Water in a Changing World A6ojooH ul sanssI
Water in a Changing World Issues in Ecology Published by the Ecological Society of America Number 9, Spring 2001
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 of the earth is vitally dependenton renewable fresh water,a resource that comprises only a tiny fraction of the global water pool.Humans rely on terorking.indsuses as weasp waterfowl,transportation In many regions of the world,the amount and quality of 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 of climate change example,accessible runoff of fresh water is unlikely to increase more than 1 percent,yet the earth's population is expected to grow by one third.Unless humans use water more efficiently,the impacts of this imbalance in supply and demand will diminish the services that freshwater ecosystems provide,increase the number of 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 of the world's accessible freshwater runoff is already appropriated for 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 g ow faster than any increase in accessible supplies of fresh water the amount of 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, urrence of storms.and significantly altering the nutrient cycles in At least 90 percent of river flows in the United States are strongly affected by dams,reservoirs. interbasin diversions,and irrigation withdrawals that fragment natural channels. Globally,20 percent of freshwater fish species are threatened or extinct,and freshwater species make up 47 percent of all federally listed endangered animals in the United States. of water-policy successes. Better monitoring.assessment,and forecasting of water resources would help government agencies allo cate water more efficiently among competing needs.Currently in the United States.at least six federal departments and twenty agencies share responsibilities for various aspects of the water ycle.We believe eithe creation of a single panel with members drawn from each department or else oversight by a central agency is needed in order to develop a well-coordinated national plan that acknowledges the diverse and competing pressures on freshwater systems and assures efficient use and equitable distribution of these resources. Cover(clockwise from top):Homestead,Kalahari Desert of South Africa (R.Jackson):Coastal zone of Serra da Arrabida The W ater (H.Be 1870 everde Cloud ica(R on USA (R. nd C ional Park.Sy and riparian zone.Gardner River of Yellowstone kson：吵and the town of Flores,Guatemala(R. Jackson)
1 Issues in Ecology Number 9 Spring 2001 SUMMARY Life on land and in the lakes, rivers, and other freshwater habitats of the earth is vitally dependent on renewable fresh water, a resource that comprises only a tiny fraction of the global water pool. Humans rely on renewable fresh water for drinking, irrigation of crops, and industrial uses as well as production of fish and waterfowl, transportation, recreation, and waste disposal. In many regions of the world, the amount and quality of 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 of climate change and increasing consumption of water by a growing human population. In the next 30 years, for example, accessible runoff of fresh water is unlikely to increase more than 10 percent, yet the earths population is expected to grow by one third. Unless humans use water more efficiently, the impacts of this imbalance in supply and demand will diminish the services that freshwater ecosystems provide, increase the number of 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 of the worlds accessible freshwater runoff is already appropriated for 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 of fresh water, the amount of fresh water available per person will decrease in the coming century. • Climate change will intensify the earths water cycle in the next century, generally increasing rainfall, evaporation rates, and the occurrence of storms, and significantly altering the nutrient cycles in land-based ecosystems that influence water quality. • At least 90 percent of river flows in the United States are strongly affected by dams, reservoirs, interbasin diversions, and irrigation withdrawals that fragment natural channels. • Globally, 20 percent of freshwater fish species are threatened or extinct, and freshwater species make up 47 percent of 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 of water-policy successes. Better monitoring, assessment, and forecasting of 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 for various aspects of the water cycle. We believe either creation of a single panel with members drawn from each department or else oversight by a central agency is needed in order to develop a well-coordinated national plan that acknowledges the diverse and competing pressures on freshwater systems and assures efficient use and equitable distribution of these resources. Water in a Changing World Cover (clockwise from top): Homestead, Kalahari Desert of South Africa (R. Jackson); Coastal zone of Serra da Arrábida, 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 Flores, Guatemala (R. Jackson).
Issues in Ecology Number 9 Spring2001☐ Water in a Changing World Robert B.Jackson,Stephen R.Carpenter,Clifford N.Dahm, Diane M.MeKnight,Robert J.Naiman.Sandra L.Postel,and Steven W.Running INTRODUCTION power,can be achieved only by damming.diverting.or creating other major changes to natural water flows.Such Life on earth depends on the continuous flow of changes often diminish or preclude other instream ben- materials through the air,water.soil,and food webs of efits of fresh water.such as providing habitat for aquatic the biosphere.The movement of water through the hy life or maintaining suitable water quality for human use drological cycle est of these Theecological, cono mic be efits tha delivering an estim neters (km reshwater ystems provide.and the trade-offs between of water to the land each year as snow and rainfall.Solar consumptive and instream values,will change dramati energy drives the hydrological cycle,vaporizing water cally in the coming century.Already,over the past one from the surface of oceans,lakes,and rivers as well as hundred years,both the amount of water humans with- from soils and plants (evapotranspiration).Water vapor draw worldwide and the land area under irrigation have rises into the atmosphere where it cools,condenses.and risen exponentially (Figure 1).Despite this greatly in hwate ad sumption the basic water eds of rld are not b eing met Currently, Renewable fresh water provides many services billion lack basic sanitation services.These deprivations essential to human health and well being.including water cause approximately 250 million cases of water-related for drinking.industrial production,and irrigation,and the diseases and five to ten million deaths each vear.also ent unmet needs limit our ability to adapt to future ch- s in ate supplies and distribu ion.Many nels nextractive or instream benefits).including flood systems des signed topr control,transportation,recreation.waste processing.hy- matic conditions may be ill prepared to adapt to future droelectric power,and habitat for aquatic plants and ani- changes in climate,consumption,and population.While a mals.Some benefits,such as irrigation and hydroelectric global perspective on water withdrawals is important for 00 45 400 30 2 20 1901p01920019001601701602000 Year Figure I-Global data for hum pulation.water withdrawals.and irrigated land area. Redrawn and updated rom Gleick()
2 Issues in Ecology Number 9 Spring 2001 Robert B. Jackson, Stephen R. Carpenter, Clifford N. Dahm, Diane M. McKnight, Robert J. Naiman, Sandra L. Postel, and Steven W. Running Water in a Changing World Figure 1 — Global data for human population, water withdrawals, and irrigated land area from 1900 to 2000. Redrawn and updated from Gleick (1998). INTRODUCTION Life on earth depends on the continuous flow of materials through the air, water, soil, and food webs of the biosphere. The movement of water through the hydrological cycle comprises the largest of these flows, delivering an estimated 110,000 cubic kilometers (km3) of water to the land each year as snow and rainfall. Solar energy drives the hydrological cycle, vaporizing water from the surface of oceans, lakes, and rivers as well as from soils and plants (evapotranspiration). Water vapor rises into the atmosphere where it cools, condenses, and eventually rains down anew. This renewable freshwater supply sustains life on the land, in estuaries, and in the freshwater ecosystems of the earth. Renewable fresh water provides many services essential to human health and well being, including water for drinking, industrial production, and irrigation, and the production of fish, waterfowl, and shellfish. Fresh water also provides many benefits while it remains in its channels (nonextractive or instream benefits), including flood control, transportation, recreation, waste processing, hydroelectric power, and habitat for aquatic plants and animals. Some benefits, such as irrigation and hydroelectric power, can be achieved only by damming, diverting, or creating other major changes to natural water flows. Such changes often diminish or preclude other instream benefits of fresh water, such as providing habitat for aquatic life or maintaining suitable water quality for human use. The ecological, social, and economic benefits that freshwater systems provide, and the trade-offs between consumptive and instream values, will change dramatically in the coming century. Already, over the past one hundred years, both the amount of water humans withdraw worldwide and the land area under irrigation have risen exponentially (Figure 1). Despite this greatly increased consumption, the basic water needs of many people in the world are not being met. Currently, 1.1 billion people lack access to safe drinking water, and 2.8 billion lack basic sanitation services. These deprivations cause approximately 250 million cases of water-related diseases and five to ten million deaths each year. Also, current unmet needs limit our ability to adapt to future changes in water supplies and distribution. Many current systems designed to provide water in relatively stable climatic conditions may be ill prepared to adapt to future changes in climate, consumption, and population. While a global perspective on water withdrawals is important for
Issues in Ecology Number 9 Spring 2001 ensuring sustainable water use.it is in sufficient for regional and local stabil- ity.How fresh ter is m ed in pa h sheds is the key to sustainable water management. The goal of this report is to describe key features of human-induced changes to the global water cycle.The on on wate availabil ity and on purifi on costs have been addressed previously in /ssues in Ecol ogy.We focus instead on current and potential changes in the cycling of wa- ter that are especially relevant for eco ng the global wate igure The renewable freshwater cycle inunits kmy cluding it current state and historic io000rnd xes (olac hPreciptationoS context. We next examine the extent er recy to which human activities currently al 40.000 evap ter the water cycle and may affect it in t15,000.0 the future.These changes include di- tive exchange wit rect actions.such as dam construction andindire ct in cts such as hos result from human-driven climate change.We examine human appropriation of fresh water of the so-called greenhouse gases (others include carbon globally.from both renewable and non-renewable sources. dioxide.nitrous oxide.and methane)that warm the earth The report ends by discussing changes in water use that by trapping heat in the atmosphere.Water vapor contrib- may be especially important in the future.We highlight utes approximately two-thirds of the total warming that greenhouse gases supply.Without these gases.ther rface te ture of the ould be well belov freezing,an THE GLOBAL WATER CYCLE the planet.Equally important for life,atmospheric water turns over every ten days or so as water vapor condenses Surface Water and rains to earth and the heat of the sun evaporates new supplies of vapor from the liquid reservoirs on earth. Most of the earth is covered by water,more than one billion km of it.The Solar energy typically vaporates about 425.000 ma water km of ocean wate ach year Most of this rain however,is in forms una ed or fres back directly to th e oceans,but approximately 10 pe water ecosystems.Less than 3 percent is fresh enough cent falls on land.If this were the only source of rainfall to drink or to irrigate crops,and of that total,more than average precipitation across the earth's land surfaces two-thirds is locked in glaciers and ice caps.Freshwater would be only 25 centimeters (cm)a vear.a value typical lakes and rivers hold 100,000 km3 globally,less than for deserts or semi-arid regions.Instead.a second.large one ten-thousandth of all water on earth (Figure 2). urce of water is rec cled fron plants and the tant ine vapor in impor through evapo thi ate and on the cyc eve source creates a direct feedback between the land face and regional climate.The cycling of other materials atmosphere at any time.This tiny fraction,however,is such as carbon and nitrogen(biogeochemical cycling)is vital for the biosphere.Water vapor is the most important strongly coupled to this water flux through the patterns
3 Issues in Ecology Number 9 Spring 2001 ensuring sustainable water use, it is insufficient for regional and local stability. How fresh water is managed in particular basins and in individual watersheds is the key to sustainable water management. The goal of this report is to describe key features of human-induced changes to the global water cycle. The effects of pollution on water availability and on purification costs have been addressed previously in Issues in Ecology. We focus instead on current and potential changes in the cycling of water that are especially relevant for ecological processes. We begin by briefly describing the global water cycle, including its current state and historical context. We next examine the extent to which human activities currently alter the water cycle and may affect it in the future. These changes include direct actions, such as dam construction, and indirect impacts, such as those that result from human-driven climate change. We examine human appropriation of fresh water globally, from both renewable and non-renewable sources. The report ends by discussing changes in water use that may be especially important in the future. We highlight some current progress and suggest priorities for research, emphasizing examples from the United States. THE GLOBAL WATER CYCLE Surface Water Most of the earth is covered by water, more than one billion km3 of it. The vast majority of that water, however, is in forms unavailable to land-based or freshwater ecosystems. Less than 3 percent is fresh enough to drink or to irrigate crops, and of that total, more than two-thirds is locked in glaciers and ice caps. Freshwater lakes and rivers hold 100,000 km3 globally, less than one ten-thousandth of all water on earth (Figure 2). Water vapor in the atmosphere exerts an important influence on climate and on the water cycle, even though only 15,000 km3 of water is typically held in the atmosphere at any time. This tiny fraction, however, is vital for the biosphere. Water vapor is the most important of the so-called greenhouse gases (others include carbon dioxide, nitrous oxide, and methane) that warm the earth by trapping heat in the atmosphere. Water vapor contributes approximately two-thirds of the total warming that greenhouse gases supply. Without these gases, the mean surface temperature of the earth would be well below freezing, and liquid water would be absent over much of the planet. Equally important for life, atmospheric water turns over every ten days or so as water vapor condenses and rains to earth and the heat of the sun evaporates new supplies of vapor from the liquid reservoirs on earth. Solar energy typically evaporates about 425,000 km3 of ocean water each year. Most of this water rains back directly to the oceans, but approximately 10 percent falls on land. If this were the only source of rainfall, average precipitation across the earths land surfaces would be only 25 centimeters (cm) a year, a value typical for deserts or semi-arid regions. Instead, a second, larger source of water is recycled from plants and the soil through evapotranspiration. The water vapor from this source creates a direct feedback between the land surface and regional climate. The cycling of other materials such as carbon and nitrogen (biogeochemical cycling) is strongly coupled to this water flux through the patterns Figure 2 — The renewable freshwater cycle in units of 103 km3 and 103 km3 /yr for pools (white numbers) and fluxes (black numbers). Total precipitation over land is about 110,000 km3 /yr. Approximately two-thirds of this precipitation is water recycled from plants and the soil (evapotranspiration = 70,000 km3 /yr) while one-third is water evaporated from the oceans that is then transported over land (40,000 km3 /yr). Ground water holds about 15,000,000 km3 of fresh water, much of it fossil water that is not in active exchange with the earths surface.
Issues in Ecology Spring 2001 of plant growth and microbial decc adtiomalecback osition,and this betwee anssthe Colorado iverr nth the size he Ar times smaller. Simila variati occurs contributes two-thirds of the 70 cm of precipitation that continental scales.Average runoff in Australia is only 4 falls over land each year.Taken together,these two cm per year,eight times less than in North America and sources account for the I 10.000 km3 of renewable fresh- orders of magnitude less than in tropical South America water available each year for terrestrial,freshwater,and As a result of these and many other disparities.freshwater estua availability varies dramatically worldwide. greater than the amount of water that evaporates from Ground Water it,the extra 40,000 km of water returns to the oceans. primarily via rivers and underground aquifers.A number Approximately 99 percent of all liquid fresh water of factors affect how much of this water is available for is in underground aquifers(Hgure 2).and at least a quarter human use on its journey to the oceans.These factors of the world's population draws its water from these include whethe the pitation falls as rair lies.Estimates of the global wate timing of precipitati relative 8代 erally trea dwate temperature and sunlight,and the regional topography as if they were balanced.In reality.however,this resource For example,in many mountain regions,most precipita is being depleted globally.Ground water typically turns tion falls as snow during winter,and spring snowmelt causes over more slowly than most other water pools.often in peak flows that flood major river sys. tems.In some tropical regions,mon- soons rather than s owmelt create s sona flo In othe pre cipitation thes to recharge ground water or is stored in wetlands.Widespread loss of wetlands and floodplains.however.reduces their ability to absorb these high flows and speeds the runoff of excess nutrie nts and con aminan ts to estua s and othe coastal e onments ore than halt of all wetlands in the U.S have already been drained,dredged,filled,or planted. Available water is not evenly distributed globally Two thirds of all pitation falls in the t latituo due to greater sola radiation and evaporation there.Daily evaporation from the oceans ranges from 0.4 cm at the equator to less than O.I cm at the poles.Typically,tropical regions also have larg noff.Roughly half of the that falls rainforests in thedeser rainfall and high evaporation rates combine to greatly reduce runoff.The Figure 3-Locations of non-renewable groundw Amazon,for example.carries 15 percent of all water returning to the global 1997
4 Issues in Ecology Number 9 Spring 2001 of plant growth and microbial decomposition, and this coupling creates additional feedbacks between vegetation and climate. This second source of recycled water contributes two-thirds of the 70 cm of precipitation that falls over land each year. Taken together, these two sources account for the 110,000 km3 of renewable freshwater available each year for terrestrial, freshwater, and estuarine ecosystems (Figure 2). Because the amount of rain that falls on land is greater than the amount of water that evaporates from it, the extra 40,000 km3 of water returns to the oceans, primarily via rivers and underground aquifers. A number of factors affect how much of this water is available for human use on its journey to the oceans. These factors include whether the precipitation falls as rain or snow, the timing of precipitation relative to patterns of seasonal temperature and sunlight, and the regional topography. For example, in many mountain regions, most precipitation falls as snow during winter, and spring snowmelt causes peak flows that flood major river systems. In some tropical regions, monsoons rather than snowmelt create seasonal flooding. In other regions, excess precipitation percolates into the soil to recharge ground water or is stored in wetlands. Widespread loss of wetlands and floodplains, however, reduces their ability to absorb these high flows and speeds the runoff of excess nutrients and contaminants to estuaries and other coastal environments. More than half of all wetlands in the U. S. have already been drained, dredged, filled, or planted. Available water is not evenly distributed globally. Two thirds of all precipitation falls in the tropics (between 30 degrees N and 30 degree S latitude) due to greater solar radiation and evaporation there. Daily evaporation from the oceans ranges from 0.4 cm at the equator to less than 0.1 cm at the poles. Typically, tropical regions also have larger runoff. Roughly half of the precipitation that falls in rainforests becomes runoff, while in the deserts low rainfall and high evaporation rates combine to greatly reduce runoff. The Amazon, for example, carries 15 percent of all water returning to the global oceans. In contrast, the Colorado River drainage, which is one-tenth the size of the Amazon, 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 worlds population draws its water from these groundwater supplies. Estimates of the global water cycle generally treat rates 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 Figure 3 — Locations of non-renewable groundwater resources (light blue) and the main locations of groundwater mining (dark gray) (Shiklomanov 1997). The inset shows the location of the High Plains (Ogallala) Aquifer