KEY ISSUES

ater is a central resource supporting human activities and ecosystems.

The hydrologic (water) cycle,a fundamental component of climate,is

likely to be altered in important ways by climate change. Precipitation is very

likely to continue to increase on average,especially in middle and high latitudes,with much of the increase coming in the form of heavy downpours.

Changes in the amount,timing,and distribution of rain,snowfall,and runoff are

very probable,leading to changes in water availability as well as in competition

for water resources. Changes are also likely in the timing,intensity, and duration of both floods and droughts,with related changes in water quality.

Snowpack serves as natural water storage in mountainous regions and northern

portions of the US, gradually releasing its water in spring and summer.

Snowpack is very likely to decrease as the climate warms,despite increasing

precipitation, for two reasons. It is very likely that more precipitation will fall

as rain,and that snowpack will develop later and melt earlier. As a result,peak

streamflows will very likely come earlier in the spring,and summer flows will

be reduced. Potential impacts of these changes include an increased possibility

of flooding in winter and early spring,a reduced possibility of flooding later in

the spring,and more shortages in summer.

Managed river systems provide opportunities to store water in reservoirs to

dampen the effects of changes in flow regimes,but this does not come without

environmental costs. Substantial infrastructure has been developed to store and

transport water supplies. There are more than 80,000 dams and reservoirs in

the US,and millions of miles of canals,pipes,and tunnels. Even in the absence

of climate change,adapting to existing stresses (such as aging infrastructure and

inadequate water supplies for growing areas) will be expensive. For a variety of

reasons,large dams are no longer viewed as a cost-effective or environmentally

acceptable solution to water supply problems,so other strategies must be

developed.

• Competition for Water

Supplies

• Surface Water Quantity

and Quality

• Groundwater Quantity

and Quality

• Floods, Droughts, and

Extreme Precipitation

Events

• Ecosystem

Vulnerabilities

WATER SECTOR

W

Water Withdrawals and Population Trends

3

2.5

2

1.5

1

0.5

300

200

100

0

Total withdrawals Population

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995

Although US population has continued to increase, total water withdrawals have stablized over the last decade. Stabilization of total withdrawals is due to increased water use efficiency and recycling in some

sectors, and a reduction in acreage of irrigated agriculture.

Total US Freshwater Consumptive Use

Irrigation

82%

Livestock

3%

Industrial

3%

Mining

Thermo Power 1%

3% Domestic

7%

Commercial

1%

96
Snowpack is very likely to

decrease as the climate

warms, despite increasing

precipitation, for two reasons.

It is very likely that more precipitation will fall as rain, and

that snowpack will develop

later and melt earlier. As a

result, peak streamflows will

very likely come earlier in the

spring, and summer flows will

be reduced. 

Columbia Basin Snow Extent

(Washington & Oregon)

4100 ft.

(Future)

3000 ft.

(Present)

In this model of

Columbia Basin snow

extent, complete loss

of snow cover is projected at lower elevations. These maps are

generated by downscaling output from

global to regional climate models for the

Columbia Basin.

Percentage change from the 1961-90 baseline in the April 1

snowpack in four areas of the western US as simulated for

the 21st century by the Canadian and Hadley models. 

April 1 snowpack is important because it stores water that

is released into streams and reservoirs later in the spring

and summer. The sharp reductions are due to rising temperatures and an increasing fraction of winter precipitation

falling as rain rather than snow. The largest changes occur

in the most southern mountain ranges and those closest to

the warming ocean waters. 

Rough estimate of how much snowlines in the Pacific Northwest

are likely to shift by 2050, assuming about 4oF warming.

Current

2025

2045

Year

Hadley Model

Year

-100

-80

-60

-40

-20

0

20

CCC Model

20

-100

-80

-60

-40

-20

0

Southern Rocky Mountains

Pacific Northwest

Sierra Nevada

Central Rocky Mountains

Changes in Western Snowpack

Canadian Model

Hadley Model

97

Southern Rocky Mountains

Pacific Northwest

Sierra Nevada

Central Rocky Mountains
98

Competition for Water Supplies

n many rivers and streams in the US,there is not enough water to satisfy

existing water rights and claims.Changing public values about preserving instream flows,protecting endangered species,and settling Indian water rights

claims have made competition for water supplies increasingly intense. Climate

change will very likely exacerbate competition in regions where fresh water

availability is reduced by increased evaporation due to rising air temperatures

and changes in precipitation. In some areas,however, an increase in precipitation could possibly outweigh these factors and increase available supplies.

Significant changes in average temperature,precipitation,and soil moisture

caused by climate change are very likely to also affect demand in most sectors,

especially in the agriculture,forestry, and municipal sectors. Irrigation water

needs are likely to change,with decreases in some places and increases in others. It is very likely that demand for water associated with electric power generation will increase due to the increasing demand for air conditioning with

higher temperatures,unless advances in technology make it possible for less

water to be used for electrical generation. Climate change is likely to reduce

water levels in the Great Lakes and summertime river levels in the central US,

thereby affecting navigation and general water supplies.

Surface Water Quantity and Quality

recipitation in the US has increased by 5-10% during the 20th century with

much of this increase attributed to heavy and very heavy precipitation

events.During this period,the relative increase in runoff has been even greater.

More data and analyses are needed to see how increases in heavy precipitation

are reflected in streamflow, but changes are likely in the future.Increases in

global temperatures have been accompanied by more precipitation in the middle and high latitudes and increases in atmospheric water vapor in many

regions of North America. These changes are significant and most apparent during spring through autumn in the contiguous US. Despite the overall increase

in precipitation,however, it is likely that many interior portions of the nation

will experience more extremes related to drought due to increased air temperatures. These changes in precipitation and evaporation are very likely to affect

the quantity of surface water, with substantial regional variation.

WATER KEY ISSUES

Palmer Drought Severity Index Change

I

P

In a warmer climate, hurricanes are likely to produce

more rainfall. The frequency

and intensity of droughts are

also likely to increase in some

areas due to higher air temperatures. 

The Palmer Drought Severity Index

(PDSI) is a commonly used measure of drought severity taking into

account differences in temperature,

precipitation, and capacity of soils

to hold water. These maps show

projected changes in the PDSI over

the 21st century, based on the

Canadian and Hadley climate scenarios. A PDSI of –4 indicates

extreme drought conditions. The

most intense droughts are in the –6

to –10 range, similar to the major

droughts of the 1930s. By the end

of the century, the Canadian scenario projects that extreme drought

will be a common occurrence over

much of the nation, while the

Hadley model projects small

changes in drought conditions.

Hadley Model 21st Centur y Canadian Model 21st Centur y

>10

8

6

4

2

0

-2

-4

-6

-8

-10
99

Rising temperatures are very likely to

affect snowfall and increase

snowmelt conditions in much of the

western and northern portions of the

US that depend on winter snowpack

for runoff. It is very likely that as the

climate warms,less precipitation will

fall as snow, the existing snowpack

will melt sooner and faster, the runoff

will be shifted from late spring and

summer to late winter and early

spring. This change in the timing of

runoff will very likely have implications for water management, flood

protection,irrigation,and planning.

Water quality is also likely to be

affected by climate change in a variety of ways. For example,more frequent heavy precipitation events will

very likely flush more contaminants

and sediments into lakes and rivers,

degrading water quality. Thus,it is

likely that pollution from agricultural

chemicals and other non-point

sources will be exacerbated. In some

regions,however, higher average

flows will likely dilute pollutants,

improving water quality. Where

streamflows are reduced,increased

salinity is a potential problem. Water

quality issues include potential

impacts on human health,such as

increased incidence of water-borne

diseases after flood events. Flooding

can cause overloading of storm and

wastewater systems,and damage

water and sewage treatment facilities,

mine tailing impoundments,and landfills,thereby increasing the risks of

contamination.

Rising water temperatures and

changes in ice cover are of particular

importance to the ecology of lakes,

streams,and their biological communities. Such changes are likely to

affect how ecosystems function,especially in combination with chemical

pollution. For example,when

warmer lake water combines with

excess nutrients from agricultural fertilizers (washed into the lake by

heavy rains),algae blooms on the lake

surface,depleting the ecosystem of

oxygen and harming the other organisms in the system.

Groundwater Quantity 

and Quality

everal regions of the US,including parts of California and the

Great Plains region,are dependent on

dwindling groundwater supplies.

Groundwater supplies are less susceptible than surface water to shortterm climate variability;they are

more affected by long-term trends.

Groundwater serves as the base flow

for many streams and rivers. In many

areas, groundwater levels are very

likely to fall,thus reducing seasonal

streamflows. Surface water temperature fluctuates more rapidly with

reduced volumes of water, likely

affecting vital habitats. Small streams

that are heavily influenced by

groundwater are more likely to have

reduced streamflows and changes in

seasonality of flows,likely damaging

existing wetland habitats.

Pumping groundwater at a faster rate

than it can be recharged is a major

concern,especially in parts of the

country that have no other supplies.

In the Great Plains, for example,

model projections indicate that

increased drought conditions are likely, and groundwater levels are already

dropping in parts of important

aquifers such as the Ogallala.

The quality of groundwater is being

diminished by a variety of factors

including chemical contamination.

Saltwater intrusion is another key

groundwater quality concern,particularly in coastal areas where changes

in freshwater flows and increases in

sea level will both occur.As groundwater pumping increases to serve

municipal demand along the coast

and less recharge occurs,coastal

groundwater aquifers are increasingly

affected by seawater. Because the

groundwater resource has been compromised by many factors,managers

are looking increasingly to surface

water supplies which are more sensitive to climate change and variability.

S

Observed Changes In Streamflow and Precipitation (1939-99)

The graph shows changes in the

intensity of precipitation and streamflow, displayed in 5% increments,

during the period 1939-99 based on

over 150 unregulated streams across

the US with nearby precipitation

measurements (with most of the colocated gages from the eastern half

of the US). As the graph demonstrates, the largest changes have

been the significant increases in the

heaviest precipitation events and the

highest streamflows. Note that

changes in streamflow follow

changes in precipitation, but are

amplified by about a factor of 3. 

Precipitation

Streamflow

0 20 40 60 80 100

Light/Low Moderate Heavy/High

22.5

20

17.5

15

12.5

10

7.5

5

2.5

0

-2.5
100

Floods, Droughts, and Extreme

Precipitation Events

hanges in climate extremes are

more likely to cause stress at

the regional level than changes in the

averages. Thus, changes in the timing

of precipitation events,as well as

increases in extreme events,are key

concerns. Climate change is likely to

increase flood frequency and amplitude in some regions,with major

impacts on infrastructure and emergency management. The 1999 North

Carolina flood, resulting from

Hurricane Floyd,offers a recent

example of the massive dislocations

and multi-billion dollar costs that

often accompany such events. In a

warmer climate,hurricanes are likely

to produce more rainfall. The frequency and intensity of droughts are

also likely to increase in some areas

due to higher air temperatures. This

is likely to have wide-ranging impacts

on agriculture, water-based transportation,and ecosystems. A recent

example of such impacts is the 1995-

96 drought in the agricultural regions

of the southern Great Plains that

resulted in about $5 billion in damages. The costs associated with

floods and droughts include those

incurred for building and managing

infrastructure to avoid damages as

well as costs associated with damages

that are not avoided. These costs are

in the billions of dollars and rising.

Ecosystem Vulnerabilities 

pecies live in the larger

context of ecosystems and

have differing environmental

needs. A change that is devastating to one species is likely to

encourage the expansion of

another to fill that niche in the system. Extreme conditions such as

floods,droughts,and fire are critical

to sustaining certain ecosystems,and

changes in the frequency of these

events are likely.

Rising temperatures in surface waters

are likely to force out some cold

water fish species such as salmon and

trout that are already near the threshold of their viable habitat. Increasing

temperatures also result in reduced

dissolved oxygen in water, reducing

ecosystem health. Temperature

increases will very likely reduce ice

cover and alter mixing and stratificaWATER KEY ISSUES

S

C

28

26

24

22

20

18

16

14

12

10

8

6

June July August September

Month

Tolerance level

for juvenile

Steelhead Trout

Simulated present-day

water temperature

Simulated water temperature

under 2xCO2 climate

Summer Stream Temperatures

Steamboat Creek, Oregon

Simulated summer stream temperatures under

present day climate (blue) and simulated temperatures under about a twice current CO2

climate

(purple). The dashed line at 24oC (75oF) on the

“water temperature” axis indicates the summer

temperature tolerance of juvenile steelhead trout.

Under doubled CO2, the model suggests that the

length of time within the year when the temperature tolerance limit is exceeded is more than twice

as long as under simulated present-day climate

conditions. Shaded area surrounding the doubled

CO2temperature curve indicates an estimate of

uncertainty.

Projected 21st Century Change in US Daily Precipitation

Percentiles HadCM2 CGCMI

Low

Moderate

Heavy

-10

0

10

20

30

40

50

60

0

20

40

60

80

95

Lightest 5% 

Percentiles HadCM2 CGCM1

Moderate

Heaviest 5% 

These projections by the Hadley and Canadian models show the changes in precipitation over the 21st century.

Each models' projected change in the lightest 5% of precipitation events is represented by the far left bar and the

change in the heaviest 5% by the far right bar. As the graph illustrates, both models project significant increases

in heavy precipitation events with smaller increases or decreases in light precipitation events.
101

tion of water in lakes,all of which are

key to the nutrient balance and habitat value. The natural ecosystems of

the Arctic,Great Lakes,Great Basin,

Southeast,and the prairie potholes of

the Great Plains appear highly vulnerable to the projected changes in climate. In regions where runoff

increases, existing stresses and threats

to biodiversity could possibly be

reduced.

Adaptation Strategies

trategies for adapting to climate

change range from changes in the

operation of dams and reservoirs,to

re-evaluating basic engineering

assumptions used in facility construction,to building new infrastructure.

Options also include water conservation,use of reclaimed wastewater,

water transfers,and increasing prices

(which encourages increases in efficiency of use). Because many of the

impacts of climate change are not

predictable,more flexible institutional arrangements are needed in order

to adapt to changing conditions

including not only climate change,

but other existing stresses as well.

Water rights systems vary from state

to state, with even more differences

at the local level. Most institutions

related to water have not responded

well to changing socioeconomic and

environmental conditions.

Some have argued that an open market in water rights would help

resolve conflict and increase efficiency because water would flow to the

highest and best use based on willingness to pay. Although major social,

equity, and environmental considerations must be addressed,market solutions appear to have great potential

to help resolve supply problems in

some parts of the US.

In considering adaptation mechanisms,it is important to point out

that humans have a great ability to

adapt to change,while natural ecosystems are likely to be more vulnerable.

Some potential adaptation options for

human water management in

response to climate change and other

stresses follow.

• Increase ability to shift water within and

between sectors (including agriculture to

urban).

• Use pricing and market mechanisms

proactively to increase efficiency of water

use.

• Incorporate potential changes in demand

and supply in long-term planning and

infrastructure design.

• Create incentives or requirements to

move people and structures out of floodplains.

• Identify ways to manage all available supplies, including groundwater, surface

water, and effluent, in a sustainable manner.

• Restore and maintain watersheds (for

example, by restoring appropriate vegetation) as an integrated strategy for managing water quality and quantity. Restoring

watersheds that have been damaged by

urbanization, forestry, or grazing can

reduce sediment loads and nutrients in

runoff, limit flooding, and reduce water

temperature.

• Reuse municipal wastewater, improve

management of urban storm water runoff,

and promote collection of rainwater for

local use to enhance urban water supplies.

• Increase the use of forecasting tools for

water management. Some weather patterns, such as those resulting from El

Niños, can now be predicted with some

accuracy, and this can help reduce damages associated with extreme events.

• Enhance monitoring efforts to improve

data for weather, climate, and hydrologic

modeling to aid understanding of waterrelated impacts and management strategies. S

The natural ecosystems of

the Arctic, Great Lakes,

Great Basin, Southeast,

and the prairie potholes of

the Great Plains appear

highly vulnerable to the projected changes in climate. 

Prairie pothole.

The Central Arizona Project brings Colorado River

water 330 miles uphill to Tucson and Phoenix, Arizona.

Land subsidence, caused by overpumping of groundwater, can result in

earth fissures such as this near Eloy,

Arizona. 

Artificial groundwater recharge in the Santa Ana

Riverbed, Orange County Water District, California.