(Chapter 14) of the Foundation document of Climate Change Impacts on the United States

31 Pages·2001·0.39 MB·English

CHAPTER 14
POTENTIAL CONSEQUENCES OF CLIMATE
VARIABILITY AND CHANGE FOR THE
WATER RESOURCES OF THE UNITED
STATES
Katharine Jacobs1,2,D.Briane Adams3,and Peter Gleick4
Contents of this Chapter
Chapter Summary
Background
Socioeconomic and Institutional Context
Climate Variability and Change
Key Issues
Competition for Water Supplies
Surface Water Quality
Groundwater Quantity and Quality
Heavy Precipitation,Floods,and Drought
Ecosystem Vulnerabilities
Adaptation Strategies
Crucial Unknowns and Research Needs
Literature Cited
Acknowledgments
1Arizona Department of Water Resources;2Coordinating author for the National Assessment Synthesis Team;3US
4
Geological Survey, Pacific Institute for Studies in Development,Environment and Security. 405
Potential Consequences of Climate Variability and Change
CHAPTER SUMMARY
Context Climate of the Past Century
Water supply conditions in all regions and sectors in • Increases in global temperatures have been
the US are likely to be affected by climate change, accompanied by more precipitation in the mid
either through increased demands associated with and high latitudes.
higher temperatures,or changes in precipitation and • Precipitation has increased an average of 10%
runoff patterns. Water sector concerns include across the US,with much of the increase attrib-
effects on ecosystems,particularly aquatic systems uted to heavy precipitation events.
such as lakes,streams,wetlands,and estuaries. • Nationally,streamflow has increased about three
Although competition for water supplies is extreme- times more than the increase in precipitation.
ly intense,particularly in the western US,substantial Regionally,the higher streamflows have
ability to adjust to changing demands for water increased in many areas,but not in the West
exists in the current water management system. It where snowmelt dominates peak flows.
is not known whether the effects of climate change • Reductions in areal extent of snowpack in the
will require dramatic changes in infrastructure to western mountains have been observed,along
control flooding and provide reliable water supplies with substantial retreat of glaciers.
during drought. However,it is known that precipita- • In snowpack-dominated streams,a shift has been
tion and temperature changes are already increasing observed in the timing of the peak runoff to ear-
runoff volumes and changing seasonal availability of lier in the season.
water supply,and that these changes are likely to be • No significant increases in the frequency of
more dramatic in the future. droughts or winter-type storms have been
observed on a national basis.
Climate of the Coming
Century
• Historic trends towards increased precipitation
are very likely to continue.
• It is possible that there will be an increase in
interannual variability,resulting in more severe
droughts in some years.
• The Canadian and Hadley climate models used in
the Assessment generally do not agree on precip-
itation impacts,with the exception of showing
an increase in precipitation in the Southwest.
• Increases in temperature,even in the context of
increases in precipitation,are likely to result in
significant loss of soil moisture in the Northern
Great Plains.
• Snowpack is very likely to be reduced even in
the context of higher precipitation.
• If the number of high intensity storm events
increases,flushing of contaminants into water-
sheds is likely to increase,causing episodic water
quality problems.
• Quality and quantity impacts are very likely to be
regionally specific.
• Surprises are likely,since many water-related
impacts cannot be predicted.
406
Chapter 14 / Water Resources
Key Findings
• More pressure on surface water supplies is likely • It is likely that aquatic and riparian ecosystems
to come from population shifts and changes in may be damaged even in the context of higher
water right allocations to accommodate endan- precipitation,due to higher air temperatures and
gered species and the water rights of Native reduced summer flows. It is also probable that
Americans. Although wetter conditions in the changes in water temperature in lakes and
Southwest may alleviate some of these stresses, streams will affect species composition.
stress is likely to increase in the Northern Great • Water managers have multiple opportunities to
Plains and in snowpack-dependent watersheds. reduce future risks by incorporating “no-regrets”
• Groundwater supplies are already over-drafted in changes into their operating strategies that are
many parts of the country,and pressure on appropriate regardless of climate change.
groundwater supplies is likely to increase to off- • Institutions governing water rights are generally
set changes in surface water supply availability. very inflexible,and are likely to prove to be
However,long-term increases in precipitation obstacles to adaptation.
will possibly increase recharge rates in some • Improvements are needed in monitoring efforts
areas. to identify key impacts related to water quantity
and quality,biological conditions of key habitats,
snowpack conditions,and groundwater supplies.
407
Potential Consequences of Climate Variability and Change
POTENTIAL CONSEQUENCES OF CLIMATE
VARIABILITY AND CHANGE FOR THE WATER
RESOURCES OF THE UNITED STATES
BACKGROUND Increases in greenhouse gas concentrations are very
likely to affect global temperature and lead to
changes in the amount,timing,and geographic dis-
Water-related concerns are central to this National
tribution of rain,snowfall,and runoff. Changes are
Assessment because the hydrologic (water) cycle is
also likely in the timing,intensity,and duration of
a fundamental component of climate and because
extreme events such as floods and droughts. Such
water plays a role in every sector and region in the
changes possibly will have greater impacts on the
US. Despite many remaining uncertainties,a signifi-
regions and sectors than changes in average temper-
cant amount of research has been done on the con-
ature or precipitation. Higher demand for water is
nections between climate change and water
probable in areas where increased temperature
resources in the US;a searchable bibliography of
results in higher evapotranspiration. Although in
almost 900 scientific articles is available at
most regions it is possible that increased streamflow
http://www.pacinst.org.
would relieve current stress,some regions are likely
to experience greater difficulty in meeting their
The US has a wide variety of tools,institutions and
water supply needs if precipitation increases do not
methods for coping with water resource problems,
offset increases in evapotranspiration. Key variables
and many of these will be useful for addressing the
in determining likely impacts and responses include
impacts of climate changes. Water managers already
changes in soil moisture and cloud cover,seasonality
deal with climate variability;reservoirs are designed
of precipitation,and the response of vegetation to
with some flexibility for extreme high and low
changes in moisture,temperature,and increased car-
flows;techniques and technologies are available for
bon dioxide availability.
managing water demands. But global climate
change raises some unresolved concerns for the
While many of the most significant impacts in the
water sector. What will be the economic costs of
agricultural,forestry,ecosystem,energy,and human
coping with climate changes imposed on top of
health sectors relate to the basic issue of water
existing variability? Are existing institutions suffi-
availability,it is likely that there will be some serious
ciently flexible to handle the additional stresses?
impacts on water quality as well. There is a direct
What might be the nature of unexpected climate
relationship between quantity of flows and dilution
“surprises”for the water sector? And will water
of pollutants in surface water;higher runoff is likely
managers be willing and able to prepare in advance
to improve water quality,but increased intensity of
for conditions different from those they are normal
rainfall will probably result in increased erosion and
ly faced with?
flushing of contaminants into watersheds. Higher
water temperature will affect the ecology of wet-
lands,lakes,and streams. Much less research has
been done on impact-related issues.
The primary water resource issue for the US is the
distribution of supply and demand,not the total
quantity of water available. The nature of water con-
cerns varies by region across the country. For much
of the western US,water resources are often sepa-
rated both by time and distance from water
demands. As a result,substantial infrastructure has
been developed to store and transport water sup-
plies (for example,from the Colorado River and
Northern California to the Southwest and Southern
Figure 1: The Central Arizona Project brings Colorado River water
330 miles uphill to Tucson and Phoenix, Arizona. Source: K. Jacobs California). There are more than 80,000 dams and
408
Chapter 14 / Water Resources
reservoirs in the US,and millions of miles of canals, Inadequate water and wastewater infrastructure,
pipes,and tunnels (Schilling et al.,1987,see Figure common along the Mexican border and in some
1). Although this infrastructure is sophisticated and rural areas,leads to high risk for health problems.
has allowed the development of urban and agricul- Because the public has very high expectations
tural areas,it is also a source of vulnerability to cli- regarding water quality,and perceptions of health
mate change,partially because it has been designed risks are not always accurate,health issues may have
based on the assumption that future conditions will a high profile and require particular attention.
be similar to the historically observed climate. Some
argue that there is substantial robustness built into Health-related issues that have been linked to
the system that provides some margin of safety,but changes in the hydrologic cycle include potential
failure to re-evaluate these assumptions and identify for increases in water-borne pathogens such as
key vulnerabilities may prove to be costly in the Cryptosporidium and vector-borne diseases such as
future. encephalitis,as well as outbreaks in marine
pathogens associated with red tide (Bernard et al.,
Water supply issues in the eastern US relate to aging 1999). Hantavirus,a disease spread by deer mice,
infrastructure and inadequate storage capacity during has also been linked to extreme climate variability
times of drought. Flood control issues and environ- associated with the El Niño Southern Oscillation
mental impacts of structural solutions are also of con- (ENSO). With higher rainfall,rodent populations
cern. In general,local surface water and groundwa- tend to increase,which increases the chance of
ter supplies are available for domestic and industrial human contact and disease.
use without major water transfers between basins,
but excess reservoir capacity to respond to drought Virtually all indices of vulnerability relative to water
is quite limited. In some areas,such as NewYork have identified the over-appropriation of western
City,reservoir function is threatened by upstream streams and rivers and over-drafting of groundwater
development. NewYork and other cities also have supplies as key issues. Gleick (1990) identified
serious problems on a regular basis with water main indicators of water resource vulnerability for the US
breaks causing flooding and other damage. and found that the most vulnerable regions were
the high irrigation areas along the eastern drainage
The initial charge of this Assessment included identi- of the Rocky Mountains,the Central Valley of
fying areas of existing stress and vulnerability and California,and Southern California. The overall
evaluating new problems that climate change may index prepared by Hurd et al.(1999a) indicates that
bring. This has necessarily resulted in an identifica- the most vulnerable watersheds are in the West,
tion of negative effects,though certain aspects of cli- Southwest,and Great Plains (see Figure 2).
mate change are likely to improve conditions in
some areas of the US. Even in the absence of climate
change,adapting to existing stresses (such as aging Current Climate Vulnerability Map, Water Supply,
infrastructure,inadequate water supplies for areas of Distribution and Consumptive Use
rapid growth,etc.) and increased pressures from
population dynamics would be expensive. Frederick
and Schwarz (1999a) estimate that the annualized
water-related costs associated with the demands of
an increasing population are likely to approach $13.8
billion by 2030. The impacts of climate change on
these costs depend on the nature of the changes.
The estimated costs include investments in new
water supplies and conservation measures as well as
the impacts on streamflows and irrigated lands. The
RelativeVulnerability
costs would be much higher if climate change were Hydrologic Unit Boundary
to significantly decrease water availability (as under State Boundary
Low Vulnerability <1.5
the Canadian climate model scenario) or increase the
Medium Vulnerability ‡1.5
magnitude and timing of extreme events. This is High Vulnerability >2.2
because the current infrastructure and management Figure 2: Assessed vulnerability based on current climate and
practices are designed based on the historical climate water resource conditions, based on data describing the following:
share of streamflow withdrawn for use, streamflow variability,
conditions.
evapotranspiration rate, groundwater overdraft, industrial use
savings potential, and water trading potential. Source: Hurd, B.J.,
N. Leary, R. Jones and J. Smith. (1999a).See Color Plate Appendix
409
Potential Consequences of Climate Variability and Change
A rise in average temperature,even in the context of The state to state variations among water rights sys-
higher precipitation,is most likely to impact aquatic tems results in substantial complexity. In general,
systems,including riparian habitat,and freshwater water rights in eastern states are not likely to be eas-
and estuarine wetlands. In some cases,this is ily quantifiable,which limits management options.
because expected changes in precipitation do not The prior appropriation doctrine of the western US
offset increased evapotranspiration,though seasonal is relatively inflexible in dealing with changing envi-
and regional impacts are likely to vary. Certain ronmental and societal needs (see box,“Major
coastal systems,prairie potholes (small ponds and Doctrines for Surface Water and Groundwater”).
lakes formed by glacial deposits),and Arctic and
alpine ecosystems are thought to be especially vul- Some innovative institutions are developing in
nerable. Stresses within the contiguous US are likely response to particular problems. For example,the
to come from changes in the distribution of precipi- “temporary”water banks in California to respond to
tation as well as increases in its intensity. drought and in Arizona to respond to long-term sup-
ply reliability issues offer some protection to exist-
ing water rights while providing much-needed flexi-
SOCIOECONOMIC AND bility. Water banks generally provide opportunities
for short-term transfers of agricultural water sup-
INSTITUTIONAL CONTEXT
plies to municipal end users on a willing buyer/will-
ing seller basis. In the case of the Arizona Water
Population pressures,including shifts towards west- Banking Authority,excess Colorado River water is
ern and coastal urban areas,land use practices,and being stored underground through recharge proj-
climate change are all likely to increase stress on ects to offset future shortfalls in supply. This oppor-
water supply systems. The need to reserve water for tunity is expected to be available on an interstate
instream uses,endangered species protection,recre- basis among the Lower Colorado Basin states in the
ation,and American Indian water rights settlements near future. Similar types of contingency planning
also places new demands on a water rights system between jurisdictions and water rights holders
that in many parts of the country is already seriously could prove beneficial in responding to short-term
stressed. As society changes,its value system also emergencies. Longer-term changes in climatic con-
evolves. Placing more value on protection of fish ditions that would require permanent changes to
and wildlife habitat and recreational values is likely legal systems could be more problematic.
to force institutional change at the same time that
new stresses are appearing due to climate variability Many have argued that an open market in water
and change. Although there is substantial uncertain- rights would help resolve conflict and increase effi-
ty in the projections of changes in runoff that are ciency because water would flow to the highest and
derived from the climate models,socioeconomic best use based on willingness to pay (National
conditions are even less predictable. Research Council,1992;WesternWater Policy
ReviewAdvisory Commission,1998). It is widely
There is a need for more flexible institutional acknowledged that market-related solutions may
arrangements and more effective ways of making relieve some water supply problems,especially in
water policy decisions in order to adapt to changing the West. However,water marketing is an imperfect
conditions (not just changes in climate,but multiple solution. Of particular concern are third party
existing stresses). The legal framework for water impacts in water transfers,and overall equity issues.
rights varies from state to state,with nearly infinite Water markets are developing in many states,but
permutations at the local level. The one characteris- they are generally regulated markets in order to pro-
tic that is typical of most institutions related to tect the public interest. Mechanisms exist to identi-
water is inability to respond efficiently to changing fy economic values for non-market goods and servic-
socioeconomic and environmental conditions. This es,but water rights for non-market values such as
is primarily because institutions tend to reflect exist- ecosystems,aesthetics,and recreation have difficulty
ing water right holders’interests,and substantial competing with major economic forces. There is
investments are made based on expectations regard- also a risk that disproportionate burdens will be
ing availability of supplies. Devising new legal and placed on the social groups that can least afford
related institutions that can introduce the necessary them (such as rural farming communities,Native
flexibility into water management without destabi- Americans,and communities along the Mexican bor-
lizing investors’expectations,while at the same time der with inadequate infrastructure) (Dellapenna,
incorporating public values (ecological,recreational, 1999b;Gomez and Steding,1998).
aesthetic,etc.) is a significant challenge.
410
Chapter 14 / Water Resources
Major Doctrines for Surface Water and Groundwater
Surface Water
Riparian doctrine – Authorization to use water in a stream or other water body is based on ownership of the
adjacent land. Each landowner may make reasonable use of water in the stream but must not interfere with
its reasonable use by other riparian landowners. The riparian doctrine prevails in the 31 humid states east of
the 100th meridian.
Prior appropriation doctrine – Users who demonstrate earlier use of water from a particular source acquire
rights over all later users of water from the same source. When shortages occur,those first in time to divert
and apply the water to beneficial use have priority. New diversions,or changes in the point of diversion or
place or purpose of use,must not cause harm to existing appropriators. The prior appropriation doctrine pre-
vails in the 19 western states.
Groundwater
Absolute ownership – Groundwater belongs to the overlying landowner,with no restrictions on use and no
liability for causing harm to other existing users. Texas is the sole absolute ownership state.
Reasonable use doctrine – Groundwater rights are incident to land ownership. Owners of overlying land are
entitled to use groundwater only to the extent that the uses are reasonable and do not unreasonably interfere
with other users. Most eastern states and California subscribe to this doctrine. Some states,such as Arizona,
have modified the reasonable use doctrine by requiring state permits to use groundwater in certain high use
areas.
Prior appropriation permit system – Groundwater rights are determined by the rule of priority,which pro-
vides that prior uses of groundwater have the best legal rights. States administer permit systems to determine
the extent to which new groundwater uses will be allowed to interfere with existing uses. Most western
states employ some form of permit system.
Sources:US Army Corps of Engineers,Volume III,Summary of Water Rights – State Laws and Administrative
Procedure report prepared for US Army,Institute for Water Resources,byApogee Research,Inc.,June 1992;
and US Geological Survey,National Water Summary 1988-89-Hydrologic Events and Floods and Droughts,
Water Supply Paper 2375 (Washington,DC,US Government Printing Office,1991).
Dellapenna (1999a) says true (unregulated) markets Drinking Water Act.Water quantity regulation
have seldom existed for water rights and there are (through rights,allocations,and permits) is primarily
good reasons for believing that they seldom will. handled by the states. An illustration of this prob-
This is because water,like air,is viewed as a “public lem is found in a study by Eheart et al.(1999),
good,”which means that people cannot realistically which evaluated the impact of reduced precipita-
be excluded from using it,at least on a subsistence tion in the Midwest on ability to meet federal dis-
basis. There is reluctance to pay the full cost of charge water quality standards. They found that a
water,including the replacement cost;people are 25% reduction in precipitation could reduce the
generally charged only the cost for capturing and critical dilution flow that determines discharge
distributing water. Key factors in developing a impact on water quality by 63%. They concluded
workable market are whether the market will that this has implications for the process of setting
enable consumers to meet their needs and whether Total Maximum Daily Loads (TMDL) for Non-Point
government regulation and assistance at the margins Discharge Elimination Permits under the Clean
can correct for market failures. Water Act. Section 303(d) requires states to identify
waters that do not meet water quality standards and
Numerous institutional issues related to responding to establish plans to achieve the TMDL standards.
to potential climate change stem from the fact that Eheart et al.(1999) noted that the present regulato-
various agencies and levels of government handle ry scheme in many midwestern states is not sophis-
water quality and water quantity issues. Water quali- ticated enough to take into account the interplay
ty regulation derives primarily from federal authori- between water quality and quantity.
ties such as the Clean Water Act and the Safe 411
Potential Consequences of Climate Variability and Change
The Tennessee Valley Authority –
Integrated Water Resources Management
A well-known example of integrated water resources management is the Tennessee ValleyAuthority (TVA),
which operates in a seven-state area in the southeastern US. Founded in 1933,TVA pioneered the concept of
"unified river basin development" within the Tennessee River Basin,integrating water resources development,
social and economic development,power production,and natural resources conservation.
TVA’s water management programs focus on the operation of a large,multipurpose reservoir system that
includes more than 50 dams and reservoirs. The system is operated as an integrated unit to provide for navi-
gation,flood control,hydropower generation,summer recreation levels,and minimum flows for the mainte-
nance of water quality and aquatic habitat. In one of the most flood-prone areas in the US,TVA has historical-
ly taken a dual approach to flood management that combines reservoir system control with a floodplain man-
agement program to encourage appropriate shoreline development. Environmental concerns are integrated
into reservoir operations,while TVA’sWatershed Teams work at grassroots levels to motivate local action to
control non-point source pollution. TVA also
maintains web-sites and special telephone sys-
tems to facilitate public access to streamflow
data,dam release information,and other system
information.
TVA has a sophisticated streamflow and rainfall
data collection and monitoring system,coupled
with a state-of-the-art simulation and optimization
modeling system. Monitoring and forecasting
occur on a continuous,24-hour basis.
Additionally,TVA utilizes 10-day and seasonal
weather forecasts to guide reservoir planning.
TVA is engaged in joint work with NOAA to bet-
ter utilize seasonal forecast information. These
capabilities will assist TVA in adapting to climate Figure 3: TVA has more than 50 dams in seven states. Source:
Tennessee Valley Authority
change and variability.
The institutions that have been successful in manag-
Sacramento River Runoff
ing water resources tend to use an integrated
approach to management,and to incorporate natu- 70.0
ral watershed boundaries rather than political
boundaries for management areas. Relatively inno- 60.0
vative water management districts have been
50.0
formed in many states to address specific resource
conditions (see box,“Tennessee ValleyAuthority – 40.0
Integrated Water Resources Management”).
30.0
CLIMATE VARIABILITY AND 20.0
y = -0.1167x + 44.308
CHANGE 10.0
0.0
Historic trends show that the surface temperature
of the Earth has increased by about 1˚F (just over
0.6˚C) over the 20thcentury,with 1998 the warmest
Figure 4: In some western watersheds, runoff timing appears to be
year on record. Higher temperatures have resulted shifting from spring to winter, suggesting a change in snowfall and
in reductions in snow cover and sea ice extent. snowmelt dynamics. Source: Gleick, P.H. and E.L. Chalecki. (1999)
JAWRA. Dec. pp. 1429-1442.
412
Chapter 14 / Water Resources
Trend analysis shows that since WWII,there has Observed Changes In Streamflow and
been significant retreat of snow cover in spring Precipitation (1939-99)
(Groisman,1999). At the same time,there appear to
22.5
be shifts in the seasonality of runoff in some west-
20
ern rivers consistent with what would be expected
17.5
from changes in snowfall and snowmelt dynamics
15
due to warming (Gleick and Chalecki,1999;Union
of Concerned Scientists,1999) (see Figure 4). 12.5
10
Increases in global temperatures have been accom- 7.5
panied by more precipitation in the mid and high 5
latitudes (based on Northern Hemisphere land-sur- 2.5
face records) and increases in atmospheric water
0
vapor in many regions of North America and Asia
-2.5
where data are adequate for analysis (IPCC,1996). 0 20 40 60 80 100
Light/Low Moderate Heavy/High
Karl et al.(1996) show that the meteorological
drought indices suggest that there have been more Figure 5: The graph shows changes in the intensity of precipitation
wet spells,but no significant changes in drought on and streamflow, displayed in 5% increments, during the period
1939-99 based on over 150 unregulated streams across the US with
a national basis. The precipitation increase in the
nearby precipitation measurements. As the graph demonstrates,
US has been attributed primarily to an increase in
the largest changes have been the significant increases in the
the heaviest precipitation events (Karl and Knight, heaviest precipitation events and the highest streamflows. Note
1998;Groisman et al.,1999;Karl et al.,1996). These that changes in streamflow follow changes in precipitation, but are
changes are statistically significant and most appar- amplified by about a factor of 3. Source: Groisman, et.al. (2001).
ent during the spring,summer,and autumn months
in the contiguous US. Based on recent work by Lins
Projected 21stCentury Change in US
and Slack (1999),the warm season precipitation
Daily Precipitation
increases may be responsible for increases in
streamflow in the low to moderate range (i.e.the
flow values that are most commonly observed dur-
ing the summer and early autumn months). Using
discharge data from a national network of stream
gages for the period 1944-1993,Lins and Slack
(1999) found statistically significant increases in the
annual median streamflow at 29% of the stream
gages nationwide and decreases at only 1% of the
stream gages. Most trends were even more positive
for the lower streamflow quantiles. Fewer signifi-
cant trends were observed in high streamflows.
Only 9% of the gages,for example,had significant
trends in the annual maximum streamflow and,of
these,more showed decreases than increases. Canadian Climate Model
Hadley Climate Model
Groisman et al.(2000) show that high streamflow in
the mountainous West has not changed despite
Figure 6: These projections from the Hadley and Canadian models
increases in heavy precipitation events. They attrib-
show the changes in precipitation over the 21stcentury. Each mod-
ute this to a trend toward reduced snow cover els’ projected change in the lightest 5% of precipitation events is
extent leading to a lower and earlier peak in the represented by the far left bar and the change in the heaviest 5% by
annual cycle of runoff. In the East and South how- the far right bar. As the graph illustrates, both models project sig-
nificant increases in heavy rain events with smaller increases or
ever,increasing trends in high and very high stream-
decreases in light rain events. Source: National Climatic Data
flow are shown to relate to increases in heavy and
Center.See Color Plate Appendix
very heavy precipitation events. In fact,there is an
amplification of the trends in precipitation across
Understanding historic changes,or projecting future
the highest precipitation and streamflow rates by a
changes in streamflow conditions will require more
factor of about three. It is well-recognized (Karl
evaluation of the complex role of changing precipi-
and Reibsame,1989) that small changes in precipita-
tation and temperature patterns as well as the role
tion can be amplified into large changes in stream-
of land-use change on streamflow. These unresolved
flow (see Figure 5).
issues further reinforce the importance of maintain- 413
Potential Consequences of Climate Variability and Change
Projected Changes in Average Annual Runoff the two models are explained in part by differences
Based on Two GCMs in the land surface models relating to soil moisture
and moisture availability in summer (Felzer and
Heard,1999) (see Figure 6 and figures on precipita-
tion change in the Climate and West Chapters).
Precipitation is a key climatic variable,but it is diffi-
cult to predict changes at the local level because
they are affected by land surface features that are at
smaller scales than the GCM outputs (Felzer and
Heard,1999). Precipitation itself is a sub-grid-scale
process,meaning that clouds and convection occur
on scales smaller than GCM grids. Both models
show increases in heavy precipitation events and
increased storminess over the eastern Pacific,off the
West Coast of the US (Lambert,1995;Carnell and
Senior,1998;Felzer and Heard,1999). However,pre-
Figure 7: The estimated percent changes in average annual runoff
cipitation patterns will vary regionally. An important
based on the Canadian and Hadley models are not well correlated.
issue for improving the utility of GCM output is the
The Canadian model predicts declines in runoff in all regions except
California, while the Hadley model projects increases in most ability to downscale the models to a regional or
regions, particularly in the Southwest. The models differ in precipi- watershed level where the information can be most
tation predictions in part due to underlying model construction. useful to water managers.
Source: Wolock, D.M. and G.J. McCabe, 1999a. See Color Plate
Appendix
Differences in temperature and moisture levels over
land and sea are crucial in determining precipitation
ing adequate nationwide networks of precipitation
levels along the coasts. Over oceans,warming leads
and streamflow gages to help describe and predict
to increased evaporation and more precipitation
changes in average streamflow and,more important-
because of the limitless supply of water. In contrast,
ly,streamflow variability.
because of the limited moisture holding capacity of
the land,warming may cause drying and less precip-
In addition to a trend analysis of climatic conditions
itation. Globally,the models show decreased storm
over the past 100 years,this Assessment has evaluat-
frequency,with increases in intensity. Over the US,
ed scenarios from two General Circulation Models
the models do not produce a consistent projection
(GCMs),one from the Canadian Centre for Climate
regarding storm frequency (Lambert,1995;Carnell
Modelling and Analysis (henceforth referred to as
and Senior,1998;Felzer and Heard,1999). No
the Canadian model),and the second,the “HadCM2”
trends have been identified in North America-wide
model from the Hadley Centre for Climate
storminess or in storm frequency variability in the
Prediction and Research of the Meteorological
period 1885-1996 (Hayden,1999a).
Office of the United Kingdom (henceforth referred
to as the Hadley model). The Canadian and Hadley
Wolock and McCabe (1999a) have used a water-bal-
models used in this Assessment both project signifi-
ance model and output from the two GCMs to esti-
cant warming (5-9˚F or 3-5˚C) in most parts of the
mate the effects of climate change on mean annual
US by 2090. However,with the exception of the
runoff for the major water resource regions of the
southwestern US,where both models show a large
US. The model includes the concepts of climatic
increase in precipitation in the future,especially in
water supply and demand,seasonality in climatic
winter (Felzer and Heard,1999),the changes in pre-
water supply and demand,and soil-moisture storage.
cipitation predicted by the two models are striking-
Inputs to the model are monthly precipitation and
ly different. In general,the Hadley model suggests
potential evapotranspiration,which is calculated
much wetter conditions than the Canadian model.
from monthly temperature using the Hamon equa-
When comparing output of multiple GCMs,the
tion (Hamon,1961). To evaluate the model’s reliabil-
Hadley model increases in precipitation are the
ity to estimate mean annual runoff for the 18 water-
most extreme. The precipitation increase in
resources regions in the coterminous US,VEMAP-
Southern California and Arizona is related to increas-
gridded monthly climate data for 1951-80 were used
es in sea surface temperatures in the eastern Pacific
in conjunction with the water-balance model to esti-
and southward shifts in the jet stream that loosely
mate mean annual runoff. These estimated runoff
resemble the El Niño pattern. Differences between
data were compared with measured data for the
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