issues in ecology - jackson lab · 1027 ecological applications, 11(4), 2001, pp. 1027–1045 q...

1027

Ecological Applications, 11(4), 2001, pp. 1027–1045q 2001 by the Ecological Society of America

TECHNICAL REPORT

Issues in Ecology

WATER IN A CHANGING WORLD

ROBERT B. JACKSON,1,8 STEPHEN R. CARPENTER,2 CLIFFORD N. DAHM,3 DIANE M. MCKNIGHT,4

ROBERT J. NAIMAN,5 SANDRA L. POSTEL,6 AND STEVEN W. RUNNING7

1Department of Biology and Nicholas School of the Environment, Duke University, Durham, North Carolina 27708 USA2Center for Limnology, University of Wisconsin, Madison, Wisconsin 53706 USA

3Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA4Institute for Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309 USA

5School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195 USA6Center for Global Water Policy, 107 Larkspur Drive, Amherst, Massachusetts 01002 USA

7School of Forestry, University of Montana, Missoula, Montana 59812 USA

Abstract. Renewable fresh water comprises a tiny fraction of the global water poolbut is the foundation for life in terrestrial and freshwater ecosystems. The benefits to humansof renewable fresh water include water for drinking, irrigation, and industrial uses, forproduction of fish and waterfowl, and for such instream uses as recreation, transportation,and waste disposal.

In the coming century, climate change and a growing imbalance among freshwatersupply, consumption, and population will alter the water cycle dramatically. Many regionsof the world are already limited by the amount and quality of available water. In the next30 yr alone, accessible runoff is unlikely to increase more than 10%, but the earth’s pop-ulation is projected to rise by approximately one-third. Unless the efficiency of water userises, this imbalance will reduce freshwater ecosystem services, increase the number ofaquatic species facing extinction, and further fragment wetlands, rivers, deltas, and estuaries.

Based on the scientific evidence currently available, we conclude that: (1) over half ofaccessible freshwater runoff globally is already appropriated for human use; (2) more than1 3 109 people currently lack access to clean drinking water and almost 3 3 109 peoplelack basic sanitation services; (3) because the human population will grow faster thanincreases in the amount of accessible fresh water, per capita availability of fresh water willdecrease in the coming century; (4) climate change will cause a general intensification ofthe earth’s hydrological cycle in the next 100 yr, with generally increased precipitation,evapotranspiration, and occurrence of storms, and significant changes in biogeochemicalprocesses influencing water quality; (5) at least 90% of total water discharge from U.S.rivers is strongly affected by channel fragmentation from dams, reservoirs, interbasin di-versions, and irrigation; and (6) globally, 20% of freshwater fish species are threatened orextinct, and freshwater species make up 47% of all animals federally endangered in theUnited States.

The growing demands on freshwater resources create an urgent need to link researchwith improved water management. Better monitoring, assessment, and forecasting of waterresources will help to allocate water more efficiently among competing needs. Currentlyin the United States, at least six federal departments and 20 agencies share responsibilitiesfor various aspects of the hydrologic cycle. Coordination by a single panel with membersdrawn from each department, or by a central agency, would acknowledge the diversepressures on freshwater systems and could lead to the development of a well-coordinatednational plan.

Key words: aquatic environment; climate change; global hydrological cycle; renewable fresh-water supply; water policy; water resources.

Manuscript received 20 October 2000; accepted 11 November2000.

Reprints of this 19-page report are available for $3.00each. Prepayment is required. Order reprints from the Eco-logical Society of America. Attention: Reprint Department,1707 H Street, N.W., Suite 400, Washington, DC 20006.

8 Address for correspondence: Department of Biology,Phytotron Building, Duke University, Durham, North Caro-lina 27708 USA. E-mail: [email protected]

INTRODUCTION

The movement of water through the hydrologicalcycle comprises the largest flow of any material in thebiosphere (Chahine 1992). Driven by solar energy, thehydrological cycle delivers an estimated 110 000 km3

of water to the land annually in precipitation (Speideland Agnew 1982, L’Vovich et al. 1990, Schwarz et al.1990). This renewable freshwater supply sustains ter-restrial, freshwater, and estuarine ecosystems.

1028 ISSUES IN ECOLOGY Ecological ApplicationsVol. 11, No. 4

FIG. 1. Global data for human population, water withdrawals, and irrigated land area from 1900 to 2000 (redrawn andupdated from Gleick [1998b]).

Renewable fresh water provides many benefits (Pos-tel and Carpenter 1997). These include water for drink-ing, industrial production, and irrigation, and the pro-duction of fish, waterfowl, and shellfish. Freshwatersystems also provide many nonextractive or instreambenefits, including flood control, transportation, rec-reation, waste processing, hydroelectric power, andhabitat for aquatic life. Some benefits, such as irrigationand hydroelectric power, are achieved only by majorchanges to flow regime and flow paths from dams andwater diversions (Rosenberg et al. 2000). These hy-drological changes often have negative consequencesor trade-offs with other instream benefits, such as sup-porting aquatic life and maintaining suitable waterquality for human use (Naiman et al. 1995; J. S. Baron,N. L. Poff, P. L. Angermeier, C. N. Dahm, P. H. Gleick,N. G. Hairston, Jr., R. B. Jackson, C. A. Johnston, B.G. Richter, and A. D. Steinman, unpublished manu-script).

The ecological, social, and economic benefits thatfreshwater systems provide, and the trade-offs betweenconsumptive and instream benefits, will change dra-matically in the coming century (Chichilnisky and Heal1998, Wilson and Carpenter 1999, Vorosmarty and Sa-hagian 2000). In the past 100 yr, the amount of waterwithdrawn globally by humans and the land area underirrigation have risen exponentially (Fig. 1). A globalperspective on water withdrawals is important for en-suring sustainable water use, but is insufficient for re-gional and local stability. How fresh water is managedin particular basins and individual watersheds is thekey to sustainable water management.

Despite the clear importance of water for humanhealth and welfare, basic water needs are not met formany people in the world. Currently, 1.1 3 109 peoplelack access to safe drinking water and 2.8 3 109 lack

basic sanitation services (World Health Organization1996). These deficiencies cause ;250 3 106 cases ofwater-related diseases and 5–10 3 106 deaths each year(Gleick 2000). Current unmet needs also limit our abil-ity to adapt to future hydrologic changes. Many currentsystems designed to provide water in relatively stableclimatic conditions may be ill prepared to adapt to fu-ture changes in climate, consumption, and population.

The goal of this report is to describe key features ofhuman-induced changes to the global water cycle. Theeffects of pollution on water availability and on puri-fication costs have been addressed previously (Car-penter et al. 1998). We focus instead on current andpotential changes in the cycling of water that are es-pecially relevant for ecological processes. We begin bybriefly summarizing the global hydrological cycle: itscurrent state and historical context. We next examinethe extent to which human activities currently alter thewater cycle and may affect it in the future. These chang-es include ‘‘direct’’ actions, such as dam construction,and ‘‘indirect’’ ones, such as climatic change. We ex-amine human appropriation of freshwater supply forrenewable and nonrenewable sources globally. The re-port ends by discussing changes in water use that maybe especially important in the future. We highlightsome current progress and suggest priorities for re-search, emphasizing examples for the United States.

THE GLOBAL WATER CYCLE

Surface water

There are .1 3 109 km3 of water on earth (Chahine1992, Schmitt 1995, Schlesinger 1997). Although mostof the planet is covered by water, the vast majority isin forms unavailable to terrestrial and freshwater eco-systems. Less than 3% is fresh enough to drink or to

August 2001 1029WATER IN A CHANGING WORLD

FIG. 2. The renewable freshwater cycle in units of thousands of km3 and thousands of km3/yr, respectively, for pools(white numbers) and fluxes (black numbers). Total precipitation over land is ;110 000 km3/yr. Approximately two-thirds ofthis precipitation is water recycled from plants and the soil (evapotranspiration is 70 000 km3/yr), whereas one-third is waterevaporated from the oceans that is transported over land (40 000 km3/yr). Total evaporation from the oceans is ;10 timeslarger than the flux pictured here (;425 000 km3/yr). Groundwater holds ;15 000 000 km3 of fresh water, much of which isnot in active exchange with the earth’s surface.

irrigate crops, and of that total, more than two-thirdsis locked in glaciers and ice caps. Freshwater lakes andrivers hold 100 000 km3 globally, less than 0.01% ofall water on earth (Schwarz et al. 1990).

Atmospheric water exerts an important influence onclimate and on the hydrological cycle (Shiklomanov1989). Only 15 000 km3 of water is typically held inthe atmosphere at any time, but this tiny fraction isvital for the biosphere (Fig. 2). Water vapor contributesapproximately two-thirds of the total warming thatgreenhouse gases supply. Without these gases, themean surface temperature of the earth would be wellbelow freezing and liquid water would be absent overmuch of the planet (Ramanathan 1988, Mitchell 1989).Equally important for life, atmospheric water turnsover every 10 d or so, and is the source of the earth’sprecipitation.

Renewable fresh water comprises a subset of thesolar-driven pools and fluxes in the earth’s hydrologicalcycle (Fig. 2). Solar energy typically evaporates;425 000 km3 of ocean water each year. Most of thiswater returns directly to the oceans, but ;10% falls onland. If this were the only source of rainfall, averageterrestrial precipitation for the earth would be only 25

cm/yr, a value typical for deserts or semiarid regions.Instead, a second, larger source of water is recycledfrom plants and the soil, a direct feedback between theland surface and regional climate (Pielke et al. 1998).Important elemental cycles in these ecosystems, suchas C and N, are strongly coupled to this freshwaterflux, creating additional feedbacks between vegetationand climate. This recycled water contributes two-thirdsof the average 70 cm/yr of precipitation that falls overland (Chahine 1992). Taken together, these two fluxesconstitute the 110 000 km3/yr of renewable freshwatersupply for terrestrial, freshwater, and estuarine eco-systems (Fig. 2).

Because precipitation is greater than evaporation onland, 40 000 km3 of water returns to the oceans, pri-marily via rivers and underground aquifers (L’Vovich1973, Schwarz et al. 1990). The availability of thisfresh water in transit to the oceans is determined bythe form of the precipitation (rain or snow), its timingrelative to patterns of seasonal temperature and sun-light, and the geomorphology of a region. For example,in many mountain regions, most precipitation falls assnow during winter, and spring snowmelt causes peakflows that propagate into major river systems. Such


river systems have often been modified to capture andstore the pulse of spring floodwater. The retention andredistribution of this water on the landscape typicallyincreases concentrations of ions, nutrients, and con-taminants through greater contact with soils and min-erals and through evaporative water losses. In sometropical regions, episodic flooding is associated withmonsoons; not as much of this floodwater is retainedin dams. In other regions, excess precipitation recharg-es ground water or is stored in wetlands. Widespreadlosses of wetlands and riparian areas reduce the atten-uation of high flows and enhances the transport of wa-terborne excess nutrients and contaminants to coastalenvironments (Peterjohn and Correll 1984). More thanhalf of all wetlands in the United States have alreadybeen drained, dredged, filled, or planted (Vileisis1997).

Important gradients in water availability exist glob-ally. Two-thirds of all precipitation falls between 308N and 308 S latitude because of greater solar radiationand evaporation there. Daily evaporation from theoceans ranges from 0.4 cm at the equator to ,0.1 cmat the poles (Mitchell 1989, Chahine 1992). Runoff intropical regions is also typically larger. Roughly halfof the precipitation in rain forests becomes runoff(Shuttleworth 1988), but the value for deserts is muchsmaller because of high evaporative demand and lowrainfall (Schlesinger et al. 1987). The Amazon, for ex-ample, carries 15% of all water returning to the oceansglobally. In contrast, the Colorado River drainage isone-tenth the size of the Amazon, but its historic annualrunoff is 300 times smaller (Loaiciga et al. 1996). Sim-ilar variation occurs at continental scales. Average run-off in Australia is only 4 cm/yr, eight times less thanin North America and orders of magnitude less than intropical South America (Tamrazyan 1989). The resultof these and many other factors is that freshwater avail-ability varies dramatically worldwide.

Ground water

At least one-fourth of the world’s population drawsits water from underground aquifers (Ford and Wil-liams 1989, White et al. 1995). Estimates of the globalhydrologic cycle generally treat rates of ground waterinflow and outflow as in balance (Hornberger et al.1998), although this resource is being depleted globally(Shiklomanov 1997). Approximately 99% of all liquidfresh water is in underground aquifers (Fig. 2). Thisground water typically turns over more slowly thanmost other water pools, often in hundreds to tens ofthousands of years, although the range in turnover ratesis large (United States Environmental Protection Agen-cy and Government of Canada 1995, Schlesinger1997). Even more dramatically, most ground water isnot in active exchange with the earth’s surface, sur-viving instead as a relic of wetter climatic conditionsand melting Pleistocene ice sheets of the past. Such

‘‘fossil water’’ accumulated over tens of thousands ofyears and, once used, cannot readily be replenished.

The distinction between renewable and nonrenew-able ground water is critical for water management andpolicy. Renewable aquifers depend on current precip-itation for refilling and are vulnerable to changes in thequantity and quality of recharge water (White et al.1995). For example, ground water pumping of the Ed-wards Aquifer, which supplies much of central Texas,USA, with drinking water, has increased fourfold sincethe 1930s and at times now exceeds annual rechargerates (Brown et al. 1992, United States Geological Sur-vey 1998). Increased water use makes aquifers moresusceptible to changes in weather, such as drought, andto contamination. Depletion of ground water can alsocause surface subsidence and compaction, permanentlyreducing aquifer storage (Sun et al. 1999). The CentralValley of California has lost ;25 km3 of storage in thisway (Bertoldi 1992), a capacity equal to .40% of thecombined storage capacity of all human-made reser-voirs in the state.

Where extraction of ground water exceeds rechargerates, the resulting lower water tables decrease summerlow-flow rates in rivers and streams, reduce perennialstream habitat, increase summer stream temperatures,and impair water quality (Jones and Mulholland 2000).Trout and salmon species select areas of ground waterupwelling in streams to moderate extreme temperaturesand to keep their eggs from freezing (Benson 1953).Dynamic exchange of surface and ground water altersthe biogeochemistry of streams (e.g., affecting dis-solved oxygen and nutrient concentrations) and reducesconcentrations of dissolved contaminants such as pes-ticides and volatile organic compounds (Kim et al.1995, Holmes 2000). Because of such links, humandevelopment of either ground water or surface wateralone often affects the quantity and quality of the other(Brunke and Gonser 1997, Slutsky and Yen 1997, Win-ter et al. 1998). Renewable ground water and surfacewater have commonly been viewed separately, bothscientifically and legally. This view is changing as stud-ies in streams, rivers, reservoirs, wetlands, and estu-aries show the importance of interactions between re-newable surface and ground waters for water supply,water quality, and aquatic habitats (Dahm et al. 1998,Winter et al. 1998).

In contrast to renewable ground water, more thanthree-fourths of underground water is nonrenewable(defined as ground water with a renewal period of cen-turies or more; Shiklomanov 1997), as shown in Fig.3. The High Plains, or Ogallala, Aquifer, arguably thelargest aquifer in the world, underlies 0.5 3 106 km2

of the central United States (Kromm and White 1990).Turbine pumps and relatively inexpensive energy sincethe 1940s have led to ;200 000 wells being drilled intothe aquifer, making it the primary water source for one-fifth of irrigated U.S. farmland (Schwarz et al. 1990,


FIG. 3. Locations of nonrenewable groundwater resources (lighter gray) and the main locations of groundwater mining(darker gray), based on Shiklomanov (1997). The inset shows the location of the High Plains (Ogallala) Aquifer.

Sahagian et al. 1994). Irrigated cropland area in theregion peaked around 1980 at 5.6 3 106 ha and atpumping rates of 22 km3/yr of water (;6 3 1012 gallonsa year), but has since declined somewhat throughgroundwater depletion and socioeconomic factors. Theaverage thickness of the Ogallala declined by .5%across one-fifth of its area in the 1980s alone (L’Vovichet al. 1990).

The links between surface water and ground waterare especially important in regions with low rainfall(Box 1 and Table 1). Arid and semiarid regions coverone-third of the earth’s lands and hold one-fifth of glob-al population. Ground water is the primary source ofwater for drinking and irrigation, and these regionshave many of the world’s largest aquifers (Sahagian etal. 1994). Limited recharge makes such aquifers highly

susceptible to groundwater depletion. For example, ex-ploitation of the Northern Sahara Basin Aquifer in the1990s was almost twice the rate of replenishment;many springs associated with this aquifer have driedup almost completely (Shiklomanov 1997).

For nonrenewable water sources, it is difficult to dis-cuss sustainable or appropriate rates of extraction. Likedeposits of coal and oil, almost any extraction is non-sustainable. Important societal questions include atwhat rate groundwater pumping should be allowed, forwhat purpose, and who, if anyone, will safeguard theneeds of future generations. For the Ogallala Aquifer,the water may be gone in as little as a century.

HUMAN APPROPRIATION OF FRESHWATER SUPPLY

Growth in global population and water consumptionwill place additional pressure on freshwater resources


FIG. 4. A projection of future changes in actual evapotranspiration (AET) and precipitation (PRCP) generated by theBIOME-BGC ecosystem model using a future climate scenario to the year 2100, derived from a General Circulation Model(GCM). In this scenario, atmospheric CO2 increased ;0.5% per year, and the terrestrial model responded with structuralecosystem changes in leaf area index as a function of changes in CO2, climate, water, and nitrogen availability. In general,these projections suggest higher precipitation and increased leaf area index in the arid West, leading to higher AET. Reducedprecipitation and the resulting effects of drought on vegetation are the primary causes of lower evapotranspiration in theSoutheast. For additional information, see Box 2. Results from VEMAP II are courtesy of P. Thornton, Numerical TerradynamicSimulation Group, University of Montana.


FIG. 5. Contrasting riparian vegetation in the Middle Rio Grande reach south of Albuquerque, New Mexico: an exoticsaltcedar-dominated site on the Sevilleta National Wildlife Refuge (top), and a native cottonwood-dominated site near LosLunas (bottom). Water management, especially dam construction and river channeling, has greatly altered this floodplainecosystem. The last major floods with significant cottonwood establishment were in 1942. Invasions by exotic phreatophytessuch as saltcedar, pictured here, and Russian olive have dramatically altered riparian forest composition. Without changesin water management, exotic species will probably dominate riparian zones in the Middle Rio Grande basin within the nexthalf century. For additional information, see Box 1.


BOX 1. A Case Study: the Middle Rio Grande

Increasing water demands create potential conflicts between human needs and those of native ecosystems(Christensen et al. 1996). Perhaps nowhere are human impacts on river and riparian ecosystems greaterthan in arid and semiarid regions of the world. The Middle Rio Grande Basin of central New Mexico isa rapidly growing area with more than half of the state’s population. The desire to balance water needsthere has led to a careful water budget for the basin (Table 1), highlighting annual variability, measurementuncertainty, and conflicting water demands for the region (Middle Rio Grande Water Assembly 1999).The goal of the water budget is to help design a sustainable water policy.

Water management has greatly altered this floodplain ecosystem (Fig. 5). Dam construction and riverchanneling now prevent spring floods. Riparian zones, limited by a system of levees, were once a mosaicof cottonwood (Populus spp.), willow (Salix spp.), wet meadows, marshes, and ponds. The last majorfloods with significant cottonwood establishment were in 1942, and the cottonwoods are declining in mostareas (Molles et al. 1998). Half of the wetlands in the drainage were lost in just 50 yr (Crawford et al.1993). Invasion by exotic phreatophytes such as saltcedar (Tamarix ramosissima) and Russian olive(Elaeagnus angustifolia) dramatically altered riparian forest composition. Without changes in water man-agement, exotic species will probably dominate the riparian zones within half a century.

The water budget of the Middle Rio Grande reflects recent changes in hydrology, riparian ecology, andgroundwater pumping (Table 1). Estimating all major water depletions in the basin is critical for managingits water. Major depletions include urban uses, irrigation, plant transpiration, open-water evaporation, andaquifer recharge (Table 1). The largest loss is open-water evaporation, comprising one-third of the total.This loss is large compared to values before management: direct evaporation from Elephant Butte Reservoiralone ranges from 50 to 280 3 106 m3/yr, depending on reservoir size and climate. The second largestdepletion is riparian plant transpiration (135–340 3 106 m3/yr). There is considerable uncertainty in thisestimate from the unknown effects of fluctuating river discharge on transpiration and differences for nativeand non-native riparian species. Irrigated agriculture in the Middle Rio Grande accounts for an estimated20% of annual average depletions, with cropping patterns, weather, and water availability contributing toannual variability. Urban consumption and net aquifer recharge are similar and account for 20–25% ofthe remaining depletion in the Middle Rio Grande. Average annual depletions are partially offset by waterfrom the San Juan-Chama Project, tributary inflows within the basin, and municipal wastewater discharge.Nonetheless, water depletions are already fully appropriated for an average water year. Municipal use ofSan Juan-Chama water, sustained drought, and continued growth will increase pressure on surface waterresources. No new water is likely to be available in the near future, so water conservation must play adominant role.

A careful water budget 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 agriculturaland urban water conservation all play an important role in a sustainable water future for the region. Otherarid and semiarid regions of the world have similar needs for fundamental data and careful water planning,where balancing diverse water demands will be a formidable and important challenge.

in the coming century (Robarts and Wetzel 2000, Vo-rosmarty et al. 2000). The solar-powered hydrologicalcycle annually makes available several times morefresh water than is needed to sustain the world’s currentpopulation of 6 3 109 people (Table 2). However, thedistribution of this water is not well matched geograph-ically or temporally to human needs (Postel et al. 1996).The large river flows of the Amazon and Zaire-Congobasins and the northern tier of undeveloped rivers(largely in tundra and taiga) in North America andEurasia are largely inaccessible for human uses andwill likely remain so for the foreseeable future. To-

gether, these remote rivers account for nearly one-fifthof the total global runoff (L’Vovich et al. 1990).

Approximately half of the global renewable supplyruns rapidly toward the sea in floods (Table 2). In man-aged river systems of North America and many otherregions, spring floodwaters from snowmelt are storedin reservoirs for later use. In tropical regions, a sub-stantial share of annual runoff occurs during monsoonflooding. In Asia, for example, 80% of runoff occursbetween May and October (Shiklomanov 1993). Thisfloodwater provides a variety of ecological services, inaddition to supporting wetlands, but is not a practical


TABLE 1. (A) Sources and (B) average annual water deple-tion from 1972 to 1997 for the Middle Rio Grande reach(the 64 000-km2 drainage between Otowi gage north ofSanta Fe, New Mexico, USA, and Elephant Butte Dam).

ParameterFresh water(106 m3/yr)

A) Water sourceAverage Otowi flowSan Juan-Chama diversion

136070

B) Water useOpen-water evaporationRiparian plant transpirationIrrigated agricultureUrban consumption (groundwater)Net aquifer recharge

270220165

8585

Notes: Flow records at the Otowi gage, the inflow pointfor the Middle Rio Grande reach, are more than a centuryold. Water supplemented from the San Juan-Chama diversionproject began in 1972 and increased Otowi flow by 70 3 106

m3/yr (average flow without this water was ;1400 3 106 m3/yr). Major municipal water systems in the basin currentlypump ground water at a rate of 85 3 106 m3/yr. Maximumallowable depletion for the reach is 500 3 106 m3/yr whenadjusted annual flow exceeds 1900 3 106 m3/yr, decreasingprogressively to 58 3 106 m3/yr in severe drought years (in-flows of 120 3 106 m3/yr at Otowi gage).

TABLE 2. Global runoff, withdrawals, and human appro-priation of freshwater supply.

Parameter Fresh water (km3/yr)

Total global runoff 40 700

Remote flow, totalAmazon basinZaire-Congo basinRemote northern rivers

Uncaptured floodwaterAccessible runoff

78005400

6601740

20 40012 500

Total human appropriation 6780Global water withdrawals, total

AgricultureIndustryMunicipalitiesReservoir losses

44302880

975300275

Instream uses 2350

Notes: Remote flow refers to river runoff that is geograph-ically inaccessible for human use, estimated to include 95%of runoff in the Amazon basin, 95% of remote northern NorthAmerican and Eurasian river flows, and 50% of the Zaire-Congo basin runoff. Runoff estimates also include renewablegroundwater. An estimated 18% (2285 km3/yr) of accessiblerunoff is consumed, compared to an estimated appropriation(including withdrawals and instream uses) of 6780 km3/yr(54%). Water that is withdrawn but not consumed is not al-ways returned to the same river or lake from which it wastaken, nor does it always provide the same natural ecosystemfunctions. Data shown are from Postel et al. (1996), basedon additional data in Czaya (1981), L’Vovich et al. (1990),and Shiklomanov (1997).

supply for irrigation, industry, and household uses thatneed water to be delivered in controlled quantities atspecific times. As a result, accessible runoff has twocomponents: (1) renewable groundwater and base riverflow, and (2) floodwater that is captured and stored inreservoirs.

L’Vovich et al. (1990) estimate that base river flowsand renewable groundwater account for ;11 100 km3/yr, or 27%, of global runoff. As long as the rate ofextraction does not exceed replenishment, these waterscan serve as a sustainable supply. Unfortunately, inmany places, including many important agricultural re-gions, ground water is chronically overpumped. Basedon data for China, India, North Africa, Saudi Arabia,and the United States, Postel (1999) estimates thatgroundwater depletion in key basins totals $160 km3/yr. Groundwater depletion is particularly serious in In-dia (National Environmental Engineering Research In-stitute 1997), and some water experts have warned thatas much as one-fourth of India’s grain harvest couldbe jeopardized by overpumping (Seckler et al. 1998).Simply because global groundwater extractions remainwell below the global recharge rate does not mean thatgroundwater use is sustainable. What matters is howwater is used and managed in particular basins, andthere are many regions of the world where current de-mand outstrips supply (Shiklomanov 1997).

Turning floodwater into an accessible supply gen-erally requires dams and reservoirs to capture, store,and control the water. Worldwide, there are ;40 000large dams .15 m high, and 20 times as many smallerdams (Oud and Muir 1997). Collectively, reservoirsworldwide are able to hold an estimated 6600 km3/yr

(Postel 1999). Considerably less water than this amountis delivered to farms, industries, and cities, however,because dams and reservoirs are also used to generateelectricity, control floods, and enhance river naviga-tion. After adjusting base flow for geographic inacces-sibility and adjusting reservoir capacity for functionsother than water supply, accessible runoff is ;12 500km3/yr, 31% of total annual runoff (Postel et al. 1996).

People use fresh water for many purposes. There arethree broad categories of extractive uses that withdrawwater from its natural channel or basin: irrigation forcrop production, industrial and commercial activities,and residential activities. In many cases, water can beused more than once after it is withdrawn. Water usedto wash dishes, for example, is not physically con-sumed and may be used again (although it sometimesrequires further treatment). In contrast, about half ofthe water diverted for irrigation of crops is consumedthrough evapotranspiration and is unavailable for fur-ther use.

Consumptive water uses can have extreme effects onlocal and regional ecosystems. In the Aral Sea basin,for example, large river diversions for irrigation havecaused the lake to shrink .75% in volume and 15 min depth over the past four decades (Kindler 1998). TheAral Sea shoreline has retreated 120 km in places, anda commercial fishery of 40 800 Mg (45 000 tons) a yearand 60 000 jobs have disappeared. Water quality has


also declined. Salinity tripled from 1960 to 1990, andthe water that remains is now saltier than the oceans(Stone 1999).

For water management, the difference between useand consumption is important. Postel et al. (1996) es-timate that global water withdrawals (including evap-orative losses from reservoirs) total ;4430 km3/yr, ofwhich 52% is consumed. Water use also modifies waterquality through increased concentration of major ions,nutrients, or contaminants, which, as in the example ofthe Aral Sea, can limit the suitability of water for futureuse.

In addition to water extracted from natural systems,human enterprise depends greatly upon water that re-mains in its natural channels. These so-called ‘‘in-stream’’ uses include pollution dilution, recreation,maintenance of navigation paths, the health of estuariesas nurseries, sustenance of fisheries, and protection ofbiodiversity (Postel and Carpenter 1997, Carpenter etal. 1998). Because instream uses of water vary geo-graphically and seasonally, it is difficult to estimatetheir global total. Using pollution dilution as a roughglobal proxy, however, Postel et al. (1996) estimateinstream uses to be 2350 km3/yr, a conservative esti-mate that does not incorporate all such uses.

Combining this figure with estimated global with-drawals places total human appropriation of freshwaterrunoff at 6780 km3/yr, or 54% of estimated accessiblerunoff (Postel et al. 1996). Although global water de-mands continue to rise with increasing population andconsumption, accessible runoff can increase principallythrough the construction of new dams or desalination.Today, desalination accounts for ,0.2% of global wateruse (Gleick 2000), and is likely to remain a minor partof global supply for the foreseeable future because ofits high energy requirements. Dams continue to bringmore water under human control, but the pace of con-struction has slowed. In developed countries, many ofthe best sites have already been used. Rising economic,environmental, and social costs, such as habitat de-struction, loss of biodiversity, and displacement of hu-man communities, make further dam construction in-creasingly difficult. About 260 new large dams nowcome on line each year, compared with ;1000/yr be-tween the 1950s and 1970s (McCully 1996). Moreover,at least 180 dams in the United States were removedin the last decade based on evaluations of safety, en-vironmental impact, and obsolescence (Born et al.1998). The destruction of the Edwards Dam on Maine’sKennebec River in 1999 marked the first time that fed-eral regulators ruled that the environmental benefits ofremoving a dam outweighed the benefits of operatingit.

As a result of these and other trends, accessible run-off is unlikely to increase by more than 5–10% overthe next 30 yr (Postel et al. 1996). During the sameperiod, the earth’s population is projected to grow by

;35% (United Nations 1998). The demands on fresh-water systems will continue to grow in the comingcentury.

THE WATER CYCLE AND CLIMATE CHANGE

A scientific consensus now exists that the buildup ofgreenhouse gases in the atmosphere is warming theearth (Intergovernmental Panel on Climate Change[IPCC] 1996). The last decade of the 20th century wasthe warmest on record, and paleo-records indicate thatthe warming of the past 50 yr has no counterpart inthe past 1000 yr (Crowley 2000). As the earth warmsin the coming century, a general intensification of thehydrological cycle is expected to occur (Miller andRussell 1992, Knox 1993, Tsonis 1996). Precipitation,evapotranspiration, and runoff are all expected to in-crease globally, and hydrologic extremes such as floodsand droughts will probably be more common and moreintense (IPCC 1996, Loaiciga et al. 1996). Some de-creases in snow and ice cover have already been ob-served. Changes in biogeochemical process controllingwater quality, especially C and N cycling, are likely tobe coupled to these hydrological changes (Murdoch etal. 2000).

Regional and local changes will likely be more var-iable and more difficult to predict. Many regions, es-pecially temperate ones, will experience increasedsummer drying from greater evaporation and, in somecases, lower summer precipitation (Fig. 4; see alsoNeilson and Marks 1994). For example, almost all gen-eral circulation models predict that southern Europewill receive less summer rainfall (IPCC 1996). In con-trast, tropical regions may experience relatively smallwarming-induced changes in the hydrologic cycle(IPCC 1996). Uncertainties for predictions at regionalscales are illustrated by large differences in future sce-narios for soil moisture in the central United States,from as much as 75% drier to 30% wetter in summer,predicted by models using different assumptions andrepresentations of hydrological processes (Hornbergeret al. 2001).

Future changes in the water cycle that will be es-pecially important for the availability of fresh waterinclude the amount and timing of precipitation and run-off, rates of evapotranspiration, and rising sea level.Evaporative demand increases exponentially with tem-perature, so evaporation from the oceans and globalmean precipitation should both increase as the earthwarms. All general circulation models examined in themost recent IPCC (1996) assessment predict an in-crease in globally averaged precipitation. Recent dataindicate that mean precipitation already may have in-creased slightly in nontropical regions (Gates 1993).Precipitation rose as much as 10–15% over the past 50yr in the United States and Canada (Bradley et al. 1987,Lettenmaier et al. 1994) and streamflow also increasedsignificantly during this period, especially in the east-


ern half of the United States (Lins and Michaels 1994).Increases in precipitation were smaller, but significant,for the former Soviet Union (;10% per 100 yr; Grois-man et al. 1991) and Scotland (Smith 1995). In contrast,tropical and arid regions show no evidence for in-creased precipitation (IPCC 1996), perhaps even dryingslightly in recent decades (Allan and Haylock 1993,Nicholson 1994).

Slight increases in average global precipitation willnot uniformly increase freshwater availability. Greaterevaporative demand on plant and soil water may morethan offset increases in precipitation, even if increasedatmospheric CO2 reduces stomatal conductance andwater use of plants (Running and Nemani 1991, Fieldet al. 1995, Miles et al. 2000). Also, because water hasa relatively high heat capacity and ocean turnover buff-ers changes in temperature, the land surface shouldwarm more quickly than the ocean surface. This im-portant transient effect increases the likelihood ofdrought in continental regions (Rind et al. 1990), andmay intensify pressure gradients and wind patterns incoastal regions, enhancing upwelling of coastal waters(Bakun 1990). In general, although some temperate andpolar regions will probably receive more precipitation,other regions will receive less, and many more regionswill be effectively drier from increased evaporative de-mand during the growing season (Mulholland et al.1997).

Feedbacks from deforestation and other forms ofvegetation change such as afforestation and shrub en-croachment will also affect climate and the hydrolog-ical cycle in the coming century (Shukla and Mintz1982, Dickinson 1991, Jackson et al. 2000). At regionalscales, deforestation reduces precipitation through in-creased albedo and decreased water recycling. Thispositive feedback of increased drying with deforesta-tion may be especially important in tropical forests andsavannas (where rates of population growth and land-use change are high), making it harder for trees tobecome reestablished (Lean and Warrilow 1989, Hoff-mann and Jackson 2000). Broadscale regional increasesin irrigation have the opposite potential feedback, in-ducing cooler and wetter regional climates (Chase etal. 1999). Agriculture uses 81% of all water consumedin the United States, and much of this water is beingapplied in drier regions where evaporative demand ishigh, especially the central Great Plains and the West(Baron et al. 1998, Solley et al. 1998). At local scales,changes in vegetation and land cover also affect runoffand water yields in individual watersheds (Bosch andHewlett 1982, Hibbert 1983).

The intensification of the hydrologic cycle under achanging climate will also influence the rates of bio-geochemical cycles coupled to hydrology (Schindler etal. 1996, Webster et al. 1996). Terrestrial productivityand plant species distributions may shift because offactors such as changing soil moisture and nutrient

availability and increased salinity. Microbial processesin soils, which control accumulation of soil organicmatter and remineralization of nutrients, are influencedby the duration of snow cover, freeze/thaw cycles, andsoil moisture (Lipson et al. 1999). In consequence, bio-geochemical processes controlling C and N cycling andsalinity can influence the feedbacks between vegetationchange and changes in hydrology (Running and Ne-mani 1991).

Changes in water quantity and quality also influencehabitat for aquatic biota (Naiman and Turner 2000). Inaquatic ecosystems, productivity of phytoplankton andperiphyton and nutrient cycling are influenced by theduration of ice and snow cover and by changes in sea-sonal flow regimes. Changes in C and N fluxes fromrivers to coastal areas can influence fisheries by de-pleting oxygen in coastal waters, and may promote haz-ardous algal blooms that threaten human health (Turnerand Rabalais 1994, National Research Council 2000a).The uncertainties in the predicted changes in hydrologyfor many regions translate to even greater uncertaintiesin how regional-scale biogeochemical cycles that areinfluenced by hydrology may change.

The water cycle will also be influenced in the comingcentury by rising sea level. Sea level increased ;18cm in the past century and is predicted to rise 30–50cm in the next 100 years (IPCC 1996). Such an increasewould push shores inland 30 m, on average, with crit-ical changes for coastal systems (Gornitz 1995, Kleinand Nicholls 1999). Increased sea level will increasesaltwater intrusion into freshwater coastal aquifers(Sherif and Singh 1999), alter the distribution and hy-drology of coastal wetlands (Michener et al. 1997, Mul-rennan and Woodroffe 1998), and displace agriculturein coastal regions and deltas (Chen and Zong 1999).Many coastal aquifers that are already being depletedface an additional threat from saltwater contamination(U.S. Environmental Protection Agency 1989, Gleick1998a). Miami, Florida and Orange County, Californiahave spent millions of dollars in recent decades in-jecting treated surface water to repel saltwater intrusioninto their aquifers (Office of Technology Assessment,U.S. Congress 1993).

ISSUES FOR THE FUTURE

Emerging problems and implications for research

Human impacts on the quality and quantity of freshwater threaten economic prosperity, social stability,and the resilience of ecosystem services and naturalcapital (Naiman et al. 1998, Wolf 1998, Meyer et al.1999, Wilson and Carpenter 1999). As society and eco-systems become increasingly dependent on static orshrinking water supplies, there is increasing risk ofsevere failures in social or ecological systems, includ-ing complete transformation of ecosystems and the pos-sibility of armed conflicts (Postel 2000). Rising de-mand for fresh water can sever ecological connections


in aquatic systems, fragmenting rivers from flood-plains, deltas, and coastal marine environments (Dy-nesius and Nilsson 1994, Harwell 1998). It also canchange the quantity, quality, and timing of the fresh-water supply on which terrestrial, aquatic, and estuarineecosystems depend (Covich 1993, Postel 2000).

Fresh water is already a limiting resource in manyparts of the world. In the next century, it will becomeeven more limiting because of increased population,urbanization, and climate change (Pringle and Barber2000). This limitation will arise from increased demandfor water and also from pollution in freshwater eco-systems. Pollution decreases the supply of usable waterand increases the cost of purifying it. Some pollutants,such as mercury or chlorinated organic compounds,contaminate aquatic resources and affect food supplies.More than 8 3 109 kg N and 2 3 109 kg P are dischargedeach year into surface waters in the United States (Car-penter et al. 1998). Such pollution and human demandfor water affect biodiversity, ecosystem functioning,and the natural services upon which society depends(Naiman and Turner 2000).

Growing demands for fresh water also dramaticallyaffect species conservation (Coyle 1993, Moyle andYoshiyama 1994). Globally, at least one-fifth of fresh-water fish species are currently threatened or extinct(Moyle and Leidy 1992, World Conservation Moni-toring Centre 1992), and aquatic species currently makeup almost half of all animals listed as federally endan-gered in the United States (Wilcove et al. 1993, Dobsonet al. 1997). The United States also has almost twiceas many threatened freshwater fish species as any othercountry and has lost more molluscs to extinction(World Conservation Monitoring Centre 1992, Flatheret al. 1998). Molluscs in the Appalachian Mountainsand freshwater fish in the Appalachians and in the Son-oran basin and range are especially vulnerable (Flatheret al. 1998). There are also many vulnerable endemicsin karst systems and aquifers, including blind catfish,crayfish, and salamanders (Bowles and Arsuffi 1993).Such examples notwithstanding, aquatic species in oth-er systems around the world are equally imperiled(Dudgeon 2000, Pringle et al. 2000).

These rapidly unfolding water trends have a numberof implications for research priorities. One is the con-tinuing need for a panel of scientists and policy analyststo clearly define realistic goals and priorities for re-search on water issues. Although a number of recentefforts have taken important steps toward such prior-ities, each is incomplete or not yet implemented. Ourbrief report can only suggest a few priorities that seemcritical to us, acknowledging the need for broader inputlinked to action.

There is an unprecedented need for cross-cutting re-search to solve existing water problems (Lubchenco1998, National Resource Council 1999). The examplespresented there have emphasized that water supply and

quality are intimately connected, yet traditional sci-entific boundaries between climatology, hydrology,limnology, ecology, and the social sciences divide ourunderstanding and treatment of water systems (Naimanet al. 1995, Sala et al. 2000). Such integration has beencalled for often, but has not typically been implementedby funding agencies, management agencies, or researchinstitutions (although the joint NSF and EPA Water andWatershed program is a notable exception). Now is anopportune time to increase incentives for this importantsynthesis (National Research Council 2000b).

Forecasting the consequences of policy scenarios forwater supply and quality has several elements, includ-ing prediction of hydrologic flows, concentrations ofsediments, nutrients, and pollutants, and changes inbiotic resources (Box 2). Watersheds are a natural spa-tial unit for such predictions (Firth 1998), but someproblems such as coastal eutrophication require inte-gration at regional scales (National Research Council2000a). The forecasts should be quantitative, shouldprovide assessments of uncertainty such as forecastprobability distributions, and should be based on clear-ly stated premises. Although the literature containsmany quantitative tools for forecasting freshwater re-sources, freshwater forecasting is not a well-organizedfield with a comprehensive set of standardized toolsand approaches. Quantitative tools for forecastingchanges in biogeochemical processes in terrestrial andfreshwater ecosystems are also lacking, especially atlarge watershed and regional scales.

In many cases, uncertainty will be the most importantfeature of freshwater forecasts. By evaluating uncer-tainties, forecasters can help decision makers to antic-ipate the range of possible outcomes and to designflexible responses. Careful analyses of uncertainty canalso help to identify promising research areas that mayimprove future decisions. Adaptive management is theprocess of designing management interventions to de-crease the variance of future forecasts and recommendalternative management options (Walters 1986). Ex-periments using adaptive management are designed tobe safe (decreasing the risk of environmental damagesor irreversible change) and informative (with clear ex-perimental design and careful scientific assessment ofeffects). Freshwater systems are increasingly the focusof adaptive management efforts (Johnson 1999, Waltersand Korman 1999, and associated articles).

Current progress and management options

Growing demands on freshwater resources presentan opportunity to link ongoing research with improvedwater management (Dellapenna 1999). Water policysuccesses of recent decades demonstrate this link clear-ly. Freshwater eutrophication and pollution have de-creased in many waterways. In the Hudson River, forexample, concentrations of copper, cadmium, nickel,and zinc have been halved since the mid-1970s (San-


BOX 2. Forecasting Water Resources

Forecasting future hydrology is important for guaranteeing human water supplies, scheduling irrigationand hydroelectric power generation, moderating flooding, and coordinating recreational activity. Hydro-logic forecasts predict future changes in hydrology using weather forecasts and current hydrologic con-ditions. Forecasting hydrologic dynamics is becoming more tractable as regular monitoring data are im-mediately available via internet connections. Current forecasts are generally 3–5 d in advance, but futureimprovements in data distribution and hydrometeorological modeling will allow 1–6 mo forecasts in thenear future.

As an example of the array of data sets required for quality forecasting, Doppler radar is now used tomap precipitation cells every 30 min, and most stream gauge data are reported daily by satellite telemetryand are posted on a USGS website. A national subset of daily surface weather observations from theNational Weather Service can also be received daily, but must be spatially interpolated and gridded forhydrologic calculations (Thornton et al. 1997). Weekly satellite data of regional snow cover are availableon a NOAA website. Snowpack models can then provide measures of snowmelt and mountain runoff thatinclude topographic variability in the estimates (Hartman et al. 1999). Regional hydroecological modelsincorporate digital topography and soils data to accurately represent hydrologic routing and watershedlatency (the lag between rainfall and runoff). Some of these hydrologic models also represent ecosystemprocesses well, including vegetation coverage, leaf area index (LAI), and the controls of canopy evapo-transpiration (Wigmosta et al. 1994). Satellite remote sensing of canopy LAI provides a critical link tospatially continuous analysis of a watershed (Band 1993). Weekly updates of surface variables like LAIand snow cover are now possible globally with the latest generation of earth-observing satellites (Runninget al. 1994).

New hydrometeorology models can ingest the daily data stream of variables discussed here and cancompute the dynamics of hydrologic discharge expected downstream in the following days (Baron et al.1998). As the quality of new 1–6 mo climate forecasts improves, longer hydrologic forecasts should bepossible. When larger regions with human impacts are considered, such human activities as irrigationwithdrawals and reservoir regulation require that hydroecological models be coupled with engineeringhydrology models and socioeconomic data.

A different type of forecasting involves analyzing long-term hydrologic responses to future scenariosof land use or climate change (Fig. 4; see also Vorosmarty et al. 2000). For example, hydrologists canpredict the increase in runoff and flood potential that might occur if portions of a watershed are clearcut(Waring and Running 1998), or the consequences of a change in grazing regime for stream sedimentation.The hydrologic consequences of changes in landscape urbanization or agricultural use can also be predictedusing a set of population and crop scenarios. Longer term forecasts of hydrological variables and freshwateravailability are also limited by uncertainties in climate projections. For predicting changes in freshwateravailability over decades and centuries, the best that can be done today is to make projections based onscenarios of climate change provided by general circulation models (Fig. 4).

Advanced hydroecological models can also calculate critical aspects of water quality, such as streamtemperatures, dissolved oxygen concentrations, nutrient loading, and aquatic productivity. Eutrophicationof lakes and reservoirs is often predictable from data on land use and fluxes of water and nutrients (Smith1998). For example, Lathrop et al. (1998) forecast probability distributions of cyanobacterial blooms ina lake under various scenarios for management of nonpoint pollution. Other chemical properties that affectwater use, such as pH and microcontaminant concentrations, are forecast by various mechanistic andstatistical models (Stow et al. 1995, Sullivan 1997). Models of freshwater chemical fluxes have an importantrole in predicting the responses of ocean biogeochemistry to anthropogenic perturbations (Doney 1999).Hydrological models also play an increasingly important role in predicting biotic interactions in freshwatersystems, including forecasts of fish stocks and the potential for invasion of freshwater habitats (Hilbornand Walters 1992, Kolar and Lodge 2000, Rose 2000).

udo-Wilhelmy and Gill 1999). Three decades ago, sci-entists and managers decisively showed that the pri-mary cause of freshwater eutrophication was not over-supply of carbon, but rather of inorganic nutrients, es-

pecially P (Smith 1998). This discovery led to widelyimplemented policies reducing inorganic pollutants inNorth America and Europe, including bans on phos-phate detergents and better sewage treatment. Rapid


BOX 3. Some Priorities for Balancing Current and Future Demands on Freshwater Supply

1. 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.†

2. Legal recognition of surface and renewable ground waters as a single coupled resource.3. Improved monitoring, assessment, and forecasting of water quantity and quality for allocating water

resources among competition needs.‡4. Protection of critical habitats such as groundwater recharge zones and watersheds.5. A more realistic valuation of water and freshwater ecosystem services.6. Stronger economic incentives for efficient water use in all sectors of the economy.7. Continued improvement in eliminating point and nonpoint sources of pollution.8. 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.§

† To 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).

‡ In the last 30 yr, more than one-fifth of gages in the United States recording flow on small, free-flowing streamshave been eliminated (United States Geological Survey 1999).

§ Currently, at least six federal departments and 20 agencies in the United States share responsibilities for variousaspects of the water cycle and for water management.

improvement in places like Lake Erie showed that thepolicies worked (Laws 1993). To build on these suc-cesses, nonpoint sources of pollution should be reducedin the future (Carpenter et al. 1998). Aggressive man-agement of N inputs will also sometimes be needed,as N is the critical nutrient in some aquatic ecosystems(Guildford and Hecky 2000, National Research Council2000a).

Habitat restoration and preservation are the focus ofmany efforts to improve water management (Redfield2000). Beginning in 1962, the 166 km long KissimmeeRiver, which once meandered south to Florida’s LakeOkeechobee, was converted to a 90-km, 9 m deep canalfor flood control (Dahm et al. 1995, Koebel 1995).Effects on biodiversity and ecosystem services wereimmediate. Wintering waterfowl declined by 90%. Eu-trophication increased in Lake Okeechobee as the wet-lands that once filtered nutrients from the river dis-appeared (Toth et al. 1998). Today, after decades ofresearch and numerous pilot studies, restoration of 70km of the river channel, 11 000 ha of wetlands, and100 km2 of floodplain has begun at a projected cost ofhalf a billion U.S. dollars.

In 1996, New York City invested more than one bil-lion dollars to buy land and restore habitat in the Cat-skill Mountains (Chichilnisky and Heal 1998). Thecity’s water supply was becoming increasingly pollutedwith sewage, fertilizers, and pesticides. A new filtrationplant would have cost eight billion dollars to build andthree hundred million dollars per year to run. In con-trast, preserving habitat in the watershed and lettingthe ecosystem do the work was just as effective as anew filtration plant. Habitat preservation and restora-tion cost one-fifth the price of a new filtration plant,

without hundreds of millions of dollars in annual main-tenance.

An impressive policy initiative is the recent cap inwater extractions from the Murray-Darling Basin inAustralia, a region where the diverse pressures of highwater demand, limited water availability, rising pop-ulation, and land-cover change all intersect. The Mur-ray-Darling Basin holds 2 3 106 people and portionsof four states, and contributes almost half of Australia’sagricultural output. Two-thirds of its 700 000 km2 ofwoodlands have been converted to crop and pasture-lands (Walker et al. 1993, Jackson et al. 2000). In recentyears, salinization from irrigation and changes in thewater table have reduced agricultural output by 20%(Anonymous 1990, Greenwood 1992). In response toincreasing evidence that the basin’s rivers were de-clining in ecological health, the Ministerial Councilrecently instituted a cap on water diversions to levelsof 1993/1994 development (Blackmore 1999). Basinstates also recently agreed to allocate one-fourth oftheir natural river flows to maintaining the ecologicalhealth of the system (Postel 2000).

Human health has also benefited from improved wa-ter availability for drinking and for sanitation. In 1994,700 3 106 fewer people were without safe drinkingwater than in 1980, even though global population in-creased by .109 (Gleick 2000). The percentage of peo-ple in developing countries with access to safe drinkingwater rose from ,50% to .75% during the same pe-riod. In the United States, the annual incidence of wa-terborne disease from 1970 to 1990 was less than halfthat from 1920 to 1940, fewer than four cases per100 000 people.

In the next half century, global population is pro-


jected to rise at least three times faster than accessiblefreshwater runoff (Postel et al. 1996, United Nations1998). As a consequence, improving the efficiency ofwater use will be important for balancing supply anddemand for fresh water and for protecting aquatic eco-systems (Box 3). Technologies such as drip irrigationhave great untapped potential to reduce water con-sumption in agriculture. Greater efficiency could beencouraged through economic incentives and a morerealistic valuation of water and freshwater ecosystemservices. More complete monitoring of the componentsof hydrological and biogeochemical cycles, includingmeasuring water quantity and quality at the same spa-tial and temporal scales, would also provide better datafor allocating water resources efficiently among com-peting needs (Rodda et al. 1993). This emphasis isespecially important, because in the last three decades,more than one-fifth of flow gages on small, free-flowingstreams in the United States have been eliminated(United States Geological Survey 1999). Additionalpriorities include securing sufficient quantity, quality,and timing of flows to natural systems, critical habitatpreservation in groundwater recharge zones and wa-tersheds, and continued improvement in pollution pre-vention (both point and nonpoint sources).

Achieving sustainable water use in the future willalso depend on continued changes in the culture ofwater management (National Research Council 1999,J. S. Baron et al., unpublished manuscript). At leastsix federal departments and 20 agencies in the UnitedStates share responsibilities for various aspects of thewater cycle. Coordinating their diverse activitiesthrough a panel with representatives from each de-partment or through one central agency would en-courage the development of a well-conceived nationalplan for water research and management. The estab-lishment of a panel of scientists and policy analystscould also help to define future research priorities andgoals for cross-cutting water issues (Box 3). A goodfirst step in this process is a plan for a new scienceinitiative on the global water cycle as part of the U.S.Global Change Research Program (Hornberger et al.2001).

ACKNOWLEDGMENTS

We wish to thank J. Baron, L. Pitelka, D. Tilman, and sevenanonymous reviewers for helpful comments and discussionson the manuscript. R. B. Jackson gratefully acknowledgessupport from the National Science Foundation, the AndrewW. Mellon Foundation, the Inter American Institute for GlobalChange Research, and the Department of Energy. This paperis a contribution to the Global Change and Terrestrial Eco-systems (GCTE) and Biospheric Aspects of the HydrologicalCycle (BAHC) Core Projects of the International GeosphereBiosphere Programme (IGBP).

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