Human Appropriation of the World's Fresh Water Supply (2024)

World's Fresh Water Supply

Over 70% of our Earth'ssurface is covered by water ( we should really call our planet "Ocean"instead of "Earth"). Although water is seemingly abundant, the real issueis the amount of freshwater available.

• 97.5% of all water on Earth is saltwater, leaving only 2.5% as fresh water

• Nearly 70% of that fresh water is frozenin the icecaps of<!AHREF="http://octopus.gma.org/surfing/antarctica/antice/images/Ice91/sequence.gif">Antarcticaand<!AHREF="http://www.geocities.com/Pentagon/5113/greenland.html">Greenland;most of the remainder is present as soil moisture, or lies in deep undergroundaquifers as groundwater not accessible to human use.

• Only ~1% of the world's fresh water is accessible for direct human uses. This is thewater found in lakes, rivers, reservoirs and those underground sourcesthat are shallow enough to be tapped at an affordable cost. Only this amountis regularly renewed by rain and snowfall, and is therefore available ona sustainable basis.

Water as a Resource

Since antiquity, irrigation, drainage,and impoundmenthavebeen the three types of water control having a major impact on landscapesand water flows. Since the dawn of irrigated agriculture at least 5000years ago, controlling water to grow crops has been the primary motivationfor human alteration of freshwater supplies. Today, principal demands forfresh water are for irrigation,household and municipal water use, and industrialuses. Most supplies come from surface runoff, although mining of "fossilwater" from underground aquifers is an important source in some areas.The pattern of water withdrawal over the past 300 years shows the dramaticincreases in this century.

A timeline of human water use:

• 12,000 yrs. ago: hunter-gatherers continuallyreturn to fertile river valleys
• 7,000 yrs. ago: water shortages spurhumans to invent irrigation
• 1,100 yrs ago: collapse of Mayan civilizationdue to drought
• Mid 1800's: fecal contamination of surfacewater causes severe health problems (typhoid, cholera) in some major NorthAmerican cities, notably Chicago
• 1858: "Year of the Great Stink" in London,due to sewage and wastes in Thames
• Late 1800s-early 1900: Dams became popularas a water management tool
• 1900s: The green revolution strengthenshuman dependency on irrigation for agriculture
• World War II: water quality impactedby industrial and agricultural chemicals
• 1972: CleanWater Act passed; humans recognize need to protect water

Figure 1: The water usage of differentregions of the world per capita in cubic meters.

Consumptive and Non-Consumptive Water Use

Consumptive water use refers to waterthat is not returned to streams after use. For the most part, this is waterthat enters the atmospheric pool of water via evaporation (from reservoirsin arid areas) and from plant transpiration (especially from "thirsty"crops such as cotton and alfalfa). Irrigated agriculture is responsiblefor most consumptive water use, and decreases surface run-off. An extremeexample is the Colorado River,which has most of its water diverted to irrigated agriculture, so thatin a normal year, no water at all reaches the river’s mouth.

Agriculture is responsible for 87% of the total water used globally. In Asia it accounts for 86% of totalannual water withdrawal, compared with 49% in North and Central Americaand 38% in Europe. Rice growing, in particular, is a heavy consumer ofwater: it takes some 5000 liters of water to produce 1 kg of rice. Comparedwith other crops, rice production is less efficient in the way it useswater. Wheat, for example, consumes 4000 m3/ha, while rice consumes 7650m3/ha.

A great deal of water use is non-consumptive,which means that the water is returned to surface runoff. Usually thatwater is contaminated however, whether used for agriculture, domestic consumption,or industry. The WHO estimates that more than 5 million people die eachyear from diseases caused by unsafe drinking water, and lack of sanitationand water for hygiene. This has economic effects as well: an outbreak ofcholera in Latin America killed hundreds of people, and cost hundreds ofmillions of dollars.

Some believe that fresh water willbe a critical limiting resource for many regions in the near future. Aboutone-third of the world's population lives in countries that are experiencingwater stress. In Asia, where water has always been regarded as an abundantresource, per capita availability declined by 40-60% between 1955 and 1990.Projections suggest that most Asian countries will have severe water problemsby the year 2025. Most of Africa historically has been water-poor.

What's the problem?:

• The population is growing rapidly, puttingmore pressure on our water supply (demand is increasing)
• The amount of water is effectively reducedby pollution and contamination (supply is decreasing)
• What does the future hold? We can bestexplore this question by looking carefully at the world's water resources.

Figure 2: This picture depictsthe global hydrological cycle adapted from Gleick. Flows are approximateestimates and are in cubic kilometers per year.

Human Appropriation of Renewable Fresh Water

The hydrological cycle:

• The water cycle on Earth is essentiallya closed system – we always have the same amount of water.
• The only parts of this cycle appropriated by humans is water held as surface water or shallow aquifers. Let us try to quantify present use.

Available renewable fresh water:

• Fossil ground water can be tapped butis non-replenishable.
• Terrestrial replenishable fw supply(RFWS _land).
• RFWS_land = ppte on land.
• P_land = evapotranspirationfrom the land (ET_land) and runoff to sea (R).
• Estimates of annual runoff range from33,500 to 47, 000 km³ (Postel uses 40,000 km³).

Runoff is the source for all humandiversions or withdrawals for irrigation, industry, municipal uses, navigation,dilution, hydropower, and maintenance of aquatic life including fisheries.

Human appropriation of runoff

Distribution of global runoff ishighly uneven and corresponds poorly to the distribution of the world population(see table 2). Asia has 69% of world population but 36% of global runoff.South America has 5% of world population, 25% of runoff.

Much of runoff is inaccessible. AmazonRiver accounts for 15% of runoff and is currently accessible to 25 millionpeople (0.4% of world’s pop). Estimate it to be 95% inaccessible. Zairemay be 50% inaccessible. The mostly untapped northern rivers have an averageannual flow of 1815 km³/yr, consider 95% to be inaccessible.

Together, this amounts to 7774 km³or 19% of total annual runoff, leaving 32,900 km³ geographicallyaccessible. Does not correct for many northern rivers with large flowsrelative to their pop sizes.

Temporal availability: about 27%of global runoff (11,100 km³) is renewable ground water andbase river flow. Remainder is flood water and harder to capture (table3). Present storage capacity of large dams totals 5500 km³,of which 3500 km³ is used to regulate river runoff. Adding togetherbase flow and surface runoff controlled by dams gives total stable flow.Correct for spatially inaccessible flows yields and estimate of availablerunoff (AR) as 12,500 km³/yr.

What fraction of AR is usedby humanity?

• Withdrawals: agricultural withdrawals= average water application rate (12,000 m³/ha) x world irrigatedarea (240 x 10^6 ha in 1990) = 2880 km³. Assuming 65% is consumed,1870 km³.

• Industrial water use is estimated at975 km³ and roughly 9% (90km³) is consumed. Remainderis discharged back into environment, often polluted.

• Municipal use is estimated at 300 km³per year, of which 50 km³ (17%) is consumed.

• Evaporation from reservoirs is estimatedto average 5% of gross storage capacity of reservoirs (5500 km³)or 275 km³/yr.

• Instream flow needs are estimated frompollution dilution, assuming that this suffices to meet instream needs.A common dilution term is 28.3 liters per second per 1000 population. Usingthe 1990 population yields a dilution requirement of 4700 km³.If half of water received adequate treatment, the dilution requirementis reduced to 2350 km³/hr.

• Combining these estimates (see table4) indicate that humans appropriate 54% of AR. Human use of ET (18,200km³) plus runoff (6780 km³) constitutes 30% of totalaccessible RFWS and 23% of unadjusted RFWS.

How much can AR be increased?

Principal options are to captureand store more flood runoff or desalinate sea water. Latter is too energy-intensivefor near future.

Worldwide, new dams (> 15 m ht) wereconstructed at rate of 885 per year during 1950-80, present rate is 500/yr,and future rate is estimated at 350/yr. Over next 30 years, assuming sizeof reservoirs is unchanged, new construction adds 1200 km3 to accessiblesupply, and raises total AR in 2025 to 13,700 km³/hr. Assumingaverage per capita water demand stays unchanged, but adjusting the pollutiondilution for additional population, the total human appropriation in 2025would be 9830 km³/yr, or 70% of estimated AR (compared to current54%). Clearly we are approaching the limit of available fresh water supply.

What are the Solutions?

Improvements in the efficiency ofwater use (ex: irrigation systems often perform poorly, wasting as muchas 60 percent of the total water pumped before it reaches the intendedcrop).

Efficient management and modern technologycan stretch even scarce water supplies much further. Israel, for example,supports its population, its growing industrial base, and intensive irrigationwith less than 500 cubic meters per person per year.

Water is often wasted because itis underpriced. Direct and indirect subsidies (especially for agriculturaluse) are still common in both developed and developing countries. Removingsuch subsidies and letting water prices rise can provide incentives forconservation and for the investments needed to spread more efficient technologies.

Water Sustainability, Water Security

The six billion people of PlanetEarth use nearly 30% of the world’s total accessible renewal supply ofwater. By 2025, that value may reach 70%. Yet billions of peoplelack basic water services, and millions die each year from water-relateddiseases. Water is a basis of international conflict. Whatis involved in achieving water sustainability and water security?The following lists some of the criteria that should help us chart ourdirection.

1. Basic human needs for water should befully acknowledged as a top international priority.
2. Water-related diseases, including Guineaworm, diarrhea, onchocerciasis, malaria and typhoid should be brought undercontrol.
3. Agricultural water should be efficientlyused and allocated.
4. Basic ecosystem water needs should beidentified and met.
5. Serious water-related conflicts shouldbe resolved through formal negotiations.

Water conservation through betterplanning, management, and technologies offers great promise.Fig. 4 shows per capita water withdrawals in the U.S. from 1900 to 1995.Per capita water withdrawals began to decline in 1985, despite continuedpopulation growth. More efficient agricultural and industrial wateruse accounts for this trend.

Figure 3. Trends in freshwaterwithdrawals, and population, for the United States, 1950-1995. Notethat per capita water use peaked in 1985, and has since declined.Source: USGS Circ. 1200)

Figure 4. Actualwater global withdrawals, 1960-200, and past forecasts of year 2000 waterwithdrawal. Past estimates have been high, because they failed toincorporate improved efficiencies in industrial and agricultural wateruse. Source: P. Gleick, The World’s Water, 1998 Island Press).

Figure 5. Global Water ScarcityMap. Source: World Resources Institute (WRI).

References

• Postel, S.L., G.C. Daily and P.R. Ehrlich.1996. Human appropriation of renewable fresh water. Science271:785
• Gleick, P. 2000. The World'swater. Island Press.
• Vitousek, P.M., P.R. Ehrlich, A.H. Ehrlichand P.A. Matson. 1986. Humanappropriation of the products ofphotosynthesis. BioScience 36:368-373.