U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3098

February 2015

Climate Change: Evaluating Your Local and Regional Water 

Resources

The Basin Characterization Model

Ongoing changes in climate, particularly rises in air 

temperature, are influencing water resources throughout the 

world, with shifts in the timing of precipitation, reductions in 

snowpack (fig. 1), and earlier springtime snowmelt among the 

most important challenges to water availability (fig. 2). These 

changes are affecting landscapes, vegetation and animal species, 

and agriculture, with longer dry seasons, which also create more 

demand on water resources, more frequent extreme storms, 

fewer chilling hours, and higher snowlines.

 To plan for future water- and land-resource demands, 

resource managers have generally relied on data from globalclimate model projections of precipitation and air temperature 

trends that lack the detail needed for precision planning at 

regional and local scales. Recently, the U.S. Geological Survey 

(USGS) has developed modeling tools that integrate climate data 

with rigorously developed regional and local environmental data 

to understand the hydrologic response to climate change and the 

effects on regional and local watersheds and landscapes.

New water-balance modeling tools can provide the types of 

information managers require to develop climate-change coping 

strategies:

Watershed recharge and runoff quantification—to assess 

water availability, seasonality, and extremes at the land surface.

Climatic water deficit estimation—to assess irrigation demand 

or landscape stress (climatic water deficit is the amount of water 

plants would use if it were available).

Spatial distribution of hydrologic processes in watersheds—

for resource planning and infrastructure development.

To develop management strategies for coping with climate 

change, land and resource managers need hydrologic derivatives 

of climate, such as recharge and runoff quantities, to assess 

water availability or climatic water-deficit estimates to assess 

projected changes in irrigation demand or landscape stress. 

Regional water-balance modeling can be used effectively to 

estimate the spatial distribution of these hydrologic variables 

and can differentiate between recharge and runoff to provide 

valuable information for resource planning and infrastructure 

development.

0 100 KILOMETERS

0 100 MILES 50

50

Change in April 1 SWE

(mm/yr)

EXPLANATION

< –500

–500 to <–100

–100 to <–50

–50 to –10

0

10 to 50

>50 to 100

>100 to 500

>500

124° 120°

42°

38°

34°

Figure 1. Change in snow water equivalent (SWE) between 1951–1980 and 

1981–2010 in California. These decreases in snowpack reflect world-wide 

trends in all but the highest elevations over the last half-century.

Runoff 1981–2010

(mm/yr)

EXPLANATION

0 to 10

>10 to 100

>100 to 200

>200 to 300

>300 to 400

>400 to 500

>500 to 600

>600 to 700

>700 to 800

>800 to 900

>900 to 1,000

>1,000 to 3,650

124° 120°

42°

38°

34°

0 100 KILOMETERS

0 100 MILES 50

50

Figure 2. Runoff in California, 1981–2010, estimated by using the Basin 

Characterization Model, a regional water-balance model used throughout 

the Western U.S. and internationally.
The Basin Characterization Model (BCM) is a mathematical computer 

model that calculates the hydrologic inputs and outputs of a specific landscape 

area (fig. 3) and is generally run at a monthly timestep for large regions (fig. 1), 

although daily models have been developed for small watersheds. Scientists divide 

the landscape into grid cells, each of which uses specific climate data inputs, such 

as precipitation and air temperature, to solve the water balance for each cell. Model 

calculations include potential evapotranspiration, calculated from solar radiation 

with topographic shading and cloudiness; snow, as it accumulates and melts; and 

excess water moving through the soil profile, which is used to calculate actual 

evapotranspiration and climatic water deficit—the difference between potential 

and actual evapotranspiration. Depending on soil properties and the permeability of 

underlying bedrock, surface water can be classified for each cell as either recharge

or runoff. Post-processing calculations are made to estimate baseflow, streamflow, 

and potential recharge to the groundwater system for watersheds (fig. 4). The model 

output can define the water balance for any size polygon representing regions or 

watersheds, or can define the distribution of the various water-balance variables 

across the landscape.

Watershed

available water

(excess water)

Solar radiation

Potential

evapotranspiration

(PET)

Precipitation

Air

temperature

Sublimation

Snow

accumulation

Snowmelt

Local

runoff

Local

recharge

Basin

discharge

Basin

groundwater

recharge

Actual

evapotranspiration

(AET)

Climatic

water deficit

(PET-AET)

Snow processes Climate inputs Energy balance

Water balance

Vegetation &

landscape

Water supply

100

Soil profile

0

–0.01

–6

–1,000

Total soil porosity about 30–70 percent

Field capacity

Fractured

bedrock

Wilting point

Infilled

fractures

Open

fractures

Plant available

water

Soil 10–60

2–20

0

Soil water content,

in percent saturation

Soil water potential, in MPa

Processes represented in the BCM

Figure 3. The Basin Characterization Model relies on climate input and the rigorous development 

of potential evapotranspiration to move water through the soil profile and into underlying bedrock to 

become recharge or runoff. [MPa, megapascal]
Global-Climate Models and Downscaling

Downscaling to 270-meters captures data with enough detail to reflect the deterministic factors influencing the water balance 

(energy loads, topographic shading) and enables the use of landscape-level mapping of soil properties, vegetation distributions, 

and land use to improve the characterization of watershed and landscape processes and climate (fig. 4).

EXPLANATION

Maximum air temperature

June 2035 (degrees in Celsius)

7–20

>20–23

>23–25

>25–28

>28–30

>30–32

>32–34

>34–37

>37–40

>40–47

0 25 50 MILES

0 5025

12-km

270-m Owens Valley

Sierra Nevada

Central Valley

Figure 4. Global Climate Models that project changes in precipitation and air temperature for the 21st century are downscaled to fine spatial scales for 

application to process-based hydrologic models.

Calibration and Uncertainty: What Does This Mean to 

Your Management Issue?

In general, models are calibrated to optimize model output 

accuracy by minimizing the differences between simulated 

model outputs and observations. BCM calibrations have 

been done for basins throughout the western U.S. The BCM 

calculates unimpaired streamflow and compares it to streamflow 

in unregulated, upstream tributaries, such as Upper Dry Creek 

in the Russian River Basin (fig. 5A). Streamflow diversions 

and estimates of agricultural demand can be included in the 

calibration for comparison with impaired mainstem streamflow, 

such as those measured downstream at the Guerneville gage. A 

final mainstem calibration at a daily timestep is shown for the 

Russian River at Hopland, California (fig. 5B), which provides 

confidence in the model extrapolating streamflow into the future.

100

1,000

10,000

100,000

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Basin discharge,

in acre-feet per day

B

A

Guerneville Upper Dry Creek

Reservoir

Agricultural 

losses

Diversions Runoff

Baseflow

Shallow groundwater flow

Discharge Runoff

Baseflow

Deep groundwater flow

Discharge 

BCM

Main stem Tributary

(impaired flows) BCM (unimpaired flows)

Recharge

Recharge

October

Observed BCM estimated flow

EXPLANATION

Figure 5. Basin Characterization Model (BCM) model calibration is based on streamflow measurements, shown as an example using A, a schematic of the flow 

processes in the Russian River basin, including unimpaired and impaired conditions considered in model calibration, and B, final flow calibration for Russian River at 

Hopland, California.
Climate Change Affects Water Availability in Several 

Ways

Timing and Amount

The timing of springtime snowmelt is controlled by air 

temperature and has been earlier in recent years (fig. 6A). 

Regardless of the amount of precipitation, less is likely to fall as 

snow, and snowpack will not maintain the water supply as long 

into the dry season as at present (fig. 6B).

Extremes

Atmospheric rivers bringing large winter storms, along 

with extended droughts, create conditions leading to flooding, to 

declines in water quality, to agricultural effects, and to threats to 

fisheries.

sac13-0515_fig 06a

100

0

200

300

400

500

600

700

800

900

1,000

200120042007201020132016201920222025202820312034203720402043204620492052205520582061206420672070207320762079208220852088209120942097

April 1 snow water equivalent, in millimeters

A

GFDL A2 PCM A2

EXPLANATION

Year

Figure 6. Change in April 1st snow water equivalent projected to the end of the 21st century is shown A, for the Indian Creek basin and B, change in recharge is shown 

for the entire Klamath River Basin.

EXPLANATION

–100 to <–60

–60 to <–40

–40 to <–20

<–20 to 0

>0 to 20

> 20

124° 123° 122° 121°

42°

41°

40°

B

Change in recharge

(GFDLA2 projection)

Change, in percent

1971 to 2000 2071 to 2100

0 50 MILES

0 50 KILOMETERS 25

25

Photo taken by S.B. Weiss.

Photo taken by L.E. Flint.
Climate Change and Infrastructure Needs 

The dominant natural hydrologic process that brings water into a watershed 

influences the kind of infrastructure that has been developed to store and move 

water to users. With climate change, these natural hydrologic processes can shift, 

possibly causing a need for new infrastructure or water-supply management 

changes. These shifts reflect changes in the seasonal timing of precipitation and 

snowmelt, which, when combined with soil thickness, influence how much water 

recharges basin groundwater rather than runs off. Loss of snow cover, along 

with the presence of deep soils, can increase recharge during climate-related 

compressed wet seasons and the longer dry seasons. In contrast, where there are 

shallow soils, higher peak flows can increase runoff. For example, the Russian, 

Tuolumne, Merced, and American River Basins all rely heavily on reservoir 

storage to maintain the water supply and to sustain it through the dry season. In the 

American River Basin, there is a projected increase in recharge relative to runoff 

(fig. 7). The Russian River, Tuolumne, and Merced could be losing recharge. 

These changes can affect downstream reservoir operations to the extent that efforts 

to implement groundwater-storage management changes are warranted.

123°30' 123°

39°

38°30'

0 10 20 MILES

0 10 20 KILOMETERS

Russian

120°30' 120° 119°30'

38°

37°30'

37°45'

Tuolumne

Merced

39°10'

39°

38°50'

38°40'

American

121° 121°30‘

EXPLANATION

Change in ratio of recharge to runoff

(1981−2010) to (2071−2100)

Recharge/runoff increases

< –2 Recharge/runoff decreases

–2 to –1

>–1 to 1

>1 to 2

>2 

Figure 7. Change in ratio of recharge to runoff for the GFDL A2 projection. Cool colors indicate an 

increase in recharge relative to runoff; warm colors indicate a decrease in recharge relative to runoff.

Effects to Landscapes, Vegetation, and Agriculture

Climatic Water Deficit (CWD)

The CWD is the annual evaporative demand that exceeds available water. It 

is calculated as potential evapotranspiration minus actual evapotranspiration and 

integrates climate, energy loading, drainage, and changes in soil moisture in a 

single variable (fig. 8A). It is strongly correlated with the distribution of vegetation 

in a landscape. Because the CWD accumulates the deficit over the season, it is 

an indicator of the irrigation demand required to make up the seasonal deficit. 

Interestingly, because the CWD is limited by the capacity of soils to hold extra 

precipitation that would become runoff or recharge during the wet season, by 

the end of the summer, it increases with all climate change scenarios, even when 

wetter years are projected (fig. 8B). 

sac13-0515_fig 08A

A. 122°30' 122°

38°

38°30'

37°30'

0 10 20 MILES 20 MILES

0 10 20 KILOMETERS 20 KILOMETERS

<775

775 to 800

>800 to 825

>825 to 850

>850 to 875

>875 to 900

>900 to 925

>925 to 950

>950 to 975

>975 to 1,000

EXPLANATION

2001 Climatic water deficit,

 in millimeters per year

N

B.

<20

20 to 50

>50 to 100

>100 to 150

>150 to 200

>200 to 300

>300 to 400

>400 to 500

>500 to 600

>600 to 700

EXPLANATION

Climatic water deficit,

 in millimeters per year

MIROC A2, 2080

40°

10°

20°

30°

50°

0 500 MILES

0 500 KILOMETERS

250

250

Figure 8. Climatic water deficit (CWD) for A, the 

San Francisco Bay area watersheds, where the CWD 

variation is clear as a result of coastal proximity, north 

and south facing hillslopes, and differing soil depths; 

and B, the Brazilian Atlantic Forest, which covers 

much of Brazil, where the CWD, as projected for 2080, 

varies from north to south.
BCM Applications and Opportunities

The BCM regional water-balance model (Flint and others, 2013) applies boundary 

conditions to groundwater-flow models and provides linkages to integrated hydrologic 

models. It has been applied to local watersheds, regional landscapes, and international 

projects.

Local

• Russian River Basin, California—water management strategies and climate change.

• San Diego watershed, California—groundwater investigations.

• Modoc National Wildlife Refuge, California—water availability and landscape 

stresses under climate change.

 Regional

• Sierra Nevada—wolverine habitat, snowmelt and water availability. 

• Western U.S.—forest die-off and wildfire severity correlations to climate and 

climatic water deficit.

• Great Basin and Upper Colorado River Basins—landscape stress for threatened 

species and distribution of groundwater recharge.

 International

• Tigris-Euphrates River Basin (Iraq)—water availability.

• Brazilian Atlantic Forest—landscape stresses and water availability.

• Central America—small coffee farm sustainability.

Regional water-balance modeling can be used to optimize or prioritize management resources, to assess the spatial distribution of landscape stress or irrigation demand 

under current or potential future climates, and to evaluate potential changes in species 

distributions as well as climatic and hydrologic refugia. 

Summary and Implications

The BCM is a fine-scale hydrologic model that uses detailed maps of soils, geology, topography, and transient monthly or daily maps of potential evapotranspiration, air 

temperature, and precipitation to generate maps of recharge, runoff, snow pack, actual 

evapotranspiration, and climatic water deficit. With these comprehensive environmental 

inputs and experienced scientific analysis, the BCM provides resource managers with 

important hydrologic and ecologic understanding of a landscape or basin at hillslope 

to regional scales. The model is calibrated using historical climate and streamflow data 

over the range of geologic materials specific to an area. Once calibrated, the model is 

used to translate climate-change data into hydrologic responses for a defined landscape, 

to provide managers an understanding of potential ecological risks and threats to water 

supplies and managed hydrologic systems. Although limited to estimates of unimpaired 

hydrologic conditions, estimates of impaired conditions, such as agricultural demand, 

diversions, or reservoir outflows can be incorporated into the calibration of the model 

to expand its utility. Additionally, the model can be linked to other models, such as 

groundwater-flow models (that is, MODFLOW) or the integrated hydrologic model 

(MF-FMP), to provide information about subsurface hydrologic processes. The model 

can be applied at a relatively small scale, but also can be applied to large-scale national 

and international river basins.

Technical Contacts:

Lorraine E. Flint, research hydrologist, lflint@usgs.gov

Alan L. Flint, research hydrologist, aflint@usgs.gov

California Water Science Center, Placer Hall, 6000 J St., Sacramento, CA 95819

James H. Thorne, environmental ecologist, jhthorne@ucdavis.edu

University of California, Davis, CA 95616

Products from the California BCM can be obtained from http://climate.calcommons.org

Future climate scenarios applied to the California BCM are described at 

http://ca.water.usgs.gov/projects/reg_hydro/projects/dataset.html

Reference Cited 

Flint, L.E., Flint, A.L., Thorne, J.H., and 

Boynton, R., 2013, Fine-scale hydrological 

modeling for climate change applications 

using watershed calibrations to assess model 

performance for landscape projections: 

Ecological Processes v. 2, no. 25, 21 p.

Flint, L.E., and Flint, A.L., 2012, Downscaling 

climate change scenarios for ecologic 

applications: Ecological Processes, v. 1, no. 2, 

15 p.

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/FS20143098