COUNTER.

 C12a-----INCREMENT BUDGET TERM COUNTER. 

 IF ( IUNITSFR.GT.0 ) THEN

 VBVL(3, MSUM) = rin2

 VBVL(4, MSUM) = 0.0

 VBVL(1, MSUM) = VBVL(1, MSUM) + rin2*DELT

 VBVL(2, MSUM) = VBVL(2, MSUM) + 0.0

 VBNM(MSUM) = txtsfrch

 MSUM = MSUM + 1

 END IF

SFR Module: gwfsfrmodule.f

Allocate new variables by including the following statements under both the MODULE GWFSFRMODULE section and the 

TYPE GWFSFRTYPE section. 

 INTEGER,SAVE, DIMENSION(:), POINTER:: DVRCH 

 INTEGER,SAVE, DIMENSION(:,:,:),POINTER:: DVRCELL 

 REAL, SAVE, DIMENSION(:,:),POINTER:: RECHSAVE 

 REAL, SAVE, DIMENSION(:),POINTER:: DVEFF 

SFR Module: gwf2sfr7.f

In specifications under SUBROUTINE GWF2SFR7AR define several variables. 

 USE GWFSFRMODULE

 USE GLOBAL, ONLY: IOUT, IBOUND, BOTM, STRT, DELR, DELC,

 + ITRSS,NCOL,NROW 

Under C5------CALCULATE SPACE NEEDED FOR TABULATED DISCHARGE VERSUS FLOW in the ALLOCATE ARRAY 

STORAGE section add the following lines to allocate and initialize several variables

 ALLOCATE (HWTPRM(nstrmar,NUMTIM))

 ALLOCATE (DVRCH(nstrmar),DVEFF(nstrmar)) 

 ALLOCATE (DVRCELL(NCOL*NROW,2,nss)) 

 ALLOCATE (RECHSAVE(NCOL,NROW)) 

 STRM = 0.0 

 HSTRM = 0.0

 QSTRM = 0.0

 HWDTH = 0.0

 HWTPRM = 0.0

 ISTRM = 0

 DVRCH = 0 

 DVEFF = 0.0 

 DVRCELL = 0 

 RECHSAVE = 0.0 

 ALLOCATE (SEG(26,nsegdim), ISEG(4,nsegdim), IDIVAR(2,nsegdim))
 DO icount = 1, DVRCH(iprvsg) 79

In subroutine GWF2SFR7FM make the following changes

 C-------SUBROUTINE GWF2SFR7FM

 SUBROUTINE GWF2SFR7FM(Kkiter, Kkper, Kkstp, Iunitlak, Iunitrch, 

 + Igrid) !cjm (added Iunitrch)

 C *****************************************************************

 C ADD STREAM TERMS TO RHS AND HCOF IF FLOW OCCURS IN MODEL CELL

 C VERSION 7.1.01: February 15, 2009

 C *****************************************************************

 USE GWFRCHMODULE,ONLY:RECH 

 USE GWFSFRMODULE

In arguments under subroutine GWF2SFR7FM add the following argument

 INTEGER Kkiter, Kkper, Iunitlak, Igrid, Kkstp, Iunitrch

In local variables under subroutine GWF2SFR7FM add the following variables

 REAL areamax, avhc, errold, fks, ha, qcnst, seep, 

 + stgon, strlen, roughch, roughbnk, widthch, deltinc, qlat, 

 + fltest, Transient_bd, dvt 

 ! real fin, fout

 INTEGER i, ibflg, ic, icalc, idivseg, iflg, iic, iic2, iic3, iic4,

 + il, ilay, iprior, iprndpth, iprvsg, ir, istsg, itot,itrib,

 + itstr, iwidthcheck, kerp, kss, l, lk, ll, nstrpts, nreach,

 + maxwav, icalccheck, iskip, iss, lsub, numdelt, irt, ii, 

 + idr, lfold

 INTEGER irr, icc, icount 

 DOUBLE PRECISION FIVE_THIRDS

Under subroutine GWF2SFR7FM insert code block below after C20-----SET FLOW INTO DIVERSION IF SEGMENT IS 

DIVERSION.

 C20B-----STORE OUTFLOW FROM PREVIOUS SEGMENT FOR REECHARGE 

 IF ( istsg.GT.1 ) THEN

 IF (Iunitrch .GT. 0) THEN

 iprvsg = ISTRM(4, ll)

 IF ( DVRCH(iprvsg) .GT. 0) THEN

 DO icount = 1, DVRCH(iprvsg)

 irr = DVRCELL(icount, 1, iprvsg)

 icc = DVRCELL(icount, 2, iprvsg)

 dvt = (SGOTFLW(iprvsg) / float(DVRCH(iprvsg)))

 RECH(icc, irr) = RECHSAVE(icc, irr) + 

 + dvt*(1.0-DVEFF(iprvsg))

 C flowin = 0.0

 END DO

 END IF

 END IF

 END IF
80 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

In subroutine GWF2SFR7BD make the following changes

 C-------SUBROUTINE GWF2SFR7BD

 SUBROUTINE GWF2SFR7BD(Kkstp, Kkper, Iunitgwt, Iunitlak, Iunitgage,

 + Iunituzf, Nsol, Iunitrch, Igrid) 

 C *****************************************************************

 C CALCULATE VOLUMETRIC GROUND-WATER BUDGET FOR STREAMS AND SUM

 C STREAMFLOWS IN MODELED AREA

 C VERSION 7.1.01: February 15, 2009

 C *****************************************************************

 USE GWFSFRMODULE

 USE GWFLAKMODULE, ONLY: VOL, LKARR1, STGNEW

 USE GLOBAL, ONLY: NCOL, NROW, NLAY, IOUT, ISSFLG, IBOUND,

 + HNEW, BUFF, BOTM

 USE GWFBASMODULE, ONLY: MSUM, ICBCFL, IBUDFL, DELT, PERTIM, TOTIM,

 + VBVL, VBNM, HDRY

 USE GWFRCHMODULE,ONLY:RECH 

In arguments under subroutine GWF2SFR7BD add the following argument

 INTEGER Kkstp, Kkper, Iunitgwt, Iunitlak, Iunitgage, Iunituzf, 

 + Iunitrch 

In local variables under subroutine GWF2SFR7BD add the following variables

 REAL areamax, avhc, fks, ha, rin, rout, strlen,

 + zero, sfrbudg_in, sfrbudg_out, qlat, deltinc, qcnst, rtime,

 + fltest, Transient_bd, Transient_bd_tot, dvt 

 INTEGER i, ibd, iblst, ibdlbl, ibdst, ibstlb, ic, icalc, idivseg, 

 + il, ilay, iout1, iout2, iprior, iprvsg, ir, istsg, itrib,

 + iwidthcheck, kss, l, lk, ll, nreach, numdelt, maxwav,

 + icalccheck, iss, lsub, irt, itstr, imassroute, lfold

 INTEGER irr, icc, icount 

Under subroutine GWF2SFR7BD insert code block below after C20-----SET FLOW INTO DIVERSION IF SEGMENT IS 

DIVERSION.

 C20B-----STORE OUTFLOW FROM PREVIOUS SEGMENT FOR REECHARGE 

 IF ( istsg.GT.1 ) THEN

 IF (Iunitrch .GT. 0) THEN

 iprvsg = ISTRM(4, ll)

 IF ( DVRCH(iprvsg) .GT. 0) THEN

 DO icount = 1, DVRCH(iprvsg)

 irr = DVRCELL(icount, 1, iprvsg)

 icc = DVRCELL(icount, 2, iprvsg)

 dvt = (SGOTFLW(iprvsg) / float(DVRCH(iprvsg)))

 RECH(icc, irr) = RECHSAVE(icc, irr) + 

 + dvt*(1.0-DVEFF(iprvsg))

 END DO

 END IF

 END IF

 END IF
 DO icount = 1, DVRCH(iprvsg) 81

In subroutine SGWF2SFR7RDSEG make the following changes

 C-------SUBROUTINE SGWF2SFR7RDSEG

 SUBROUTINE SGWF2SFR7RDSEG(Nlst, Lstbeg, In, Iunitgwt, Ischk, 

 + Nischk, Ichk, Kkper, Nsol)

 C ******************************************************************

 C READ STREAM SEGMENT DATA -- parameters or non parameters

 C VERSION 7.1.01: February 15, 2009

 C ******************************************************************

 USE GWFSFRMODULE, ONLY: NSS, MAXPTS, ISFROPT, IDIVAR, IOTSG, ISEG,

 + SEG, XSEC, QSTAGE, CONCQ, CONCRUN,CONCPPT,

 + DVRCH, DVRCELL, RECHSAVE, DVEFF

 USE GLOBAL, ONLY: IOUT

 USE GWFRCHMODULE,ONLY: RECH 

In local variables under subroutine SGWF2SFR7RDSEG add the following variables

 INTEGER icalc, idum, ii, iqseg, isol, iupseg, jj, jk, lstend, n, 

 + noutseg, nseg, nstrpts, numcell, i 

Under C2------ONLY READ FIRST 4 VARIABLES TO DETERMINE VALUE OF IUPSEG. Change the following

 IF ( n.GT.NSS .OR. n.LT.1 ) THEN

 IF ( n.GT.NSS ) THEN 

Under subroutine SGWF2SFR7RDSEG insert code block below after C2------ONLY READ FIRST 4 VARIABLES TO 

DETERMINE VALUE OF IUPSEG.

 C2a-----DETERMINE IF SEGMENT OUTFLOW WILL BE DIVERTED TO RECHARGE MF CELLS 

 IF ( N.LT.0 ) THEN

 N = ABS(N)

 DVRCH(N) = 1

 ELSE 

 DVRCH(N) = 0 

 END IF

Under subroutine SGWF2SFR7RDSEG insert code block below after C10-----READ DATA SET 4G FOR SEGMENT IF SOLUTES SPECIFIED.

 C10b----READ CELL INDICES THAT RECEIVE RECHARGE: i,1 = ROW, i,2 = COL 

 IF ( DVRCH(N).GT.0 ) THEN

 READ(In, *)DVRCH(N),DVEFF(N)

 DO i = 1, DVRCH(N)

 READ(In, *) DVRCELL(i,1,N),DVRCELL(i,2,N)

 END DO

 END IF

In arguments under subroutine GWF2SFR7DA add the following 

 DEALLOCATE (GWFSFRDAT(IGRID)%IOTSG)

 DEALLOCATE (GWFSFRDAT(IGRID)%DVRCH) 

 DEALLOCATE (GWFSFRDAT(IGRID)%DVEFF) 

 DEALLOCATE (GWFSFRDAT(IGRID)%DVRCELL) 

 DEALLOCATE (GWFSFRDAT(IGRID)%RECHSAVE) 

 DEALLOCATE (GWFSFRDAT(IGRID)%NSEGCK)
82 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

In arguments under subroutine GWF2SFR7PNT add the following 

 IOTSG=>GWFSFRDAT(IGRID)%IOTSG

 DVRCH=>GWFSFRDAT(IGRID)%DVRCH 

 DVEFF=>GWFSFRDAT(IGRID)%DVEFF 

 DVRCELL=>GWFSFRDAT(IGRID)%DVRCELL 

 RECHSAVE=>GWFSFRDAT(IGRID)%RECHSAVE 

 NSEGCK=>GWFSFRDAT(IGRID)%NSEGCK

In arguments under subroutine SGWF2SFR7PSV add the following 

 GWFSFRDAT(IGRID)%IOTSG=>IOTSG

 GWFSFRDAT(IGRID)%DVRCH=>DVRCH 

 GWFSFRDAT(IGRID)%DVEFF=>DVEFF 

 GWFSFRDAT(IGRID)%DVRCELL=>DVRCELL 

 GWFSFRDAT(IGRID)%RECHSAVE=>RECHSAVE 

 GWFSFRDAT(IGRID)%NSEGCK=>NSEGCK

SFR Input Modification

Details of how this dataset is read by MODFLOW’s SFR2 package are described below. Original documentation (Niswonger 

and Prudic,2005). Note this has modification has not been coded to allow for the use of parameters in the SFR package.

Changes to the existing instructions in the SFR2 documentation are shown below in bold type:

For each Stress Period

Part 6a. Data: NSEG ...

NSEG An integer value of the stream segment for which information is given to identify inflow, outflow, and computation of 

stream depth. A negative NSEG utilizes the diverted recharge routines. 

The following instructions are added to the SFR2 documentation after instructions for line 6f:

Part 6g. Data: NCELL EFACT

 Data: NROWi NCOLi

 Data: NROWj NCOLj

 Data: NROWk NCOLk (repeat NCELL times)

NCELL An integer value representing the number of model cell to evenly apply diverted recharge from this segment

EFACT A real value between 0-1 that represents the efficiency factor to be applied to the diverted recharge

NROW An integer value representing the model row of the cell to receive diverted recharge

NCOL An integer value representing the model column of the cell to receive diverted recharge
Appendix 2. Modifications to PCGN Solver 83

Appendix 2. Modifications to PCGN Solver

The PCGN solver (Naff and Banta, 2008) uses the preconditioned, conjugate-gradient iterative method to solve a 

symmetric matrix as obtained from the application of the cellcentered finite-difference scheme to a regular, three-dimensional grid. In the current version of PCGN, preconditioning is performed by means of either a modified, incomplete 

Cholesky algorithm with no additional fill required (MIC(0)), 

or one additional level of fill required (MIC(1)). The solver 

is attached to the MODFLOW simulator by means of an 

interface that, among other things, institutes an outer Picard 

iteration when the flow problem contains nonlinear elements. 

In the simplest case of nonlinear approximation, the Picard 

iteration is accompanied by a constant damping factor that 

functions to diminish overshoot in the new head estimates. For 

a moderately nonlinear problem, the combination of Picard 

iteration with constant damping usually converges rather 

efficiently. However, for problems with more severe nonlinear elements, Picard iteration may not converge readily to a 

solution. For these instances, the PCGN solver has an adaptive 

damping scheme whereby the amount of damping applied is 

made proportional to the degree of nonlinearity. That is, from 

measures of convergence difficulty obtained from the preceding Picard iteration, the damping can either be increased, 

decreased, or left unchanged; the adjusted damping is then 

applied to the new head solution from the current Picard iteration. This adaptive scheme, however, is rather costly in terms 

of execution time—as it is heavily biased toward obtaining 

convergence by increasing damping, thus requiring more 

iterations to reach convergence. For the Carson Valley model 

described herein, some stress-period and time-step combinations require adaptive damping in order to converge. Rather 

than force every stress-period and time-step combination to 

undergo Picard iteration with adaptive damping, a reserve 

adaptive damping procedure was instituted. Reserve adaptive 

damping is selected by setting the damping-option parameter, 

ADAMP, to a negative integer one. 

The reserve adaptive damping procedure, as its name 

implies, is only called upon if Picard iteration with a constant 

damping does not result in convergence of the nonlinear problem in any given time step. An a priori method does not exist 

for determining the convergence characteristics of the Picard 

iteration. To quantify the difficulty of convergence, the number 

of Picard iterations in a time step is used; if Picard iterations 

become larger than a predetermined value, a switch is made 

from constant damping to adaptive damping. With ADAMP 

= −1, the maximum outer-iteration parameter, ITER_MO, is 

given a new function: ITER_MO becomes the number of Picard 

iterations after which the switch to adaptive damping is made. 

The maximum allowed number of outer (Picard) iterations 

under the reserve damping option becomes 4×ITER_MO. That 

is, Picard iteration with constant damping is given ITER_MO 

iterations to coverage. If convergence isn’t obtained within 

ITER_MO iterations, then the adaptive damping procedure is 

instituted and an additional 3×ITER_MO iterations are allowed 

for convergence. If after 4×ITER_MO Picard iteration, the 

problem hasn’t converged, then the MODFLOW computation 

is stopped.

Two operations are undertaken to facilitate the switch 

to adaptive damping under the reserve algorithm. First, the 

Picard iteration is restarted with a head distribution that is the 

average of the head distribution at the beginning of the time 

step and the head distribution obtained after ITER_MO Picard 

iterations with constant damping. This procedure results from 

the consideration that the head distribution after ITER_MO 

Picard iterations could be the result of a problematic direction 

taken in the fixed-damping Picard iteration. Averaging the 

head distribution insures that the adaptive damping initiated 

by the reserve algorithm has an acceptable starting point. The 

second operation adjusts the maximum damping factor. When 

initiated with ADAMP=1, the value of DAMP is the upper limit 

of the damping factor under favorable convergence conditions. 

Under constant damping (ADAMP=0), the value damping factor is DAMP. Because the reserve adaptive-damping scheme 

incorporates both concepts, the initial constant damping factor 

is specified as DAMP. When a switch to adaptive damping 

occurs, a new upper limit is set using an additional input 

parameter, MAX_DAMP_MOD, that has been incorporated 

into PCGN for this purpose. When reserve adaptive-damping 

is selected (ADAMP=−1) and ITER_MO Picard iterations are 

exceeded, a new upper limit MAX_DAMP is set by: MAX_

DAMP=MAX_DAMP_MOD x DAMP. For fixed damping under 

the reserve algorithm, it is expected that a value of DAMP 

close to but less than 1.0 will be selected (typically, 0.9). 

For the adaptive damping, it is generally desirable to have 

MAX_DAMP be a value less than 1.0 (typically, 0.5). Setting 

MAX_DAMP_MOD equal to MAX_DAMP/DAMP will produce 

the desired value of MAX_DAMP in the event that adaptive 

damping is initiated under the reserve algorithm.

Inputs to this modified version of PCGN are similar to the 

original. The following data items are required in the PCGN 

input file to utilize the reserve adaptive-damping option:

1. ITER_MO, ITER_MI, CLOSE_R, CLOSE_H

2. RELAX, IFILL, UNIT_PC, UNIT_TS

3. ADAMP, DAMP, DAMP_LB, RATE_D, CHGLIMIT, MAX_DAMP_MOD

4. ACNVG, CNVG_LB, MCNVG, RATE_C, IPUNIT
84 Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley

An explanation of other parameters unrelated to reserve adaptive damping is described in Naff and Banta (2008). If fixed 

format is selected, then the corresponding format types for the preceding variables are required:

1. 2I10,2F10.0

2. F10.0,3I10

3. I10,5F10.0

4. I10,F10.0,I10,F10.0,I10

Yager and others—Assessing Potential Effects of Changes in Water Use With a Numerical Groundwater-Flow Model of Carson Valley, Douglas County, 

Nevada, and Alpine County, California—Scientific Investigations Report 2012–5262