Introduction Study Area

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3124

March 2012

Introduction

General Circulation Model (GCM) simulations of future 

climate through 2099 project a wide range of possible scenarios 

(Intergovernmental Panel on Climate Change, 2007). To determine the sensitivity and potential effect of long-term climate 

change on the freshwater resources of the United States, the 

U.S. Geological Survey Global Change study, “An integrated 

watershed scale response to global change in selected basins 

across the United States” was started in 2008. The long-term 

goal of this national study is to provide the foundation for 

hydrologically based climate-change studies across the nation.

Fourteen basins for which the Precipitation Runoff Modeling System (PRMS) has been calibrated and evaluated were 

selected as study sites. PRMS is a deterministic, distributedparameter watershed model developed to evaluate the effects 

of various combinations of precipitation, temperature, and land 

use on streamflow and general basin hydrology. Output from 

five GCMs and four emission scenarios were used to develop 

an ensemble of climate-change scenarios for each basin. These 

ensembles were simulated with the corresponding PRMS 

model. This fact sheet summarizes the hydrologic effect and 

sensitivity of the PRMS simulations to climate change for South 

Fork Flathead River Basin in northwestern Montana (U.S. 

Geological Survey streamflow-gaging station 12362500; fig. 1) 

presented in the project summary report (Markstrom and others, 

2012) and a journal article (Hay and others, 2011).

Study Area

The South Fork Flathead River Basin is located on the west 

side of the Continental Divide in northwestern Montana. It has 

a drainage area of 4,307 square kilometers(km2) at South Fork 

Flathead River near Columbia Falls (streamflow-gaging station 

12362500) downstream from Hungry Horse Reservoir (Berkas 

and others, 2004). This mountainous basin (elevation ranges 

from 1,045 to 2,078 meters) is undeveloped and covered mostly 

with forests that were affected by wildfires and mountain pine 

beetles during the 1998–2008 drought. The South Fork Flathead River flows into the Flathead River and ultimately into 

the Clark Fork of the Columbia River. Hungry Horse Reservoir, 

which was completed in 1952, on the South Fork Flathead 

River stores water that is used primarily for hydroelectric power 

generation, both onsite and at federal reservoirs downstream 

along the Columbia River system. Hungry Horse Reservoir also 

provides flood control as well as water for use in Montana and 

several downstream states for recreation, irrigation, fish protection, and fish migration.

The Bureau of Reclamation (BOR) maintains and operates 

the Hungry Horse Dam and Reservoir. Downstream requirements of stored water from the reservoir limit the use of this 

water in Montana. The U.S. Geological Survey, in cooperation 

with BOR, is using PRMS to simulate runoff in the South Fork 

Flathead Basin. The model will enable the BOR to forecast 

inflow to the reservoir and allow water-resource managers to 

optimize reservoir operations (Chase, 2011).

Watershed Scale Response to Climate Change—South Fork 

Flathead River Basin, Montana
High 4,314

Low -79

EXPLANATION

Base from U.S. Geological Survey digital data, 2011 1:100,000

Albers Equal-Area Conic projection

Standard parallels 29°30'N and 45°30'N

Central meridian 96°00'W, Latitude of origin 23°00'N

Horizontal coordinate information is referenced to the

World Geodetic System of 1984 (WGS 84)

Elevation in meters

12362500 U.S. Geological Survey streamflow-

 gaging station and identifier

South

Fork

Flathead

River

Hungry Horse

Reservoir

0 4 8 12 16 MILES

0 4 8 12 16 KILOMETERS

114°

48°20'

113°

40'

30'

10'

50'

20'

10'

50'

47°20'

40'

30'

48°

12362500

INDEX MAP

Study area

MONTANA

Figure 1. Precipitation Runoff Modeling System study locations, South Fork of the Flathead River Basin, 

Montana, and location of U.S. Geological Survey streamflow-gaging station 12362500 with a drainage area 

of 4,307 square kilometers and elevation range from 1,045 to 2,078 meters.
General Circulation Models

Results

Climate-change fields were derived by calculating the 

change in climate from current (water years 1988–1999) to 

future conditions simulated by each GCM. The 20C3M simulation for water years 1988–1999 was used to represent current 

climatic conditions. This 12-year period of record was chosen 

based on the overlap of the available historical records from 

the 14 basins included in the national study. Climate change 

fields (percentage changes in precipitation and degree changes 

in temperature) were computed for 12-year moving window 

periods (from 2001–2099) using the 20C3M (1988–1999) and 

the A1B, B1, and A2 emission scenarios. A 12-year moving 

window, starting in 2001 and ending in 2099, results in 1,320 

future scenarios [(88, 12-year climatologies, 1 per year starting 

with 2001–2012 and ending with 2088–2099) x (3 emission 

scenarios) x (5 GCMs)].

Climate-change scenarios were generated for PRMS by 

modifying PRMS precipitation and temperature inputs with the 

mean monthly climate change fields derived from the GCMs, 

resulting in 1,320 PRMS-input files. Table 3 shows the change 

(slope) and adjusted R2 (adjR2) for the least squares fit to the 

trend line for selected output variables from the PRMS projections. The slope indicates the change in the selected variable by 

year. The adjusted R2 value gives an indication of the variability 

in the central tendency of the trend line.

Figure 2 shows a summary of the projected range in 11-year 

moving mean daily values of maximum temperature (fig. 2A), 

minimum temperature (fig. 2B), and precipitation (fig. 2C) by 

emission scenario. The first year of each 12-year simulation 

was used as PRMS initialization and is not included in the 

results. The three solid-colored lines indicate the 11-year moving mean values (x-axis indicates center of 11-year window) 

for the three future emission scenarios (central tendency of the 

five GCMs for each emission scenario). The projected range 

shown for each emission scenario indicates the range of potential future climatic conditions simulated by the five GCMs. 

All GCM simulations project steady increases in maximum 

and minimum temperature (table 3), with uncertainties associated with these GCM projections increasing with time. Both 

minimum and maximum temperatures show the smallest projected changes for the B1 emission scenario. Projected changes 

in mean annual precipitation for the South Fork Flathead River 

watershed are variable, with an overall positive trend in the 

central tendencies projected for the A1B emission scenario 

(table 3). The wide range in the precipitation projections indicates a large amount of uncertainty. 

Results

PRMS simulates spatially distributed streamflow, components of flow (surface, subsurface, and groundwater), snowpack 

conditions, and many other hydrologic components of interest. 

Figure 3 shows the projected range in 11-year moving mean 

daily values of streamflow, precipitation that falls as snow, 

snowpack water equivalent, and soil moisture by emission 

scenario. The central tendency of the five GCMs for each of the 

three emission scenarios (indicated by the solid-colored lines) 

The GCM outputs were obtained from the World Climate Research Programme’s Coupled Model Intercomparison Project 

phase 3 multi-model dataset archive, which was referenced in the Intergovernmental Panel on Climate Change Fourth Assessment 

Special Report on Emission scenarios (Intergovernmental Panel on Climate Change, 2007). For each GCM, one current (water 

years 1988–1999) and three future emission scenarios were used and are described in table 2. 

General Circulation Models

Given the uncertainty in climate modeling, it is desirable to use more than one GCM to obtain a range of potential future climatic 

conditions. Monthly precipitation and temperature output from five GCMs was processed (table 1). 

Table 2. Climate-change emission scenarios simulated by the General Circulation Models in this study.

Emission scenario Description/assumptions

20C3M 20th century climate used to determine baseline (1989–1999) conditions

A1B Rapid economic growth, a global population that peaks in mid-21st century and rapid introduction of new and 

more efficient technologies with a balanced emphasis on all energy sources

B1 Convergent world, with the same global population as Emission scenario A1B, but with more rapid changes in 

economic structures toward a service and information economy that is more ecologically friendly

A2 Heterogeneous world with high population growth, slow economic development, and slow technological change

Table 1. General Circulation Model (GCM) projections used in this study.

GCM Center and country of origin

BCC–BCM2.0 Bjerknes Centre for Climate Research, Norway

CSIRO–Mk3.0 Australia’s Commonwealth Scientific and Industrial Research Organization, Australia

CSIRO–Mk3.5 Australia’s Commonwealth Scientific and Industrial Research Organization, Australia

INM–CM3.0 Institute for Numerical Mathematics, Russia

MIROC3.2 National Institute for Environmental Studies, Japan
projects an increase in mean annual streamflow for the A1B 

emission scenario only (table 3), though the uncertainties associated with each of the streamflow projections are large (fig. 3A). 

The streamflow projections follow the same oscillating trends as 

the precipitation projections shown in figure 2C. Because of the 

increase in minimum and maximum temperatures, the percentage of precipitation that falls as snow is projected to decrease 

in the basin (fig. 3B, table 3), with a corresponding decrease in 

snowpack water equivalent (fig. 3C, table 3). Figure 3D shows 

decreasing soil moisture for the A1B and A2 emission scenarios 

(fig. 3D, table 3), indicating drier conditions in the basin by the 

end of the 21st century.

Projected changes in streamflow are displayed on a monthly 

basis in figure 4. The solid-red lines in figure 4 show PRMSsimulated mean monthly streamflow for current conditions 

(1989–1999). The boxplots represent the range in the projected 

mean monthly streamflow for the five GCMs and three scenarios for 2030 (green, 2025–2035), 2060 (tan, 2055–2065) 

and 2090 (blue, 2085–2095). The range of values indicated by 

the boxplots in figure 4 illustrate the high degree of uncertainty 

associated with the magnitude of these projected streamflow 

changes, especially during the months of April, May, and June. 

By the end of the 21st century, streamflow is projected to 

increase from November through April and decrease in May, 

Center of 11-year window

Maximum temperature,

degrees Celsius

A. Basin mean maximum temperature

2020 2040 2060 2080

7

8

9

10

11

12

13

7

8

9

10

11

12

13

Center of 11-year window

Minimum temperature,

degrees Celsius

B. Basin mean minimum temperature

2020 2040 2060 2080

-4

-3

-2

-1

0

1

2

-4

-3

-2

-1

0

1

2

Center of 11-year window

Precipitation, millimeters per day

C. Basin mean precipitation

2020 2040 2060 2080

2.8

3.0

3.2

3.4

2.8

3.0

3.2

3.4

EXPLANATION

Emission Scenarios

A2 A1B B1

Maximum

Mean from five General Circulation Models

Minimum

Figure 2. Projected range in 11-year moving mean daily values 

of (A) maximum temperature, (B) minimum temperature, and 

(C) precipitation by emission scenario. 

Table 3. Projected change by year (slope) and adjusted R2 (adjR2) based on the central tendencies of the five General Circulation Models for the 

three carbon emission scenarios for selected Precipitation Runoff Modeling System (PRMS)output variables.

[Blue indicates a significant negative trend and yellow indicates a significant positive trend (p<0.05) accounting for lag-1 autocorrelation].

PRMS output variable

Emission scenario

A1B

Emission scenario

A2

Emission scenario

B1

slope adjR2 slope adjR2 slope adjR2

Maximum temperature in degrees Celsius 0.031 0.98 0.040 0.99 0.024 0.98

Minimum temperature in degrees Celsius 0.033 0.99 0.040 0.98 0.024 0.98

Precipitation in millimeters per day 0.0040 0.87 0.0049 0.69 0.0022 0.51

Streamflow in cubic meters per second 0.0547 0.41 0.0676 0.22 -0.0010 -0.01

Percent snow in percent per day -0.18 0.97 -0.21 0.95 -0.13 0.94

Snowpack water equivalent in millimeters per day -1.21 0.95 -1.45 0.95 -1.04 0.92

Snowmelt in millimeters per day -0.0008 0.30 -0.0010 0.17 -0.0010 0.18

Soil moisture in millimeters per day -0.0456 0.69 -0.0760 0.86 -0.0322 0.38
June, and July. For current conditions (1989–1999, red line) 

peak timing of streamflow occurs in both May and June. The 

timing of peak streamflow is projected to shift by 2030 to May 

only, and remain in May through the end of the 21st century. 

These changes in mean monthly streamflow correspond to 

changes in mean monthly snowmelt (fig. 5). On an annual 

basis there are no significant trends projected for snowmelt 

(table 3). On a monthly basis, warmer temperatures will result 

in less snowfall and increased snowmelt in November through 

April, leaving less snow available to provide streamflow in May 

through July. The range of values indicated by the monthly boxplots illustrate the high degree of uncertainty associated with the 

magnitude of these projected changes in snowmelt (fig. 5) and 

streamflow (fig. 4), especially during June.

Center of 11-year window

Streamflow,

cubic meters per second

A. Basin mean streamflow

2020 2040 2060 2080

90

100

110

120

90

100

110

120

Center of 11-year window

Percent snow

B. Basin mean percent snow

2020 2040 2060 2080

45

50

55

60

65

70

75

45

50

55

60

65

70

75

Center of 11-year window

Snowpack water, millimeters

C. Basin mean snowpack water

2020 2040 2060 2080

50

100

150

200

250

300

50

100

150

200

250

300

Center of 11-year window

Soil moisture, millimeters

D. Basin mean soil moisture

2020 2040 2060 2080

5.5

6.0

6.5

7.0

7.5

5.5

6.0

6.5

7.0

7.5

Month

Streamflow,

cubic meters per second

Basin mean streamflow

 Feb. Apr. June Aug. Oct. Dec.

0

100

200

300

400

500

0

100

200

300

400

500

EXPLANATION

Baseline

Conditions

(1989-1999)

2030

(2025-2035)

2060

(2055-2065)

2090

(2085-2095)

Whisker, 90th percentile

Upper quartile line, 75th percentile

Median line, 50th percentile

Lower quartile line, 25th percentile

Whisker, 10th percentile

Outlier

Figure 3. Projected range in 11-year moving mean daily values of (A) streamflow, (B) precipitation that falls as snow, (C) snowpack 

water equivalent, and (D) soil moisture by emission scenario.

Figure 4. Mean daily streamflow values by month for baseline conditions and projected range (2030, 2060, and 2090) using the five General 

Circulation Models and three emission scenarios.
Conclusion and

 Discussion Selected References

By Katherine J. Chase, Lauren E. Hay and 

Steven L. Markstrom

For more information visit the following Web sites:

http://wwwbrr.cr.usgs.gov/projects/SW_MoWS/

http://mt.water.usgs.gov/

http://www.usgs.gov/climate_landuse/

Conclusion and Discussion

Hungry Horse Reservoir, on the South Fork Flathead River, 

is an important component in the Columbia River system of 

reservoirs operated by the Bureau of Reclamation. A demand 

for the water stored in the reservoir is increasing and climate 

projections indicate that the quantity and timing of streamflow 

entering the reservoir could be changing. PRMS simulations 

using GCM scenarios indicate that flows could increase in the 

winter and early spring and decrease in late spring and through 

the summer. Uncertainty associated with the magnitude of these 

changes in streamflow is large. These results did not consider 

potential future land-cover dynamics; changes caused by forest 

fire or pine beetle damage could be important. 

The combined effects of climate change and land-cover 

changes in the South Fork Flathead River Basin could affect 

water available for use in Montana and in downstream states. 

The scientific techniques described in the fact sheet can be augmented with other techniques in developing the science needed 

to address the combined effects of climate and land-cover 

dynamics on streamflow regimes.

Selected References

Berkas, W.R., White, M.K., Ladd, P.B., Bailey, F.A., and Dodge, 

K.A., 2004, Water resources data, Montana, water year 2004, 

Volume 2, Yellowstone and Upper Columbia River Basins 

and ground water levels: U.S. Geological Survey Water-Data 

Report MT–04–02, 55 p.

Chase, K.J., 2011, Development of a precipitation-runoff model 

to simulate unregulated streamflow in the South Fork Flathead River Basin, Montana: U.S. Geological Survey Scientific Investigations Report 2011–5095, 39 p.

Hay, L. E., Markstrom, S.L., and Ward-Garrison, C.D., 2011, 

Watershed-scale response to climate change through the 

twenty-first century for selected basins across the United 

States, Earth Interactions, v. 15, 37 p.

Intergovernmental Panel on Climate Change, 2007, Summary 

for policymakers, in Climate change 2007—The physical 

science basis, Contributions of Working Group 1 to the Fourth 

Assessment Report of the Intergovernmental Panel on Climate 

Change: Cambridge and New York, Cambridge University 

Press, 18 p.

Markstrom, S.L., Hay, L.E., Ward-Garrison, C.D., Risley, J.C., 

Battaglin, W.A., Bjerklie, D.M., Chase, K.J., Christiansen, 

D.E., Dudley, R.W., Hunt, R.J., Koczot, K.M., Mastin, M.C., 

Regan, R.S., Viger, R.J., Vining, K.C., and Walker, J.F., 2012, 

An integrated watershed scale response to climate change 

for selected basins across the United States: U.S. Geological 

Survey Scientific Investigations Report 2011–5077, 142 p.

Month

Snowmelt, millimeters per day

Basin mean snowmelt

 Feb. Apr. June Aug. Oct. Dec.

0

2

4

6

8

10

12

0

2

4

6

8

10

12

EXPLANATION

Baseline

Conditions

(1989-1999)

2030

(2025-2035)

2060

(2055-2065)

2090

(2085-2095)

Whisker, 90th percentile

Upper quartile line, 75th percentile

Median line, 50th percentile

Lower quartile line, 25th percentile

Whisker, 10th percentile

Outlier

Figure 5. Mean daily snowmelt values by month for baseline conditions and projected range (2030, 2060, and 2090) using the five General 

Circulation Models and three emission scenarios.