+ In this section you'll learn about the hydrosphere and hydrological (water) cycle.

Water and life on Earth Water is what makes Earth - the Blue Planet -

unique and different from other planets in the solar

system. Without it, life could not exist. lt is more

important than anything else on Earth. You can live for

three weeks without food, but without water you'll be

dead in three days!

The hydrosphere consists of all the water on the

planet- in seas, oceans, rivers and lakes, in rocks

and soil, in living things and in the atmosphere. Water

exists on the Earth's surface and in the atmosphere in

three states: as a liquid (water); as a solid (ice); and as

a gas (water vapour).

Water - a continuous cycle Water flows in a never-ending cycle between the

atmosphere, land, and oceans. The hydrological

cycle, or water cycle, is a closed system. The water

goes round and round - none is added or lost so the

Earth gets neither wetter nor drier. Think of the global

water cycle as having a number of stores, such as

lakes, oceans, soil and rocks underground.

Water flows between stores via transfers, such as

runoff (overland flow), infiltration, throughflow, and

groundwater flow.

Sometimes this involves a change of state.

• Heat energy can change liquid (water) to a gas

(water vapour). The process is called evaporation.

• Air can only hold so much water vapour before

it becomes saturated. As air cools, water vapour

turns back into liquid, known as condensation.

• Water can freeze to a solid (ice) as the temperature

cools, or melt back to liquid as it warms.

The table below shows how important the oceans

are as a store, but remember that oceans store salt

water which cannot be used by people unless it is

converted into fresh water. This can be done but

it is hugely expensive. Water stays in the stores

for varying amounts of time, from around a day,

to thousands of years! Most of the fresh water

is stored in ice sheets and glaciers (especially in

Antarctica), and these stores are gradually being

reduced as a result of global warming. Relatively

small amounts of water are stored in rocks as

groundwater, and in lakes and rivers, and these are

in huge demand as sources of water.

Just think what happens after a heavy storm -

water will be dripping off trees, and the grass

will be wet for about 12 hours afterwards. The

puddles dry up after a day or so but water

which finds its way underground can stay there

for years. If rainfall is heavy and goes on for a

long time, some of the smaller stores fill up and

flooding occurs.

Store Size (km3 x 10 000 000) % of all water

Oceans 1370.0 97.0

Polar ice and glaciers 29.0 2.0

Groundwater 9.5 0.7

Lakes 0.125 0.01

Soils 0.065 0.005

Atmosphere 0.013 0.001

Rivers 0.0017 0.0001

Living things 0.0006 0.00004

The photos show some water stores:

A Lakes

B Oceans and atmosphere

C Polar ice and glaciers

D Trees and vegetation

your questions

1 a Make a copy of the diagram below.

b Label A and B to show water changing state. c Label the water stores C, D, E and F.

0

0 .... ·;:::~>/! t c

D

2 Exam-style question Explain why the hydrological system on Earth is a 'closed system'. (3 marks)

+ In this section you'll find out about the role of the biosphere and lithosphere in the hydrological (water) cycle.

The global hydrological cycle The diagram on the right shows how the global

hydrological (water) cycle works. The stores of

water are linked by processes which transfer

water into and out of them. These processes

regulate the water cycle.

• Evaporation from oceans and rivers, and

evapotranspiration from trees, condenses to

cause precipitation (rainfall).

• This precipitation follows a number of routes:

- Some runs off over the surface.

- Some seeps into the soil or rock.

- Some collects as snow or ice.

The biosphere and lithosphere The biosphere and lithosphere play a vital role

in the water cycle, and act as sub-cycles. The

diagram opposite shows these sub-cycles in

a river basin system (part of the water cycle

which operates on land). In the biosphere, trees

intercept precipitation, and over half of it is then

evaporated and transpired without ever reaching

the ground. This water is known as green water.

If the storm or rainfall is very heavy, or goes

on for a long time, precipitation drips from the

leaves and stems and slowly makes its way into

the river system. Precipitation infiltrates into the

soil, where it flows down hill as throughflow, or if the underlying rock is permeable, into the

ground, to be stored as groundwater - a vital

supply of water. Only after many hours is the

water released into the river basin. Both the

biosphere and the lithosphere help to regulate the water cycle.

stored as groundwater

Key

.... Transfer flows

D Fresh water storage

D Salt water storage

.6. The global hydrological cycle.

97% of the world's water is in oceans and seas

+ The biosphere is the part of the Earth and atmosphere in which living organisms exist.

+ The lithosphere is the outer layers of the Earth's surface (the crust and upper mantle).

Precipitation Key

Transpiration D Inputs

c::::J outputs

- Transfers

c::::J storage Biosphere sub-cycle

Lithosphere sub-cycle

rock/soil

~ Water table ...__

Saturated rock/soil

c

.&. A river basin system showing the biosphere and lithosphere.

Infiltration - movement of water into the soil from the surface

Percolation - movement of water into underlying rocks Groundwater storage- water stored in rocks following percolation Saturation - when soi l is full of moisture

Water table - the level at which saturation occurs in the ground or soil Inputs - things which enter the system

Outputs - things which leave the system Transfers or flows- movements within the system Stores - held within the system

your questions

Evaporation

Surface runoff (overland f low)

Groundwater flow

River carrying water to the sea

+ Evapotranspiration is the

combined process of; Evaporation -the changing of water from a liquid to a gas (water vapour) due to the heat of the sun, and, Transpiration -the movement of water through a plant

(from roots to leaves) and its loss into the atmosphere as water vapour.

1 Make a copy of the river basin system above, and complete the terms in boxes ·A, Band C.

0"' yo~r pfCih.ef

2 Define these words: evaporation, evapotranspiration

3 Using the diagram and definitions, explain the passage of water from the time it falls as precipitation, to the time it reaches the river.

4 Exam-style question Describe how the hydrological cycle links the biosphere, atmosphere and lithosphere. (4 marks)

+ Did you know that some of the groundwater deep down in the Earth

has been stored there for over 1 0 000 years?

+ In this section you'll learn about effects of climate change on the hydrological cycle.

Water crisis The world is currently facing a freshwater

crisis. Demand is soaring as population

increases, and supplies are becoming

increasingly unpredictable. Many economists

and experts predict that in the future we will

have water wars, where countries fight over

water resources- especially in the Middle

East. As you can see from the graph, our

use of water is increasing, and much of the

increasing demand is from agriculture. Modern

l...

Cll Q) >. l... Q) 0.

"' E ~

• Reservoir losses

• Industrial

• Domestic

• Agriculture

2000

1000

0~--~~~T===~~~~--~~ 1900 1920 1940 1960 1980 2000

Year farming often requires irrigation, which uses

vast quantities of water. Jt.. Increasing global water use.

As if that was not enough, the world now faces

major change caused by climate change. In 2007,

Oxfam published a report 'Africa- Up in smoke'

which showed the kinds of threats faced by Africa

as a result of climate change. How can this be?

Most global authorities are agreed -that climate

change will affect water vapour, cloud formation,

precipitation patterns, surface run-off, and river flow.

T The increased likelihood of storms (shown in b lue)

-1.5 - 1 -.5 0 .5 1.5

The impacts on precipitation lt is difficult to predict whether

precipitation amounts will increase or

decrease as a result of climate change.

However, some changes are already

occurring:

• Warmer temperatures are likely to lead

to more precipitation falling as rain,

instead of snow.

• If there is more water vapour in the

atmosphere, then there should be an

increase in the amount of precipitation.

• However, the most likely impact of

climate change is an increase in

precipitation intensity, that is, a larger

proportion of rain will fall in a shorter

amount of time. There are more likely

to be more intense storms over land

areas, as the map shows.

I Water world

The impacts on evaporation As the atmosphere gets warmer, evaporation

rates increase, leading to an increase in

the amount of moisture in the atmosphere.

Warmer temperatures would also lead to

greater evaporation from soil surfaces in

some areas, making drought more likely.

The impacts on drought With a combination of increased

temperatures, increased evaporation,

and reduced river flow, one very likely

outcome of climate change is increased

drought. This is likely to vary, as the

map from NASA shows. The map

shows a drought severity index, where

-4 is most likely and +4 least likely. lt is

most likely that areas already suffering

drought will suffer worse droughts

than now; areas with little likelihood of

drought at present are likely to be much

less affected.

your questions

1 Make a sketch of the hydrological cycle from page 60, and change the labels to show which parts of the hydrological cycle will increase, decrease, or stay the

same. 2 Write sentences beneath the diagram to explain why

these will change in the ways that you have shown.

The impacts on river flow The Northern Hemisphere has more land

surface, and so most changes in river flow will

occur there. A warming climate would lead to

an earlier arrival of spring. Mountain snows

would melt earlier, increasing river flow in the

spring, and therefore reducing river flow in the

summer. NASA, the US Earth Observatory,

fears that the impact will be the reduced

availability of fresh water in the hot, summer

months, when water demands are highest.

T The increased likelihood of drought, shown in orange and brown

Change in Palmer Drought Severity Index (1900-2002)

-4 - 2 0 2 4

3 Exam-style question Use the map on the page opposite to describe those parts of the world where rainfall will become more intense. (4 marks).

4 Exam-style question Using examples, explain how climate change is likely to alter the hydrological cycle.

(6 marks)