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Water in a Changing World

Robert B. Jackson

Stephen R. Carpenter

Clifford N. Dahm

Diane M. McKnight

Robert J. Naiman

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Jackson, R. B., Carpenter S. R., Dahm C. N., McKnight D. M., Naiman R. J., Postel S. L., and Running S. W.
(Spring 2001). Water in a Changing World. Issues in Ecology, 9, 1-16.

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Authors
Robert B. Jackson, Stephen R. Carpenter, Clifford N. Dahm, Diane M. McKnight, Robert J. Naiman, Sandra
L. Postel, and Steven W. Running

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 A b o u t Issues m Ecology

Issues in E cology \%designed to report, in language understandable by non-scientists, the

consensus o f a panel o f scientific experts on issues relevant to the environment. Issues in
E cology supported by the Pew Scholars in Conservation Biology program and by the
Ecological Society o f America. It is published at irregular intervals, as reports are com­
pleted. All reports undergo peer review and must be approved by the Editorial Board
before publication. No responsibility fo r the views expressed by authors in ESA publica­
tions is assumed by the editors or the publisher, the Ecological Society o f America.

Issues in E cobgy\s an official publication o f the Ecological Society o f America, the nation’s
leading professional society o f ecologists. Eounded in 1915, ESA seeks to promote the
responsible application o f ecological principles to the solution o f environmental problems.
Eor more information, contact the Ecological Society o f America, 1707 H Street, NW,
Suite 400, Washington, DC, 2 0 0 0 6 . ISSN 1092-8987
 Water in a Changing World

00

I
Issues in Ecology Number 9 Spring 2001

Water in a Changing World

SUMMARY

Life on land and in the lakes, rivers, and other freshwater habitats o f the earth is vitally dependent on renewable
fresh water, a resource that comprises only a tiny fraction o f the global water pool. Humans rely on renewable fresh water
fo r drinking, irrigation o f crops, and industrial uses as well as production o ffish and waterfowl, transportation, recreation,
and waste disposal.
In many regions o f the world, the amount and quality o f water available to meet human needs are already limited.
The gap between freshwater supply and demand will widen during the coming century as a result o f climate change and
increasing consumption o f water by a growing human population. In the next 30 years, fo r example, accessible runoff o f
fresh water is unlikely to increase more than 10 percent, yet the earth’s population is expected to grow by one third.
Unless humans use water more efficiently, the impacts o f this imbalance in supply and demand will diminish the services
that freshwater ecosystems provide, increase the number o f aquatic species facing extinction, and further fragment
wetlands, rivers, deltas, and estuaries.
Based on the scientific evidence currently available, we conclude that:
More than half o f the world’ s accessible freshwater runoff is already appropriated fo r human use.
More than a billion people currently lack access to clean drinking water, and almost three billion lack basic
sanitation services.
Because human population will grow faster than any increase in accessible supplies o f fresh water, the
amount o f fresh water available per person will decrease in the coming century.
Climate change will intensify the earth’ s water cycle in the next century, generally increasing rainfall,
evaporation rates, and the occurrence o f storms, and significantly altering the nutrient cycles in land-
based ecosystems that influence water quality.
A t least 90 percent o f river flows in the United States are strongly affected by dams, reservoirs, interbasin
diversions, and irrigation withdrawals that fragment natural channels.
Globally, 20 percent o f freshwater fish species are threatened or extinct, and freshwater species make up
47 percent o f all federally listed endangered animals in the United States.
Growing demands on freshwater resources are creating an urgent need to link research with improved water management,
a need that has already resulted in a number o f water-policy successes.
Better monitoring, assessment, and forecasting o f water resources would help government agencies allocate
water more efficiently among competing needs. Currently in the United States, at least six federal departments and twenty
agencies share responsibilities fo r various aspects o f the water cycle. We believe either creation o f a single panel with
members drawn from each department or else oversight by a central agency is needed in order to develop a well-coordi­
nated national plan that acknowledges the diverse and competing pressures on freshwater systems and assures efficient
use and equitable distribution o f these resources.

Cover (clockwise from top): Homestead, Kalahari Desert of South Africa (R. Jackson); Coastal zone of Serra da Arrabida,
Portugal (R. Jackson); “ The Water Seller” (H. Bechard, Egypt ca. 1870); Monteverde Cloud Forest, Costa Rica (R. Jackson);
Little Colorado River, Grand Canyon National Park, USA (R. Jackson); Elk and riparian zone, Gardner River of Yellowstone
National Park, USA (R. Jackson); and the town of Elores, Guatemala (R. Jackson).
Issues in Ecology Number 9 Spring 2 0 0 1

Water in a Changing World

Robert B. Jackson, Stephen R. Carpenter, Clifford N. Dahm,
Diane M. McKnight, Robert J. Naiman, Sandra L. Postel, and Steven W. Running

INTRODUCTION The ecological, social, and economic benefits that

freshwater systems provide, and the trade-offs between con­
Life on earth depends on the continuous flow of sumptive and instream values, will change dramatically in
materials through the air, water, soil, and food webs of the the coming century. Already, over the past one hundred
biosphere. The movement of water through the hydrological years, both the amount of water humans withdraw world­
cycle comprises the largest of these flows, delivering an esti­ wide and the land area under irrigation have risen exponen­
mated I 10,000 cubic kilometers (km^> of water to the land tially (Figure I). Despite this greatly increased consump­
each year as snow and rainfall. Solar energy drives the hy­ tion, the basic water needs of many people in the world are
drological cycle, vaporizing water from the surface of oceans, not being met. Currently, I . I billion people lack access to
lakes, and rivers as well as from soils and plants (evapotrans- safe drinking water, and 2.8 billion lack basic sanitation ser­
piration). Water vapor rises into the atmosphere where it vices. These deprivations cause approximately 250 million
cools, condenses, and eventually rains down anew. This re­ cases of water-related diseases and five to ten million deaths
newable freshwater supply sustains life on the land, in estu­ each year. Also, current unmet needs limit our ability to
aries, and in the freshwater ecosystems of the earth. adapt to future changes in water supplies and distribution.
Renewable fresh water provides many services es­ Many current systems designed to provide water in relatively
sential to human health and well being, including water for stable climatic conditions may be ill prepared to adapt to
drinking, industrial production, and irrigation, and the pro­ future changes in climate, consumption, and population. While
duction of fish, waterfowl, and shellfish. Fresh water also a global perspective on water withdrawals is important for
provides many benefits while it remains in its channels ensuring sustainable water use, it is insufficient for regional
(nonextractive or instream benefits), including flood control, and local stability. Flow fresh water is managed in particular
transportation, recreation, waste processing, hydroelectric basins and in individual watersheds is the key to sustainable
power, and habitat for aquatic plants and animals. Some water management.
benefits, such as irrigation and hydroelectric power, can be The goal of this report is to describe key features of
achieved only by damming, diverting, or creating other ma­ human-induced changes to the global water cycle. The ef­
jor changes to natural water flows. Such changes often di­ fects of pollution on water availability and on purification
minish or preclude other instream benefits of fresh water, costs have been addressed previously in Issues in Ecology.
such as providing habitat for aquatic life or maintaining suit­ We focus instead on current and potential changes in the
able water quality for human use. cycling of water that are especially relevant for ecological

505

exna- ■^SO

40)
1
■350
J
m
■i UJ
3.TSW- ■E
■?50

-» Q
1
■190

.100

■M

•Q

Figure I — Global data for human population, water withdrawals, and irrigated land area from 1900 to 2000.
Redrawn and updated from Gleick (1998).
Issues in Ecology Number 9 Spring 2 0 0 1

processes. We begin by briefly describing

the global water cycle, including its cur­
rent state and historical context. We next AlnnciBphMft
examine the extent to which human ac­
tivities currently alter the water cycle and
may affect it in the future. These changes Ldket
a i'd
include direct actions, such as dam con­ Rtvam
IID PrwaptiBDon
struction, and indirect impacts, such as
those that result from human-driven cli­
mate change. We examine human appro­
priation of fresh water globally, from both
renewable and non-renewable sources.
The report ends by discussing changes in Ewa<»ratKir¥ GrountMolsr
water use that may be especially impor­
tant in the future. We highlight some cur­
rent progress and suggest priorities for
research, emphasizing examples from the
United States. Figure 2 — The renewable freshwater cycle in units of 10^ km^ and 10^ kmVyr for
pools (white numbers) and fluxes (black numbers). Total precipitation over land is
THE GLOBAL WATER CYCLE about 110,000 kmVyr. Approximately two-thirds of this precipitation is water re­
cycled from plants and the soil (evapotranspiration = 70,000 kmVyr) while one-
Surface Water third is water evaporated from the oceans that is then transported over land (40,000
kmVyr). Ground water holds about 15,000,000 km^ of fresh water, much of it
Most of the earth is covered by “ fossil water” that is not in active exchange with the earth’s surface.
water, more than one billion km^ of it. The
vast majority of that water, however, is in
forms unavailable to land-based or freshwater ecosystems. centimeters (cm) a year, a value typical for deserts or semi-
Less than 3 percent is fresh enough to drink or to irrigate arid regions. Instead, a second, larger source of water is
crops, and of that total, more than two-thirds is locked in recycled from plants and the soil through evapotranspira­
glaciers and ice caps. Freshwater lakes and rivers hold tion. The water vapor from this source creates a direct feed­
100,000 km^ globally, less than one ten-thousandth of all back between the land surface and regional climate. The
water on earth (Figure 2). cycling of other materials such as carbon and nitrogen (bio­
Water vapor in the atmosphere exerts an important geochemical cycling) is strongly coupled to this water flux
influence on climate and on the water cycle, even though through the patterns of plant growth and microbial decom­
only I 5,000 km^ of water is typically held in the atmosphere position, and this coupling creates additional feedbacks be­
at any time. This tiny fraction, however, is vital for the bio­ tween vegetation and climate. This second source of recycled
sphere. Water vapor is the most important of the so-called water contributes two-thirds of the 70 cm of precipitation
greenhouse gases (others include carbon dioxide, nitrous ox­ that falls over land each year. Taken together, these two
ide, and methane) that warm the earth by trapping heat in sources account for the I 10,000 km^ of renewable freshwa­
the atmosphere. Water vapor contributes approximately two- ter available each year for terrestrial, freshwater, and estua-
thirds of the total warming that greenhouse gases supply. rine ecosystems (Figure 2).
Without these gases, the mean surface temperature of the Because the amount of rain that falls on land is
earth would be well below freezing, and liquid water would greater than the amount of water that evaporates from it,
be absent over much of the planet. Equally important for the extra 40,000 km^ of water returns to the oceans, prima­
life, atmospheric water turns over every ten days or so as rily via rivers and underground aquifers. A number of factors
water vapor condenses and rains to earth and the heat of affect how much of this water is available for human use on
the sun evaporates new supplies of vapor from the liquid res­ its journey to the oceans. These factors include whether the
ervoirs on earth. precipitation falls as rain or snow, the timing of precipitation
Solar energy typically evaporates about 425,000 relative to patterns of seasonal temperature and sunlight,
km^ of ocean water each year. Most of this water rains back and the regional topography. For example, in many moun­
directly to the oceans, but approximately 10 percent falls on tain regions, most precipitation falls as snow during winter,
land. If this were the only source of rainfall, average precipi­ and spring snowmelt causes peak flows that flood major river
tation across the earth’s land surfaces would be only 25 systems. In some tropical regions, monsoons rather than
 Issues in Ecology Number 9 Spring 2 0 0 1

snowmelt create seasonal flooding. In other regions, excess The distinction between renewable and non-renewable
precipitation percolates into the soil to recharge ground water ground water is critical for water management and policy.
or is stored in wetlands. Widespread loss of wetlands and More than three-quarters of underground water is non­
floodplains, however, reduces their ability to absorb these renewable, meaning it has a replenishment period of centuries
high flows and speeds the runoff of excess nutrients and con­ or more (Figure 3). The Fligh Plains or Ogallala Aquifer that
taminants to estuaries and other coastal environments. More underlies half a million km^ of the central United States is
than half of all wetlands in the U. S. have already been drained, arguably the largest aquifer in the world. The availability of
dredged, filled, or planted. turbine pumps and relatively inexpensive energy has spurred
Available water is not evenly distributed globally. the drilling of about 200,000 wells into the aquifer since the
Two thirds of all precipitation falls in the tropics (between 30 1940s, making the Ogallala the primary water source for a
degrees N and 30 degree S latitude) due to greater solar fifth of irrigated U.S. farmland. The extent of irrigated cropland
radiation and evaporation there. Daily evaporation from in the region peaked around 1980 at 5.6 million hectares
the oceans ranges from 0.4 cm at the equator to less than and at pumping rates of about 6 trillion gallons of water a
0 . 1 cm at the poles. Typically, tropical regions also have year. That has since declined somewhat due to groundwater
larger runoff. Roughly half of the precipitation that falls in depletion and socioeconomic changes in the region. Flowever,
rainforests becomes runoff, while in the deserts low rainfall the average thickness of the Ogallala declined by more than
and high evaporation rates combine to greatly reduce runoff. 5 percent across a fifth of its area in the 1980s alone.
The Amazon, for example, carries I 5 percent of all water In contrast, renewable aquifers depend on current
returning to the global oceans. In contrast, the Colorado rainfall for refilling and so are vulnerable to changes in the
River drainage, which is one-tenth the size of the Amazon, quantity and quality of recharge water. For example, ground­
has a historic annual runoff 300 times
smaller. Similar variation occurs at
continental scales. Average runoff in
Australia is only 4 cm per year, eight times
less than in North America and orders of
magnitude less than in tropical South
America. As a result of these and many
other disparities, freshwater availability
varies dramatically worldwide.

Ground Water

Approximately 99 percent of all

liquid fresh water is in underground aquifers
(Figure 2), and at least a quarter of the TfiB OgBllaLa A q u ih ir
world’s population draws its water from
these groundwater supplies. Estimates of
the global water cycle generally treat rates m T

of groundwater inflow and outflow as if

they were balanced. In reality, however,
this resource is being depleted globally.
Ground water typically turns over more
slowly than most other water pools, often
in hundreds to tens of thousands of years,
although the range in turnover rates is
large. Indeed, a majority of ground water
is not actively turning over or being
recharged from the earth’s surface at all.
Instead, it is “ fossil water,” a relic of wetter
ancient climatic conditions and melting
Pleistocene ice sheets that accumulated Figure 3 — Locations of non-renewable groundwater resources (light blue) and
over tens of thousands of years. Once used, the main locations of groundwater mining (dark gray) (Shiklomanov 1997). The
it cannot readily be replenished. inset shows the location of the High Plains (Ogallala) Aquifer.
Issues in Ecology Number 9 Spring 2 0 0 1

water pumping of the Edwards Aquifer, which supplies much HUMAN APPROPRIATION OF FRESHWATER SUPPLY
of central Texas with drinking water, has increased four fold
since the 1930s and at times now exceeds annual recharge Global Renewable W ater Supplies
rates. Increased water withdrawal makes aquifers more sus­
ceptible to drought and other changes in weather and to Growth in global population and water consump­
contamination from pollutants and wastes that percolate into tion will place additional pressure on freshwater resources in
the ground water. Depletion of ground water can also cause the coming century. Currently, the water cycle makes avail­
land subsidence and compaction of the porous sand, gravel, able several times more fresh water each year than is needed
or rock of the aquifer, permanently reducing its capacity to to sustain the world’s population of six billion people (Table
store water. The Central Valley of California has lost about 2). However, the distribution of this water, both geographi­
25 km^ of storage in this way, a capacity equal to more than cally and temporally, is not well matched to human needs.
40 percent of the combined storage capacity of all human- The large river flows of the Amazon and Zaire-Congo basins
made reservoirs in the state. and the tier of undeveloped rivers in the northern tundra and
Renewable ground water and surface waters have taiga regions of Eurasia and North America are largely inac­
commonly been viewed separately, both scientifically and le­ cessible for human uses and will likely remain so for the fore­
gally. This view is changing, however, as studies in streams, seeable future. Together, these remote rivers account for
rivers, reservoirs, wetlands, and estuaries show the impor­ nearly one-fifth of total global runoff.
tance of interactions between renewable surface and ground Approximately half of the global renewable water
waters for water supply, water quality, and aquatic habitats. supply runs rapidly toward the sea in floods (Table 2). In
Where extraction of ground water exceeds recharge rates, managed river systems of North America and many other
the result is lower water tables. In summer, when a high regions, spring floodwaters from snowmelt are captured in
water table is needed to sustain minimum flows in rivers and reservoirs for later use. In tropical regions, a substantial
streams, low groundwater levels can decrease low-flow rates, share of annual runoff occurs during monsoon flooding. In
reduce perennial stream habitat, increase summer stream tem­ Asia, for example, 80 percent of runoff occurs between May
peratures, and impair water quality. Trout and salmon spe­ and October. Although this floodwater provides a variety of
cies select areas of groundwater upwelling in streams to ecological services, including sustaining wetlands, it is not a
moderate extreme seasonal temperatures and to keep their practical supply for irrigation, industry, and household uses
eggs from overheating or freezing. Dynamic exchange of that need water to be delivered in controlled quantities at
surface and ground waters alters the dissolved oxygen and specific times.
nutrient concentrations of streams and dilutes concentrations Thus, there are two categories of accessible runoff
of dissolved contaminants such as pesticides and volatile or­ available to meet human water needs: ( I ) renewable ground
ganic compounds. Because of such links, human develop­ water and base river flow, and (2) floodwater that is cap­
ment of either ground water or surface water often affects tured and stored in reservoirs.
the quantity and quality of the other. Base river flows and renewable ground water ac­
The links between surface and ground waters are count for about 27 percent of global runoff each year. As
especially important in regions with low rainfall (see Box I , long as the rate of water withdrawals does not exceed re­
Table I , and Figure 4). Arid and semi-arid regions cover a plenishment by rainfall, these sources can serve as a sustain­
third of the earth’s lands and hold a fifth of the global popu­ able supply. Unfortunately in many places, including many
lation. Ground water is the primary source of water for drink­ important agricultural regions, ground water is chronically
ing and irrigation in these regions, which possess many of overpumped. Data for China, India, North Africa, Saudi
the world’s largest aquifers. Limited recharge makes such Arabia, and the United States indicate that groundwater
aquifers highly susceptible to groundwater depletion. For ex­ depletion in key basins totals at least 160 km^ per year.
ample, exploitation of the Northern Sahara Basin Aquifer in Groundwater depletion is particularly serious in India, and
the 1990s was almost twice the rate of replenishment, and some water experts have warned that as much as one-fourth
many springs associated with this aquifer are drying up. of India’s grain harvest could be jeopardized by overpumping.
For non-renewable groundwater sources, discussing The fact that global groundwater extractions remain well
sustainable or appropriate rates of extraction is difficult. As below the global recharge rate does not mean that ground­
with deposits of coal and oil, almost any extraction is non- water use in a specific region is sustainable. What matters is
sustainable. Important questions for society include at what how water is used and managed in particular basins, and
rate groundwater pumping should be allowed, for what pur­ there are many regions of the world where current demand
pose, and who if anyone will safeguard the needs of future outstrips supply.
generations. In the Ogallala Aquifer, for example, the wa­ Turning floodwater into an accessible supply gener­
ter may be gone in as little as a century. ally requires dams and reservoirs to capture, store, and con-
Issues in Ecology Number 9 Spring 2 0 0 1

Box I : A Case Study - the Middle Rio Grande

Increasing water demands create potential conflicts between human needs and those of native ecosystems. Perhaps
nowhere are human impacts on river and floodplain ecosystems greater than in arid and semi-arid regions of the world. The
Middle Rio Grande Basin of central New Mexico is a rapidly growing area that holds more than half of the state’s population.
The desire to balance water needs there has led to development of a careful water budget for the basin (Table I ), highlighting
annual variability, measurement uncertainty, and conflicting water demands for the region. The goal of the water budget is
to help design a sustainable water policy.
Water management has already greatly altered this floodplain ecosystem (Rgure 4). Dams and constructed river
channels prevent spring floods. Riparian zones, now limited by a system of levees, once hosted a mosaic of cottonwood and
willow woodlands, wet meadows, marshes, and ponds. The last major floods with significant cottonwood establishment
occurred in 1942, and cottonwoods are declining in most areas. Half of the wetlands in the drainage were lost in just 50
years. Invasion by nonnative deep-rooted trees such as saltcedar and Russian-olive has dramatically altered riparian forest
composition. Without changes in water management, exotic species will likely dominate riparian zones within half a century.
The water budget of the Middle Rio Grande reflects recent changes in hydrology, riparian ecology, and groundwater
pumping. Estimating all major water depletions in the basin is critical for managing its water. Major depletions include urban
uses, irrigation, plant transpiration, open-water evaporation, and aquifer recharge. The largest loss is open-water evapora­
tion, comprising one-third of the total. This loss is large compared to pre-dam values — direct evaporation from Elephant
Butte Reservoir alone ranges from 50 to 280 million cubic meters (m^) per year depending on reservoir size and climate. The
second largest depletion is riparian plant transpiration (135 to 340 million mVy). There is considerable uncertainty in this
estimate because of the unknown effects of fluctuating river discharge on transpiration and differences between native and

Table I — Sources and average annual water depletion from

1972 to 1997 for the Middle Rio Grande reach (the 64,000-
km^ drainage between Otowi Gage north of Santa Fe and
Average Otowi Flow 1360 Elephant Butte Dam). Flow records at the Otowi gage, the
San Juan-Chama Diversion 70 inflow point for the Middle Rio Grande reach, are more than
a century old. Water supplemented from the San Juan-Chama
diversion project began in 1972 and increased Otowi flow
by 70 million mVy (average flow without this water was about
1400 million mVy). Major municipal water systems in the
Open-water evaporation 270
Riparian plant transpiration basin currently pump ground water at a rate of 85 million mV
220
y. Maximum allowable depletion for the reach is 500 million
Irrigated agriculture 165
mVy when adjusted annual flow exceeds 1900 million mVy
Urban consumption (ground water) 85
decreasing progressively to 58 million mVy in severe drought
Net aquifer recharge 85
years (inflows of 120 million mVy at Otowi Gage).

trol the water. Worldwide, there are approximately 40,000 Human W ater Use
large dams more than I 5 meters (m) high and twenty times
as many smaller dams. Collectively, the world’s reservoirs People use fresh water for many purposes. There
can hold an estimated 6,600 km^ of water each year. Con­ are three broad categories of extractive uses for which people
siderably less water than this is delivered to farms, industries, withdraw water from its natural channel or basin: irrigation
and cities, however, because dams and reservoirs are also of crops, industrial and commercial activities, and residential
used to generate electricity, control floods, and enhance river life. In many cases, water can be used more than once after
navigation. it is withdrawn. Water that is used but not physically con­
Finally after subtracting remote rivers from base flows sumed — to wash dishes, for example — may be used
and discounting reservoir capacity allocated to functions other again, although it sometimes requires further treatment. In
than water supply, the total accessible runoff available for contrast, about half the water diverted for irrigation is lost
human use is about 12,500 km^ per year, or 3 I percent of through evapotranspiration and is unavailable for further use.
total annual runoff.
Issues in Ecology Number 9 Spring 2 0 0 1

non-native plants in transpiration rates. Irrigated agriculture in the Middle Rio Grande accounts for an estimated 20 percent
of annual average depletions, with cropping patterns, weather, and water availability contributing to annual variations.
Urban consumption and net aquifer recharge are similar and account for 20-25 percent of the remaining depletion in the
Middle Rio Grande.
Average annual depletions are partially offset by water from the San Juan-Chama Project, inflows from tributaries
within the basin, and municipal wastewater discharge. Nonetheless, water depletions are already fully appropriated for an
average water year. Municipal use of San Juan-Chama water, sustained drought, and continued population growth will
increase pressure on surface water resources. No new water will litely be available in the near future, so water conservation
must play a dominant role.
A careful water budget such as the one described here is essential in designing sustainable water policy. For the
Middle Rio Grande, accurate long-term measurements of surface flows, evapotranspiration, net aquifer recharge, and ground­
water levels are necessary. Reservoir operations, exotic species control, land use planning, and agricultural and urban water
conservation will all play an important role in a sustainable water future for the region. Other arid and semi-arid regions of
the world, where balancing diverse water demands will be a formidable and important challenge, have similar needs for
fundamental data and careful water planning.

Figure 4 — Contrasting riparian vegetation in the Middle Rio Grande reach south of Albuquerque, New Mexico: a native
cottonwood-dominated site (A) near Los Lunas and an exotic saltcedar-dominated site (B) on the Sevilleta National Wildlife
Refuge. Water management, especially dam construction and river channeling, has greatly altered this floodplain ecosystem. The
last major floods with significant cottonwood establishment were in 1942. Invasions by exotic deep-rooted plants such as
saltcedar pictured here and Russian-olive have dramatically altered riparian forest composition. Without changes in water manage­
ment, exotic species will likely dominate riparian zones in the Middle Rio Grande basin within the next half century.

Excessive rates of consumptive water use can have drawals of water (including evaporative losses from reser­
extreme effects on local and regional ecosystems. In the Aral voirs) total 4,430 km^ a year, and 52 percent of that is
Sea Basin, for example, large river diversions for irrigation consumed. Water use or withdrawal also modifies the qual­
have caused the lake to shrink more than three quarters in ity of the remaining water in a basin or channel by increas­
volume and fifteen meters in depth over the past four de­ ing concentration of major ions, nutrients, or contaminants.
cades. The shoreline of the Aral Sea has retreated 120 km in As the example of the Aral Sea showed, this effect can limit
places, and a commercial fishery that once landed 45,000 the suitability of water for future use.
tonnes a year and employed 60,000 people has disappeared. In addition to water removed from natural systems,
Water quality has also declined. Salinity tripled from 1960 human enterprises depend heavily on water that remains in
to 1990, and the water that remains is now saltier than the its natural channels. These instream uses include dilution of
oceans. pollutants, recreation, navigation, maintenance of healthy
For purposes of water management, the difference estuaries, sustenance of fisheries, and protection of biodiversity.
between use and consumption is important. Global with­ Because instream uses of water vary by region and season, it
Issues in Ecology Number 9 Spring 2 0 0 1

is difficult to estimate their global total. If pollution dilution THE WATER CYCLE AND CLIMATE CHANGE
is taken as a rough global proxy, however, instream uses
may total 2,350 km^ a year, a conservative estimate that A scientific consensus now exists that the continu­
does not incorporate all instream uses. ing buildup of human-generated greenhouse gases in the at­
Combining this instream use figure with estimated mosphere is warming the earth. The last decade of the twen­
global withdrawals puts the total at 6,780 km^ a year. That tieth century was the warmest on record, and paleoclimate
means humans currently are appropriating 54 percent of the records indicate that the warming of the past 50 years had
accessible freshwater runoff of the planet. no counterpart in the past thousand years. As the earth
Global water demands continue to rise with increases continues to warm in the coming century, a general intensi­
in human population and consumption. Increases in acces­ fication of the water cycle is expected to occur. In a warmer
sible runoff, however, can only be accomplished by construc­ climate, greater volumes of water will evaporate from plants,
tion of new dams or desalination of seawater. Today, desali­ soils and water bodies globally, lofting more vapor into the
nation accounts for less than 0.2 percent of global water use atmosphere to rain out and in turn, increasing runoff and
and, because of its high energy requirements, it is likely to making hydrologic extremes such as floods and droughts more
remain a minor part of global supply for the foreseeable fu­ common and more intense. Some decreases in snow and ice
ture. Dams continue to bring more water under human con­ cover have already been observed. Changes in the tempera­
trol, but the pace of construction has slowed. In developed ture and water cycle will necessarily affect plant growth and
countries, many of the best sites have already been used. decomposition processes in the soil, including the cycling of
Rising economic, environmental, and social costs — includ­ carbon, nitrogen, and other nutrients whose concentrations
ing habitat destruction, loss of biodiversity, and displacement influence water quality.
of human communities — are making further dam construc­ Regional and local changes will likely be more vari­
tion increasingly d ifficu lt. able and more difficult to pre­
About 260 new large dams dict than global changes.
now come on line worldwide Total Global Runoff 40,700 Many regions, especially tem­
each year compared w ith Remote Flow perate ones, will experience
1,000 a year between the Amazon Basin 5,400 increased summer drying
1950s and 1970s. Moreover, Zaire-Congo Basin 660 from greater evaporation
at least I 80 dams in the Remote northern rivers 1,740 and, in some cases, lower
United States were removed in Total Remote Flow 7,800 summer rainfall (Figure 5).
the past decade based on Uncaptured Floodwater 20,400 Eor example, almost all of the
evaluations of safety, environ­ Accessible Runoff 12,500 General Circulation Models
mental impact, and obsoles­ (GCMs) of global climate pre­
cence. The destruction of the Global Water Withdrawals dict that southern Europe will
Edwards Dam on Maine’ s Agriculture 2,880 receive less summer rainfall.
Kennebec River in 1999 Industry 975 In contrast, tropical regions
marked the first time that fed­ Municipalities 300 may experience relatively
eral regulators ruled that the Reservoir Losses 275 small warming-induced
environmental benefits of re­ Total Global Withdrawals 4,430 changes in the water cycle.
moving a dam outweighed the Instream Uses 2,350 The level of uncertainty that
economic benefits of operat­ Total Human Appropriation 6,780 remains in climate predictions
ing it. at regional scales is illus­
As a result of these Table 2 — Global runoff, withdrawals, and human appropria­ trated by the wide range of
and other trends, accessible tion of freshwater supply (kmVyr). Remote flow refers to river future scenarios predicted for
runoff that is geographically inaccessible, estimated to include
runoff is unlikely to increase soil moisture in the central
95% of runoff in the Amazon basin, 95% of remote north­
by more than 5-10 percent United States — from as
ern North American and Eurasian river flows, and half of the
over the next 30 years. Dur­ much as 75 percent drier to
Zaire-Congo basin runoff. The runoff estimates also include
ing the same period, the 30 percent wetter in summer
renewable ground water. An estimated 18% (or 2285 kmV
earth’s population is projected — by models using different
yr) of accessible runoff is consumed, although humans with­
to grow by approximately 35 assumptions and representa­
draw 6,780 kmVyr or 54% of accessible runoff. Water that
percent. The demands on is withdrawn but not consumed is not always returned to the tions of water processes.
freshwater systems will con­ same river or lake from which it was taken. From Postel et al. Future changes in
tinue to grow throughout the (1996), based on additional data in Czaya (1981), L’Vovich the water cycle that will be
coming century. et al. (1990), and Shiklomanov (1997). especially important for fresh-
Issues in Ecology Number 9 Spring 2 0 0 1

Projected Future CKartgeS kt A b t Hwjected Future C llafl^ei in HRCP

% change from fSG1-19B0 baseline to2!06l-ZQao scenario climate % change from 19G1-19S0 baseline toZDGl-ZQSO scenario climate

-56 5 io ^56

Figure 5 — A projection of future changes in actual evapotranspiration (AET) and precipitation (PRCP) generated by an ecosystem
model (BIOME-BGC) using a future climate scenario to the year 2 100 derived from a global climate model. In this scenario,
atmospheric carbon dioxide (CO^) increased approximately 0.5%/yr, and the ecosystem model responded with changes in leaf
area index (a measure of plant productivity) based on changes in CO^, climate, water, and nitrogen availability. In general, these
projections suggest higher rainfall and increased plant growth in the arid West, leading to higher AET. Reduced rainfall and the
resulting effects of drought on vegetation are the primary causes of lower evapotranspiration projected for the Southeast. For
additional information, see Box 2 (Results from VEMAP II, courtesy of P Thornton, Numerical Terradynamic Simulation Group,
Univ. of Montana).

water availability include the amount and timing of rainfall next century (because water turnover and the relatively high
and runoff, rates of evapotranspiration from plants and soils, heat capacity of the oceans buffer changes in temperature),
and rises in sea level. As temperatures get hotter, evapora­ and this will increase the likelihood of drought over the con­
tion increases exponentially, so both evaporation from the tinents. This difference in warming rate may also intensify
oceans and, consequently, global average rainfall should in­ pressure gradients and wind patterns in coastal regions, en­
crease as the earth warms. All GCMs examined in the most hancing upwelling of coastal waters.
recent assessment by the Intergovernmental Panel on Cli­ All of this indicates that the changes in the water
mate Change predict increased rainfall for the earth. In fact, cycle that accompany climate warming will be felt quite dif­
recent data indicate that average rainfall may already have ferently from one region to the next. In general, although
increased slightly in non-tropical regions. In the United States some temperate and polar regions will likely receive more
and Canada, precipitation rose as much as 10 to 15 percent precipitation, other regions will receive less, and many more
over the past fifty years, and stream flow also increased sig­ regions will be effectively drier from increased evaporative
nificantly during this period, especially in the eastern half of demand during the growing season.
the United States. Increases in precipitation were smaller but Atmospheric changes will not be the only forces driv­
still significant for the former Soviet Union (about 10 per­ ing the evolving climate in the next century. Human land use
cent in a century) and Scotland. In contrast, tropical and changes will also play an important role, since the nature of
arid regions show no evidence of increased precipitation, and plant cover on the land affects the rate of evapotranspira­
perhaps have even been drying slightly in recent decades. tion and also the albedo of the surface, meaning how much
Slight increases in average global rainfall, of course, sunlight it reflects. Thus activities such as deforestation, re­
will not uniformly increase available fresh water in all re­ forestation, and even desertification processes such as shrub
gions. Regional effects will depend in part on complex feed­ encroachment into grasslands will also feed back to affect
backs between plants and soils and the atmosphere in a climate and the water cycle. At regional scales, deforesta­
warmer, wetter environment. For instance, increased atmo­ tion reduces rainfall by decreasing water recycling and in­
spheric carbon dioxide can increase the efficiency of plant creasing the albedo. The increased drying that follows tree
water use, and that effect combined with increased rainfall clearing may be especially important in tropical forests and
would tend to increase water availability. Yet those effects savannas, making it harder to reestablish trees on burned or
may be more than offset by greater evapotranspiration rates cut-over land. Region-wide increases in irrigation could have
in a warmer climate. Also, the land surface can be expected an opposite feedback effect, inducing cooler and wetter re­
to warm much more quickly than the ocean surface over the gional climates. Agriculture uses 8 1 percent of all water
Issues in Ecology Number 9 Spring 2 0 0 1

consumed in the United States, and much of this water goes transformations of ecosystems. Rising demand for fresh wa­
to irrigate crops in drier regions where evaporation rates are ter can sever ecological connections in aquatic systems, frag­
high, especially the central Great Plains and the West. Land menting rivers from floodplains, deltas, and coastal marine
use changes also have impacts on water cycling at smaller environments. It also can change the quantity, quality, and
scales. Changes such as deforestation, for instance, can sig­ timing of freshwater supplies on which terrestrial, aquatic,
nificantly alter runoff and water yields in individual water­ and estuarine ecosystems depend .
sheds. Fresh water is already a limiting resource in many
Changes in the water cycle that affect soil moisture, parts of the world. In the next century, it will become even
nutrient availability, and increased salinity will also alter plant more limiting due to increased population, urbanization, and
growth and productivity and the distribution of plant spe­ climate change. This limitation will be caused not just by
cies. Furthermore, the rate of microbial processes in the soil, increased demand for water, but also by pollution in fresh­
which control accumulation of soil organic matter and the water ecosystems. Pollution decreases the supply of usable
release of nutrients such as nitrogen, are strongly influenced water and increases the cost of purifying it. Some pollut­
by the duration of snow cover, freeze/thaw cycles, and soil ants, such as mercury or chlorinated organic compounds,
moisture. In turn, climate and water-driven changes in plant contaminate aquatic resources and affect food supplies. More
growth and microbial activities will influence the biogeochemi­ than 8 billion kilograms of nitrogen and 2 billion kilograms
cal processes that affect water quality. of phosphorus are discharged each year into surface waters
Changes in water quality and quantity also influ­ in the United States. This nutrient pollution, combined with
ence habitat for aquatic life. In aquatic ecosystems, just as human demand for water, affects biodiversity, ecosystem func­
on land, plant productivity and nutrient cycling are influ­ tioning, and the natural services of aquatic systems upon
enced by the duration of ice and snow cover and by changes which society depends.
in seasonal water flow. Because river runoff carries carbon, Growing demands for fresh water also dramatically
nitrogen, and other nutrients from upstream systems into affect species conservation. Globally, at least a fifth of fresh­
coastal waters, increases in these fluxes can damage coastal water fish species are currently threatened or extinct, and
fisheries by depleting oxygen, or even threaten human health aquatic species currently make up almost half of all animals
by promoting hazardous algal blooms. listed as federally endangered in the United States. The United
The water cycle will also be influenced in the coming States also has almost twice as many threatened freshwater
century by rising sea level. Sea level increased by about 8 fish species as any other country and has lost more molluscs
cm in the past century and is predicted to rise another 30 to to extinction. Molluscs in the Appalachian Mountains and
50 cm over the next hundred years. This rise would push freshwater fish in the Appalachians as well as the arid Sonoran
shores inland 30 m on average, creating dramatic changes basin and range are especially vulnerable. There are also
in coastal systems. For example, increased sea level will worsen many vulnerable endemics in karst systems (limestone caves
saltwater intrusion into freshwater coastal aquifers, alter the and tunnels) and aquifers, including blind catfish, crayfish,
distribution and hydrology of coastal wetlands, and displace and salamanders. Aquatic species in other systems around
agriculture in coastal regions and deltas. Many coastal aqui­ the world are equally imperiled. Current rapidly unfolding
fers that are already being depleted for agriculture and ur­ trends in water resources have a number of implications for
ban water supplies face an additional threat from saltwater research priorities. For one thing, they highlight the con­
contamination. Miami, Florida and Orange County, Califor­ tinuing need for a panel of scientists and policy analysts to
nia have spent millions of dollars in recent decades injecting define realistic goals and priorities for research on water is­
treated surface water into their aquifers to keep water tables sues. While a number of recent efforts have taken important
high and repel saltwater intrusion. steps toward delineating such priorities, each is incomplete
or not yet implemented. Our brief report can only suggest a
ISSUES FOR THE FUTURE few priorities that seem critical to us, acknowledging the
need for broader input linked to action.
Emerging Problems and Implications for Research There is an unprecedented need, for instance, for
multidisciplinary research to solve existing water problems.
Human impacts on the quality and quantity of fresh The examples presented above have emphasized that water
water can threaten economic prosperity, social stability, and supply and quality are intimately connected, yet traditional
the resilience of ecological services that aquatic systems pro­ scientific boundaries between climatology, hydrology, limnol­
vide. As societies and ecosystems become increasingly depen­ ogy, ecology, and the social sciences fragment our under­
dent on static or shrinking water supplies, there is a height­ standing and treatment of water systems. The need for inte­
ened risk of severe failures in social systems, including the grated research has been cited often, but funding agencies,
possibility of armed conflicts over water, and also complete management agencies, and research institutions have seldom
Issues in Ecology Number 9 Spring 2 0 0 1

implemented these recommendations. (A notable exception and freshwater ecosystems are also lacking, especially at the
is the joint National Science Foundation and Environmental scale of large watersheds and regions.
Protection Agency Water and Watershed program.) Now In many cases, uncertainty will be the most impor­
is an opportune time to increase incentives for such critically tant feature of freshwater forecasts. By evaluating uncer­
needed efforts at synthesis. tainties, forecasters can help decision-makers anticipate the
Several elements must be taken into consideration in range of possible outcomes and design flexible responses.
forecasting the consequences of various policy scenarios for Careful analyses of uncertainty can also help identify promis­
water supply and quality. These include predicted changes in ing research areas th a t may improve future
water flows, in concentrations of sediments, nutrients, and decisions. Freshwater systems are increasingly the focus of
pollutants, and in biotic resources (Box 2). Watersheds are adaptive management efforts, which are designed to be safe
a natural spatial unit for such predictions, but some prob­ (decreasing the risk of environmental damages or irreversible
lems such as coastal eutrophication require integration of change) and informative (with clear experimental design and
predictions at regional scales. Such forecasts should be quan­ careful scientific assessment of effects).
titative, provide assessments of uncertainty such as probabil­
ity distributions, and be based on clearly stated premises. Current Progress and Management Options
Although the literature contains many quantitative tools for
forecasting freshwater resources, freshwater forecasting is Growing demands on freshwater resources present
not a well-organized field with a comprehensive set of stan­ an opportunity to link ongoing research with improved wa­
dardized tools and approaches. Quantitative tools for fore­ ter management. Water-policy successes of recent decades
casting changes in biogeochemical processes in land-based clearly demonstrate this link. Because of such links, in fact.

Box 2 - Forecasting Water Resources

Forecasting our water future is important for guaranteeing human water supplies, scheduling irrigation and hydro­
electric power generation, moderating flooding, and coordinating recreational activity. Flydrologic forecasts predict future
changes in hydrology using weather forecasts and current hydrologic conditions. Forecasts of hydrologic dynamics are
improving now that regular monitoring data are immediately available via the internet. Current forecasts are generally three
to five days in advance, but improvements in data distribution and hydrometeorological modeling will allow one- to six-month
forecasts in the near future.
As an example of the array of datasets required for quality forecasting, Doppler radar is now used to map precipita­
tion cells every half hour, and most stream gauge data are reported daily by satellite telemetry and posted on a U. S.
Geological Survey website. Weekly updates of surface variables such as snow cover and Leaf Area Index (a measure of the
greenness of the landscape) are now possible globally with the latest generation of earth observing satellites. Various com­
puter models use the data on weather observations, rainfall, snowpack, topography, soils, plant cover, and stream flows to
predict trends in levels and timing of runoff in specific watersheds. New hydrometeorology models can use the daily data
stream to compute the levels of river runoff expected downstream in the following days. As the quality of new one- to six-
month climate forecasts improves, longer hydrologic forecasts should be possible. In larger regions, where human activities
such as water withdrawals for irrigation and regulation of reservoir flows affect runoff, hydrology models must be coupled
with other types of models that can take these factors into account.
A different type of water forecasting involves analyzing long term hydrologic responses to future scenarios of land-
use or climate change (Figure 5). For example, hydrologists can predict the increase in runoff and flood potential that might
occur if portions of a watershed are clearcut, or the changes in stream sedimentation that would occur with increased levels
of cattle grazing in a watershed. The consequences for water quality and flows that might follow from increasing urbaniza­
tion or agricultural use of a landscape can also be predicted using a set of population and crop scenarios. For predicting
changes in freshwater availability over decades and centuries, however, the best that can be done today is to make projections
based on scenarios of climate change provided by General Circulation Models.
Advanced hydroecological models can also calculate critical aspects of water quality, such as stream temperatures,
dissolved oxygen concentrations, nutrient loading, and aquatic plant productivity. Eutrophication of lakes and reservoirs is
often predictable from data on land use and water and nutrient flows. Other chemical properties that affect water use, such
as pFI and microcontaminant concentrations, can be forecast using various mechanistic and statistical models. Models are
also playing increasingly important roles in predicting impacts of human activities on nutrient cycles in the ocean, forecasting
fish stocks, and gauging the potential for invasion of freshwater habitats by nonnative species.
Issues in Ecology Number 9 Spring 2 0 0 1

freshwater eutrophication and pollution have decreased in percent. As a result of mounting evidence that the ecologi­
many waterways. In the Hudson River, for example, concen­ cal health of the basin’s rivers was declining, the Ministerial
trations of heavy metals such as copper, cadmium, nickel, Council recently capped water diversions at 1993/94 levels.
and zinc have been halved since the mid 1970s. Three de­ Basin states also recently agreed to allocate one quarter of
cades ago, scientists and managers decisively showed that natural river flows to maintaining the ecological health of
the primary cause of freshwater eutrophication was not over­ the system.
supply of carbon but rather of inorganic nutrients, especially Progress has also been made in water availability
phosphorus. This discovery led to widely implemented poli­ for human health. Seven hundred million fewer people were
cies reducing inorganic pollutants in North America and Eu­ without safe drinking water in 1994 than in 1980, even
rope, including bans on phosphate detergents and better sew­ though global population increased by more than a billion.
age treatment. Rapid improvement in water bodies such as The proportion of people in developing countries with access
Lake Erie showed that the policies worked. To build on these to safe drinking water rose from fewer than half to more
successes, nonpoint sources of nutrient pollution should be than three quarters during the same period. In the United
reduced in the future. Aggressive management of nitrogen States, the annual incidence of waterborne disease from 1970
inputs will also sometimes be needed, since nitrogen is the to 1990 was less than half of the value from 1920 to 1940,
critical nutrient in some aquatic ecosystems. fewer than four cases per 100,000 people.
Habitat restoration and preservation are the focus
of many efforts to improve water management. Beginning CONCLUSIONS
in 1962, for example, the 166-kilometer-long Kissimmee River
that once meandered south to Elorida’s Lake Okeechobee In the next half century, global population is pro­
was converted to a 90-km, 9-meter-deep canal for flood con­ jected to rise at least three times faster than accessible fresh­
trol. Damages to biodiversity and ecosystem services oc­ water runoff. As a result, it will be necessary to improve the
curred immediately. Wintering waterfowl declined by 90 efficiency of water use if we are to balance freshwater supply
percent. Eutrophication increased in Lake Okeechobee as with demand and also protect the integrity of aquatic eco­
the floodplain wetlands that once filtered nutrients from the systems (Table 3). Technologies such as drip irrigation have
river disappeared. Today, after decades of research and nu­ great but underused potential to reduce water consumption
merous pilot studies, restoration of 70 km of the river chan­ in agriculture. Greater efficiency in all water uses could be
nel, I 1,000 hectares of wetlands, and 100 km^ of floodplain encouraged through economic incentives and a more realis­
has begun at a projected cost of half a billion dollars. tic valuation of both water supplies and freshwater ecosys­
In 1996, New York City invested more than a billion tem services. More complete monitoring of water chemistry
dollars to buy land and restore habitat in the Catskill Moun­ and water flows, including measuring water quantity and
tains, the source of the city’s fresh water supply. The water­ quality at the same spatial and temporal scales, would also
shed was becoming increasingly polluted with sewage, fertil­ provide better data for efficient allocation of water resources
izers, and pesticides. A filtration plant to treat the water among competing needs. This emphasis is especially impor­
was projected to cost $8 billion dollars to build and $300 tant because in the past three decades, more than one-fifth
million dollars annually to run. In contrast, preserving habi­ of flow gauges on small, free-flowing streams in the United
tat in the watershed and letting the ecosystem do the work States have been eliminated. Additional priorities include
of cleansing the water was judged to be just as effective as a assuring that natural aquatic systems retain sufficient quan­
new filtration plant. Habitat preservation and restoration tity, quality, and timing of instream flows, that critical habi­
costs one-fifth the price of a new filtration plant, avoids hun­ tat is preserved in groundwater recharge zones and water­
dreds of millions of dollars in annual maintenance costs, and sheds, and that pollution prevention efforts for both point
provides many other ecological and social benefits to the and non-point sources continue to improve.
region. Achieving sustainable water use in the future will
An impressive policy initiative is also taking place in also depend on continued changes in the culture of water
the Murray-Darling Basin in Australia, a region under pres­ management. At least six federal departments and twenty
sure from high water demand, limited water availability, ris­ agencies in the United States share responsibilities for vari­
ing population, and land use changes. The Murray-Darling ous aspects of the water cycle. Coordinating their diverse
Basin contains two million people, covers portions of four activities through a panel with representatives from each
Australian states, and contributes almost half of Australia’s department or through one central agency would encour­
agricultural output. Two-thirds of its 700,000 km^ of wood­ age the development of a well-conceived national plan for
lands have been converted to crop and pasturelands. In re­ water research and management. The establishment of an
cent years, salinization of heavily irrigated soils and changes advisory panel of scientists and policy analysts is also needed
in the water table have reduced agricultural output by 20 to help define future research priorities and goals for cross-
Issues in Ecology Number 9 Spring 2 0 0 1

Priorities for Balancing Current and Future Demands on Freshwater Supply

• Promotion of an “ environmental water reserve” to ensure that ecosystems receive the quantity, quality, and
timing of flows needed to support their ecological functions and their services to society t
• Legal recognition of surface and renewable ground waters as a single coupled resource
• Improved monitoring, assessment, and forecasting of water quantity and quality for allocating water resources
among competing needs^
• Protection of critical habitats such as groundwater recharge zones and watersheds
• A more realistic valuation of water supplies and freshwater ecosystem services
• Stronger economic incentives for efficient water use in all sectors of the economy
• Continued improvement in eliminating point and nonpoint sources of pollution
• A well-coordinated national plan for managing the diverse and growing pressures on freshwater systems and for
establishing goals and research priorities for cross-cutting water issues. §

Table 3 — Some priorities for balancing current and future demands on freshwater supplytTo our knowledge.South Africa
is the only country currently attempting to implement such a policy nationally (e.g.. South Africa’s National Water Act of
1998). tin the past thirty years, more than a fifth of gauges that recorded flow on small, free-flowing streams in the U.S.
have been eliminated (USGS 1999). §Currently, at least six federal departments andtwenty agencies in the U.S. share
responsibilities for various aspects of the water cycle and for water management.

cutting water issues. A good first step in this process would Jones, J. B., and P. J. Mulholland (eds.) 2000. Streams and
be a new science initiative on the global water cycle as part Ground Waters. Academic Press. San Diego, California. 425
of the Global Change Research Program. pp.
L’Vovich, M. L, G. F. White, A. V. Belyaev, J. Kindler, N. I.
ACKNOWLEDGMENTS Koronkevic, T. R. Lee, and G. V. Voropaev. 1990. Use and
transformation of terrestrial water systems. Pages 235-252
The panel thanks J. Baron, L. Pitelka, D. Tilman, and 7 />7B. L. Turner II, W. C. Clark, R. W. Kates, J. F. Richards, J.
anonymous reviewers for helpful comments and discussions on T. Mathews, and W. B. Meyer, (eds.). The Earth as Trans­
the manuscript. RBJ gratefully acknowledges support from the formed by Human Action. Cambridge University Press,
National Science Foundation, the Andrew W. Mellon Founda­ Cambridge, UK.
tion, the Inter American Institute for Global Change Research, Naiman, R.J., J.J. Magnuson, D.M. McKnight, and J. A. Stanford
and the Department of Energy. This paper is a contribution to (eds.). 1995. The Freshwater Imperative: A Research
the Global Change and Terrestrial Ecosystems (GCTE) and Bio­ Agenda. Island Press, Washington, D.C.
spheric Aspects of the Flydrological Cycle (BAFIC) Core Projects National Research Council. 1999. New Strategies for America’s
of the International Geosphere Biosphere Programme (IGBP). Watersheds. National Academy Press, Washington, D.C.
Postel, S. L., G. C. Daily, and R R. Ehrlich. 1996. Human appro­
SUGGESTIONS FOR FURTHER READING priation of renewable fresh water. Science 271: 785-788.
Rostel, S. 1999. Rillar of Sand: Can the Irrigation Miracle Last?
This report summarizes the findings of our panel. Our W.W Norton S Co., New York.
full report, which is being published in the journal Ecological Shiklomanov, L A . 1997. Comprehensive assessment of the
Applications (Volume 11, Number 4, August 2001) discusses freshwater resources of the world. World Meteorological
and cites extensive references to the primary scientific literature Organization, Stockholm, Sweden.
on this subject. From that list we have chosen those below as United States Geological Survey. 1999. Streamflow informa­
illustrative of the scientific publications and summaries upon tion for the next century: a plan for the national streamflow
which our report is based. information program of the U.S. Geological Survey. USGS
Open-File Report 99-456.
Czaya, E. 19 8 1. Rivers of the World. Van Nostrand Reinhold, Vorosmarty C. J., R. Green, J. Salisbury, and R. B. Lammers.
New York. 2000. Global water resources: vulnerability from climate
Gleick, R H. 1998. The World’s Water 1998-1999. Island change and population growth. Science 289:284-288.
Press. Washington, D.C. Wilson, M. A. and S. R. Carpenter. 1999. Economic valuation
Jackson, R. B., S. R. Carpenter, C. N. Dahm, D. M. McKnight, of freshwater ecosystem services in the United States, 1977-
R. j. Naiman, S. L. Postel, S. W. Running. 2001. Water in 1997. Ecological Applications 9:772-783.
a Changing World. Ecological Applications I I (4): in press.
Issues in Ecology Number 9 Spring 2 0 0 1

ABOUTTHE PANEL Dr. Gordon Orians, Department of Zoology, University of

Washington, Seattle, WA 9 8 195
This report presents a consensus reached by a panel of Dr. Lou Pitelka, Appalachian Environmental Laboratory, Gunter
seven scientists chosen to include a broad array of expertise in Hall, Frostburg, MD 21532
this area. This report underwent peer review and was approved Dr. William Schlesinger, Departments of Botany and Geology,
by the Board of Editors of Issues in Ecology. The affiliations of Duke University, Durham, NC 27708-0340
the members of the panel of scientists are:
Previous Reports
Dr. Robert B. Jackson, Panel Chair, Department of Biology and Previous Issues in Ecology reports available from the
Nicholas School of the Environment, Duke University, Ecological Society of America include:
Durham, NC, 27708 Vitousek, P.M., J. Aber, R.W Howarth, G.E. Likens, P.A. Matson,
Dr. Stephen R. Carpenter, Center for Limnology, University of D.W Schindler, W.H. Schlesinger, and G.D. Tilman. 1997.
Wisconsin, Madison, Wl, 53706 Human Alteration of the Global Nitrogen Cycle: Causes and
Dr. Clifford N. Dahm, Department of Biology, University of New Consequences, Issues in EcohgyHo. I .
Mexico, Albuquerque, NM, 87131 Daily G.C., S. Alexander, PR. Ehrlich, L. Goulder, J. Lubchenco,
Dr. Diane M. McKnight, Institute for Arctic and Alpine Research, RA. Matson, H.A. Mooney, S. Postel, S.H. Schneider, D. Tilman,
University of Colorado, Boulder, CO, 80309 andG.M.Woodwell. 1997. Ecosystem Services: Benefits Supplied
Dr. Robert J. Naiman, School of Aquatic and Fishery Sciences, to Human Societies by Natural Ecosystems, Issues in EcohgyHo.
University of Washington, Seattle, WA, 9 8 195 2.
Dr. Sandra L. Postel, Center for Global Water Policy, 107 Lark­ Carpenter, S., N. Caraco, D. L. Correll, R. W. Howarth, A. N.
spur Drive, Amherst, MA, 0 1002 Sharpley, and Y H. Smith. 1998. Nonpoint Pollution of Surface
Dr. Steven W. Running, School of Forestry, University of Mon­ Waters with Phosphorus and Nitrogen, Issues in EcohgyHo. 3.
tana, Missoula, MT, 59812 Naeem, S., F.S. Chapin III, R. Costanza, RR. Ehrlich, F.B. Golley,
D.U. Hooper, J.H. Lawton, R.V. O’Neill, H.A. Mooney, G.E.
About the Science W riter Sala, A.J. Symstad, and D. Tilman. 1999. Biodiversity and
Yvonne Baskin, a science writer, edited the report of Ecosystem Functioning: Maintaining Natural Life Support Pro­
the panel of scientists to allow it to more effectively communi­ cesses, Issues in EcohgyHo. 4.
cate its findings with non-scientists. Mack, R., D. Simberloff, W.M. Lonsdale, H. Evans, M. Clout, and
F. Bazzaz. 2000. Biotic Invasions: Causes, Epidemiology, Global
About Issues In Ecology Consequences and Control, Issues in EcohgyHo. 5.
Issues in Ecology is designed to report, in language Aber, J., N. Christensen, I. Fernandez, J. Franklin, L. Hidinger, M.
Hunter, J. MacMahon, D. Mladenoff, J. Pastor, D. Perry, R.
understandable by non-scientists, the consensus of a panel of
Slangen, H. van Miegroet. 2000. Applying Ecological Principles
scientific experts on issues relevant to the environment. Issues
to Management of the U.S. National Forests, Issues in Ecology
in Ecology is supported by a Pew Scholars in Conservation
No. 6.
Biology grant to David Tilman and by the Ecological Society of
Howarth, R., D. Anderson, J. Cloern, C. Elfring, C. Hopkinson, B.
America. All reports undergo peer review and must be
LaPointe, T. Malone, N. Marcus, K. McGlathery, A. Sharpley,
approved by the editorial board before publication. No
and D. Walker. Nutrient Pollution of Coastal Rivers, Bays, and
responsibility for the views expressed by authors in ESA Seas, Issues in EcohgyHo. 1.
publications is assumed by the editors or the publisher, the Naylor, R., R. Goldburg, J. Primavera, N. Kautsky, M. Beveridge, J.
Ecological Society of America. Clay, C. Folke, J. Lubchenco, H. Mooney, and M. Troell. Effects
of Aquaculture on World Fish Supplies, Issues in EcohgyHo. 8.
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Dr. Stephen Carpenter, Center for Limnology, University of 1707 H Street, NW, Suite 400
Wisconsin, Madison, Wl 53706 Washington, DC 20006
Dr. Deborah Jensen, The Nature Conservancy, 4245 North Fairfax (202) 833-8773, esahq@esa.org
Drive, Arlington, VA 22203.
Dr. Simon Levin, Department of Ecology and Evolutionary
Biology, Princeton University, Princeton, NJ 08544-1003
Dr. Jane Lubchenco, Department of Zoology, Oregon State
University, Corvallis, OR 97331-2914
Dr. Judy L. Meyer, Institute of Ecology, University of Georgia, The Issues in Ecology series is also available
Athens, GA 30602-2202 electronically at http://esa.sdsc.edu/.