Prepared in cooperation with the ClearWater Conservancy 

and Pennsylvania Department of Environmental Protection

Water-Budget and Recharge-Area Simulations for Spring 

Creek and Nittany Creek Basins and Parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, Pennsylvania, 

Water Years 2000–06

Scientific Investigations Report 2015–5073

U.S. Department of the Interior

U.S. Geological Survey
Cover. Map showing location of study area.
Water-Budget and Recharge-Area 

Simulations for Spring Creek and Nittany 

Creek Basins and Parts of the Spruce Creek 

Basin, Centre and Huntingdon Counties, 

Pennsylvania, Water Years 2000–06

By John W. Fulton, Dennis W. Risser, Robert S. Regan, John F. Walker, 

Randall J. Hunt, Richard G. Niswonger, Scott A. Hoffman, 

and Stephen L. Markstrom

Prepared in cooperation with the ClearWater Conservancy 

and Pennsylvania Department of Environmental Protection

Scientific Investigations Report 2015–5073

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Fulton, J.W., Risser, D.W., Regan, R.S., Walker, J.F., Hunt, R.J., Niswonger, R.G., Hoffman, S.A., and Markstrom, S.L., 

2015, Water-budgets and recharge-area simulations for the Spring Creek and Nittany Creek Basins and parts of the 

Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06: U.S. Geological Survey 

Scientific Investigations Report 2015–5073, 86 p, http://dx.doi.org/10.3133/sir20155073.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors thank Jennifer Shuey (ClearWater Conservancy), Kathryn Ombalksi (ClearWater 

Conservancy), and Michael Hill (Pennsylvania Department of Environmental Protection) for 

collaborating on this project. We thank William Werkheiser [U.S. Geological Survey (USGS)], 

Ronald Thompson (USGS, retired), and George Leavesley (USGS, retired) for having the vision 

to undertake this effort. Matt Ely (USGS), David Bjerklie (USGS), and Daniel Goode (USGS) 

provided constructive criticism and review of the manuscript. Thank you to Lauren Hay (USGS) 

for assisting with model development and PRMS calibration. Roland Viger (USGS) assisted 

with model parameterization and ArcMap datasets. Elizabeth Hittle (USGS) assisted with 

implementing MISTE to estimate missing daily discharge values. Thank you to Kevin Hlywiak 

(National Weather Service) and Paul Knight and his staff (Pennsylvania Climatologist Office) 

for providing meteorological data used to parameterize the GSFLOW model. The Spring Creek 

Watershed Technical Steering Committee and in particular Todd Giddings (Giddings and 

Associates) and Lawrence Fennessey (Pennsylvania State University), Robert Carline (USGS, 

retired), Robert Cooper (Pennsylvania State University), Robert Donaldson (Centre County 

Planning Office, Retired), Walter Ebaugh (Nittany Geoscience, retired), Christopher Finton 

(Meiser & Earl), John Gaudlip (Pennsylvania State University), Jay Lynch (Meiser & Earl), and 

Mark Ralston (Converse Consultants) offered valuable local knowledge about the basin. Thank 

you to the Spring Creek Watershed Commission and, in particular, Dennis Haemeister.
iv

Contents

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

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

Purpose and Scope .............................................................................................................................2

Previous Studies ...................................................................................................................................2

Study Area..............................................................................................................................................2

Simulation Methods and the GSFLOW Model ..........................................................................................5

Model Overview....................................................................................................................................5

Model Development ............................................................................................................................5

PRMS-Only Model Development ..............................................................................................6

Overview...............................................................................................................................6

Digital Elevation Model......................................................................................................6

Modeled Area......................................................................................................................6

Stream Network and Hydrologic Response Unit Delineation .....................................6

Hydrologic Response Unit Cascades ..............................................................................8

Hydrologic Response Unit Parameterization .................................................................8

Climate Data ......................................................................................................................14

PRMS-Only Model Calibration and Results...........................................................................15

MODFLOW-Only Model Development....................................................................................15

Spatial and Temporal Discretization..............................................................................15

Boundary Conditions........................................................................................................15

Lateral Boundaries ...........................................................................................................17

Infiltration from Precipitation..........................................................................................17

Streams...............................................................................................................................20

Wells ...................................................................................................................................20

Evapotranspiration ..........................................................................................................24

Aquifer Properties ............................................................................................................24

MODFLOW-Only Model Steady-State Calibration and Results..........................................24

Weighting of Observations..............................................................................................27

Adjusted Aquifer Properties ...........................................................................................28

Comparison of Simulated and Observed Steady-State Groundwater Levels 

and Base Flows ...................................................................................................28

Calibration of the Hydrologic Model and Simulation Results...............................................................37

Parameter Estimation with PEST ............................................................................................37

Calibration Approach .......................................................................................................38

Observation Weights........................................................................................................38

Comparison of Simulated Results with Observations..........................................................40

Streamflow and Base Flow .............................................................................................40

Groundwater Levels .........................................................................................................42
v

Water Budget and Groundwater Recharge ............................................................................................43

Basin Water Budget..................................................................................................................43

Groundwater Recharge ............................................................................................................46

Model Limitations.........................................................................................................................................51

Summary and Conclusions.........................................................................................................................51

References Cited..........................................................................................................................................52

Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 

2000–06.............................................................................................................................................66

Figures

1. Map showing location of Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania....................3

2. Map showing delineation of subbasins in Spring Creek and Nittany Creek 

Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania .................................................................................................................................4

3. Flow chart of the strategy for development of the GSFLOW model for Spring 

Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, 

Centre and Huntingdon Counties, Pennsylvania.....................................................................7

4. Map showing stream network and locations of streamgages used to derive 

streamflow and base-flow observations for calibrating the MODFLOW-NWT, 

PRMS, and GSFLOW models in Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania .................................................................................................................................9

5. Map showing bedrock geologic units of Spring Creek and Nittany Creek 

Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................10

6. Map showing location of 829 hydrologic response units in Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre 

and Huntingdon Counties, Pennsylvania ................................................................................11

7. Map showing general soil types in Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................12

8. Map showing land cover in Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania..................13

9. Map showing mean infiltration rate simulated by PRMS for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, 

Centre and Huntingdon Counties, Pennsylvania, water years 2000–06 ............................18
vi

10. Map showing finite-difference grid and boundaries of the MODFLOW-NWT 

model for Spring Creek and Nittany Creek Basins and parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, Pennsylvania ...........................................19

11. Map showing location of surface-water withdrawals and discharges simulated 

by the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania........21

12. Map showing location of groundwater withdrawals simulated by the 

MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania..................22

13. Map showing locations of groundwater wells with groundwater-level data used 

for calibrating the MODFLOW-NWT model for Spring Creek and Nittany Creek 

Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................26

14. Map showing horizontal hydraulic conductivity zone parameters for layer 1 

of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................29

15. Map showing horizontal hydraulic conductivity zone parameters for layer 2 

of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................30

16. Map showing horizontal hydraulic conductivity zone parameters for layer 3 

of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................31

17. Map showing areas of horizontal anisotropy in layer 1 of the MODFLOW-NWT 

model for Spring Creek and Nittany Creek Basins and parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, Pennsylvania ...........................................32

18. Map showing areas of specific yield in layers 1 and 2 of the MODFLOW-NWT 

model for Spring Creek and Nittany Creek Basins and parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, Pennsylvania ...........................................33

19. Map showing areas of specific yield in layer 3 of the MODFLOW-NWT model 

for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, 

Centre and Huntingdon Counties, Pennsylvania...................................................................34

20. Map showing water-table elevations simulated by the MODFLOW-NWT model 

for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, 

Centre and Huntingdon Counties, Pennsylvania...................................................................35

21. Diagram showing hydraulic conductivities through column 119 of the 

MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania..................36

22. Graphs showing observed steady-state groundwater levels in relation to 

A, MODFLOW-NWT simulated elevations and B, the difference between 

simulated and observed elevations for 257 wells in Spring Creek and Nittany 

Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon 

Counties, Pennsylvania..............................................................................................................36

23. Graph showing observed steady-state base flows and steady-state base 

flows simulated by the MODFLOW-NWT model at 17 sites in Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre 

and Huntingdon Counties, Pennsylvania, water years 2000–06 .........................................37
vii

24. Graph showing summary of annual water-budget terms from the GSFLOW 

simulation, Spring Creek and Nittany Creek Basins and parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, water years 2000–06...............................43

25. Graph showing daily fluxes of precipitation (basinppt), evapotranspiration 

(basinactet), and streamflow (basinstrmflow) from the GSFLOW Comma 

Separated Value (CSV) output file............................................................................................45

26. Graph showing daily fluxes of precipitation (basinppt) and recharge from 

the unsaturated zone to the saturated zone (uzf_recharge) from the GSFLOW 

Comma Separated Value (CSV) output file.............................................................................47

27. Graph showing daily fluxes of streamflow (basinstrmflow) and recharge from 

the unsaturated zone to the saturated zone (uzf_recharge) derived from the 

GSFLOW Comma Separated Value (CSV) output file............................................................47

28. Map showing net recharge simulated by GSFLOW in Spring Creek and Nittany 

Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon 

Counties, Pennsylvania, water year 2001 ...............................................................................48

29. Map showing net recharge simulated by GSFLOW in Spring Creek and Nittany 

Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon 

Counties, Pennsylvania, water year 2004 ...............................................................................49

30. Map showing difference in net recharge between water years (2004 minus 2001) 

simulated by GSFLOW in Spring Creek and Nittany Creek Basins and parts of the 

Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania .............................50

Tables

1. Precipitation Runoff Modeling System parameter values assigned to hydrologic 

response units for Spring Creek and Nittany Creek Basins and parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, Pennsylvania. ..........................................58

2. Meteorological stations used to generate the climate data for the PrecipitationRunoff Modeling System-only simulations for Spring Creek and Nittany Creek 

Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania ...............................................................................................................................14

3. Description of manual calibration parameters with ranges for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania, water years 2000–06 .................................................16

4. Water budget components from the Precipitation-Runoff Modeling System-only 

simulation to generate the steady-state infiltration distribution for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania, water years 2000–06 .................................................17

5. Discharges to streams and withdrawals from streams simulated by using the 

Streamflow-Routing Package of MODFLOW-NWT model for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania........................................................................................20

6. Groundwater withdrawals simulated using the MODFLOW-NWT steady-state 

model for Spring Creek and Nittany Creek Basins and parts of the Spruce 

Creek Basin, Centre and Huntingdon Counties, Pennsylvania ...........................................23
viii

7. Parameters used to represent the hydraulic conductivity of geologic units 

in the MODFLOW-NWT steady-state model and horizontal hydraulic conductivity 

values from previous studies for Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania........63

8. Sources of streamflow data and groundwater levels used to adjust aquifer 

properties in the MODFLOW-NWT steady-state model for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania........................................................................................25

9. Base-flow values and weights used to adjust the MODFLOW-NWT steady-state 

model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek 

Basin, Centre and Huntingdon Counties, Pennsylvania.......................................................27

10. Adjusted values and sensitivities of parameters used in the steady-state 

MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania..................28

11. Parameters used in the calibration of the coupled model for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania........................................................................................39

12. Final weights used for groundwater-level data in the PEST transient calibration 

of the coupled model for Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania..................40

13. Final weights used for the streamflow data in the PEST transient calibration 

of the coupled model for Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania..................41

14. Nash-Sutcliffe coefficients of model efficiency for streamflow calibration 

targets in the Spring Creek and Nittany Creek Basins and parts of the 

Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, 

water years 2000–06...................................................................................................................42

15. Annual water budgets from the GSFLOW simulation for Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre 

and Huntingdon Counties, Pennsylvania, water years 2000–06 .........................................44

16. Simulated flux of recharge from the unsaturated zone to the saturated zone 

(uzf_recharge) derived from the GSFLOW Comma Separated Value (CSV) 

output file .....................................................................................................................................45
ix

Conversion Factors

[Inch/Pound to International System of Units[

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

square foot (ft2

) 0.09290 square meter (m2)

square inch (in2

) 6.452 square centimeter (cm2)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 0.003785 cubic meter (m3

) 

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

)

cubic inch (in3

) 16.39 cubic centimeter (cm3) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Flow rate

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

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]

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)

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

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

inch of mercury at 60oF (in Hg) 3.377 kilopascal (kPa) 

pound per square inch (lb/in2

) 6.895 kilopascal (kPa) 

Hydraulic conductivity

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

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

°F = (1.8 × °C) + 32.

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

°C = (°F – 32) / 1.8.
x

Datum

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

(NAVD 88).

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

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

Abbreviations

CWC ClearWater Conservancy

DEM digital elevation model

EPA U.S. Environmental Protection Agency

ET evapotranspiration

GIS geographic information system

GSFLOW Groundwater and Surface-water FLOW

HRU hydrologic response unit

IDE Inverse Distance and Elevation

MISTE MIssing STreamflow Estimation

MODFLOW-NWT Modular Groundwater Flow Model

NED National Elevation Dataset

NHD National Hydrography Dataset

PADEP Pennsylvania Department of Environmental Protection

PEST Parameter ESTimation

PRMS Precipitation-Runoff Modeling System

RMSD root mean squared difference

SSURGO Soil Survey Geographic Database

SCWC Spring Creek Watershed Committee

SRBC Susquehanna River Basin Commission

USGS U.S. Geological Survey

UTM Universal Transverse Mercator

VSA variable source area
Water-Budget and Recharge-Area Simulations for the 

Spring Creek and Nittany Creek Basins and Parts of the 

Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania, Water Years 2000–06

By John W. Fulton, Dennis W. Risser, Robert S. Regan, John F. Walker, Randall J. Hunt, 

Richard G. Niswonger, Scott A. Hoffman, and Stephen L. Markstrom 

Abstract

This report describes the results of a study by the U.S. 

Geological Survey in cooperation with ClearWater Conservancy and the Pennsylvania Department of Environmental 

Protection to develop a hydrologic model to simulate a water 

budget and identify areas of greater than average recharge 

for the Spring Creek Basin in central Pennsylvania. The 

model was developed to help policy makers, natural resource 

managers, and the public better understand and manage the 

water resources in the region. The Groundwater and Surfacewater FLOW model (GSFLOW), which is an integration of 

the Precipitation-Runoff Modeling System (PRMS) and the 

Modular Groundwater Flow Model (MODFLOW-NWT), was 

used to simulate surface water and groundwater in the Spring 

Creek Basin for water years 2000–06. Because the groundwater and surface-water divides for the Spring Creek Basin do 

not coincide, the study area includes the Nittany Creek Basin 

and headwaters of the Spruce Creek Basin. 

The hydrologic model was developed by the use of a 

stepwise process: (1) develop and calibrate a PRMS model 

and steady-state MODFLOW-NWT model; (2) re-calibrate 

the steady-state MODFLOW-NWT model using potential recharge estimates simulated from the PRMS model, 

and (3) integrate the PRMS and MODFLOW-NWT models into GSFLOW. The individually calibrated PRMS and 

MODFLOW-NWT models were used as a starting point for 

the calibration of the fully coupled GSFLOW model. The 

GSFLOW model calibration was done by comparing observations and corresponding simulated values of streamflow from 

11 streamgages and groundwater levels from 16 wells.

The cumulative water budget and individual water 

budgets for water years 2000–06 were simulated by using 

GSFLOW. The largest source and sink terms are represented 

by precipitation and evapotranspiration, respectively. For the 

period simulated, a net surplus in the water budget was computed where inflows exceeded outflows by about 1.7 billion 

cubic feet (0.47 inches per year over the basin area); storage 

increased by about the same amount to balance the budget. 

The rate and distribution of recharge throughout the 

Spring Creek, Nittany Creek, and Spruce Creek Basins is variable as a result of the high degree of hydrogeologic heterogeneity and karst features. The greatest amount of recharge was 

simulated in the carbonate-bedrock valley, near the toe slopes 

of Nittany and Tussey Mountains, in the Scotia Barrens, and 

along the area coinciding with the Gatesburg Formation. 

Runoff extremes were observed for water years 2001 

(dry year) and 2004 (wet year). Simulated average recharge 

rates (water reaching the saturated zone as defined in 

GSFLOW) for 2001 and 2004 were 5.4 in/yr and 22.0 in/yr, 

respectively. Areas where simulations show large variations 

in annual recharge between wet and dry years are the same 

areas where simulated recharge was large. Those areas where 

rates of groundwater recharge are much higher than average, 

and are capable of accepting substantially greater quantities 

of recharge during wet years, might be considered critical 

for maintaining the flow of springs, stream base flow, or the 

source of water to supply wells. The slopes of the Bald Eagle, 

Tussey, and Nittany Mountains are relatively insensitive to 

variations in recharge, primarily because of reduced infiltration rates and steep slopes.

Introduction

The Spring Creek Basin, Nittany Creek Basin, and headwaters of the Spruce Creek Basin in central Pennsylvania (collectively termed the “study basin”) have experienced growth 

and development resulting in land-use change and increased 

water use in parts of the basins. These changes influence the 

(1) quantity and availability of surface water and groundwater, (2) surface-water and groundwater interactions, and (3) 

aquatic resources in the basin. The study basin was identified 

as one of seven potentially stressed areas in the Susquehanna 
2 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

River Basin because of stormwater issues, groundwater 

contamination, mine dewatering, and diminished streamflow 

caused by groundwater withdrawals (Susquehanna River 

Basin Commission, 2005, p. 18–20). To assist water-management agencies in assessing the effects of increased water 

use and land-use change on surface-water and groundwater 

resources, the U.S. Geological Survey (USGS) in cooperation 

with ClearWater Conservancy (CWC) and the Pennsylvania 

Department of Environmental Protection (PADEP) developed 

a computer simulation model of the groundwater and surfacewater system by using the coupled groundwater and surfacewater flow model GSFLOW (Markstrom and others, 2008). 

Because of the variability in hydrologic connectivity between 

surface-water and groundwater across the study area, a model 

capable of simulating interactions between the systems was 

needed. GSFLOW is a useful tool (Mejia and others, 2012; 

Huntington and Niswonger, 2012) for assessing the effects of 

land-use change on surface-water and groundwater resources. 

Purpose and Scope 

This report describes the development of a GSFLOW 

computer model and presents the climate, physiographic, geologic, land-use, hydrologic, and streamflow data used to drive 

the computer model. The data were used to develop a model 

of the hydrology of the study area capable of accounting for 

water through the land surface, soils, subsurface, and stream 

network. Calibration of the model is discussed, and results of 

the simulations are presented. 

The specific objectives of this project were to (1) establish a water budget for the study basin using GSFLOW for 

water years1 2000–06 and (2) identify areas of greater than 

average recharge that can be used to assist decision makers in 

managing water resources. By documenting the steps needed 

to develop a GSFLOW model, the approach described in this 

report could be used to develop a baseline water budget and 

to estimate water volume and distribution for various uses 

or processes, such as drinking water, surface-water runoff, 

streamflow, and ecological flows for other basins in the Commonwealth of Pennsylvania.

Previous Studies

Substantive investigations related to the study basin 

are referenced in Fulton and others (2005), who summarize 

the hydrologic and physical characteristics of the basins and 

present a conceptual model for surface-water and groundwater flow. The data and the conceptual model reported by 

Fulton and others (2005) were used to demonstrate the need 

for a coupled-regions model, which dynamically accounts 

for the spatial and temporal distribution of surface water 

and groundwater. 

1

 A water year is the 12-month period that begins October 1 and ends September 30. It is designated by the year in which it ends. 

Study Area

The study basin is 229 square miles (mi2) in Centre and 

Huntingdon Counties in central Pennsylvania (fig. 1). The 

study basin incorporates all of Spring Creek and Nittany Creek 

Basins and the headwaters of the Spruce Creek Basin. The 

study basin is equivalent to the domain of the GSFLOW model. 

The Spring Creek Basin is 147 mi2, the Nittany Creek Basin is 

17 mi2, and the headwaters of Spruce Creek Basin in the study 

area is about 65 mi2. The study basin is divided into 14 subbasins in the GSFLOW model (fig. 2).

Water-level maps indicate that groundwater and surfacewater divides do not coincide in the study area (Giddings, 

1974; Taylor, 1997). The Spring Creek groundwater basin is 

approximately 175 mi2 (Fulton and others, 2005), which is 

about 28 mi2 larger than the Spring Creek surface-water basin. 

The study basin includes the groundwater divide that delineates the area of groundwater contribution to the Spring Creek 

Basin (fig. 1). The contributing area for groundwater flow 

from Nittany and Spruce Creek Basins, which is captured by 

Spring Creek, is not a fixed area; thus, it may vary based on 

hydrologic conditions and nearby groundwater withdrawals 

(Brachet, 2004). 

Two principal hydrologic settings define the study area—a 

forested, siliciclastic (sandstone and shale) -bedrock upland 

and a carbonate (limestone and dolomite) -bedrock valley with 

agricultural, suburban, and urban land uses. The settings differ 

in physiography, geology, and land use but are linked hydrologically by climate, evapotranspiration, surface runoff, interflow, 

snowmelt, recharge, and groundwater flow from the uplands, 

which provide and limit the water available for streamflow and 

groundwater recharge and discharge within the valley.

Forested, siliciclastic-bedrock uplands bound the study 

basin to the north and south. The carbonate-bedrock valley is 

dominated by agricultural and urban land uses and is described 

by Fulton and others (2005). Direct runoff from upland subbasins is characterized by variable source-area (VSA) hydrology, 

which is common in well-vegetated, humid areas that contain 

thin soil layers with large infiltration capacities and laterally 

continuous, low-permeability zones (Fulton and others, 2005). 

The carbonate-bedrock units in the valley are highly fractured 

and contain sinkholes and conduits, which promote rapid 

recharge and streamflow response to precipitation and snowmelt events and influence the rate and direction of groundwater 

flow. However, in areas where the residual soil veneer is substantial (greater than 100 feet), wetting fronts are attenuated, 

and the water-table response to precipitation and snowmelt can 

be delayed. The carbonate-bedrock units are capable of providing groundwater to wells that withdraw in excess of 1 million 

gallons of water per day (Fulton and others, 2005). Groundwater in the carbonate-bedrock units ultimately discharges to 

streams, large springs, wells, and mines, and as evapotranspiration to riparian vegetation.
Introduction 

3

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Streams from U.S. Geological Survey, National Hydrography Dataset, 2009Shaded relief from PAMAP Program, 2008

EXPLANATION

Study basinScotia Barrens (Pennsylvania Game Commission, 2011)Municipal boundaryCounty boundarySurface-water divideLiving Filter project boundary (Lin, 2011)Groundwater divide (Giddings, 1974)Groundwater divide (Taylor, 1997)

0 5 MILES

0 5 KILOMETERS

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Halfmo

o

n

C reek Beaver Branch

Spruce

Creek

Spring

Creek

Ni

ttany Creek

Roaring

Run

Buf af ol Run

Logan

B

r

a

nch

Cedar Run

S al b

Cabin

Run Galbraith Gap Ru n

Roaring Run

Nittany 

Mountain

Bald Eagle Mountain

Tussey Mountain

Gatesburg Ridge

Nittany Valley

Penns Valley

Figure 1. Location of Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
4 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Streams from U.S. Geological Survey, National Hydrography Dataset, 2009 Shaded relief from PAMAP Program, 2008 EXPLANATION Study basin Scotia Barrens (Pennsylvania Game Commission, 2011) Municipal boundary County boundary Surface-water divide Subbasin boundary Living Filter project boundary (Lin, 2011) Groundwater divide (Giddings, 1974) Groundwater divide (Taylor, 1997) Spring

0 5 MILES

0 5 KILOMETERS

Nittany Creek 

Basin

Spring Creek 

Basin

Spruce Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Halfmoon C reek

Beaver Branch

Spruce Creek

Spring Creek

Nittany

Creek

Roaring

Run

Buf af ol Run

Logan Branch

Cedar Run

S al b

Cabin Run

Galbraith Gap Ru n

Roaring Run

Figure 2. Delineation of subbasins in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 5

Groundwater recharge typically is greatest during 

November through May when evapotranspiration rates are 

minimal. Recharge to the carbonate-bedrock aquifer occurs 

along six pathways as conceptualized by Parizek (1984): 

(1) direct infiltration of precipitation into soils and exposed 

bedrock; (2) concentrated stormwater runoff from the valley 

into sinkholes; (3) concentrated surface runoff from uplands 

into sinkholes; (4) diffuse surface runoff from uplands; (5) 

streamflow losses from perched or intermittent streams on 

karst terrain; and (6) leakage from underground pipes, disposal 

of on-lot sewage effluent, and irrigation. 

Simulation Methods and the GSFLOW 

Model

To simulate the hydrologic cycle, a modified version 

of GSFLOW (Markstrom and others, 2008; enhanced version 1.1.5) was used in the model domain. It is a physically 

based, distributed model that simulates coupled groundwater 

and surface-water flow across the land surface, in the stream 

network, and within the subsurface variably saturated and 

saturated materials in single or multiple basins (Markstrom 

and others, 2008). GSFLOW simulates the timing, feedback, 

and rates of exchange of water and energy in the atmosphere, 

canopy, snowpack, pervious and impervious areas of the land 

surface, soil, unsaturated and saturated zones, streams, lakes, 

and the effects of wells and surface-water diversions using 

daily time series climate and water-use data.

The version of GSFLOW (enhanced version 1.1.5) used 

for this project includes the Newton solution method and the 

Inverse Distance and Elevation (IDE) module. The Newton solution method is a formulation of MODFLOW-2005, 

designed to solve problems involving drying and rewetting in 

hydrogeologic settings, which are dominated by nonlinearity. 

This feature is particularly important to the study basin, which 

is characterized by unconfined conditions, steep topography, 

and surface-water/groundwater interactions. 

IDE uses a combination of inverse distance and elevation 

weighting to interpolate maximum and minimum precipitation 

and temperature data for each hydrologic response unit (HRU) 

in the PRMS model. HRUs are spatial discretizations of the 

land surface, which are commonly represented as polygons 

or cascading-flow networks to stream segments and possess 

similar parameters, such as soil type, land cover, land use, 

and geology.

Model Overview

GSFLOW integrates two previously documented USGS 

models—the Precipitation Runoff Modeling System (PRMS) 

and the three-dimensional, modular groundwater-flow model 

MODFLOW-NWT (Niswonger and others, 2011). PRMS 

simulates the distribution of water from the top of the plant 

canopy to the bottom of the soil zone on the basis of hydrologic and climate variables, such as precipitation, air temperature, potential solar radiation, and evapotranspiration. 

MODFLOW-NWT simulates the distribution of water from 

the base of the soil zone, through the unsaturated zone, to the 

saturated zone, and the discharges of water to streams and the 

land surface. GSFLOW couples these models within the soil 

veneer on the basis of soil moisture content, hydrogeologic 

characteristics, and hydraulic-head differences.

Surface-water/groundwater interactions can occur 

between the (1) PRMS simulated soil zone and MODFLOWNWT simulated unsaturated zone, (2) PRMS simulated 

surface runoff and shallow lateral subsurface flow (interflow) 

to MODFLOW-NWT simulated streams and lakes, and (3) 

MODFLOW-NWT simulated unsaturated and saturated flow 

in subsurface areas below the soil zone to streams and lakes. 

The governing equations of each region are solved separately 

in an iterative process to balance the dependent variables and 

conserve mass throughout the model. Flow in the unsaturatedzone beneath the soil zone, streams, and lakes is based on 

a one-dimensional kinematic-wave approximation to the 

Richards equation solved by using the UZF Package within 

MODFLOW-NWT (Niswonger and others, 2007). Streamflow 

routing is simulated by using the Streamflow Routing Package 

within MODFLOW-NWT (Niswonger and Prudic, 2005).

The states and fluxes among the soil zone, unsaturated 

zone, and saturated zone are available to compute a water 

budget and components of flow in and out of each spatial unit: 

(1) HRUs, (2) MODFLOW-NWT cells, (3) stream segments, 

(4) intersection of HRUs and MODFLOW-NWT cells, and (5) 

intersections between stream segments and MODFLOW-NWT 

cells or reaches. These spatial units are connected topologically such that simulated flows cascade on the basis of hydrologic gradients. Hydrologic processes are simulated by using a 

daily time step, and a water budget can be generated by region 

or stream segment, and for specified time period. 

Model Development 

Model development involved a three-step process: 

(1) develop and calibrate a PRMS model and steady-state 

MODFLOW-NWT model; (2) re-calibrate the steady-state 

MODFLOW-NWT model using potential recharge estimates 

simulated from the PRMS-only model, and (3) integrate the 

PRMS and MODFLOW-NWT models in the GSFLOW structure and calibrate the coupled model using the individually 

calibrated PRMS and MODFLOW-NWT models as a starting 

point. This calibration process was an iterative process occurring as new information and enhancements to GSFLOW were 

added to improve the model. Calibration was accomplished 

by comparing observed streamflow and groundwater levels to 

simulated values with a combination of best professional judgment, manual adjustment of model parameters, and automated 
6 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

methods including Parameter Estimation (PEST) (Doherty and 

Hunt, 2010). Model development is illustrated schematically 

in figure 3. Note that the MODFLOW-NWT model was calibrated for steady-state conditions (step 1); initial adjustments 

of aquifer properties were done with a model constructed with 

a uniform spatial distribution of infiltration, then was recalibrated by using spatially variable estimates of infiltration from 

the PRMS model output (step 2). The infiltration of water 

below the root zone simulated by PRMS has been used as 

input for MODFLOW models in various applications (Bjerklie 

and others, 2010; Jeton and Maurer, 2007; Lee and Risley, 

2001; Steuer and Hunt, 2001; Hunt and Steuer, 2000; Vaccaro, 

1992; and Ely and others, 2011). The simulated rate of infiltration below the root zone in PRMS-only can be thought of as 

potential recharge that will either be added to the saturated 

zone or be rejected by MODFLOW-NWT. 

PRMS-Only Model Development

An overview of the development of the PRMS model is 

described in the following section. The digital elevation model 

(DEM) developed for the PRMS, the modeled area, HRU 

delineation, HRU cascades, HRU parameterization, climate 

data, calibration, and results prior to running GSFLOW 

are discussed.

Overview

PRMS (Leavesley and others, 1983; Leavesley and Stannard, 1995; Leavesley and others, 2005) is a modular, deterministic, distributed-parameter, physical-process-based model 

developed to evaluate the hydrologic response to various combinations of climate, land use, topography, and hydrogeology. 

It is capable of simulating the temporal and spatial distribution 

and routing of water in the model domain, which comprises 

single or multiple subbasins of any size and spatial discretization. The phrase “hydrologic response” refers to simulated 

water flow to and from the atmosphere, canopy, land surface, 

shallow subsurface, deep aquifers, stream segments, and lakes. 

A response to normal and extreme precipitation and snowmelt 

is simulated by basin and is a function of the temporal and 

spatial variability of hydrologic parameters, water sources and 

sinks, and storage in a subbasin. Simulated results include a 

water budget and values for snow dynamics, evapotranspiration, infiltration, streamflow, overland flow, soil-moisture 

relations, and vertical (recharge), lateral (interflow), and 

subsurface flows.

Simulation processes are based on physical laws, empirical relations, and associated parameters and attributes of the 

modeled area. Because these parameters vary spatially and 

temporally, each subbasin is partitioned into a series of HRUs. 

Each HRU represents a single, lumped area that is assumed to 

be homogeneous (with respect to hydrology, physical characteristics, and response) and responds instantaneously and 

uniformly, when precipitation and snowmelt are added to the 

HRU. A comprehensive description of PRMS is found in the 

GSFLOW documentation and software descriptions (Markstrom and others, 2008).

Digital Elevation Model

A digital elevation model (DEM) was generated by using 

the 30-meter grid USGS National Elevation Dataset (NED) 

(U.S. Geological Survey, 2007), resampled with a 10-meter 

grid, and processed in accordance with methods described by 

Viger and Leavesley (2007). Elevations were combined with 

the USGS National Hydrography Dataset (NHD) (U.S. Geological Survey, 2011) to incorporate known stream locations 

into the DEM prior to establishing the modeled area.

Modeled Area

The modeled area was based on the DEM developed 

for the study basin, including Spring Creek and Nittany 

Creek Basins, which drain north through water gaps in Bald 

Eagle Mountain, and the headwaters of Spruce Creek, which 

is drained by Halfmoon Creek and Beaver Branch (fig. 2). 

Spruce Creek was included in the domain so that the groundwater divide (fig. 1) near the headwaters of Spring Creek and 

Spruce Creek could be simulated dynamically as hydrologic 

conditions varied temporally. The Nittany Creek Basin was 

included to allow an exchange of groundwater between Spring 

Creek and Nittany Creek Basins.

Stream Network and Hydrologic Response Unit 

Delineation

The stream network and HRU delineation was simplified 

by using a geographic information system (GIS) database. 

Spatial variations in basin characteristics were determined by 

using the database and standard spatial processing techniques 

similar to those described by Battaglin and others (1993), 

Jeton (2000), and Jeton and others (1996). Coordinates in 

the database were assumed to refer to a Cartesian coordinate 

system with Universal Transverse Mercator (UTM), zone 18, 

Northern Hemisphere, with North American Datum (NAD) 

1983 as the horizontal control datum.

A stream network was generated by using the DEM 

to create stream segments based on contributing area. The 

intersection of stream segments and MODFLOW-NWT cells 

were used in conjunction with the streamflow-routing package 

(SFR2) in MODFLOW-NWT to create the routing network 

used in GSFLOW. Surface runoff and interflow were added to 

stream reaches by connecting HRUs to stream segments. Each 

stream segment was associated with an HRU representing the 

riparian area approximated by a 656-foot (200-meter) distance 

on each side of the segment. The riparian areas (or stream buffers) were needed to optimize the surface-water and groundwater interaction along the stream network and to assure that 

groundwater discharge in riparian areas was not routed to 

upslope HRUs.
Simulation Methods and the GSFLOW Model 7

Construct

steady-state MODFLOW-NWT

model

STEP 1

Calibrate MODFLOW model—

Adjust parameters for 

average conditions

for water years 2000–06

(calibration)

STEP 2

Recalibrate MODFLOW model—

Adjust MODFLOW steady-state

parameters using PRMS

infiltration

for water years 2000–06

STEP 1

Calibrate PRMS model—

Adjust parameters for

water years 1997–2006

(calibration)

Construct

PRMS

model

Process spatial

distribution of

infiltration from

output of PRMS

for water years

2000–06

Construct GSFLOW model from

MODFLOW and PRMS input datasets

Calibrate GSFLOW with PEST

STEP 3

GSFLOW simulations for Spring Creek Basin

Figure 3. The strategy for development of the GSFLOW model for Spring 

Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre 

and Huntingdon Counties, Pennsylvania.
8 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Flow direction and flow accumulation were computed 

for the outlet of each HRU from the DEM and used to compute contributing areas. The headwater point of each stream 

segment was established on the basis of a contributing-area 

threshold of approximately 15 mi2; however, additional headwater streams were added manually at locations of particular 

interest, such as locations where streams flow into the Spring 

Creek Basin that do not meet the contributing-area threshold. 

The stream network (fig. 4) consisted of stream segments with 

an upper boundary as a headwater point, confluence point, or 

USGS or CWC streamgage and a lower boundary as a confluence point, USGS or CWC streamgage, or subbasin outlet. 

The stream network was used to define the left- and rightbank contributing areas and delineate initial HRUs associated 

with each stream segment. The initial HRU delineations were 

refined on the basis of soil type, geology (fig. 5), elevation, 

stream buffers, toe-slope boundaries, and the active MODFLOW boundary. The resulting HRUs were parameterized on 

the basis of areally weighted averages of subbasin characteristics—soil type, land cover, land use, and geology (Markstrom 

and others, 2008). GIS processing was used to remove small 

HRUs by merging them into adjacent, hydrologically similar 

HRUs (fig. 6). The Spring Creek model is composed of 14 

subbasins, 829 HRUs, and 387 stream segments. The 14 subbasins are shown in figure 2.

Hydrologic Response Unit Cascades

The HRU map was analyzed to determine the cascadingflow paths for routing flow. There are multiple paths. Some 

HRUs cascade to one or more adjacent HRUs (upslope and 

downslope), some cascade to a stream segment, some terminate in swale HRUs, and some cascade to multiple stream 

segments. Swale HRUs were used to simulate sink holes and 

closed depressions on the land surface. Routines were selected 

to determine parameters that describe the cascade (routing 

surface runoff and interflow from upslope HRUs to downslope 

HRUs) connectivity of HRUs with the stream network. The 

cascade flow network was determined topologically by using 

GIS processing of the DEM. HRUs were grouped to define 

each subbasin as the contributing area to the terminus of any 

stream segment of interest, such as at a streamgage. Each 

HRU is included in a single subbasin; thus, HRUs included 

in a subbasin upstream from another subbasin are treated as 

a contributing subbasin to a downstream subbasin. HRU-toHRU and HRU-to-stream segment cascade assignments are 

described in Markstrom and others (2008, p. 33–34). Flow 

between HRUs (cascades) that is not directly connected to a 

stream segment is based on topological parameters for routing 

surface runoff and interflow from upslope HRUs to downslope 

HRUs and stream segments. For PRMS-only simulations, 

groundwater was routed by using a similar cascading flow 

network. Groundwater PRMS cascades differ from HRU cascades in that groundwater reservoirs (GWR) cannot be swales. 

GWR cascades were the initial cascade flow paths, whereas 

the HRU cascade network was modified to account for swales.

Hydrologic Response Unit Parameterization

Initial HRU parameterization, including flow coefficients (gwflow_coef, slowcoef_lin, ssflow_coeff, ssr2gw_exp, 

ssr2gw_rate), was based on the physical characteristics of 

the subbasins and PRMS model defaults. Geospatial datasets 

related to elevation, slope, sinks, HRU area, hydraulic conductivity, aspect, vegetation type, soil type, land use, hydrology, aspect, and precipitation distribution were processed, 

and PRMS parameters were assigned to each HRU. Data 

in raster format (a gridded data structure made of rows and 

columns) and vector format (discrete coordinates that can be 

used as points or connected to create lines and polygons) were 

compiled to produce a database of input geospatial datasets. 

Sources of geospatial data include the USGS NED for elevation (U.S. Geological Survey, 2007) where slope and aspect 

calculations were derived by using GIS software ArcGIS 

version 9.2; state geology data from Pennsylvania (Pennsylvania Department of Conservation and Natural Resources, 

2001); land-cover and impervious-surface data from the 2001 

National Land Cover Database (Multi-Resolution Land Characteristics Consortium, 2001); and soils data (fig. 7), which 

were originally from the Soil Survey Geographic (SSURGO) 

database (Natural Resources Conservation Service, 2006) 

and were further processed with value-added soil characteristics (Miller and White, 1998). Selected reclassification 

tables (Viger and Leavesley, 2007) were used with land-cover 

data (fig. 8) to assist in refining the HRU characteristics. The 

sources of values used for selected distributed PRMS model 

parameters are listed in table 1 (at end of report), along with 

ranges of values used.
Simulation Methods and the GSFLOW Model 9

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Streams from U.S. Geological Survey, National Hydrography Dataset, 2009Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

Map 

identifier

Station 

identifier Station name PRMS and GSFLOW MODFLOW

1 404057078015901 Spruce Creek at Grayville, PA Yes Yes

2 01547100 Spring Creek at Milesburg, PA Yes Yes

3 01546500 Spring Creek near Axemann, PA Yes Yes

4 01546400 Spring Creek at Houserville, PA Yes Yes

5 BUL Buffalo Run Lower Yes Yes6 BUU Buffalo Run Upper No No7 LOL Logan Branch Lower Yes Yes

8 LOU Logan Branch Upper Yes Yes

9 CEL Cedar Run Lower Yes Yes

10 THL Thompson Run Lower No Yes

11 SLL Slab Cabin Run Lower Yes Yes

12 SLU Slab Cabin Run Upper Yes No

13 SPU Spring Creek Upper Yes Yes

EXPLANATION

Study basinMunicipal boundaryCounty boundarySurface-water divideLocation of streamgage 

and map identifier

U.S. Geological SurveyClearWater ConservancyU.S. Fish and Wildlife Service

451

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN COUNTY

Halfmoon C reek

Beaver Branch

Spruce Creek

Spring Creek

Nittany

Creek

Roaring

Run

Buf af ol Run

Logan Branch

Cedar Run

S al b

Cabin Run

Galbraith Gap Ru n

Roaring Run

1

2

3

4

5

6

7

8

9

10

12

11

13

Figure 4. Stream network and locations of streamgages used to derive streamflow and base-flow observations for calibrating the MODFLOW-NWT, PRMS, and GSFLOW models in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
10 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Streams from U.S. Geological Survey, National Hydrography Dataset, 2009 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

Geology by Parizek and Filley, 1982

EXPLANATION

Geologic unit

Silurian and Ordovician SiliciclasticsCoburn and Salona FormationsNealmont Limestone through Loysburg 

Formation , undivided

Bellefonte DolomiteAxemann LimestoneNittany DolomiteStonehenge FormationGatesburg Formation (Mines Dolomite and 

upper sandy members)

Gatesburg Formation (Ore Hill and lower sandy 

members, and Warrior Formation)

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Halfmoon C reek

Beaver Branch

Spruce Creek

Spring Creek

Nittany

Creek

Roaring

Run

Buf af ol Run

Logan Branch

Cedar Run

S al b

Cabin Run

Galbraith Gap Ru n

Roaring Run

Figure 5. Bedrock geologic units of Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 11

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Study basinMunicipal boundaryCounty boundaryLiving Filter project boundary (Lin, 2011)Hydrologic response unit boundaryPRMS-generated streams

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 6. Location of 829 hydrologic response units in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
12 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

General soil type (Natural Resources 

Conservation Service, 2006)

ClayLoamSandSiltFragipanWater

Municipal boundaryCounty boundary

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 7. General soil types in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 13

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

National Land Cover Dataset 

(U.S. Geological Survey, 2001)

Open waterDeveloped open spaceDeveloped low intensityDeveloped medium intensityDeveloped high intensityBarren land (rock, sand, clay)Deciduous forestEvergreen forestMixed forestPasture/hayCultivated cropsWoody wetlands

Municipal boundaryCounty boundary

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 8. Land cover in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
14 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Climate Data

The forcing functions for PRMS are based on climate 

time-series data and include (1) precipitation, (2) maximum 

and minimum air temperatures, (3) potential evapotranspiration, and (4) solar radiation. Because GSFLOW operates in 

a daily mode only, the time-series data were organized by 

using daily time steps and provided as input to GSFLOW in 

climate-by-HRU (CBH) files, which have the same format as 

PRMS data files (Markstrom and others, 2008, p. 139–140). 

Five CBH files were input, one for each climate-forcing type: 

daily precipitation, maximum air temperature, minimum air 

temperature, potential evapotranspiration, and actual solar 

radiation. The CBH files provided a value for each HRU for 

each day.

Precipitation data for meteorological stations were 

obtained from multiple sources, including 13 meteorological stations (table 2) in and near the modeled area. Data were 

acquired by using the Downsizer, a computer application that 

locates and downloads time-series data from environmental 

databases that are used to parameterize models such as PRMS 

(Ward-Garrison and others, 2009) and were augmented with 

data from the Pennsylvania State Climatologist office. The 

meteorological data were preprocessed by using an IDE 

weighting algorithm to generate the precipitation, minimum 

air temperature, and maximum air temperature CBH files for 

the simulation period. The IDE method distributes precipitation and maximum and minimum temperatures to each HRU 

using the closest climate-station elevations above and below 

a given HRU’s elevation and linearly interpolates climate 

values for the HRU on the basis of the data from these 

two stations.

Wastewater effluent to the canopy has been included in 

the model from the Living Filter (Pennsylvania State University, 2012), which is a water re-use project that began in 1963 

(figs. 1 and 2). Approximately 2.7 million gallons per day 

(Mgal/d) of wastewater from the Pennsylvania State University (PSU) is applied to approximately 600 acres of forests, 

croplands, and fields as tertiary treatment (Richardson, 2011). 

In addition to distributed precipitation (38.4 inches on average), approximately 62.4 inches of water per year (in/yr) 

related to reuse was added in GSFLOW to the application 

area to simulate the operation of the Living Filter.

Potential evapotranspiration was simulated by PRMS 

using a modified Jensen-Haise formulation (Markstrom and 

others, 2008, p. 43). Solar radiation was estimated by using 

computed daily clear-sky short-wave solar radiation for each 

HRU (Markstrom and others, 2008, p. 41–42). Modification 

of a degree-day method was used to compute actual solar 

radiation for each HRU (Markstrom and others, 2008, p. 42).

Table 2. Meteorological stations used to generate the climate data for the 

Precipitation-Runoff Modeling System-only simulations for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania.

[NAVD 88, North American Vertical Datum of 1988]

Station 

number Station name Latitude Longitude

Elevation, 

in feet above 

above NAVD 88 

361480 Clarence 41.049 -77.941 1,390

360482 Beavertown 1 NE 40.774 -77.157 540

360132 Altoona 40.523 -78.369 1,280

369022 Tyrone 40.664 -78.219 890

369714 Williamsburg 40.467 -78.200 845

365109 Lock Haven Sewage Plant 41.117 -77.450 566

364159 Huntingdon 40.515 -78.003 685

367409 Renovo 41.330 -77.738 660

368449 State College 40.793 -77.867 1,170

364992 Lewistown 40.587 -77.570 460

364853 Laurelton Center 40.902 -77.214 800

366921 Philipsburg 2 S 40.872 -78.215 1,720

365790 Millheim 40.884 -77.474 1,120
Simulation Methods and the GSFLOW Model 15

PRMS-Only Model Calibration and Results

Initial values of most PRMS parameters were computed 

by using methods documented by Viger and Leavesley (2007), 

Battaglin and others (1993), Jeton (2000), and Koczot and 

others (2005). Other parameters were estimated on the basis 

of GIS analysis, PRMS default values, and other datasets 

available for the modeled area. Initial parameter values were 

modified during the model calibration procedure by using 

manual and automated methods to minimize the difference 

between simulated and measured (or independently computed) 

potential evapotranspiration, solar radiation, and streamflow. 

Streamflow was recorded at USGS and CWC streamgages 

(fig. 4). Manual calibration methods were based on previous 

PRMS calibration experience and are summarized in table 3.

The water budget components for the PRMS-only simulation were computed for water years 2000–06. To generate 

budgets for those years, the PRMS simulation period was 

initialized three years prior to WY 2000, which provided time 

for the simulated hydraulic system to adjust from the initial 

conditions that were assigned to the model. The major water 

budget components are summarized by water year in table 4. 

During water years 2000–06, 2001 was the driest year and 

2004 the wettest year on the basis of both simulated precipitation and runoff amounts.

The simulated mean infiltration rate for water years 

2000–06 from PRMS was 14.7 inches per year (in/yr) for the 

basin as a whole. PRMS-derived infiltration (vertical flows 

from the PRMS soilzone) is a downward vertical flux that 

originates at the land surface. This process differs from gravity drainage (potential recharge from the PRMS soilzone by 

HRU) and recharge from the UZF Package to MODFLOW 

generated by GSFLOW, which is a vertical flux from the 

unsaturated zone to the saturated zone for each model cell.

Simulated mean infiltration rates ranged throughout the 

basin from approximately 2.1 to 72.9 in/yr (fig. 9). The greatest amount of infiltration was simulated in the carbonate-bedrock valley near the Living Filter, along Logan Branch, along 

the toe slopes of Nittany and Tussey Mountains, along the 

Gatesburg Ridge near the Scotia Barrens, along Buffalo Run 

and Spruce Creek, and in Penns Valley. The variation is attributed to the distribution of precipitation, variable slopes, and 

the runoff characteristics of land-use types in the study basin.

MODFLOW-Only Model Development

The finite-difference computer code MODFLOW-NWT 

(Niswonger and others, 2011) was used to simulate threedimensional (3D), steady-state, groundwater flow of the study 

basin. The MODFLOW-NWT model was constructed in units 

of meters and days. The length unit of meters was converted 

into feet for this report, resulting in some values that may 

seem unusual or may convey more precision than is warranted. 

MODFLOW-NWT is a code based on MODFLOW-2005 

(Harbaugh, 2005) and includes a Newton solver that can be 

used to solve the groundwater-flow equation for situations 

where the fluctuating water table causes drying and rewetting 

of model cells (Hunt and Feinstein, 2012). A graphical user 

interface linked to Argus Numerical Environments was used 

for pre- and post-processing of data (Winston, 2000).

Spatial and Temporal Discretization

The study area was divided into a finite-difference grid 

(fig. 10) with three layers, 104 rows, and 216 columns. The 

horizontal dimensions of the cells were uniform 656 by 

656 feet (200 by 200 meters) in horizontal dimension. The 

model grid was constructed with rows oriented N. 55° E. to 

align with the general strike of geologic units in the area. The 

active model area coincides with the HRU boundary. The 

orientation of model rows along the strike of dipping units 

is important because fractures and solution openings that 

enhance permeability are better developed parallel to strike, 

and shaley beds tend to impede the movement of water across 

the strike of beds (Parizek and others, 1971, p. 40).

The steady-state MODFLOW-NWT model was based 

on water years 2000–06 and was assumed to represent the 

average groundwater conditions during that period. Recharge 

for initial simulations was assumed to be spatially uniform 

(calibration step 1, fig. 3), then was refined to use the average infiltration at each PRMS HRU for water years 2000–06 

to represent a spatially varied recharge distribution (calibration step 2, fig. 3). Changes caused by seasonal variations in 

recharge or pumping were not simulated in the steady-state 

model but were incorporated in GSFLOW. For water years 

2000–06, groundwater withdrawals, surface-water withdrawals, and discharges to surface water were varied by stress 

period on a monthly basis in GSFLOW.

Boundary Conditions

The elevation of the top of each cell in the uppermost 

model layer (layer 1) was set equal to the elevation of land 

surface determined as the mean of all data points from the 

USGS 30-meter DEM falling within the cell. The landsurface elevations assigned to model cells ranged from 680 

to 2,399 feet, and the maximum elevation difference between 

adjacent cells was about 200 ft. The thickness of each model 

layer was set to a uniform value of 300 feet, so the total model 

thickness was 900 feet throughout the study area.

For this study, groundwater movement is assumed to 

be minimal below the base of the model (900 ft below land 

surface). The depth of active flow is not well known, but in 

Centre County, most high yielding zones in wells are generally encountered within weathered bedrock at depths of less 

than 400 ft below land surface, and evidence of secondary 

permeability in limestone mines was almost entirely absent 

at depths greater than about 500 ft (Wood, 1980, p. 31–32). 

Important water-bearing zones have been reported as deep 

as 600 ft in production wells drilled in the Gatesburg Formation, and Parizek and others (1971, p. 61) hypothesized that 

groundwater flows at depths greater than 1,000 feet below 

land surface within the Dale Sandstone Member of the 
16 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 Table 3. Description of manual calibration parameters with ranges for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06. [PRMS, Precipitation-Runoff Modeling System; Min, minimum; Max, maximum; NRMSE, normalized root mean square error; HRU, hydrologic response unit; GWR, groundwater reservoir] PRMS calibration data set Objective function NRMSE PRMS parameters used to calibrate model state Range Parameter description Comments Min Max Water budget Monthly mean Mean monthly Annual mean rain_adjust (nhru, nmonths) 0.6 1.4 Precipitation adjust factor for rain days (unitless) Calibrate the mean. snow_adjust (nhru, nmonths) 0.6 1.4 Precipitation adjust factor for snow days (unitless) Daily flow timing (all flows) Daily mean Monthly mean adjmix_rain (nmonths) 0.6 1.4 Factor to adjust rain proportion in mixed rain/snow event (unitless) Calibrate mean for spatial parameters. Weight the daily mean more than the monthly mean. slowcoeff_lin (nhru) 0.001 0.5 Linear coefficient in equation to route gravity-reservoir storage down slope for each HRU (1/day) soil_moist_max (nhru) 2 10 Maximum available water holding capacity of soil profile (inches) soil_rechr_max (nhru) 1.5 5 Maximum available water holding capacity for soil recharge zone (inches) Daily flow timing (all flows) Daily mean Monthly mean fastcoef_lin (nhru) 0.001 0.8 Coefficient to route preferential-flow storage down slope (1/day) Calibrate mean for spatial parameters. Weight the daily mean more than the monthly mean. Subdivide high flow. sat_threshold (nhru) 1 15 Water holding capacity of the grav- ity and preferential flow reser- voirs; 2x soil_moist_max initial value (inches) smidx_coef (nhru) 0.001 0.05 Coefficient in non-linear surface runoff contributing area algorithm (1/day) Daily flow timing (all flows) Daily mean Monthly mean gwflow_coef (ngw) 0.001 0.5 Linear coefficient to compute groundwater discharge from each GWR (1/day) Calibrate the mean for spatial parameters. Data subdivide: low flows soil2gw_max (nhru) 0 0.5 Maximum amount of capillary reser- voir excess routed directly to the GWR (inches) ssr2gw_rate (nssr) 0.05 0.8 Linear coefficient used to route water from the gravity reservoir to the GWR (1/day)
Simulation Methods and the GSFLOW Model 17

Table 4. Water budget components from the Precipitation-Runoff Modeling Systemonly simulation to generate the steady-state infiltration distribution for Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon 

Counties, Pennsylvania, water years 2000–06.

[in, inches; surface water and energy budgets simulated by using Precipitation-Runoff Modeling 

System (PRMS), Version 3.4050 2011-12-12; start time: 1996/10/01 00:00:00; end time: 2006/09/30 

00:00:00; sum of hydrologic response unit (HRU) areas: 228.7 square miles (mi2); active basin area: 

228.7 mi2; impervious basin area: 0.3 mi2; pervious basin area: 228.4 mi2]

Water 

year

Precipitation 

(in)

Evapotranspiration 

(in)

Storage 

(in)

Simulated runoff 

(in)

Measured runoff 

(in)

2000 35.008 26.786 1.314 10.782 10.935

2001 30.946 22.412 2.779 7.056 9.511

2002 39.666 28.228 4.139 10.057 10.437

2003 55.849 31.678 6.446 21.822 15.138

2004 58.629 34.406 6.434 24.191 20.548

2005 34.732 21.813 1.265 18.058 16.571

2006 44.189 27.586 3.338 14.502 11.891

Bellefonte Dolomite. However, for the purpose of simulating 

basin-wide surface-water and groundwater budgets, accounting for groundwater down to 900 ft below land surface is 

probably sufficient.

Lateral Boundaries

The lateral extent of the modeled area was defined with 

no-flow, general-head, and specified-head boundaries (fig. 10). 

No-flow (inactive) cells were placed around the perimeter of 

the modeled area except (1) along the topographic divide separating Halfmoon Creek (within the Spruce Creek Basin) from 

Warriors Mark Run (outside of the modeled area), which was 

simulated by the use of a general-head boundary, and (2) along 

the divide separating Cedar Run (within the Spring Creek 

Basin) from Penns Creek Basin (outside of the modeled area), 

which was simulated by the use of specified heads (fig. 10). 

No-flow cells prohibit transfer of water across Bald Eagle 

Mountain to the northwest and Tussey Mountain to the southeast. No-flow cells also follow topographic divides separating 

parts of study basin from Penns Creek, Little Fishing Creek, 

and Lick Run Basins to the east and northeast.

General-head cells allow a transfer of groundwater across 

the topographic divide between Halfmoon Creek and Warriors Mark Run Basins. The general-head boundary allows 

groundwater within the carbonate rocks to cross the divide and 

move out of the modeled area to discharge to Warriors Mark 

Run, owing to its lower elevation and the high permeability 

of the carbonate rocks. The magnitude of the flow is related to 

the difference between the elevation of the groundwater levels 

simulated by the model at the general-head boundary and the 

reference elevations assigned to Warriors Mark Run, and to the 

hydraulic conductance between those locations. Reference elevations were determined by temporarily extending the model 

grid to Warriors Mark Run, simulating groundwater flow paths 

from the divide to the creek, and assigning reference values 

to the divide equal to the elevation of the flow path terminus 

where groundwater discharged to Warriors Mark Run.

Specified-head cells separating parts of the Cedar Run 

and Penns Creek watersheds also allow the transfer of water 

across the divide in the carbonate-rock units. The direction 

and magnitude of flow depend upon the difference between 

the groundwater levels specified at the boundary and adjacent 

cells. Groundwater levels along the boundary of the Spring 

Creek model were set to the elevation of the water table 

mapped by Taylor (1997).

Infiltration from Precipitation

Groundwater recharge (water reaching the water table) 

is customarily assigned in the recharge (RCH) package of 

MODFLOW-NWT. In this study, however, the PRMS routine in GSFLOW computes infiltration through the root zone 

from each HRU, passing that flux to the UZF package of 

MODFLOW-NWT. Recharge to the water table is computed 

in the UZF package, so the RCH package is not needed. Thus, 

the MODFLOW-NWT model was constructed by assigning 

the spatially variable distribution of average infiltration to 

the UZF package that was derived from PRMS simulations 

for water years 2000–06. The infiltration rate from PRMS, 

simulated as the sum of PRMS vertical fluxes from the soil 

zone from each HRU (fig. 9), is assigned as infiltration to 
18 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

PRMS-derived infiltration, in inches per year

2.1–6.06.1–9.09.1–12.012.1–15.015.1–18.018.1–24.024.1–73.0

Scotia Barrens (Pennsylvania Game Commission, 2011)Municipal boundaryCounty boundaryLiving Filter project boundary (Lin, 2011)Surface-water divide

Nittany Creek 

Basin

Spring Creek 

Basin

Spruce Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Nittany 

Mountain

Bald Eagle Mountain

Tussey Mountain

Gatesburg Ridge

Nittany Valley

Penns Valley

Figure 9. Mean infiltration rate simulated by PRMS for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Simulation Methods and the GSFLOW Model 19

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

HUNTINGDON COUNTY

MIFFLIN COUNTY

CLINTON COUNTY

JUNIATA COUNTY

BLAIR COUNTY

CENTRE COUNTY

CLEARFIELD COUNTY

Spruce Creek

Warr oi Msr ark R nu

Halfmoon Creek

Penns Creek

Litt el F si h ni g

Creek Lick R nu

PENNSYLVANIA

Location of study area

77°30' 77°40' 77°50' 78°0' 78°10' 78°20'

40°50'40°40'

EXPLANATION

Model-cell type

ActiveNo flowGeneral-head boundarySpecific-head boundaryStreamflow routing

Municipal boundaryCounty boundarySurface-water divide

0 5 MILES

0 5 KILOMETERS

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Streams from U.S. Geological Survey, National Hydrography Dataset, 2009Shaded relief from PAMAP Program, 2008

Bald Eagle Mountain

Tussey Mountain

1

104

1

216

COLUMNS

ROWS

Figure 10. Finite-difference grid and boundaries of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
20 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

MODFLOW cells on the basis of the percentage of each 

HRU in the cell. The simulated rate of infiltration averaged 

14.7 in/yr for the study basin and ranged from about 2.1 to 

72.9 in/yr. 

The average infiltration rate for the study basin of 

14.7 in/yr is slightly less than the amount needed to provide 

the base flow computed at the USGS streamflow-gaging 

station 01547100 Spring Creek near Milesburg. During 

water years 2000–06, average base flow determined by the 

local minimum method of hydrograph separation (Pettyjohn 

and Henning, 1979) by use of the HYSEP program (Sloto 

and Crouse, 1996) was 196 cubic feet per second (ft3/sec). 

Recharge of 15.2 in/yr is needed to provide this base flow, 

assuming a contributing groundwater basin of 175 mi2. 

Streams

Streamflow is simulated by using the SFR2 package (Niswonger and Prudic, 2005), which allows streams to gain or 

lose water and accounts for the flow in each stream cell so that 

losses cannot exceed the simulated streamflow. Streams were 

represented in model layer 1 by 387 segments made up of 

3,444 reaches (fig. 10). Unsaturated flow beneath the streams 

was also simulated. The elevation of the top of the streambed 

in each SFR reach was assigned 3.28 ft (1 m) less than the 

nearest elevation derived from the USGS 10-meter DEM and 

was adjusted to insure that the streambed elevation always 

decreased downstream. Thickness of the streambed was set to 

3.28 ft for all stream segments. Stream width was varied by 

segment from 3.3 to 66 ft on the basis of 39 locations where 

the stream width was measured in the field. Hydraulic conductivity of the streambed was assigned an initial value of 2 feet 

per day (ft/d) on non-carbonate rocks and 13.1 ft/d on carbonate rocks, which are based on the approximate difference in 

the horizontal hydraulic conductivity between the non-carbonate and carbonate rocks; hydraulic conductivity was adjusted 

during model calibration. Stream slope for each segment was 

determined as the difference between the upstream and downstream elevations divided by segment length. Unsaturatedzone properties beneath all stream reaches were set to constant 

values. Vertical hydraulic conductivity was set equal to the 

vertical hydraulic conductivity of layer 1 beneath the stream, 

and the saturated water content was set equal to 0.03.

Water was added at the upstream end of stream segments 

in the SRF2 package to simulate major wastewater discharges 

to streams from municipalities and industry. Water was 

subtracted from SFR2 segments to simulate major withdrawals from streams for water supply and other uses. Simulated 

discharges to streams totaled about 20.5 ft3

/sec, and withdrawals from streams totaled about 9.4 ft3

/sec (fig. 11 and table 5). 

Wastewater discharge from PSU is a source of recharge. It was 

not included in the steady-state MODFLOW-NWT simulation 

but was included in the GSFLOW model. The average withdrawal from Roaring Run (fig. 2) of about 330,000 gallons per 

day by the State College Borough Water Authority (SCBWA) 

was not simulated because the withdrawal is from a reservoir 

that was not included in the model. Without the reservoir, 

simulated streamflow was insufficient to supply a withdrawal 

of this magnitude, so the model, as constructed for this study, 

is not designed to accurately simulate flow in Roaring Run.

Wells

Forty-five groundwater withdrawals were simulated by 

the steady-state model using the multi-node well (MNW) 

package (Halford and Hanson, 2002). Wells were used to 

represent groundwater withdrawals from 5 mines and 40 production wells totaling about 14.5 Mgal/d (fig. 12 and table 6). 

Wells were incorporated into the model to represent the mean 

withdrawals during water years 2000–06 from the PSU and 

SCBWA production wells, along with other water purveyors 

with withdrawals greater than 50 gallons per minute (gal/min). 

The average withdrawal rate was assigned to each well for the 

steady-state simulation, and the MNW package computed the 

contribution from each model layer on the basis of the transmissivity and groundwater levels in each layer.

Table 5. Discharges to streams and withdrawals from 

streams simulated by using the Streamflow-Routing Package 

of MODFLOW-NWT model for Spring Creek and Nittany Creek 

Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania.

[m3/d, cubic meters per day; Mgal/d, million gallons per day; ft3/s, cubic 

feet per second; WWTP, wastewater treatment plant; UAJA, University 

Area Joint Authority; TWP, township]

Identifier

Rate

m3

/d Mgal/d ft3/s

Wastewater discharges1

 to streams in model layer 1

Hanson Quarry 1,750 0.46 0.72

Corning 3,540 0.94 1.45

Bellefonte WWTP 10,721 2.83 4.38

Graymont Mine 14,030 3.71 5.73

UAJA WWTP 20,150 5.32 8.24

Total discharge 50,191 13.26 20.51

Withdrawals2

 from streams in model layer 1

Diamond Spring 78 0.02 0.03

Elks Country Club 414 0.11 0.17

College Twp Spring 1,075 0.28 0.44

Bellefonte Borough 21,450 5.67 8.77

Total withdrawal 23,017 6.08 9.41

1

Penn State University wastewater is discharged as a land application, so 

it is not included in the list of surface-water discharges.

2

The average withdrawal of 330,000 gallons per day from Roaring Run is 

not simulated in the model.
Simulation Methods and the GSFLOW Model 21

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Streams from U.S. Geological Survey, National Hydrography Dataset, 2009Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Study basinMunicipal boundaryCounty boundarySurface-water divideSurface-water withdrawal site

—

Upper number is site identifier. Lower number is withdrawal, in million gallons per day

Wastewater discharge site

—

Upper number is site identifier. Lower number is wastewater discharge, in million gallons per day

20.1160.94

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

Halfmoon C reek

Beaver Branch

Spruce Creek

Spring Creek

Nittany

Creek

Roaring

Run

Buf af ol Run

Logan Branch

Cedar Run

S al b

Cabin Run

Galbraith Gap Ru n

Roaring Run

20.11

30.28

50.46

95.32

45.67

83.71

72.83

60.94

10.02

Map 

identifier Site name

1 Diamond Spring2 Elks Country Club3 College Township Spring4 Bellefonte Borough5 Hanson Quarry6 Corning7 Bellefonte Wastewater Treatment Plant8 Graymont Mine9 University Area Joint Authority 

 Wastewater Treatment Plant

Figure 11. Location of surface-water withdrawals and discharges simulated by the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
22 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Streams from U.S. Geological Survey, National Hydrography Dataset, 2009 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Study basinMunicipal boundaryCounty boundarySurface-water divideGroundwater withdrawal site and identifier 

(identifiers are listed in table 6)

Mine dewatering, in million gallons per day

Less than or equal to 0.12Greater than 0.12 and less than or equal to 1.23Greater than 1.23 and less than 1.94

Production well, in million gallons per day

Less than 0.120.12 to 0.320.33 to 0.64Greater than 0.64

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN

COUNTY

Halfmoon C reek

Beaver Branch

Spruce Creek

Spring Creek

Nittany

Creek

Roaring

Run

Buf af ol Run

Logan Branch

Cedar Run

S al b

Cabin Run

Galbraith Gap Ru n

Roaring Run

1 3

2

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24 25

26

27

28 29

30 31

32

33

34

35

36

37

38

39

45

44

41

40

43

42

Figure 12. Location of groundwater withdrawals simulated by the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 23

Table 6. Groundwater withdrawals simulated using the MODFLOW-NWT steady-state model for Spring Creek and Nittany Creek 

Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.

[m3/d, cubic meters per day; Mgal/d, million gallons per day; ft3/s, cubic feet per second]

Map 

identifier Local name of well

Rate

m3

/d Mgal/d ft3/s

Pennsylvania State University

1 UN14 1,211 0.320 0.50

2 UN16 664 0.175 0.27

3 UN17 1,528 0.404 0.62

4 UN24 1,662 0.439 0.68

5 UN26 3,115 0.823 1.27

6 PS33 1,485 0.392 0.61

7 PS35 181 0.048 0.07

8 PS34 902 0.238 0.37

9 PS37 13 0.003 0.01

State College Borough Water Authority

10 SCBWA 07 75 0.020 0.03

11 SCBWA 08 75 0.020 0.03

12 SCBWA 11 75 0.020 0.03

13 SCBWA 14 43 0.011 0.02

14 SCBWA 17 1,041 0.275 0.43

15 SCBWA 19 1,041 0.275 0.43

16 SCBWA 22 1,671 0.441 0.68

17 SCBWA 24 141 0.037 0.06

18 SCBWA 25 5,492 1.451 2.24

19 SCBWA 41 1,027 0.271 0.42

20 SCBWA 43 890 0.235 0.36

21 SCBWA 53 890 0.235 0.36

22 SCBWA 55 30 0.008 0.01

23 SCBWA 57 814 0.215 0.33

24 SCBWA 62 55 0.015 0.02

25 SCBWA 63 100 0.026 0.04

26 SCBWA 64 2,411 0.637 0.99

27 SCBWA 65 42 0.011 0.02

28 SCBWA 71 166 0.044 0.07

29 SCBWA 73 267 0.070 0.11

30 SCBWA 78 178 0.047 0.07

31 SCBWA 79 6 0.002 0.00

Map 

identifier Local name of well

Rate

m3

/d Mgal/d ft3/s

Other water purveyors

32 CE162 Lemont 3,103 0.820 1.27

33 Centre Hall Well 8 81 0.022 0.03

34 Centre Hall Well 9 815 0.215 0.33

35 College Township 

Rogers Silo

773 0.204 0.32

36 College Township Spring 

Creek Well

3,468 0.916 1.42

37 Spring Township PW1 

Carles Well

852 0.225 0.35

38 Upper Halfmoon 

Township Well 5

542 0.143 0.22

39 Walker Township Zion 2 454 0.120 0.19

Industrial and mineral use

40 Gentzel Deep Mine 4,636 1.225 1.89

41 Gentzel Quarry 7,357 1.944 3.01

42 Hanson Oak Hall Quarry 288 0.076 0.12

43 Hawbaker Pleasant Gap 

White Rock Quarry

432 0.114 0.18

44 HRI—Curtin Gap 

Quarry

4,186 1.106 1.71

45 Spectrum Control RW-1 452 0.119 0.18

Total withdrawals 54,728 14.458 22.37
24 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Evapotranspiration 

Evapotranspiration from groundwater was simulated 

with the UZF package. A potential evapotranspiration rate of 

25.5 in/yr was used, as computed for State College by Waltman and others (1997, p. 217) using a modified Thornthwaite 

approach. An extinction depth of 6.56 ft (2 meters) below land 

surface was assumed. When linked to the GSFLOW model, 

the potential evapotranspiration (PET) from MODFLOWNWT is replaced by values for each HRU simulated by PRMS 

on the basis of daily climate conditions. Actual evapotranspiration is computed in GSFLOW as the sum of each component of evapotranspiration (ET) in the sequence (1) canopy 

storage evaporation, (2) sublimation, (3) impervious storage 

evaporation, (4) evapotranspiration in the PRMS soilzone, 

and (5) transpiration below the PRMS soilzone. The available 

evapotranspiration at each step is the unsatisfied PET not used 

by other ET components (canopy-storage evaporation, sublimation, impervious-storage evaporation, transpiration, and 

soil-storage evaporation) after the previous computation step.

Aquifer Properties

Aquifer properties were assigned by parameters in the 

Upstream Weighting (UPW) Package (Niswonger and others, 

2011) of the steady-state MODFLOW-NWT model to represent the horizontal hydraulic conductivity (K) along model 

rows, ratio of horizontal to vertical hydraulic conductivity 

(VANI), and ratio of hydraulic conductivity along model 

columns (dip direction of bedding) to hydraulic conductivity 

along model rows—the strike direction of bedding (HANI). 

Parameters for specific yield and specific storage also were 

specified for use in the transient GSFLOW model but were not 

needed for the MODFLOW-NWT steady-state simulation. 

Ten parameters were used in the UPW package to represent the spatial distribution of hydraulic conductivity for the 

geologic units in the study area. Six parameters represent the 

groups of geologic units shown in table 7 (at end of report). 

Groupings of geologic units with similar hydraulic conductivity were based on previous studies, particularly the geologic 

mapping (fig. 5) and relative rankings by Parizek and Filey 

(1982) in table 7. Two additional parameters were used to 

represent zones of high horizontal hydraulic conductivity not 

restricted to a particular geologic unit. The parameter Kfract 

represents a zone of high hydraulic conductivity along the 

western margin of carbonate rocks in the Spring Creek Valley, 

and ThomK represents a similar zone of hypothesized high 

hydraulic conductivity near Thompson Run. Horizontal and 

vertical anisotropy with respect to hydraulic conductivity were 

assigned by parameters HANIcarb and VANI. Initially, all 

sandstone and shale units were assigned a hydraulic conductivity of 0.66 ft/d (0.2 m/d), and carbonate rocks were assigned 

9.8 ft/d (3 m/d); then the values were adjusted during the 

model-calibration procedure.

Values of hydraulic conductivity for all parameters, 

except those representing the Mines Dolomite and upper sandy 

members of the Gatesburg Formation (Kgates), were assumed 

to decrease with depth because few high yielding fractures 

were reported below 300 feet, except in the Gatesburg Formation (Wood, 1980, p. 31). Values assigned to parameters in 

layer 1 were reduced in layer 2 by a factor of 0.2 for shale 

units and 0.7 for carbonate units. The hydraulic conductivity 

for all units in layer 3 was reduced by a factor of 0.04 from 

layer 1 values.

The ratio of horizontal to vertical hydraulic conductivity is specified by the parameter VANI, which was assigned 

a value of 1.0 for all geologic units in all layers. Its value 

was changed during model calibration only in a small area 

near Bellefonte, Pa., to control the quantity of groundwater 

discharge to the lower reaches of Logan Branch and Spring 

Creek. The ratio of hydraulic conductivity along model rows 

(along strike) to that along model columns (across strike) is 

specified by the parameter HANI. The value of HANI was 

adjusted during model calibration. Its value was computed as 

a base value multiplied by a factor ranging from 0.1 to 1.0 that 

represented the approximate effect of dipping beds on HANI. 

The factor was small for beds dipping steeply in the direction 

of model columns and was 1.0 for horizontal beds.

MODFLOW-Only Model Steady-State Calibration 

and Results

Aquifer properties in the MODFLOW-NWT model were 

adjusted by use of the parameter-estimation program UCODE2005 (Poeter and others, 2005) and by trial and error. Values 

of hydraulic conductivity and horizontal anisotropy were 

adjusted by trying to match observations of average, steadystate, groundwater levels and computed base flow during 

water years 2000–06. The sources of groundwater levels and 

base flows used as observations for model adjustments are 

given in table 8. Locations of the wells used for groundwaterlevel data are shown in figure 13, and locations of streamgages 

used for base-flow computations are shown in figure 4.

Groundwater-level observation data came from multiple 

sources. For wells with multiple groundwater levels, the mean 

value of the observations during water years 2000–06 was 

used. Groundwater levels from Taylor (1997) represent the 

largest synoptic dataset of groundwater levels, but they were 

measured during 1994, prior to the calibration period of this 

model. Regardless, because of its large areal coverage, the 

dataset was used, but the values were assigned a small weighting factor in the regression.

Base-flow observations used to adjust the model were 

either the base flow at a streamgage or the base-flow gain 

between two stations (table 9). Mean base-flow values for 

water years 2000–06 were computed for the local minimum 
Simulation Methods and the GSFLOW Model 25

Table 8. Sources of streamflow data and groundwater levels used to adjust aquifer properties in the MODFLOW-NWT 

steady-state model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and 

Huntingdon Counties, Pennsylvania.

[USGS, U.S. Geological Survey; USFWS, U.S. Fish and Wildlife Service; CWC, ClearWater Conservancy; SCBWA, State College Borough 

Water Authority; SRBC, Susquehanna River Basin Commission; NAWS, North American Water Systems; PSU, Penn State University; C, Continuous; D, Daily; CMP, Continuous with missing periods; P, Periodic; S, Single observations; --, none; X, missing record]

Streamgage name for stations used 

to compute base flow or source of 

groundwater levels

Station 

abbreviation Agency Type of observation

Missing 

record Period of record

Streamflow data

Spring Creek at Houserville, PA SPH USGS C -- 10/1999–09/2006

Spring Creek near Axemann, PA SPA USGS C -- 10/1999–09/2006

Spring Creek at Milesburg, PA SPM USGS C -- 10/1999–09/2006

Spruce Creek at Graysville, PA SPG USFWS and USGS D X 10/1999–09/2006

Slab Cabin Run at Rt. 26 at Lemont, PA 

(Slab Cabin Run Lower) SLL CWC C X 10/1999–09/2006

Cedar Run at Oak Hall, PA (Cedar Run 

Lower) CEL CWC C X 10/1999–09/2006

Spring Creek at Oak Hall, PA (Spring 

Creek Upper) SPU CWC C X 10/1999–09/2006

Buffalo Run near Bellefonte, PA (Buffalo 

Run Lower) BUL CWC C X 10/1999–09/2006

Logan Branch near Pleasant Gap, PA 

(Logan Branch Upper) LOU CWC C X 10/1999–09/2006

Logan Branch above Big Spring at 

Bellefonte, PA (Logan Branch Lower) LOL CWC C X 02/2005–09/2006

Thompson Run near State College, PA 

(Thompson Run Lower) THL PSU C -- 2007

Groundwater-level data

7 observation wells -- CWC and USGS CMP -- 10/1999–09/2006

33 domestic-supply wells -- NAWS P -- 01/1999–06/2005

9 unused supply wells -- SCBWA P -- 01/2003–12/2004

180 domestic-supply wells -- SRBC S -- October 1994

28 domestic-supply wells -- USGS S -- 1984
26 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Streams from U.S. Geological Survey, National Hydrography Dataset, 2009 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Study basinMunicipal boundaryCounty boundarySurface-water divideObservation well—Identifier indicates 

well with continuous recorder

Clearwater ConservancyState College Borough Water AuthorityU.S. Geological SurveyNorth American Water SystemsSusquehanna River Basin Commission

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN

COUNTY

CE 118 CE 686

"Big Hollow"

Nittany 1

Filmore 1

Centre Hall 1

Pine Grove 2

Figure 13. Locations of groundwater wells with groundwater-level data used for calibrating the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 27

Table 9. Base-flow values and weights used to adjust the MODFLOW-NWT steady-state model for 

Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon 

Counties, Pennsylvania.

[ft, feet; ft3/s, cubic feet per second]

Station name1 Station 

abbreviation

Base-flow value 

(ft3

/s)

Weighting value 

[ft/(ft3/s)]

Thompson Run Lower THL 4.9 55.7

Cedar Run Lower CEL 16.0 17.1

Logan Run Upper LOU 21.0 13.0

Spring Creek Upper SPU 19.0 14.4

Logan Run Lower LOL 76.0 3.6

Slab Cabin Run Lower SLL 10.7 25.5

Spruce Creek at Graysville, PA SPG 38.0 5.4

Spring Creek at Houserville, PA SPH 55.3 11.9

Buffalo Run Lower BUL 14.0 19.5

Spring Creek—gain between Houserville and 

Axemann streamgages AXE_GAIN 32.0 6.4

Logan Branch—between Upper and Lower 

streamgages LOL_GAIN 55.0 2.7

Spring Creek—gain in main stem below streamgages 

near Axemann, PA and at Milesburg, PA MILES_GAIN 20.0 10.3

1

Slab Cabin Run Upper (SLU) and Buffalo Run Upper (BUU) were not included because of the uncertainty in the datasets.

method (Pettyjohn and Henning, 1979) by use of the HYSEP 

program (Sloto and Crouse, 1996). Because the streamflow 

record was not complete for some of the stations, daily 

streamflow values were estimated by using MISTE (MIssing 

STreamflow Estimation), a tool for estimating missing daily 

discharge values that relies on daily values that have been 

determined for other (index) sites (U.S. Geological Survey, 

2003). The program uses stepwise regression analysis to correlate daily discharge data at the study site with daily discharge 

data from one or more index sites.

Weighting of Observations

A weighting factor was used in the parameter-estimation 

program UCODE to adjust observations to account for the 

difference in units between groundwater levels and base flows, 

and to incorporate differences in the perceived accuracy of 

observations. Weights for groundwater levels were assigned 

different values depending upon the accuracy of the measuring 

point elevation and frequency of groundwater-level observations. Groundwater levels measured once were assigned a 

lower weight than groundwater levels measured continuously and recorded for the study period. Weights assigned to 

groundwater levels ranged from 0.69 feet for discrete observations at 180 domestic-supply wells to 4.3 feet for the average 

groundwater level for the study period at seven observation 

wells with continuous recorders. Weights for base-flow observations relied on the estimated accuracy of the streamflow 

record compiled by USGS, CWC, and others, as described in 

Hill and Tiedeman (2007, p. 291). All base-flow weights were 

then increased by a factor of 100 to yield weighted residuals 

that were in the same range as the groundwater-level residuals. The greater weights increased the weighted residuals and 

importance of the base-flow observations in the parameter 

estimation process. The base-flow weighting values are shown 

in table 9.
28 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Adjusted Aquifer Properties

Aquifer properties were adjusted during model calibration, and the final values for each parameter are given in 

table 10. Horizontal hydraulic conductivity zones for layers 

1 to 3 are illustrated in figures 14, 15, and 16, and values 

for areas of horizontal anisotropy applicable to all layers 

are shown in figure 17. The storage properties of the aquifer—specific storage and specific yield—were not adjusted 

during steady-state MODFLOW-NWT simulations because 

those parameters were not applicable. Storage properties were 

needed for transient GSFLOW simulations, so they were 

adjusted from their initial assigned values during GSFLOW 

calibration. Specific storage was initially assigned a value of 

0.3048E-6 per foot (1.0 E-6 per meter) in all layers; the initial 

distribution of specific-yield values is illustrated in figures 18 

and 19.

During model calibration, results were not sensitive 

(composite scaled sensitivity of less than 0.4) to the ratio of 

horizontal to vertical hydraulic conductivity (VANI) so its 

value was set to 1.0 and was only adjusted beneath the lower 

reaches of Spring Creek and Logan Run. For example, beneath 

Spring Creek, VANI was increased to 100; and beneath Logan 

Run, it was decreased to 0.01 in order to increase the groundwater discharge to Logan Run in a reach of known large 

stream gains and to reduce the simulated discharge to Spring 

Creek near Houserville.

To simulate the groundwater trough along the northwestern side of the study basin, a zone of large hydraulic conductivity was inserted into layer 2 (parameter Kfract in table 10 

and fig. 15) to depress simulated groundwater levels in the 

area of the trough. The axis of the trough trends northeastsouthwest, parallel to model rows. Formation of the trough has 

been explained as an expression of a zone of large hydraulic 

conductivity associated with the Birmingham Thrust Fault 

(Siddiqui, 1969, p. 404; Parizek and others, 1971, p. 36). The 

water-table configuration simulated by the model is shown 

in figure 20. An area of large hydraulic conductivity was also 

simulated in layer 1 near Thompson Run (parameter ThomK 

in table 10 and fig. 14) to allow the groundwater flow to discharge to that stream in quantities indicated by the streamgage.

Spatial differences in hydraulic conductivity with depth 

are illustrated in a section across the study basin along model 

column 119 at the nose of Nittany Mountain (fig. 21). Note the 

decrease in hydraulic conductivity with depth and the large 

hydraulic conductivity cell in layer 2 representing the highly 

permeable fracture (shown in red).

Comparison of Simulated and Observed Steady-State 

Groundwater Levels and Base Flows

Steady-state groundwater levels simulated by the model 

were compared to groundwater levels from 257 wells (fig. 22). 

There is considerable scatter associated with the data. The 

root mean squared difference (RMSD) is 43 ft, and RMSD 

normalized to the range of groundwater-level observations is 

5.1 percent. About 72 percent of the simulated groundwater 

Table 10. Adjusted values and sensitivities of parameters used 

in the steady-state MODFLOW-NWT model for Spring Creek 

and Nittany Creek Basins and Parts of the Spruce Creek Basin, 

Centre and Huntingdon Counties, Pennsylvania.

Formation 

parameter name

Adjusted value of 

horizontal hydraulic 

conductivity along rows 

(feet/day)

Composite scaled 

sensitivity

Kshale 0.26 1.2

Kcarb 9.0 1.8

Kgates 61.0 2.4

Knit 33.1 1.9

Kfract 1,417 3.9

Kaxe_ston 3.4 0.69

ThomK 1,247 1.9

LemontK 60.4 1.6

Formation 

parameter name

Adjusted ratio of 

horizontal anisotropy along 

columns to along rows 

(dimensionless)

Composite scaled 

sensitivity

HANIcarb 0.33 2.78

levels were within 40 ft of the observed values. However, for 

the wells with surveyed measuring points [levels provided by 

Spring Creek Watershed Community (SCWC), N.A. Water 

Systems, and SCBWA], 96 percent of the residuals were less 

than 40 ft. Simulated groundwater levels were lower than 

observed levels at 39 percent of the observations, so the model 

more frequently simulated levels that were too high rather than 

too low. Simulated groundwater levels with the greatest error 

tended to be those for single groundwater-level observations 

from the Susquehanna River Basin Commission (SRBC) and 

USGS sources. Those observations were historical in nature 

made many years prior to the simulation period and, for the 

SRBC sources, were only approximately located on the land 

surface. For these reasons, observations from those datasets 

were assigned low weights in the overall parameter estimation. 

Steady-state base-flow values simulated by the model 

were compared to base flows computed from streamflow at 

16 sites in figure 23. Three of the 16 base-flow values were 

for the base-flow gains between streamgages on Spring 

Creek. The simulated values are biased low and are less than 

the base-flow estimates at most locations. The low values of 

simulated base flow were caused by an insufficient application of infiltration to the unsaturated zone (UZF package) of 

the MODFLOW-NWT model. Because the infiltration was 

derived from output of the calibrated PRMS model, infiltration values were not increased in MODFLOW. The infiltration 

rates were estimated independently during calibration of the 

coupled GSFLOW model.
Simulation Methods and the GSFLOW Model 29

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Horizontal hydraulic-conductivity 

zone parameters—Layer 1

KshaleKaxe_stonKcarbKnitKgatesKlemontThomK

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 14. Horizontal hydraulic conductivity zone parameters for layer 1 of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
30 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Horizontal hydraulic-conductivity 

zone parameters—Layer 2

KshaleKaxe_stonKcarbKnitKgatesKlemontKfract

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 15. Horizontal hydraulic conductivity zone parameters for layer 2 of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 31

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Horizontal hydraulic-conductivity 

zone parameters—Layer 3

KsilKaxe_stonKcarbKnitKgatesKlemont

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 16. Horizontal hydraulic conductivity zone parameters for layer 3 of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
32 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Area of horizontal anisotropy, dimensionless 

(ratio of hydraulic conductivity along columns to along rows)

0.095–0.0810.08–0.0660.067–0.0450.046–0.0320.033–0.0150.016–0.010

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 17. Areas of horizontal anisotropy in layer 1 of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 33

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Area of specific yield, dimensionless—

Layers 1 and 2

0.0050.010.02

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 18. Areas of specific yield in layers 1 and 2 of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
34 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Area of specific yield, dimensionless

—

Layer 3

0.0050.010.02

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 19. Areas of specific yield in layer 3 of the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
Simulation Methods and the GSFLOW Model 35

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Water-table elevation, in feet below North 

American Vertical Datum of 1988

671–850851–950951–1,0501,051–1,1501,151–1,2501,251–1,3501,351–1,4501,451–1,5501,551–1,6501,651–2,152

Municipal boundaryCounty boundarySurface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Halfmo

o

n

C reek Beaver Branch

Spruce

Creek

Spring

Creek

Ni

ttany Creek

Roaring

Run

Buf af ol Run

Logan

B

r

a

nch

Cedar Run

S al b

Cabin

Run Galbraith Gap Ru n

Roaring Run

Figure 20. Water-table elevations simulated by the MODFLOW-NWT model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.
36 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Spring Creek

Valley Penns

Valley

Nittany

Mountain

Bald Eagle

Mountain

Tussey

Mountain

Model

layer

1

2

3

Hydraulic conductivity, in feet per day

0.025 0.37 5.4 80 1,178

EXPLANATION

Figure 21. Hydraulic conductivities through column 119 of the MODFLOW-NWT model for Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.

600

800

1,000

1,200

1,400

1,600

A

-200

-150

-100

-50

0

50

100

150

200

600 700 800 900 1,000 1,100 1,200 1,300 1,400 1,500 1,600 1,800

B

Observed water-level elevation, in feet above NAVD 88

Simulated water-level elevation,

in feet above NAVD 88 

Difference between simulated and observed

water-level elevation, in feet above NAVD 88

Source of water-level data

Susquehanna River Basin Commision

U.S. Geological Survey

State College Borough Water Authority

North American Water Systems

Spring Creek Watershed Community

EXPLANATION

Figure 22. Observed steady-state groundwater levels in relation to A, MODFLOW-NWT simulated elevations and B, the 

difference between simulated and observed elevations for 257 wells in Spring Creek and Nittany Creek Basins and parts of the 

Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.)
Calibration of the Hydrologic Model and Simulation Results 37

0

20

40

60

80

100

120

140

160

180

200

Base flow, in cubic feet per second

Site identifier

THL

CEL

LOU

SLU

SPU

BUU

LOL

SLL

SPG

SPH

BUL

SPA

SPM

SPA_GAIN

LOL_GAIN

SPM_GAIN

Observed

Simulated

Simulated, but not used 

for parameter estimation

EXPLANATION

Figure 23. Observed steady-state base flows and steady-state base flows simulated by the MODFLOW-NWT model at 

16 sites in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, 

Pennsylvania, water years 2000–06. See tables 8 and 9 for description of site identifiers.

Calibration of the Hydrologic Model 

and Simulation Results

Surface-water and groundwater budgets were calculated 

for daily time steps in GSFLOW. Results from the GSFLOW 

model are reported for water years 2000–06, but the simulation period included the 3-years prior to water year 2000, 

which provided time for the simulated hydraulic system 

to adjust from the initial conditions that were assigned to 

the model. 

Calibration of the GSFLOW model was conducted in 

three steps. First, the parameters of the PRMS rainfall-runoff 

model and MODFLOW-NWT groundwater models were 

adjusted separately as stand-alone simulations that were not 

linked. Next, the simulated infiltration rates from PRMS were 

added to the MODFLOW-NWT model, and the MODFLOWNWT input parameters were readjusted using professional 

judgement and UCODE. Finally, after these preliminary 

adjustments, the separate models were linked in the GSFLOW 

code, and the parameters of both models were adjusted 

together for a final calibration by using the parameter estimation software PEST (Doherty, 2010, see http://pesthomepage.

org for description of PEST). Hydrologic processes were 

computed in a fixed computation sequence that included four 

procedures as described in Markstrom and others (2008). 

Parameter Estimation with PEST

In addition to issues of parameter insensitivity and correlation of observation data for constraining a coupled model, 

there are also concerns with measurement noise and redundant 

information in the large number of transient observations used 

to calibrate the fully coupled model. This is a primary concern 

here because areal surface-water data sets typically include 

many observations, especially with respect to the temporal 

density of the observations in a spatially distributed network; 

many of these data carry redundant insight into the system, but 

each contributes to the measurement noise that is encountered 

during calibration. In order to enhance the signal-to-noise ratio 

within the observation data, a time-series processing approach 

to the time-series observations was employed whereby the raw 

observations were processed and distilled into characteristic 

aspects of the system (Westenbroek and others, 2012). The 

simulated GSFLOW output was then processed in the same 

way as the raw observations and compared directly in the 

parameter estimation process. The processing was performed 

by using the Time-Series Processor TSPROC (Doherty, 2008; 
38 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Westenbroek and others, 2012). TSPROC has the ability 

to read and process native PRMS and MODFLOW-NWT 

output files. 

Calibration Approach

Whereas the sequentially linked calibration used representative steady-state water levels for groundwater model 

calibration, the fully coupled model is transient on a daily time 

step. Therefore, the time-series groundwater-levels measured 

during the study were included in the coupled model. In 

addition to the daily or intermittent values, the annual mean 

groundwater-levels were used for calibration.

In addition to the groundwater-level data, the following 

streamflow calibration targets were processed:

1. Log of daily streamflow—the natural log of daily 

streamflow was used to mitigate the undue influence of 

extremely high daily discharges on the calibration,

2. Annual mean streamflow—the average streamflow 

for each water year during the simulation period and 

represents the streamflow portion of the annual hydrologic budget,

3. Monthly mean streamflow—the average streamflow 

for each month during the simulation and represents the 

total monthly volume of streamflow,

4. Mean monthly streamflow—the streamflow for each 

month averaged over the entire simulation period and 

represents the seasonal variation of streamflow, and 

5. Monthly mean base flow—the average base flow for 

each month during the simulation and represents the 

groundwater contribution to streamflow.

Because forward run times for the fully coupled model 

were multiple hours long, all parameters varied during the 

separate PRMS and MODFLOW-NWT calibrations could not 

be evaluated in the fully coupled model calibration. Thus, it is 

possible that undesirable artifacts from the uncoupled model 

calibration may not be completely addressed in the coupled 

model calibration. After some initial calibration tests, the final 

fully coupled model was calibrated by using the selected set of 

parameters listed in table 11. Some parameters not available 

in the steady-state MODFLOW-NWT-only calibration (for 

example, aquifer storage), parameters important for simulating 

the interface between the MODFLOW-NWT only and PRMSonly models, and parameters having utility for calibration of 

the coupled model (for example, vertical hydraulic conductivity of the unsaturated zone) were included. All but three of the 

parameters were used for final calibration control routing of 

infiltrated water through the subsurface; those three parameters affect precipitation storage in the form of snow. The fully 

coupled model calibration used singular value decomposition 

on the entire base parameter set. Of these parameters, the 

information content of the multi-objective function observation data supported approximately linear combinations of the 

base parameters (singular values) by using stability criteria (PEST variable EIGTHRESH= 5.0 x 107

) proposed by 

Doherty and Hunt (2010).

Observation Weights

In general, an estimate of uncertainty in the streamflow 

observations was used as a starting point for the weights for 

each observation group. The weight (wg )was assigned to be 

the reciprocal of the uncertainty for each group (σg); thus,

wg = 1

σ g

 .

The uncertainties were estimated by using the coefficient 

of variation (standard deviation divided by the mean) and an 

average value for each observation group. Thus, the weight is 

estimated as

wg = 1

μgCVg

 ,

where μg and CVg are the mean and coefficient of variation of the observations for the group, respectively. For a 

log-transformed normally distributed variable, the standard 

deviation in log space was determined by rearranging the 

equations relating log-space (y) moments to real-space (x) 

moments (Miller and Freund, 1977):

σ y = log(1+ CVx

2

) .

Because the groups contained observations at different 

time scales, there was a considerable difference in the number 

of observations within each group and from station to station. 

To compensate for the number of observations, the weights 

were adjusted to represent an equivalent number of annual, 

monthly, and mean monthly observations. This reasoning follows from the basic identity that the standard deviation of the 

mean (m) from a random sample of size n is given by

σ m = σ g

n .

Because the weights are equal to the inverse of the standard deviation, the weight for a mean statistic becomes

wm = 1

σ m

= n

σ g

= wg n .

The UCODE weights for the steady-state MODFLOWNWT calibration were used as a starting point for the weights 

for the fully coupled calibration. On the basis of experimentation, it was determined that the initial weights for the waterlevel data needed to be adjusted to assure that the water-level 

data were seen in the resulting objective function. The final 

non-zero weights used for the water-level data are given in 

table 12. The water-level data for well CE118 (fig. 13) was set 

to 0.0 in the final calibration, which removes the observation 
Calibration of the Hydrologic Model and Simulation Results 39

Table 11. Parameters used in the calibration of the coupled model for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.[PRMS, Precipitation-Runoff Modeling System]

PRMS 

parameter name Description of PRMS parameter Default value Lower bound Upper bound Average calibrated parameter Minimum calibrated parameter Maximum calibrated parameter

adjmix_rain Adjustment factor for rain in a rain/snow mix (decimal fraction) 1 0 3 1.0644 0.6 1.4fastcoef_lin Linear preferential-flow routing coefficient (1/day) 0.1 0 1 0.0107 0.0004 0.2452rain_sub_adj Rain adjustment factor for each subbasin and each month (decimal fraction) 1 0 4 1.0571 0.8 1.2sat_threshold Soil saturation threshold, above field-capacity threshold (inches) 999 1 999 5.3311 1 14.9996slowcoef_lin Linear gravity-flow reservoir routing coefficient (1/day) 0.015 0 1 0.012 0.001 0.1395smidx_coef Coefficient in contributing area computations (decimal fraction) 0.01 0.0001 1 0.0049 0.0007 0.0263snow_sub_adj Snow adjustment factor for each subbasin and each month (decimal fraction) 1 0 4 1.1 1 1.2soil_moist_max Maximum value of water for soil zone (inches) 6 0 20 5.8882 2 10soil_rechr_max Maximum value for soil recharge zone (inches) 2 0 10 1.1107 0.7018 1.4658soil2gw_max Maximum value for soil water excess to groundwater (inches) 0 0 5 0.0683 0 0.5ssr2gw_rate Coefficient to route water from subsurface to groundwater (1/day) 0.1 0 1 0.1159 0.05 0.7364

MODFLOW 

parameter name 

or multiplier

Description of MODFLOW parameter or multiplier Calibrated value

Kshale

Horizontal hydrologic conductivity value along rows in model layer 1 for geologic units assigned to each parameter in table 7 (feet/day)

0.07887

Kcarb 10.52024Kgates 75.41289Knit 27.29462Kfract 2,430.89140Kaxe_ston 11.31514ThomK 1,493.16273

LemontK 60.71178HANIcarb Ratio of horizontal hydraulic conductivity along columns to along rows for carbonate-rock aquifers in table 7 (dimensionless) 0.10305vani Ratio of horizontal to vertical hydraulic conductivity (dimensionless) 0.70002sycarb Specific yield of all carbonate-rock units in table 7 except the upper sandy and Mines Members of the Gatesburg Formation (dimensionless) 0.01287sygates Specific yield of the upper sandy and Mines Members of the Gatesburg Formation in table 7 (dimensionless) 0.18254sysil Specific yield of siliciclastic aquifers in table 7 (dimensionless) 0.05000surfdep Average undulation depth of the soil-zone base elevation (feet) 2.12507kmult_lay2 Multiplication factors for multiplier arrays of hydraulic conductivity values in layers 2 and 3 (dimensionless) 2.00000 kmult_lay3 1.67659vani_mult Multiplication factor for multiplier array of the ratio of horizontal to vertical hydraulic conductivity (dimensionless) 1.56919hani_mult Multiplication factor for multiplier array for the ratio of horizontal hydraulic conductivity along columns to along rows (dimensionless) 0.76602ss1

Specific storage multiplier for arrays of specific-storage values for all parts of layers 1, 2, and 3 (dimensionless)

0.89850

ss2 0.80800ss3 1.12800vksat Multiplier for the array of saturated vertical hydraulic conductivity in the unsaturated zone (dimensionless) 0.94377
40 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

from consideration for the evaluation of model fit because 

it was causing instability in estimation of parameters; therefore, well CE118 is not listed in table 12. The final non-zero 

weights for the streamflow data are given in table 13. The 

streamflow data for upper Buffalo Run (BUL) and Thompson 

Run (THL) were zeroed out in the final calibration and, thus, 

are not shown in table 13.

Comparison of Simulated Results with 

Observations

Simulated values of streamflow and groundwater 

levels from GSFLOW are compared to observations from 

11 streamgages and 16 wells for water years 2000–06 in 

figures A1–A14 (see appendix). Model results are compared 

to calibration targets of the natural log of daily mean streamflow (fig. A1); annual streamflow, monthly mean streamflow, 

monthly mean base flow, and mean monthly streamflow 

(figs. A2–A12); and groundwater levels and annual mean 

groundwater-levels in observation wells (figs. A13–A14).

Simulated and observed results are compared graphically 

and with the Nash-Sutcliffe coefficient. The Nash-Sutcliffe 

coefficient is a widely used statistic to describe the accuracy of 

model simulations (Nash and Sutcliffe, 1970). The coefficient 

can vary between 1 and negative infinity, with 1 representing 

a perfect correspondence between simulated and observed 

values and values less than zero indicating that the mean 

value of the observations is a better predictor than the model 

results. Although the Nash-Sutcliffe coefficient has biases that 

affect the magnitude of the statistic, Moriasi and others (2007) 

consider a coefficient of greater than 0.5 to be one indicator of 

acceptable model performance.

Streamflow and Base Flow

Comparison of model simulations to observed values for 

the calibration targets indicates the calibrated GSFLOW model 

represents an acceptable simulation of streamflow and base 

flow for most sites, as shown graphically (figs. A1–A12) and 

by Nash-Sutcliffe coefficients that were generally greater than 

0.5. The exception was for the site Buffalo Run Lower where 

Nash-Sutcliffe coefficients were less than 0.5 for most of the 

calibration targets. The Nash-Sutcliffe coefficients for the 

streamflow and base-flow calibration targets are summarized 

in table 14. 

Simulations of daily streamflow are considered acceptable for most sites (fig. A1). The model fit to daily streamflow 

was best (Nash-Sutcliffe coefficients of 0.70 and 0.73) for 

Spring Creek at Milesburg and for Spruce Creek at Graysville, 

respectively. Those two streamgages measured the major outflows from the study basin. Overall, simulation of streamflow 

appeared to be poorest for Buffalo Run Lower as indicated 

by Nash-Sutcliffe coefficients of less than 0.5 for most of the 

calibration targets. Simulation of low streamflows at Buffalo 

Run Lower seemed most problematic, which was also was 

the case at Logan Run Upper, and at the two streamgages 

on Slab Cabin Run. These streams have substantial losing 

reaches during low-flow periods, which probably are not being 

adequately simulated.

The simulations of annual mean streamflow (graph A 

in figs. A2–A12) are considered acceptable (Nash-Sutcliffe 

coefficients 0.65 to 0.96), except at the gaging station Buffalo 

Run Lower (Nash-Sutcliffe coefficient 0.41). During wet years 

of high streamflow, the model slightly overestimated annual 

streamflow at most sites, and during dry years streamflow generally was slightly underestimated. However, at streamgages 

Spring Creek Upper and Spruce Creek at Graysville, simulations underestimated annual streamflow for all years except 

for water year 2006, the last year of the simulation. The 

underestimates of streamflow at these sites are probably 

caused by simulating too much groundwater underflow from 

these subbains.

Table 12. Final weights used for groundwater-level data in 

the PEST transient calibration of the coupled model for Spring 

Creek and Nittany Creek Basins and parts of the Spruce Creek 

Basin, Centre and Huntingdon Counties, Pennsylvania.

Well name Weight1

Continuous groundwater levels

BIG HOLLOW 2.60

CE686 1.25

CENTRE HALL 2.60

FILMORE1 2.60

NITTANY 2.60

PINEGROVE 2 1.25

Intermittent groundwater levels

CE 446 1.25

CE 690 1.25

CE 691 1.25

CE 692 1.25

CE 693 1.25

CE 694 1.25

CE 695 1.25

CE 696 1.25

CE 697 2.60

CE 698 2.60

Annual mean groundwater levels

CE 446 1.25

CE 690 1.25

CE 691 1.25

CE 692 1.25

CE 693 1.25

CE 694 1.25

1

Higher weight equals higher importance in the parameter estimation.
Calibration of the Hydrologic Model and Simulation Results 41

Table 13. Final weights used for the streamflow data in the PEST transient calibration of the coupled model for Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.

Stream name Weight

Log of daily streamflow

Buffalo Run lower 2.53811

Cedar Run lower 2.53811

Logan Run lower 2.53811

Logan Run upper 2.53811

Slab Cabin Run lower 2.53811

Slab Cabin Run upper 2.53811

Spring Creek at Houserville 6.07350

Spring Creek at Milesburg 6.07350

Spring Creek near Axemann 6.07350

Spring Creek upper 2.53811

Spruce Creek at Graysville 1.90883

Annual average streamflow

Buffalo Run lower 22.75233

Cedar Run lower 19.90946

Logan Run lower 4.19137

Logan Run upper 15.16822

Slab Cabin Run lower 29.69118

Slab Cabin Run upper 0.00093

Spring Creek at Houserville 13.84030

Spring Creek at Milesburg 0.00012

Spring Creek near Axemann 0.00028

Spring Creek upper 16.76733

Spruce Creek at Graysville 6.28705

Monthly average streamflow

Buffalo Run lower 6.56803

Cedar Run lower 5.74736

Logan Run lower 1.20994

Logan Run upper 4.37869

Slab Cabin Run lower 8.57111

Slab Cabin Run upper 0.00027

Spring Creek at Houserville 3.99535

Spring Creek at Milesburg 0.00004

Spring Creek near Axemann 0.00008

Spring Creek upper 4.84031

Spruce Creek at Graysville 1.81491

Stream name Weight

Monthly average base flow

Buffalo Run lower 6.56803

Cedar Run lower 5.74736

Logan Run lower 1.20994

Logan Run upper 4.37869

Slab Cabin Run lower 8.57111

Slab Cabin Run upper 0.00027

Spring Creek at Houserville 3.99535

Spring Creek at Milesburg 0.00004

Spring Creek near Axemann 0.00008

Spring Creek upper 4.84031

Spruce Creek at Graysville 1.81491

Mean monthly streamflow

Buffalo Run lower 17.37738

Cedar Run lower 15.20610

Logan Run lower 3.20121

Logan Run upper 11.58492

Slab Cabin Run lower 22.67701

Slab Cabin Run upper 0.00071

Spring Creek at Houserville 10.57070

Spring Creek at Milesburg 0.00009

Spring Creek near Axemann 0.00021

Spring Creek upper 12.80626

Spruce Creek at Graysville 4.80181
42 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

The simulations of monthly mean streamflow (graph B 

in figs. A2–A12) are acceptable (Nash-Sutcliffe coefficients 

0.75 to 0.92), except at Buffalo Run Lower. The timing and 

magnitude of the simulated monthly mean streamflow corresponds reasonably well to the observed streamflow. During dry periods of low flow, the fit of simulated to observed 

monthly mean streamflow was better than the fit to daily flow 

for the same periods. 

Mean monthly streamflow is the mean of all monthly 

mean flows for a particular month for the 7-year simulation 

period. Simulated values of mean monthly streamflow are 

shown in graph D (figs. A2–A12), where observation numbers 

1 through 12 correspond to months of the calendar year. Comparisons of simulated and observed mean monthly flow highlight the seasonal errors in flow that are difficult to notice in 

graphs of daily or monthly mean streamflow. At seven of the 

streamgages, simulations underestimated high flows in March 

and April. At four streamgages, the Nash-Sutcliffe coefficient 

for mean monthly streamflow was less than 0.5 (table 14); 

at those sites streamflow tended to be underestimated during 

most months. 

Similar to the monthly means, simulations of monthly 

mean base flows (graph C in figs. A2–A12) are considered 

acceptable (Nash-Sutcliffe coefficients 0.54 to 0.87) except at 

the gaging station Buffalo Run Lower (Nash-Sutcliffe coefficient 0.38). The magnitude of simulated base-flow fluctuations is less than was observed at many of the streamgages 

during water years 2000–02, yet the timing of the highs and 

lows is correct (observations 1 through 36 on graph C in 

figs. A2–A12). For 2003–06, both the timing and magnitude 

of the simulated and observed streamflow is predictive and 

consistent with monthly mean streamflows.

Groundwater Levels

Transient groundwater levels, a lesser objective of 

calibration, were not well simulated as shown graphically and 

by Nash-Sutcliffe coefficients of less than zero at most of the 

16 wells (figs. A13 and A14). The timing and magnitude of 

simulated groundwater-level fluctuations can only be considered acceptable for three wells used for calibration—CE 686, 

Centre Hall, and CE 692. At some wells, the magnitude of the 

simulated fluctuation was reasonable, but the starting elevation 

was too high or low (CE 691 and CE 697); at some wells the 

simulated elevation was reasonable, but the fluctuations were 

not adequately simulated (Nittany, CE 693, and CE 695); and 

at some wells, neither the simulated fluctuations nor elevation 

were acceptable (CE 698, Pine Grove 2). 

The poor fit of the model to observed groundwater levels 

relative to the fit of the model to observed streamflows was not 

unexpected because the zone-based parameterization of the 

GSFLOW model is best suited for predictions that integrate 

large areas of the model domain (such as flux targets), whereas 

water-level targets are more local. Therefore, the structural 

error associated with the simplified zone-based model makes 

it difficult to accurately simulate of groundwater levels in the 

complex geologic setting of the study area.

Table 14. Nash-Sutcliffe coefficients of model efficiency for streamflow calibration targets in the Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water 

years 2000–06.

[--, not available]

Streamgage site

Calibration target and Nash-Sutcliff coefficient

Natural log of 

daily streamflow

Annual mean 

streamflow

Monthly mean 

streamflow

Monthly mean 

base flow

Mean monthly 

streamflow

Spring Creek near Axemann, PA 0.65 0.93 0.92 0.87 0.84

Buffalo Run Lower 0.47 0.41 0.48 0.28 0.38

Cedar Run Lower 0.61 0.65 0.75 0.75 0.55

Spring Creek at Houserville, PA 0.58 0.90 0.91 0.82 0.83

Logan Branch Lower 0.58 -- 0.78 0.74 0.54

Logan Branch Upper 0.52 0.71 0.75 0.64 0.73

Spring Creek at Milesburg 0.70 0.91 0.90 0.86 0.79

Spring Creek Upper 0.53 0.72 0.75 0.54 0.23

Spruce Creek at Graysville, PA 0.73 0.89 0.82 0.85 -0.16

Slab Cabin Run Lower 0.55 0.81 0.83 0.80 0.71

Slab Cabin Run Upper 0.50 0.96 0.84 0.74 0.35
Water Budget and Groundwater Recharge 43

Water Budget and Groundwater 

Recharge

The principal objectives of this project were to simulate 

the water budget and estimate the distribution of groundwater 

recharge for the study basin for water years 2000–06 by the 

use of GSFLOW. GSFLOW provides two output files that can 

be used to view water budgets for the major components of 

the hydrologic cycle. An example of the annual water budgets 

compiled from the main GSFLOW output file (gsflow_FINALBUDGET.out) is presented in table 15. Values of other 

water-budget terms are provided in the GSFLOW CommaSeparated Values (CSV) output file (gsflow.csv). These 

output files are available from the GSFLOW model archive, 

available upon request from the USGS Pennsylvania Water 

Science Center.

Basin Water Budget

Water budgets simulated by GSFLOW for water years 

2000–06 are summarized in table 15 as volumes of inflow, 

outflow, and storage change for each water year and for the 

entire period. The largest source and sink terms are represented by precipitation and evapotranspiration, respectively. 

For water years 2000–06, precipitation was more than 99 

percent of all inflow; evapotranspiration and streamflow 

together accounted for 98 percent of all outflow. Withdrawals 

by wells and quarries represented about 2 percent of the outflow. For water years 2000–06, total inflows of water exceeded 

total outflows by about 1.7 billion cubic feet (0.47 inches per 

year over the basin area), so the simulated total storage of 

water in the basin increased by an amount about equal to the 

surplus inflow. Contributions from precipitation, streamflow 

gains, inter-basin groundwater exchanges such as those from 

the northeast and southwest portions of the study basin, and 

well withdrawals/injection exceeded those outflows from 

evapotranspiration, streamflow losses, inter-basin losses, and 

well withdrawals/injection. 

The major terms of the water budget simulated by 

GSFLOW are shown for water years 2000–06 in figure 24. 

Water year 2001 was the driest year during the study period 

and 2004 was the wettest year in terms of both simulated 

annual precipitation and streamflow (table 15 and fig. 24). 

Annual outflow as evaporation was relatively constant, however, averaging about 14 billion cubic feet per year, indicating 

that water is generally available to satisfy the environmental 

demand for evapotranspiration as simulated by the model. The 

volume of water in storage increased most in water year 2003, 

which was a wet year preceded by a dry year, and decreased 

most during water year 2005, which was a dry year preceded 

by a wet year. 

-10,000

-5,000

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

2000 2001 2002 2003 2004 2005 2006

Water year

Volume, in millions of cubic feet 

EXPLANATION

Precipitation

Evapotranspiration

Net streamflow

Net wells and quarries

Net boundary flow

Storage change

Figure 24. Summary of annual water-budget terms from the GSFLOW simulation, Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, water years 2000–06.
44 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 Table 15. Annual water budgets from the GSFLOW simulation for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06. [Values in the table were computed as differences between volumetric budgets for the last day of each water year given in the GSLFOW main output file (gsflow_FINALBUDGET.out) and converted from cubic meters to cubic feet. All values are reported as thousands of cubic feet except percent discrepancy is dimensionless.] Water budget term Water year Totals for 2000–06 2000 2001 2002 2003 2004 2005 2006 Inflows Precipitation 19,917,491 17,533,749 22,467,212 31,698,475 33,213,475 19,903,365 25,151,129 169,884,896 Streamflow 41,070 19,216 31,250 115,393 124,025 175,161 247,803 753,919 Groundwater boundary flow 20,790 21,656 22,386 15,993 13,386 14,748 19,645 128,604 Wells 136 117 125 188 213 129 98 1,005 Outflows Evapotranspiration 15,224,167 13,380,740 15,750,356 13,744,481 15,464,307 13,027,593 15,001,685 101,593,329 Streamflow 6,654,702 5,093,792 5,883,429 11,198,291 14,782,733 11,039,375 8,786,297 63,438,621 Groundwater boundary flow 61,196 57,945 55,033 73,984 86,556 75,785 60,000 470,500 Wells and quarries 419,892 532,440 509,097 448,497 444,824 563,269 610,238 3,528,256 Inflow minus outflow1 -2,379,575 -1,492,823 327,544 6,360,354 2,574,442 -4,612,488 957,947 1,735,400 Storage change2 Land surface -10,939 96 91 11,626 -602 1,283 266 1,821 Soil zone -1,234,955 482,469 549,638 1,167,398 48,381 -2,373,430 1,219,417 -141,082 Unsaturated zone -42,484 -195,043 362,611 1,741,862 -242,683 -1,606,959 177,103 194,407 Saturated zone -1,086,109 -1,777,948 -589,402 3,438,840 2,741,092 -623,375 -432,888 1,670,209 Total -2,374,490 -1,490,422 322,918 6,359,577 2,546,543 -4,602,671 963,879 1,725,335 Overall budget error3 -5,071 -2,403 4,620 516 28,062 -9,716 -5,967 10,041 Percent discrepancy -0.03 -0.01 0.02 0.00 0.08 -0.05 -0.02 0.01

1The “Inflow minus outflow” budget term does not exactly equal the difference between inflow and outflows shown in this table because of rounding in the GSFLOW output file.2Minus is loss of water from storage.3Inflows - outflows - change in storage.
Water Budget and Groundwater Recharge 45

Simulated daily fluxes for the major budget terms 

(precipitation, evapotranspiration, and streamflow) from the 

GSFLOW Comma Separated Value (CSV) output file are 

shown in figure 25. The regular seasonal variability of evapotranspiration is apparent—high rates of water loss in the summer and low rates of loss in the winter. Simulated streamflow 

was low many days during water years 2000 and 2001, as well 

as during the summer months of water years 2005 and 2006. 

An estimate of annual groundwater recharge from 

GSFLOW was summarized in table 16 from simulated values of daily flux of water from the unsaturated zone to the 

saturated zone (uzf_recharge). Recharge from the unsaturated 

zone, expressed as a depth over the study area, ranged from 

5.4 inches in 2001 (the driest year) to 22 inches in 2004 (the 

wettest year) and averaged about 12.4 inches. The GSFLOW 

model average for water years 2000–06 is less than the estimate of 15.2 inches of groundwater recharge derived from 

base-flow separation of the streamflow hydrograph. Recharge, 

expressed as a percentage of precipitation, ranged from 16 percent in water years 2001 and 2002 to 42 percent in water year 

2005. The large amount of recharge relative to precipitation 

in water year 2005 was caused by infiltration of precipitation 

during previous wet years that did not reach the saturated zone 

until water year 2005.

Variability in the simulated daily flux of recharge from 

the unsaturated zone to the saturated zone (uzf_recharge) is 

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

2000 2001 2002 2003 2004 2005 2006

Water year

Precipitation, in cubic feet per second

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

Evapotranspiration and streamflow, in cubic feet per second

EXPLANATION

Precipitation

Evapotranspiration

Streamflow

Figure 25. Daily fluxes of precipitation (basinppt), evapotranspiration (basinactet), and streamflow 

(basinstrmflow) from the GSFLOW Comma Separated Value (CSV) output file.

Table 16. Simulated flux of recharge from the unsaturated zone to the saturated zone (uzf_recharge) derived 

from the GSFLOW Comma Separated Value (CSV) output file.

Mean recharge rate from 

unsaturated to saturated zone 

(uzf_recharge)

Water year

2000 2001 2002 2003 2004 2005 2006

In cubic feet per second 132 91 112 308 370 265 182

As inches per year 7.8 5.4 6.6 18.3 22.0 15.8 10.8

As percent of precipitation 21 16 16 31 35 42 23
46 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

shown in figure 26. The amount of recharge varied seasonally in water years 2000, 2001, 2005 and 2006—recharge was 

greater in the winter and spring than in the summer and fall. 

However, in water years 2002–04 substantial amounts of water 

recharged the saturated zone regardless of season. 

The recharge from the unsaturated zone to the saturated 

zone (uzf_recharge) is compared to basin streamflow in 

figure 27. Recharge was much less than the base streamflow 

for dry periods in water years 2000–02 and 2005–06; during 

those times the streamflow was being sustained by depletion of groundwater storage. This comparison illustrates why 

base flows determined by the use of hydrograph-separation 

methods are not usually a good approximation of groundwater 

recharge at time scales of less than one year.

Groundwater Recharge

The distribution of net recharge derived from GSFLOW 

across the basin is illustrated for the driest water year of the 

study period (2001) in figure 28 and the wettest water year 

(2004) in figure 29. Areas in the basin where rates of groundwater recharge are much higher than average, especially 

during wet years, might be considered critical for maintaining 

the flow of springs, stream base flow, or the source of water to 

supply wells. 

Net recharge derived from GSFLOW differs from 

the infiltration flux from PRMS in that net recharge from 

GSFLOW is the net difference in the exchange of water 

between the saturated zone and land surface by MODFLOWNWT grid cell, whereas infiltration from PRMS is a downward 

vertical flux that originates at the land surface summarized by 

HUC. Thus, the resulting GSFLOW net recharge distributions 

for 2001 and 2004 in figures 28 and 29 differ from the PRMSonly infiltration (fig. 9) and are attributed to a more complete 

accounting of water in the GSFLOW model. The net recharge 

is useful for assessing the areal distribution of locations where 

recharge exceeds groundwater discharge. The sum of net 

recharge for all cells does not equal total recharge for the basin 

because of the large negative net recharge values computed for 

model cells having gaining streams and surface leakage. Net 

recharge rate for each cell is computed as

Net recharge = (Recharge – Groundwater ET – Surface 

leakage – Stream seepage),

where

Recharge is water added to the saturated zone from 

the UZF package, 

Groundwater ET is evapotranspiration from the saturated 

zone,

Surface leakage is groundwater discharge to the land 

surface where the water table intercepts 

the land surface, and 

Stream seepage at model cells with streams, is 

groundwater discharge to stream or 

streamflow loss to groundwater. 

The maps of the simulated distribution and magnitude of 

annual net recharge differ for water years 2001 (fig. 28) and 

2004 (fig. 29). In the dry water year 2001, the net recharge for 

most model cells was 0 to 1,000 cubic feet per day (red and 

orange cells). Net recharge was greater than 1,500 cubic feet 

per day (green, blue, and purple cells) in a few model cells 

in valleys along losing streams. Net recharge was less than 

zero (white cells) in areas where water from the saturated 

zone was discharging to land surface on mountains and along 

gaining streams. In the wet water year 2004, except for the 

mountain areas, net recharge in model cells mostly ranged 

from 1,500 to 5,000 cubic feet per day (green and blue cells) 

and was even greater (purple cells) in a few valleys along losing streams (fig. 29). 

The differences between the wet and dry water years 

are highlighted in figure 30, which shows the simulated net 

recharge for water year 2004 minus net recharge for water 

year 2001. The difference was positive in most parts of the 

basin, indicating that net recharge was greater in 2004 than 

in 2001. Areas with the greatest increase in recharge include 

model cells in the Scotia Barrens and valley floor between 

there and Bellefonte, along the toe slopes of Tussey and Bald 

Eagle Mountains, and some cells along Nittany Mountain. 

The Scotia Barrens, located west of State College, are approximately 200 ft higher than the Nittany Valley floor. The ridge 

is underlain by the Gatesburg Formation, which is characterized by a coarse-grained dolomite, interbedded orthoquartzite 

and sandy dolomite (Fulton and others, 2005). The substantially greater net recharge in that area for 2004 compared to 

2001 is a function of the thick unsaturated zone, large specific 

yield, and large hydraulic conductivity associated with the 

Gatesburg Formation. 

The difference between net recharge in 2004 and 2001 

was negative in a few areas on the mountains and along 

streams (orange and red cells). Simulated net recharge was 

greater during 2001 than during 2004 in a few parts of Tussey, Nittany, and Bald Eagle Mountains because the water 

table rose during the wet year 2004, causing discharge of 

water from the saturated zone to land surface to increase, 

which reduced net recharge. Net recharge was also less in 

2004 than in 2001 in model cells along some streams because 

substantially greater amounts of water discharged along 

stream valleys during the wet year 2004 than during the dry 

year 2001. 
Water Budget and Groundwater Recharge 47

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

2000 2001 2002 2003 2004 2005 2006

Water year

Precipitation, in cubic feet per second

0

500

1,000

1,500

2,000

Recharge flux, in cubic feet per second

EXPLANATION

Precipitation

Recharge flux

Figure 26. Daily fluxes of precipitation (basinppt) and recharge from the unsaturated zone to the saturated 

zone (uzf_recharge) from the GSFLOW Comma Separated Value (CSV) output file. 

10

100

1,000

10,000

2000 2001 2002 2003 2004 2005 2006

Water year

Flux, in cubic feet per second

EXPLANATION

Streamflow flux

Recharge flux

Figure 27. Daily fluxes of streamflow (basinstrmflow) and recharge from the unsaturated zone 

to the saturated zone (uzf_recharge) derived from the GSFLOW Comma Separated Value (CSV) 

output file.
48 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Simulated net recharge water year 2001, 

in cubic feet per day

Less than 00 to 500Greater than 500 to 1,000Greater than 1,000 to 1,500Greater than 1,500 to 2,500Greater than 2,500 to 5,000Greater than 5,000

Scotia Barrens (Pennsylvania Game Commission, 2011)Municipal boundaryCounty boundaryLiving Filter project boundary (Lin, 2011)Surface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 28. Net recharge simulated by GSFLOW in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water year 2001.
Water Budget and Groundwater Recharge 49

77°40' 77°50' 78° 78°10'

40°50'40°40'

PENNSYLVANIA

Location of study area

Base from U.S. Geological Survey digital data, 1972Universal Transverse Mercator projection, Zone 18, NAD 1983Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Simulated net recharge water year 2004, 

in cubic feet per day

Less than 00 to 500Greater than 500 to 1,000Greater than 1,000 to 1,500Greater than 1,500 to 2,500Greater than 2,500 to 5,000Greater than 5,000

Scotia Barrens (Pennsylvania Game Commission, 2011)Municipal boundaryCounty boundaryLiving Filter project boundary (Lin, 2011)Surface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 29. Net recharge simulated by GSFLOW in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water year 2004.
50 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 77°40' 77°50' 78° 78°10' 40°50' 40°40' PENNSYLVANIA Location of study area Base from U.S. Geological Survey digital data, 1972 Universal Transverse Mercator projection, Zone 18, NAD 1983 Shaded relief from PAMAP Program, 2008

0 5 MILES

0 5 KILOMETERS

EXPLANATION

Difference in simulated net recharge (water year 

2004 minus water year 2001), in cubic feet per day

Less than or equal to -10Greater than -10 to 0Greater than 0 to 1,000Greater than 1,000 to 2,000Greater than 2,000 to 5,000Greater than 5,000

Scotia Barrens (Pennsylvania Game Commission, 2011)Municipal boundaryCounty boundaryLiving Filter project boundary (Lin, 2011)Surface-water divide

Nittany Creek 

Basin

Spruce Creek 

Basin

Spring Creek 

Basin

State College

Bellefonte

Milesburg

CLEARFIELD COUNTY

BLAIR COUNTY

HUNTINGDON COUNTY

CENTRE COUNTY

MIFFLIN 

COUNTY

Figure 30. Difference in net recharge between water years (2004 minus 2001) simulated by GSFLOW in Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania. 
Summary and Conclusions 51

Model Limitations

The hydrologic model of the Spring Creek and Nittany 

Creek Basins, and parts of the Spruce Creek Basin was developed to simulate a basin-scale water budget and groundwater 

recharge for water years 2000–06. The model is probably 

adequate for simulating the regional water budget; however, 

substantial errors between simulated and observed groundwater levels suggest that use of the maps of groundwater recharge 

should be limited to providing general information about the 

overall pattern and magnitude of spatial variability in recharge. 

The GSFLOW model developed for the basin is limited 

by the availability and reliability of data used for its construction and calibration, but the principal limitations are probably 

the result of model simplifications and spatial resolution. 

Development of a numerical model requires simplification of 

the regional groundwater/surface-water system; however, for 

this study basin in karst terrain, simplification of the extreme 

heterogeneity with respect to hydraulic conductivity may 

not allow accurate representation of groundwater levels and 

flows, especially at the local scale. Thus, simulated results 

from GSFLOW at the local scale may not compare closely 

to observed fluxes or levels. Comparisons of simulated and 

observed daily streamflows and groundwater levels show 

the degree to which the GSFLOW model was able to match 

local observations. Some locations are well characterized by 

the GSFLOW model; in other areas, the GSFLOW model 

is a poor representation of the natural system. Although the 

model provides a reasonable assessment of the basin water 

budget and streamflow hydrographs for the period 2000–06, 

caution should be used when examining model results at 

specific locations. 

The spatial resolution that could be simulated in the 

GSFLOW model was limited by the horizontal and vertical 

model discretization. The topographically and hydrogeologically delineated HRUs, with stream buffers based on a width 

of 1,312 feet (400 meters), may have introduced surfacewater/groundwater interaction zones that are not based on 

the true riparian areas for each stream segment and do not 

include the full extent of the riparian areas of stream segments. 

In addition, the resolution of the MODFLOW-NWT cells, 

656 by 656 feet (200 by 200 meters) in horizontal dimension, allowed only a coarse representation of the interaction 

between groundwater and streams because the groundwater 

head at the stream was averaged over the large area of the cell, 

and because the physical stream-segment properties may not 

have been accurately represented. The timing of streamflow 

routing between stream segments might have been improved 

by using a kinematic wave method in the SFR2 package. 

However, because the model uses a daily time step, use of 

advanced streamflow routing methods was determined to be 

unnecessary. The thick vertical layers in the model of 300 feet 

were used to help mitigate computational difficulties caused 

by the steep relief on the mountains, affecting the ability to 

accurately simulate shallow groundwater levels, especially in 

areas where vertical gradients are greatest, such as beneath the 

mountains and streams.

The hydrologic model could be used to compare simulations of historic conditions of climate or land use during water 

years 2000–06 to simulations with alternative values for those 

conditions. Evaluation of differences between the simulations 

could be used to help understand the effects of those alternative conditions on the water budget and groundwater recharge. 

As a planning tool, the model could be used to simulate the 

effects of future hypothetical changes at the basin scale.

Summary and Conclusions

Rapid growth and development in the study basin 

(Spring Creek and Nittany Creek Basins and the headwaters 

of Spruce Creek Basin) has resulted in land-use changes and 

increased water use, influencing (1) the quantity and availability of runoff and groundwater, (2) surface-water/groundwater 

interactions, and (3) aquatic resources in the study basin. 

Because of the hydrologic connectivity between surface-water 

and groundwater in parts of the basins, a hydrologic model 

that accounts for groundwater and surface-water components 

(GSFLOW model) was constructed to simulate the interactions between both systems. 

ClearWater Conservancy and the Pennsylvania Department of Environmental Protection identified a number of 

priorities for the project, two of which were to (1) create a 

coupled-regional model to compute the water budget for the 

study basin using GSFLOW for water years 2000–06 and (2) 

identify areas of greater than average recharge, which can be 

used by decision makers in managing water resources in the 

study basin. 

The cumulative and annual water budgets for water years 

2000–06 for each of the storage reservoirs were simulated 

by GSFLOW. The largest source and sink terms are represented by precipitation and evapotranspiration, respectively. 

For water years 2002–06, a net surplus in the water budget of 

about 1.7 billion cubic feet was computed, where the inflows 

(precipitation, streamflow gains, inter-basin groundwater 

exchanges, well withdrawals/injection) exceeded outflows 

(evapotranspiration, streamflow losses, inter-basin losses, and 

well withdrawals/injection). The surplus inflow was balanced 

in the budget by an increase in storage by about the same 

amount. Groundwater withdrawals accounted for about 2 percent of the simulated outflow. 

Simulated values of streamflow and groundwater levels 

from GSFLOW were compared to observations from 11 

streamgages and 16 wells for water years 2000–06 during model calibration. Simulations of daily streamflow are 

considered acceptable for most streamgages except at the 
52 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

station Buffalo Run Lower. During wet years of high streamflow, the model slightly overestimated annual streamflow at 

most streamgages, and during dry years streamflow generally 

was slightly underestimated. The timing and magnitude of 

the simulated monthly mean streamflow corresponds closely 

to the observed streamflow. Simulations of mean monthly 

flow do not match the observations as closely as the daily 

and monthly mean streamflow calibration targets; at many of 

the streamgages, mean monthly flow was underestimated in 

March and April. 

Transient groundwater levels, a lesser objective of calibration, were not adequately simulated at most observation 

wells. The timing and magnitude of the simulated groundwater-level fluctuations can only be considered acceptable for 

three wells used for calibration—CE 686, Centre Hall, and 

CE 692. The poor fit of the model to observed groundwater 

levels relative to the fit of the model to observed streamflows 

was not unexpected because the zone-based parameterization 

of the GSFLOW model is best suited for predictions that integrate large areas of the model domain (such as flux targets), 

whereas water-level targets are more local. 

Differences in the magnitude and distribution of simulated net recharge between wet and dry periods were evaluated by comparing net recharge from water year 2001 (driest) 

to water year 2004 (wettest). Areas in the basin where rates 

of groundwater recharge are much higher than average and 

are capable of accepting substantially greater quantities of 

recharge during wet years might be considered critical for 

maintaining the flow of springs, stream base flow, or the 

source of water to supply wells. Areas where simulated rates 

of net recharge increased the most between the driest water 

year (2001) and wettest water year (2004) include the Scotia 

Barrens and valley floor between there and Bellefonte, along 

the toe slopes of Tussey and Bald Eagle Mountains, and some 

cells along Nittany Mountain.

The model is judged adequate for simulating the regional 

water budget; however, substantial errors between simulated 

and observed groundwater levels suggest that use of the 

maps of groundwater recharge should be limited to providing 

general information about the overall pattern and magnitude 

of spatial variability in recharge. The model could be used to 

compare simulations of historic conditions of climate or land 

use to alternative values for those conditions to help understand the effects of those alternative conditions on the water 

budget and groundwater recharge. As a planning tool, the 

model could be used to simulate the effects of future hypothetical changes at the basin scale.

An archive of the hydrologic model (including all 

GSFLOW files) is stored in the U.S. Geological Survey Pennsylvania Water Science Center digital model repository. The 

files are available upon request.

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56 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06
Tables 1 and 7
58 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 Table 1. Precipitation Runoff Modeling System parameter values assigned to hydrologic response units for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued [PRMS, Precipitation Runoff Modeling System; HRU, hydrologic response unit; gm/cm3, grams per cubic centimeter; id, identifier; precip, precipitation; GWR, groundwater reservoir; elev, elevation in feet above the North American Vertical Datum of 1988; temp, temperature; precip, precipitation; mm, millimeters; cfs, cubic feet per second; cms, cubic meters per second] PRMS parameter name Module Description of model parameter Units Minimum value Maximum value Mean value adj_by_hru climate_hru Flag to indicate whether to adjust precipitation and air temperature by HRU or subbasin (0=subbasin; 1=HRU) None 0 0 0 adjmix_rain climateflow Monthly (January to December) factor to adjust rain proportion in a mixed rain/snow event Decimal fraction 0.6 1.4 1.06436 albset_rna snowcomp Albedo reset—rain, accumulation stage. Fraction of rain in a mixed precipitation event above which the snow albedo is not reset, applied during the snowpack accumulation stage Decimal fraction 0.8 0.8 0.8 albset_rnm snowcomp Albedo reset—rain, melt stage. Fraction of rain in a mixed precipitation event above which the snow albedo is not reset, applied during the snowpack melt stage Decimal fraction 0.6 0.6 0.6 albset_sna snowcomp Albedo reset—snow, accumulation stage. Minimum snowfall, in water equivalent, needed to reset snow albedo during the snowpack accumulation stage Inches 0.05 0.05 0.05 albset_snm snowcomp Albedo reset—snow, melt stage. Minimum snowfall, in water equivalent, needed to reset snow albedo during the snowpack melt stage Inches 0.2 0.2 0.2 basin_area basin Area of basin Acres 146,350 146,350 146,350 basin_tsta climateflow Index of temperature station used to compute basin temperature values None 0 0 0 carea_max climateflow Maximum possible area contributing to surface runoff expressed as a portion of the HRU area Decimal fraction 0.25 0.25 0.25 cascade_flg cascade Flag to indicate cascade type (0=allow many to many; 1=force one to one) None 0 0 0 cascade_tol cascade Cascade area below which a cascade link is ignored Acres 0.5 0.5 0.5 cecn_coef snowcomp Monthly (January to December) convection condensation energy coefficient Calories per degree Celsius above 0 13.04718 13.04718 13.04718 circle_switch cascade Switch to check for circles (0=no check; 1=check) None 0 0 0 cov_type intcp_dev Vegetation cover type for each HRU (0=bare soil, 1=grasses, 2=shrubs, 3=trees) None 0 4 1.87093 covden_sum intcp_dev Summer vegetation cover density for the major vegetation type on each HRU Decimal fraction 0.14671 1 0.57692 covden_win intcp_dev Winter vegetation cover density for the major vegetation type on each HRU Decimal fraction 0.03631 0.76499 0.25523

den_init snowcomp Initial density of new-fallen snow gm/cm3 0.1 0.1 0.1
Table 1 59

Table 1. Precipitation Runoff Modeling System parameter values assigned to hydrologic response units for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued[PRMS, Precipitation Runoff Modeling System; HRU, hydrologic response unit; gm/cm3, grams per cubic centimeter; id, identifier; precip, precipitation; GWR, groundwater reservoir; elev, elevation in feet 

above the North American Vertical Datum of 1988; temp, temperature; precip, precipitation; mm, millimeters; cfs, cubic feet per second; cms, cubic meters per second]

PRMS parameter 

name

Module Description of model parameter Units Minimum value Maximum value Mean value

den_max snowcomp Average maximum snowpack density gm/cm3 0.6 0.6 0.6

elev_units basin Flag to indicate the units of the elevation values (0=feet, 1=meters) None 0 0 0

emis_noppt snowcomp Average emissivity of air on days without precipitation Decimal fraction 0.76024 0.76024 0.76024

epan_coef intcp_dev Monthly (January to December ) evaporation pan coefficient None 1 1 1

fall_frost transp_frost The solar date (number of days after winter solstice) of the first killing 

frost of the fall

Solar date 299 315 307.80941

fastcoef_lin soilzone Linear coefficient in equation to route preferential-flow storage 

downslope for each HRU

1/day 0.0004 0.24519 0.01073

fastcoef_sq soilzone Non-linear coefficient in equation used to route preferential-flow 

storage down slope for each HRU

None 0.02 0.02 0.02

freeh2o_cap snowcomp Free-water holding capacity of snowpack expressed as a decimal 

fraction of the frozen water content of the snowpack (pk_ice)

Decimal fraction 0.15314 0.15314 0.15314

gvr_cell_id gsflow_prms2mf Finite-difference cell associated with a gravity reservoir None 411 22,364 9,514.81453gvr_cell_pct gsflow_prms2mf Decimal fraction of HRU area associated with a finite-difference cell None 0 1.00195 0.49626

gvr_hru_id gsflow_prms2mf HRU associated with a gravity reservoir None 1 829 423.48547

gvr_hru_pct gsflow_prms2mf Decimal fraction of HRU area associated with a gravity reservoir None 0 0.39636 0.02778

hru_area basin Area of each HRU Acres 22.2394 6,081.58 176.54226

hru_aspect soltab Aspect of each HRU Degrees 0 315 166.26659

hru_deplcrv snowcomp Index number for the snowpack areal depletion curve associated with 

each HRU

None 1 1 1

hru_down_id cascade Index number of the downslope HRU to which the upslope HRU 

contributes flow

None 0 829 245.22263

hru_elev basin Mean elevation for each HRU Elev_units 208.00001 688.00002 378.49097

hru_frac_apply1 intcp_dev Portion of HRU that an application rate is applied Decimal fraction 0.00349 0.69496 0.28876

hru_lat soltab Latitude of each HRU Degrees 40.67987 40.9718 40.81942

hru_pct_up cascade Fraction of HRU area used to compute flow contributed to a down 

slope HRU or stream segment for cascade area.

decimal fraction 0 1 0.60406

hru_percent_imperv basin Fraction of each HRU area that is impervious Decimal fraction 0 0.19126 0.0015

hru_slope soltab Slope of each HRU, specified as change in vertical length divided by 

change in horizontal length

Decimal fraction 0.01711 0.42313 0.1137

hru_strmseg_down_id cascade Index number of the stream segment that cascade area contributes flow None 0 387 69.74818
60 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 Table 1. Precipitation Runoff Modeling System parameter values assigned to hydrologic response units for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued [PRMS, Precipitation Runoff Modeling System; HRU, hydrologic response unit; gm/cm3, grams per cubic centimeter; id, identifier; precip, precipitation; GWR, groundwater reservoir; elev, elevation in feet above the North American Vertical Datum of 1988; temp, temperature; precip, precipitation; mm, millimeters; cfs, cubic feet per second; cms, cubic meters per second] PRMS parameter name Module Description of model parameter Units Minimum value Maximum value Mean value hru_subbasin climate_hru Index of subbasin assigned to each HRU None 1 14 5.56333 hru_type basin Type of each HRU (0=inactive; 1=land, 2=lake, 3=swale) None 1 3 1.17612 hru_up_id cascade Index of HRU containing cascade area None 1 829 405.65985 id_obsrunoff gsflow_sum Identifier for streamflow-gaging station at outlet 4 4 4 imperv_stor_max climateflow Maximum impervious area retention storage for each HRU Inches 0.00321 0.07934 0.02132 irr_type1 intcp_dev Method of application of water for each application time series (0=sprinkler (intercep applies), 1=furrow/drip (no intercept), 2=ignore) None 1 1 1 melt_force snowcomp Julian date to force snowpack to spring snowmelt stage; varies with region depending on length of time that permanent snowpack exists Julian day 120 120 120 melt_look snowcomp Julian date to start looking for spring snowmelt stage. Varies with region depending on length of time that permanent snowpack exists Julian day 45 45 45 mnsziter gsflow_prms Version identifiers associated with the Parameter File String 25 25 25 mxsziter hru_sum Switch for HRU monthly and yearly summary (0=off, 1=on) None 75 75 75 napp_to_hru1 intcp_dev HRU id to apply an observed application rate None 256 366 328.33333 potet_sublim intcp_dev Fraction of potential ET that is sublimated from the snow surface Decimal fraction 0.18293 0.18293 0.18293 ppt_rad_adj climateflow Monthly minimum precipitation, if basin precipitation exceeds this value, radiation is mutiplied by radj_sppt or radj_wppt adjustment factor Inches 0.02 0.02 0.02 precip_units climateflow Units for measured precipitation (0=inches; 1=mm) None 0 0 0 pref_flow_den soilzone Preferential-flow pore density for each HRU Decimal fraction 0.00247 0.05826 0.01574 rad_trncf snowcomp Transmission coefficient for short-wave radiation through the winter vegetation canopy Decimal fraction 0.1659 0.68029 0.52575 radj_sppt climateflow Adjustment factor for computed solar radiation for summer day with greater than ppt_rad_adj inches precip Decimal fraction 0.44 0.44 0.44 radj_wppt climateflow Adjustment factor for computed solar radiation for winter day with greater than ppt_rad_adj inches precip Decimal fraction 0.5 0.5 0.5 radmax climateflow Maximum fraction of the potential solar radiation that may reach the ground due to haze, dust, smog Decimal fraction 0.8 0.8 0.8 rain_sub_adj climate_hru Monthly (January to December) adjustment factor to measured precipitation determined to be rain for each subbasin Decimal fraction 0.8 1.2 1.05711

runoff_units obs_dev Measured runoff units (0=cfs; 1=cms) None 0 0 0
Table 1 61

Table 1. Precipitation Runoff Modeling System parameter values assigned to hydrologic response units for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued[PRMS, Precipitation Runoff Modeling System; HRU, hydrologic response unit; gm/cm3, grams per cubic centimeter; id, identifier; precip, precipitation; GWR, groundwater reservoir; elev, elevation in feet 

above the North American Vertical Datum of 1988; temp, temperature; precip, precipitation; mm, millimeters; cfs, cubic feet per second; cms, cubic meters per second]

PRMS parameter 

name

Module Description of model parameter Units Minimum value Maximum value Mean value

sat_threshold soilzone Water holding capacity of the gravity and preferential-flow reservoirs; 

difference between field capacity and total soil saturation for each HRU

Inches 0.99995 14.99963 5.33114

settle_const snowcomp Snowpack settlement time constant Decimal fraction 0.1 0.1 0.1

slowcoef_lin soilzone Linear coefficient in equation to route gravity-reservoir storage down 

slope for each HRU

1.0/day 0.001 0.13949 0.012

slowcoef_sq soilzone Non-linear coefficient in equation to route gravity-reservoir storage 

down slope for each HRU

None 0.015 0.015 0.015

smidx_coef srunoff_smidx Coefficient in non-linear contributing area algorithm Decimal fraction 0.0007 0.02628 0.00491

smidx_exp srunoff_smidx Exponent in non-linear contributing area algorithm 1.0/inch 0.3 0.3 0.3

snarea_curve snowcomp Snow area depletion curve values, 11 values for each curve (0.0 to 1.0 

in 0.1 increments)

Decimal fraction 0.05 1 0.58318

snarea_thresh snowcomp Maximum threshold snowpack water equivalent below which the 

snow-covered-area curve is applied; varies with elevation

Inches 5.77157 48.45613 20.46169

snow_intcp intcp_dev Snow interception storage capacity for the major vegetation type in 

each HRU

Inches 0 0.07155 0.01336

snow_sub_adj climate_hru Monthly (January to December) adjustment factor to measured 

precipitation determined to be snow for each subbasin

Decimal fraction 1 1.2 1.1

snowinfil_max climateflow Maximum snow infiltration per day Inches/day 1.61992 2.76008 2.48885

soil2gw_max soilzone Maximum amount of the capillary reservoir excess that is routed 

directly to the GWR for each HRU

Inches 0 0.5 0.06834

soil_moist_init soilzone Initial value of available water in capillary reservoir for each HRU Inches 1 1 1

soil_moist_max climateflow Maximum available water holding capacity of capillary reservoir from 

land surface to rooting depth of the major vegetation type of each HRU

Inches 2.00003 9.99998 5.88824

soil_rechr_init soilzone Initial storage for soil recharge zone (upper part of capillary reservoir 

where losses occur as both evaporation and transpiration) for each HRU; must be less than or equal to soil_moist_init

Inches 0.1 0.1 0.1

soil_rechr_max climateflow Maximum storage for soil recharge zone (upper portion of capillary 

reservoir where losses occur as both evaporation and transpiration); must be less than or equal to soil_moist

Inches 0.70175 1.46576 1.11072

soil_type soilzone Soil type of each HRU (1=sand; 2=loam; 3=clay) None 2 2 2
62 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 Table 1. Precipitation Runoff Modeling System parameter values assigned to hydrologic response units for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued [PRMS, Precipitation Runoff Modeling System; HRU, hydrologic response unit; gm/cm3, grams per cubic centimeter; id, identifier; precip, precipitation; GWR, groundwater reservoir; elev, elevation in feet above the North American Vertical Datum of 1988; temp, temperature; precip, precipitation; mm, millimeters; cfs, cubic feet per second; cms, cubic meters per second] PRMS parameter name Module Description of model parameter Units Minimum value Maximum value Mean value spring_frost transp_frost The solar date (number of days after winter solstice) of the last killing frost of the spring Solar date 107 119 113.69843 srain_intcp intcp_dev Summer rain interception storage capacity for the major vegetation type in each HRU Inches 0 0.05 0.03138 ssr2gw_exp soilzone Non-linear coefficient in equation used to route water from the gravity reservoir to the GWR for each HRU None 1 1 1 ssr2gw_rate soilzone Linear coefficient in equation used to route water from the gravity reservoir to the GWR for each HRU 1/day 0.05 0.73644 0.11593 ssstor_init soilzone Initial storage of the gravity and preferential-flow reservoirs for each HRU Inches 2 2 2 subbasin_down subbasin Index number for the downstream subbasin whose inflow is outflow from this subbasin None 0 14 8.35714 szconverge gsflow_prms2mf Convergence criterion for checking soil-zone flows Inches 0.005 0.005 0.005 temp_units climateflow Flag to indicate the units of measured air-temperature values (0=Fahrenheit, 1=Celsius) None 0 0 0 tmax_allrain climateflow Monthly (January to December) maximum air temperature when precipitation is assumed to be rain; if HRU air temperature is greater than or equal to this value, precipitation is rain Temp_units 39 39 39 tmax_allsnow climateflow Monthly (January to December) maximum air temperature when precipitation is assumed to be snow; if HRU air temperature is less than or equal to this value, precipitation is snow Temp_units 35 35 35 tmax_hru_adj climateflow Adjustment to minimum temperature for each HRU, estimated based on slope and aspect Temp_units 0 0 0 tmin_hru_adj climateflow Adjustment to minimum temperature for each HRU, estimated based on slope and aspect Temp_units 0 0 0 tstorm_mo snowcomp Monthly flag (January to December) for prevalent storm type (0=frontal storms, 1=convective storms) None 0 1 0.5 wrain_intcp intcp_dev Winter rain interception storage capacity for the major vegetation type in the HRU Inches 0 0.03768 0.01798

1The parameters irr_type, napp_to_hru, and hru_frac_apply are not included in GSFLOW version 1.1.5 and are used to apply the time series of effluent data to the canopy of the Living Filter on an areaweighted basis to all HRUs affected by the Living Filter. Application of the effluent uses the same computation methods as used for rain on the canopy.
Table 7 63

Table 7. Parameters used to represent the hydraulic conductivity of geologic units in the MODFLOW-NWT steady-state model and horizontal hydraulic conductivity values from previous studies for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued[ft/d, feet per day; ft, foot; max, maximum; —, no data]

Geologic 

period Formation1 Description Aquifer type

Parameter 

name 

used for 

hydraulic 

conductivity 

of the 

formation, 

in this study

K Ranks 

(based on 

productivity 

estimates 

of Parizek 

and Filley, 

1982)

Hydraulic conductivity (K) values from other studies (ft/d)

Model 

values 

from Curtin 

Gap study 

(Gannett 

Fleming, 

2008) 

Model 

values from 

Brachet 

(2004)

Reported 

median 

aquifer-test 

values in 

Fulton and 

others (2005)

Model 

values from 

study in Blair 

Co (Lindsey, 

2004)

K Parizek 

(assume 500 

ft thick)

Model 

values 

K(max) 

from Todd 

Giddings and 

Associates 

(1994) 

From aquifertest data 

assuming 

200-ft aquifer 

thickness in Becher 

(1990, 

table 7) 

Silurian Rose Hill Formation Sandstone and 

limestone

Fracture 

Siliciclastic

Kshale Low 0.23

Tuscarora Formation Sandstone Fracture 

Siliciclastic

Kshale Low 0.23

Ordovician Juniata Sandstone and 

shale

Fracture 

Siliciclastic

Kshale Low 0.23

Bald Eagle Formation Sandstone Fracture 

Siliciclastic

Kshale Low 0.23 0.33

Reedsville Shale Shale Fracture 

Siliciclastic

Kshale Low 0.23 0.13

Coburn Formation Limestone Conduit 

Carbonate

Kcarb Low 0.23 0.16 3.3

Salona Formation Limestone with 

shale

Conduit 

Carbonate

Kcarb Low 0.23 0.16 3.3

Nealmont Limestone Limestone Conduit 

Carbonate

LemontK High 0.23 22 3.3

Benner Formation Limestone Conduit 

Carbonate

LemontK High 4.9 0.23 3.3

Valentine Member of 

Benner Formation1

Limestone Conduit 

Carbonate

LemontK High 4.9 0.23 0.72 3.3

Snyder Formation Limestone and 

dolomite

Conduit 

Carbonate

LemontK High 4.9 0.23 11 3.3

Hatter Formation Limestone and 

dolomite

Conduit 

Carbonate

LemontK High 4.9 0.23 11 3.3

Loysburg Formation Limestone and 

dolomite

Conduit 

Carbonate

LemontK High 4.9 0.23 2.6 3.3

Bellefonte Dolomite Dolomite Fracture 

Carbonate

Kcarb Moderate 4.6 1.7 1.3 0.4–2.6 1.6

Axemann Limestone Limestone and 

dolomite

Conduit 

Carbonate

Kaxe_ston Moderate 6.6 1.7 3.3

Nittany Dolomite Dolomite Fracture 

Carbonate

Knit High 11 1.7 9.5 85–490 1.0 300

Stonehenge Formation Limestone and 

dolomite

Conduit 

Carbonate

Kaxe_ston Moderate 2.6 4.7 1.8 36
64 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06 Table 7. Parameters used to represent the hydraulic conductivity of geologic units in the MODFLOW-NWT steady-state model and horizontal hydraulic conductivity values from previous studies for Spring Creek and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania.—Continued [ft/d, feet per day; ft, foot; max, maximum; —, no data] Geologic period Formation1 Description Aquifer type Parameter name used for hydraulic conductivity of the formation, in this study K Ranks (based on productivity estimates of Parizek and Filley, 1982) Hydraulic conductivity (K) values from other studies (ft/d) Model values from Curtin Gap study (Gannett Fleming, 2008) Model values from Brachet (2004) Reported median aquifer-test values in Fulton and others (2005) Model values from study in Blair Co (Lindsey, 2004) K Parizek (assume 500 ft thick) Model values K(max) from Todd Giddings and Associates (1994) From aquifer- test data assuming 200-ft aquifer thickness in Becher (1990, table 7) Cambrian Mines Dolomite Member of Gatesburg Formation Dolomite Conduit/ Fracture Carbonate Kgates High 12 4.7 117 60–120 20 Upper sandy member of Gatesburg Formation Dolomite Diffuse-flow Carbonate Kgates High 3.9 4.7 19 117 60–120 20 Ore Hill Dolomite Member of Gatesburg Formation Dolomite Diffuse-flow Carbonate Kcarb Moderate 3.9 4.7 117 60–120 20 Lower sandy member of Gatesburg Formation Dolomite and sandstone Diffuse-flow Carbonate Kcarb Moderate 3.9 4.7 28 117 60–120 20 Warrior Formation Limestone and dolomite Fracture Carbonate Kcarb Moderate 4.7 60–120

1Formation name is from the Pennsylvania Geological Survey and may not conform to usage by the U.S. Geological Survey.
Appendix
66 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

Appendix. Results for All of the Calibration Targets for 

GSFLOW Simulations for Water Years 2000–06

The results for all of the calibration targets for GSFLOW simulations for water years 

2000–06 are presented in figures A1–A14. Comparisons are shown for natural log of daily 

mean streamflow (fig. A1), and annual and monthly streamflow and base flow (figs. A2–A12). 

Groundwater levels and annual mean water levels in observation wells are shown in figures A13 and A14.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 67

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Spring Creek near Axemann

Nash−Sutcliffe coefficient: 0.65

−0.5

 0.0

 0.5

 1.0

 1.5

 2.0

 2.5

 3.0

 3.5

Buffalo Run Lower

Nash−Sutcliffe coefficient: 0.47

0.5

1.0

1.5

2.0

2.5

3.0

Cedar Run Lower

Nash−Sutcliffe coefficient: 0.61

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Spring Creek at Houserville

Nash−Sutcliffe coefficient: 0.58

Observation number

Natural log of streamflow, in cubic feet per second

0 500 1,000 1,500 2,000 2,500 3,000 0 500 1,000 1,500 2,000 2,500 3,000

0 500 1,000 1,500 2,000 2,500 3,000 0 500 1,000 1,500 2,000 2,500 3,000

EXPLANATION

Observed value

Simulated value

Figure A1. Observations and GSFLOW simulations of the natural log of daily streamflow at 11 streamgages in the Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 

2000–06.
68 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 100 200 300 400 500 600

1.6

1.8

2.0

2.2

2.4

2.6

Logan Run Lower

Nash−Sutcliffe coefficient: 0.58

0 500 1,000 1,500 2,000 2,500 3,000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Logan Run Upper

Nash−Sutcliffe coefficient: 0.52

1.5

2.0

2.5

3.0

3.5

4.0

Spring Creek at Milesburg

Nash−Sutcliffe coefficient: 0.7

−1

 0

 1

 2

 3

 4

Spring Creek Upper

Nash−Sutcliffe coefficient: 0.53

Observation number

Natural log of streamflow, in cubic feet per second

0 500 1,000 1,500 2,000 2,500 3,000 0 500 1,000 1,500 2,000 2,500 3,000

EXPLANATION

Observed value

Simulated value

Figure A1. Observations and GSFLOW simulations of the natural log of daily streamflow at 11 streamgages in the Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 

2000–06.—Continued
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 69

0 500 1,000 1,500 2,000 2,500

1.0

1.5

2.0

2.5

3.0

Spruce Creek at Graysville

Nash−Sutcliffe coefficient: 0.73

−2

−1

 0

 1

 2

 3

 4

Slab Cabin Run Lower

Nash−Sutcliffe coefficient: 0.55

−2

−1

 0

 1

 2

 3

Slab Cabin Run Upper

Nash−Sutcliffe coefficient: 0.5

Observation number

Natural log of streamflow, in cubic feet per second

0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500

EXPLANATION

Observed value

Simulated value

Figure A1. Observations and GSFLOW simulations of the natural log of daily streamflow at 11 streamgages in the Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 

2000–06.—Continued
70 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 2 4 6 8

 40

 60

 80

100

120

140

160

180

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.93

0 20 40 60 80 100

 0

100

200

300

400

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.92

0 20 40 60 80 100

 0

 50

100

150

200

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.87

024 6 8 10 12

 70

 80

 90

100

110

120

130

140

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.84

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A2. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Spring Creek near Axemann in the Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 71

0 2 4 6 8

 5

10

15

20

25

30

35

40

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.41

0 20 40 60 80 100

 0

10

20

30

40

50

60

70

80

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.48

0 20 40 60 80 100

 0

10

20

30

40

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.28

024 6 8 10 12

 5

10

15

20

25

30

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.38

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A3. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Buffalo Run Lower in the Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
72 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 2 4 6 8

 5

10

15

20

25

30

35

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.65

0 20 40 60 80 100

 0

10

20

30

40

50

60

70

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.75

0 20 40 60 80 100

 0

 5

10

15

20

25

30

35

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.75

024 6 8 10 12

10

15

20

25

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.55

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A4. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Cedar Run Lower in the Spring Creek and Nittany Creek Basins and parts of 

the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 73

0 2 4 6 8

10

20

30

40

50

60

70

80

90

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.89

0 20 40 60 80

 0

 50

100

150

200

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.82

0 20 40 60 80

 0

 20

 40

 60

 80

100

120

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.85

024 6 8 10 12

20

30

40

50

60

70

80

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: −0.16

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A5. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Spruce Creek at Graysville in the Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
74 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 2 4 6 8

 20

 40

 60

 80

100

120

140

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.9

0 20 40 60 80 100

 0

 50

100

150

200

250

300

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.91

0 20 40 60 80 100

 0

 20

 40

 60

 80

100

120

140

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.82

024 6 8 10 12

 40

 50

 60

 70

 80

 90

100

110

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.83

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A6. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Spring Creek at Houserville in the Spring Creek and Nittany Creek Basins 

and parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 75

Figure A7. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Logan Branch Lower in the Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.

1.0 1.2 1.4 1.6 1.8 2.0

73

74

75

76

77

78

79

A. Annual mean streamflow

Nash−Sutcliffe coefficient: −7.8

0 5 10 15 20

 50

 60

 70

 80

 90

100

110

120

130

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.78

0 5 10 15 20

 50

 60

 70

 80

 90

100

110

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.74

024 6 8 10 12

 60

 70

 80

 90

100

110

120

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.54

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value
76 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 2 4 6 8

10

15

20

25

30

35

40

45

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.71

0 20 40 60 80 100

 0

 20

 40

 60

 80

100

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.75

0 20 40 60 80 100

 0

10

20

30

40

50

60

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.64

024 6 8 10 12

10

15

20

25

30

35

40

45

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.73

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A8. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Logan Branch Upper in the Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 77

0 2 4 6 8

100

150

200

250

300

350

400

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.91

0 20 40 60 80 100

100

200

300

400

500

600

700

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.9

0 20 40 60 80 100

100

150

200

250

300

350

400

450

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.86

024 6 8 10 12

160

180

200

220

240

260

280

300

320

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.79

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A9. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Spring Creek at Milesburg in the Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
78 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 2 4 6 8

 0

 5

10

15

20

25

30

35

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.81

0 20 40 60 80 100

 0

10

20

30

40

50

60

70

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.83

0 20 40 60 80 100

 0

 5

10

15

20

25

30

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.8

024 6 8 10 12

 5

10

15

20

25

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.71

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A10. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Slab Cabin Run Lower in the Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 79

0 2 4 6 8

 0

 5

10

15

20

25

30

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.96

0 20 40 60 80

 0

10

20

30

40

50

60

70

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.84

0 20 40 60 80

 0

 5

10

15

20

25

30

35

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.78

024 6 8 10 12

 5

10

15

20

25

30

35

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.35

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A11. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Slab Cabin Run Upper in the Spring Creek and Nittany Creek Basins and 

parts of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
80 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 2 4 6 8

 0

10

20

30

40

50

A. Annual mean streamflow

Nash−Sutcliffe coefficient: 0.72

0 20 40 60 80 100

 0

 20

 40

 60

 80

100

120

B. Monthly mean streamflow

Nash−Sutcliffe coefficient: 0.75

0 20 40 60 80 100

 0

10

20

30

40

50

C. Monthly mean base flow

Nash−Sutcliffe coefficient: 0.54

024 6 8 10 12

10

15

20

25

30

35

40

D. Mean monthly streamflow

Nash−Sutcliffe coefficient: 0.23

Observation number

Flow, in cubic feet per second

EXPLANATION

Observed value

Simulated value

Figure A12. Observations and GSFLOW simulations of A, annual mean streamflow, B, monthly mean streamflow, C, monthly 

mean base flow, and D, mean monthly streamflow at Spring Creek Upper in the Spring Creek and Nittany Creek Basins and parts 

of the Spruce Creek Basin, Centre and Huntingdon Counties, Pennsylvania, water years 2000–06.
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 81

0 500 1,000 1,500

945

950

955

960

965

FILMORE 1

Nash−Sutcliffe coefficient: −39

0 500 1,000 1,500 2,000 2,500 3,000

1,080

1,100

1,120

1,140

1,160

1,180

1,200

CE686

Nash−Sutcliffe coefficient: 0.84

0 500 1,000 1,500

1,590

1,595

1,600

1,605

1,610

1,615

1,620

PINEGROVE 2

Nash−Sutcliffe coefficient: −7.4

0 500 1,000 1,500

930

940

950

960

970

980

990

1,000

BIG HOLLOW

Nash−Sutcliffe coefficient: −1.5

Observation number

Water-level elevation, in feet above NAVD 88

EXPLANATION

Observed value

Simulated value

Figure A13. Observations and GSFLOW simulations of groundwater levels in selected wells in the Spring Creek and Nittany 

Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon counties, Pennsylvania, water years 2000–06.
82 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 500 1,000 1,500

1,000

1,020

1,040

1,060

1,080

1,100

NITTANY

Nash−Sutcliffe coefficient: −0.069

0 200 400 600 800 1,000

1,160

1,180

1,200

1,220

1,240

1,260

CENTRE HALL

Nash−Sutcliffe coefficient: 0.49

0 20 40 60

1,080

1,090

1,100

1,110

1,120

1,130

1,140

1,150

CE696

Nash−Sutcliffe coefficient: −0.0021

0 5 10 15

1,140

1,150

1,160

1,170

1,180

1,190

1,200

CE695

Nash−Sutcliffe coefficient: −2

Observation number

Water-level elevation, in feet above NAVD 88

EXPLANATION

Observed value

Simulated value

Figure A13. Observations and GSFLOW simulations of groundwater levels in selected wells in the Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon counties, Pennsylvania, water 

years≈2000–06.—Continued
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 83

0 20 40 60 80

880

900

920

940

960

980

1,000

CE698

Nash−Sutcliffe coefficient: −16

0 5 10 15

1,100

1,120

1,140

1,160

1,180

1,200

CE692

Nash−Sutcliffe coefficient: 0.67

0 20 40 60 80

1,150

1,160

1,170

1,180

1,190

1,200

CE691

Nash−Sutcliffe coefficient: −3.2

0 2 4 6 8 10

1,030

1,040

1,050

1,060

1,070

1,080

1,090

CE693

Nash−Sutcliffe coefficient: 0.17

Observation number

Water-level elevation, in feet above NAVD 88

EXPLANATION

Observed value

Simulated value

Figure A13. Observations and GSFLOW simulations of groundwater levels in selected wells in the Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon counties, Pennsylvania, water 

years≈2000–06.—Continued
84 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

0 5 10 15 20 25 30

1,035

1,040

1,045

1,050

1,055

1,060

1,065

1,070

1,075

CE446

Nash−Sutcliffe coefficient: −9.8

0 10 20 30 40 50

890

900

910

920

930

940

CE697

Nash−Sutcliffe coefficient: −2

0 5 10 15 20

1,150

1,160

1,170

1,180

1,190

1,200

CE690

Nash−Sutcliffe coefficient: −1.3

0 5 10 15

1,080

1,090

1,100

1,110

1,120

1,130

1,140

1,150

CE694

Nash−Sutcliffe coefficient: −4.1

Observation number

Water-level elevation, in feet above NAVD 88

EXPLANATION

Observed value

Simulated value

Figure A13. Observations and GSFLOW simulations of groundwater levels in selected wells in the Spring Creek and 

Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon counties, Pennsylvania, water 

years≈2000–06.—Continued
Appendix. Results for All of the Calibration Targets for GSFLOW Simulations for Water Years 2000–06 85

Figure A14. Observations and GSFLOW simulations of annual mean groundwater levels in selected wells in the Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon counties, Pennsylvania, water years 

2000–06.

1 2 3 4

954

955

956

957

958

959

960

 FILMORE 1

Nash−Sutcliffe coefficient: −28

0 2 4 6 8

1,080

1,100

1,120

1,140

1,160

1,180

 CE686

Nash−Sutcliffe coefficient: 0.88

1 2 3 4

1,598

1,600

1,602

1,604

1,606

1,608

1,610

 PINEGROVE 2

Nash−Sutcliffe coefficient: −37

1 2 3 4

940

945

950

955

960

965

970

975

 BIG HOLLOW

Nash−Sutcliffe coefficient: −2.7

Observation number

Mean water-level elevation, in feet above NAVD 88

EXPLANATION

Observed value

Simulated value
86 Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06

1 2 3 4

1,015

1,020

1,025

1,030

1,035

1,040

1,045

1,050

 NITTANY

Nash−Sutcliffe coefficient: 0.23

1 2 3 4

1,180

1,185

1,190

1,195

1,200

1,205

1,210

1,215

 CENTRE HALL

Nash−Sutcliffe coefficient: 0.43

Observation number

Mean water-level elevation, in feet above NAVD 88

EXPLANATION

Observed value

Simulated value

Figure A14. Observations and GSFLOW simulations of annual mean groundwater levels in selected wells in the Spring Creek 

and Nittany Creek Basins and parts of the Spruce Creek Basin, Centre and Huntingdon counties, Pennsylvania, water years 

2000–06.—Continued
For more information concerning this publication contact:

Director, Pennsylvania Water Science Center

U.S. Geological Survey

215 Limekiln Road

New Cumberland, PA 17070

Or visit:

http://pa.water.usgs.gov/

Document prepared by the West Trenton Publishing Service Center
Fulton and others—Water-Budget and Recharge-Area Simulations, Centre and Huntingdon Counties, Pennsylvania, Water Years 2000–06—Scientific Investigations Report 2015–5073

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155073