STATEMENT OF

CHRISTOPHER MILLY

RESEARCH HYDROLOGIST

U.S. GEOLOGICAL SURVEY

U.S. DEPARTMENT OF THE INTERIOR

BEFORE THE

COMMITTEE ON ENERGY AND NATURAL RESOURCES,

SUBCOMMITTEE ON WATER AND POWER,

UNITED STATES SENATE,

HEARING ON

IMPACTS OF CLIMATE CHANGE ON

WATER SUPPLY AND AVAILABILITY IN THE UNITED STATES,

AND RELATED ISSUES FROM A WATER USE PERSPECTIVE

JUNE 6, 2007

# Introduction

Mr. Chairman and Members of the Subcommittee, thank you for this
opportunity to present testimony on the impacts of climate change on
water supply and water availability in the United States.

Water is the life-blood of the Nation. Water keeps our bodies hydrated
and clean and sanitizes our living spaces. Water in the soil grows the
food we eat. We use water in the processing of food and fuel and the
manufacture of products. Water flowing through our rivers produces
electricity and transports cargo. Water is habitat and highway for fish
and fowl. Water, liquid or frozen, is the Nation's playground in summer
and winter.

The distribution of water across the Nation depends largely on climate.
Changing climate is now affecting the availability of water in the
United States.

Water availability can be measured in many ways: precipitation,
streamflow, reservoir levels, snow pack, soil moisture, glaciers, and
water tables. Precipitation is the gross income of our Nation's water
budget; streamflow is the net income---what remains after the
evapotranspiration tax has been extracted. Snow pack, reservoir levels,
soil moisture and water tables represent the contents of our water bank
accounts. The bank accounts are important for getting us through hot dry
summers or the inevitable years of drought, but ultimately streamflow is
the single best measure of disposable income in our national water
budget.

How might water availability be expected to respond to a general
climatic warming? The behavior of the water substance is very sensitive
to temperature variations. Warm ice melts. Warm water expands. Warm air
can hold more water vapor. Together with some more advanced atmospheric
physics, which predicts subtle shifts in atmospheric circulation, these
facts suggest the changes in water availability that can result from
warming:

-   Systematic regional increases and decreases of total annual
    streamflow.

-   Rising sea level, resulting in increasing risk of contamination of
    coastal water supplies.

-   Loss of snow pack, resulting in increased winter streamflow and
    winter flood risk and decreased summer streamflow.

# Observed Streamflow Changes

During the last several decades, annual streamflow in the United States
fluctuated widely over time. In 1988, the Ohio River gave 115 million
acre feet of water to the Mississippi; the next year it gave 270 million
acre feet. Such wide variation is a normal state of affairs.

The normal ups and downs of annual streamflow are superimposed upon more
subtle, longer-term changes. In recent decades, the U.S. Midwest and
Alaska became wetter, while the U.S. Southwest became drier. For
example, the flow of the Ohio River at Metropolis, Ohio, during the last
30 years was 12 percent higher than during the preceding 48 years of
observations. The flow of the Colorado River at Lees Ferry, Arizona, was
3 percent lower than during the preceding 71 years (after making
adjustments for flow decreases associated with water withdrawals). The
flow of the Yukon River at Eagle, Alaska, was 3 percent higher than
during the preceding 26 years.

Long-term changes in seasonal timing of streamflow, possibly related to
warming-induced changes in snowfall and snowmelt, have also been
observed. As the western United States has warmed during recent decades,
a tendency toward earlier timing of streamflow has been noted. Similar
trends toward earlier streamflow have been seen in the northern tier of
the eastern United States. In both regions, seasonal streamflows are
typically rising and falling about a week earlier in the year during
recent decades than in the prior period of record.

**Causes of Observed Streamflow Changes: Normal Variability vs. Forced
Climate Chang**e

On the basis of statistical analyses of streamflow measurements,
tree-ring records, and models, it appears that the recent long-term
changes in annual streamflow observed over large areas of the United
States were not unprecedented. Consequently, taken alone, these
streamflow changes are not unequivocal evidence of forced climate
change, but might be explained as mere manifestations of natural,
internal variability in the climate system.

However, these data need not be taken alone. We have other sources of
information, including streamflow measurements from around the world and
computer simulations of changing climate in the United States and the
rest of the world. The observed pattern of a wetter Midwest, a drier
Southwest, and a wetter Alaska is also the pattern that emerges from
climate models when they try to simulate streamflow during the 20^th^
century. And, when we look at a global comparison of observed and
climate-modeled changes in annual streamflow during the 20^th^ century,
this rough agreement for the United States is repeated over and over on
the other continents. Analysis suggests that such a level of agreement
across the globe would be very unlikely to arise simply by chance. On
the basis of this global perspective, we conclude that the same factors
causing global warming are changing the global water cycle. The change
in the global water cycle, in turn, contributes to the observed changes
in streamflow and water availability in the

United States.

The earlier streamflow timing observed in the western and northeastern
United States has been correlated with rising temperatures, but changes
in precipitation amounts and timing have also played a role. Changes in
streamflow timing have not been clearly attributed to forced climate
change. However, we can say that the observed changes in streamflow
timing are qualitatively consistent with expected impacts of forced
climate change.

# Predicting Future Water Availability

It is not valid simply to extrapolate the observed past changes in water
availability forward into the future. However, the demonstrated skill of
climate models in simulating the global pattern of 20^th^-century change
in annual streamflow means that those models are credible, though far
from perfect, tools for looking into the future. Given best assumptions
about future atmospheric carbon dioxide concentrations and other drivers
of climate change, these models project a long-term drying trend in the
Southwest and moistening trends in the Midwest and Alaska. The drying
trend in the Southwest can be expected to imply also an increasing
probability of occurrence of Southwestern drought.

![](media/image1.png){width="3.85625in"
height="2.248611111111111in"}![](media/image2.png){width="3.1708333333333334in"
height="0.3611111111111111in"}

**Figure. Model-projected percentage change in production of streamflow,
by State, from 1900-1970 to 2041-2060. Colored States: more than 66
percent of 12 climate models agreed on the direction (increase vs.
decrease) of change. Diagonal stippling: more than 90 percent of models
agreed.**

These projections, at best, are only crude caricatures of the real
future. Are they the best that we can realistically hope for? Not at
all. There is much room for improvement:

-   Climate models typically represent conditions over areas larger than
    the State of Maryland by a single point. Such an approach has been
    adequate to assess global warming. However, climate varies
    geographically on a much finer scale, especially in mountainous
    regions. Therefore, to assess practical impacts on water and to
    design, plan, and implement needed adaptations, water managers need
    information on a much finer spatial scale, more like that of a
    county. To deliver this, much-higher-resolution climate models are
    needed.

-   The Nation has no comprehensive network of streamflow measurement
    stations dedicated to monitoring long-term changes in streamflow in
    natural, developed, and developing environments across the national
    landscape. The available measurements, assembled from stations
    established for other purposes, have proven critical for the
    progress that has been made in detecting global changes in water
    availability. However, keeping higher-resolution models honest and
    tracking ongoing changes in water availability will require
    higher-resolution measurements.

-   Climate models currently ignore the effects of water-resource
    development, land use, and land-cover change on climate. This has
    not been identified as a crucial impediment for global analyses, but
    it probably matters at the finer spatial scale of water management.

-   Water shortages come about when supply falls short of demand.
    Increased demand can create shortage, even when supply is stable. A
    change in climate causes a change in water demand, e.g., for
    irrigation and for natural ecosystems. Our understanding of this
    relation between climate and water demand needs improvement.

-   Production of better climate information is necessary but not
    sufficient to assess future impacts. Climate information needs to
    come in a form that is relevant to water management. In order to
    ensure the relevance of climate-model information to water managers,
    accelerated and continuing dialogue will be needed between climate
    science and water managers.

-   To make best use of available information in a changing climate,
    water management will need to adopt more flexible tools than those
    that have sufficed in the past. These new tools, unlike those that
    currently do the lion's share of water-system planning and design,
    must recognize that climate will change during the lifetime of a
    project and that estimates of the changing climate are uncertain.
    This will require a sea change in the field of water management.
    Such a change will not be accomplished without a concerted effort by
    government, academia, and professional societies.

Mr. Chairman, thank you for this opportunity to present testimony. I
will do my best to answer any questions that you or other Members of the
Subcommittee may have on this topic.