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Newly published for IB GeographyOur brand new Course Companion for Geography takes a rigorous approach,

stretching your students and challenging their perceptions. It aims to foster

well-rounded learners with a holistic understanding of the subject material,

to help your students get the best results.

Our approach to a lesson on freshwater

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Part 2 Optional themes

Freshwater – issues and conflicts

By the end of this chapter you should be able to:

l understand the physical geography of freshwater, and why water

on the land is a scarce resource

l consider the human impacts on water quality and the ways in which

humans respond to the challenges of managing the quantity and

quality of freshwater

l explain the consequences of water management, positive and negative.

Precipitation – the transfer of moisture

(as dew, hail, rain, sleet or snow) to the

earth’s surface from the atmosphere.

Interception – the capture of raindrops

by plant cover, which prevents direct

contact with the soil.

Runoff – precipitation that does not

soak into the ground but flows over it

into surface waters.

Groundwater – water held

underground in soil or porous rock,

often feeding springs and wells.

Evapotranspiration (EVT) – the loss

of water from vegetation and water

surfaces to the atmosphere.

The global hydrological cycle

The global hydrological cycle is the transfer of water between sea, air

and land. It comprises evaporation from oceans, water vapour,

condensation, precipitation, runoff, groundwater and evapotranspiration.

l If 100 units represent global precipitation (on average 860 millimetres

per annum), 77% falls over the oceans, 23% on to the land.

l About 84 units enter the atmosphere by evaporation via the

oceans; thus there is a horizontal

transfer of seven units from

the land to the sea.

l Of precipitation over the

land, 16 units are

evaporated or

transpired;

seven units are

runoff to the

oceans.

The seven optional themes explore the

interaction between human and physical factors

and processes, using contemporary case studies

drawn from a variety of environments.

Whichever of the themes you choose, each will

raise your awareness of social injustice and

uneven access to resources and how such

problems may be overcome through sustainable

management practices, from the global to the

individual level.

Land precipitation 11

Land 35 978

Oceans 1 350 000

All volumes in thousands

of cubic kilometres. Boxed

figures are storage; all

others are transfer values.

Groundwater

outflow 12

Surface runoff 41.5 Lakes and rivers 27

Atmosphere 13

Ice meltwater 2.5

Evaporation and

transpiration from

plants 71

Evaporation

from rivers,

lakes and soil 71

Evaporation

from oceans 425

Oceanic precipitation 385

Figure 5.1 The global hydrological cycle

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103

5 ●Freshwater – issues and conflicts

5 ●Freshwater – issues and conflicts

Eustatic change refers to a global change in sea level. At the height of glacial advance, 18 000 years ago, sea level was 100–150 metres below current sea level. The level of the land also varies in relation to the sea. Land may rise as a result of tectonic uplift or the removal of an ice sheet. The change in the level of the land relative to the level of the sea is known as isostatic adjustment, or isostacy. Parts of Scandinavia and Canada are continuing to rise at rates of up to 20 millimetres a year. As the world’s temperature rises, the amount of water stored in the world’s glaciers and ice sheets decreases and global sea levels rise. In addition, there is also the Steric effect. This is the phenomeneon whereby seawater expands with higher temperatures. Thus, even if ice sheets and ice caps did not melt, sea levels would rise in a warmer world.

Some scientists have predicted that global worming may push the Greenland ice sheet over a threshold where the entire ice mass would melt within a few hundred years, causing sea level to rise by 7.2m. This would flood many of the world’s coastal cities and many islands, such as the Maldives.

ReservoirValue (km 3 3 103) Percentage of total

Ocean1 350 000.0 97.403

Atmosphere 13.00.00094

Land35 977.8 2.596 Of which

l Rivers1.7

0.00012l Freshwater lakes 100.0

0.0072l Inland seas 105.0

0.0076l Soil water 70.0

0.0051l Groundwater 8 200.0

0.592l Ice caps/glaciers 27 500.0 1.984l Biota1.1

0.00008Annual exchange Values (km3 3 103) Values (km3 3 103)Evaporation 496.0 Of which l Ocean

425.0l Land

71.0Precipitation 496.0 Of which l Ocean

385.0l Land

111.0Runoff to oceans 41.5 Of whichl Rivers

27.0l Groundwater12.0l Glacial meltwater2.5

Table 5.2 Global water reservoirs and exchanges

Maximum sustainable yield (MSY) – the maximum level of extraction of water that can be maintained indefinitely for a region.

TOK LinkWhat is the maximum sustainable yield?The sustainable yield (SY) may be calculated as the rate of increased use of a natural resource, that is, that which can be exploited without depleting the original stock or its potential for replenishment. Thus, maximum sustainable yield (MSY) is the largest yield that can be taken from a resource over an indefinite period. MSY aims to maintain the resource size at the point of maximum growth rate by harvesting the amount that would normally be replenished, allowing the resource to continue to be productive indefinitely. MSY is often difficult to determine.

a Comment on the stores of freshwater as shown in Table 5.2. What are the implications for human use of water resources?

b Identify and explain two ways in which water can be temporarily stored on the surface.c Identify and explain three ways in which vegetation influences the hydrological cycle.

d Explain the difference in value between evaporation from oceans and precipitation into oceans.e Explain how sea levels may rise without any melting of ice sheets and ice caps.

To do:

l There may be a time lag between precipitation and eventual runoff. About 98% of all free water on the globe is stored in the oceans.

EvaporationThis is the physical process by which a liquid becomes a gas. It is a function of:l vapour pressurel air temperaturel windl rock surface (e.g. bare soils and rocks have higher rates of

evaporation than surfaces with a protective tilth, where rates are low).

High rates of evaporation are recorded in hot deserts. For example,

in Atbara (Sudan) the potential evapotranspiration – the rate of

water loss if there were no shortage of water – is 6,250 millimetres per annum, and in Helwan (Egypt) it is 2,390 millimetres. Rates are

much lower in tropical rainforests because of the high humidity (500–750 millimetres) and in cold climates such as the UK (London’s

rate is 330 millimetres per year). In parts of Egyypt, evapotranspiration rates – the rates at which water transfers from the earth into the atmosphere – are less than 250 millimetres per year, because Egypt’s annual rainfall is less than 250 millimetres. However, if Egypt received, for example, 2,000 millimetres of rain each year, the evapotranspiration rate would increase because of the very high temperatures there. Thus, if there were no shortage of water in Egypt, potential evapotranspiration could be as high as 2,000 millimetres per year.CondensationCondensation is the process by which a gas (water vapour) becomes a liquid. It occurs when air cools to its dew point or becomes saturated by evaporation into it. Further cooling leads to condensation on nuclei, to form water droplets or frost. Precipitation occurs when so much water has condensed that the air can no longer hold it, so the water falls to earth as rain, hail, sleet or snow.Interception

When raindrops fall on plant cover, the plants intercept the rain, which prevents its direct contact with the soil. If rain is prolonged, the retaining capacity of leaves will be exceeded and water will drop to the ground (throughfall). Some will trickle along branches and down the stems or trunk (stemflow). The water retained on the leaves later evaporates.

The world’s changing water balanceThe amount of water stored in the oceans and in ice varies with temperature change. This can be on a long-term scale – over millions of years – or on a short-term scale, such as with accelerated global warming. On a long-term scale, sea levels change in connection with the growth and decay of ice sheets.

Permanent snow 9,700 yearsOceans2,500 yearsGroundwater 1,400 yearsLakes

17 yearsSwamp water 5 yearsSoil moisture 1 yearStreams16 daysAtmospheric moisture 8 days

Table 5.1 Average water renewal cycles for different water bodies

Figure 5.2 Condensation

To researchVisit http://geography.about.com for articles about the hydrological cycle in the physical and cultural section (water and ice), and for links to some excellent sites on hydrology and rivers.

Try the matching quiz (for the hydrological cycle) athttp://highered.mcgraw-hill.com/sites/0072402466/student_view0/chapter10/matching_quiz.html

To researchDiscuss the causes and consequences of changes in the balance of water stored in oceans and ice.

Potential evapotranspiration (pEVT) – the rate of water loss from an area if there were no shortage of water.

IB Course Companion: Geography

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right approach.

Authors Garrett NagleBriony Cooke

I B d I p l o m a p r o g r a m m e

Environmental Systems and SocietiesUniquely developed with the IB

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