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nts
nt
1 Hydrology and fluvial geomorphology 1
2 Atmosphere and weather 17
3 Rocks and weathering 31
4 Population 41
5 Migrati on 58
6 Settlement dynamics 69
7 Tropica! environments 87
8 Coastal environrnents 102
9 Hazardous environments 118
10 Hot arid and semi-arid environrnents 134
11 Production, location and change 146
12 Environmental management 161
13 Global interdependence 180
14 Economie transition 193
lndex 210
iii
Hydrology and fluvial
m r I
he drainage sin s ste
A drainage basin is the area drained by a river and its • Evaporation is the loss of water from the lanci
tributaries. A drainage basin system is an open system surface and bodies of water as it transfers frorn
as water can be added as an input or lost as an output. a liquici to a gaseous state (water vapour) by
Within the system water can flow/be transferred in a application of heat. The rate at which it will take
number of ways from a variety of stores. piace will ciepend on the temperature; the higher
the temperature the higher the rate of evaporation.
• Evapotranspiration refers to the combined
Evapotranspiration
loss from both evaporation and transpiration.
Transpiration is the loss of water vapour from the
stornata in the leaves of plants and trees. Rates of
transpiration will depend on the amount anci type
of vegetation. The typical tropica! forest ecosystern
has very high rates of transpiration. More
precipitation than evaporation or evapotranspiration
occurs over the lanci but most of the Earth's
evaporation (86 per cent) and precipitation
(78 per cent) take piace aver the oceans.
• River discharge is the volume of water being
discharged by a river. lt is normally expressed as
the volume of water passing a point in the river
channel in a given unit of time. This is commonly
the nurnber of cubie metres per second -
abbreviateci as "cumecs".
Channel Stores
flow out
of basin
The stores in a drainage basin are where water is held
in some part of the cirainage basin for any length of
time 'from seconds - on a leaf, to hours and days in
depression stores such as puddles and lakes or in
the soil, to severa I thousand years in an underground
Key
411119 Input aquifer. Stores include soil stores - water retained
f::~;$;'':i'.~5~1'1 Store within the pore spaces of the soil (called interstices)
e:::>
Flow and grounciwater stores - water that has percolateci
C==:)output
and is held in rocks below the top of the water table.
lnterception and interception stores - this is
Fig 1.1 The processes operating within drainage basin precipitation which is intercepted on its way to
systems the lanci surface by leaves, plants and trees. lt
slows up the arrivai of rainwater at the surface
Inputs and outputs
and reduces the amount that reaches the ground
as some or all will evaporate as it lies anci flows
lnputs are the addition of water to a drainage basin
over the leaves, stems and trunks of the trees
in the form of precipitation. The inputs can occur in a
and plants. lt is important in that it reduces the
number of forms (rain, snow, hail, etc.) and at different
amount of water available for overland flow/
times, intensities anci frequencies throughout the year.
surface runoff and therefore helps reduce the
In some parts of the world, such as high mountainous
possibility of soil erosion and flooding.
areas and in polar and temperate climates, snow and
• Surface water or depression stores - this is
glacial meltwater is an important input, especially in
water that is stored on the surface in the form of
spring and summer when it can produce floods.
puddles, ponds, strearns and lakes - often above
Outputs are the losses of water from a drainage
an impermeable surface or where the ground or
basin in terrns of evaporation, evapotranspiration
soil below is fully saturateci.
anci river/channel flow.
1
1 Hydrology and fluvial geomorphology
or mn;1~1:,ure - this is water impermeable surface, such as a clay soil, which
retained within the gaps and pore spaces in has a very slow infiltration capacity as it contains
the soil. Antecedent moisture is the moisture very tiny pore spaces (while a sandy soil rnay
retained in the soil before a rainfall event. exhibit a fast infiltration capacity as it has large
pore spaces). A clay soil may therefore r1ave water
• Groundwater stores - this is water that has
percolateci downwards and is held in gaps such as quickly building up on its surface and this may
cracks, joints, bedding planes, fault lines and pore then start to flow over the surface, as overland
spaces in rocks in the underground aquifer. The flow, possibly causing soil erosion. Without a
top of the aquifer is called the water table. The protective cover of trees and vegetation soil
factors that influence the amount of water that erosion is a distinct possibility. The occurrence of
reaches the groundwater store are precipitation overland flow will be increased on slopes, or when
amount and intensity, surface flow and there is saturation overland flow - when all the
throughflow. The time it takes is controlled mainly open/pore spaces in the underlying soil and rock
by the speed of infiltration and percolation which, are filled with water, which rneans that water is
in turn, are controlled by porosity and permeability forced to flow over the land surface.
of both the soil and the underlying bedrock.
Below ground
• Channel store - this is the volume of water
contained in the river channel. Once in the river • lnfiltration - when water enters small openings
channel water will flow to the sea or lake and be and pores in the ground from the surface. Every
lost from the drainage basin system. land surface has its own individuai infiltration
capacity i.e. the speed at which water enters that
Flows
land surface. Areas with a low infiltration capacity
can be very prone to flooding after heavy rain.
The main flows found within a drainage basin system
• Percolation - when water flows down through
can be either above ground or below ground:
the soil and underlying rock pulled down by
gravity. The rate at which the water percolates
Above ground
will depend on the porosity of the soil or rock -
• Throughfall - this is precipitation that makes depending on the size and number of open pore
it directly to the land surface without being spaces in the soil or rock and the permeability
intercepted by the plant canopy. Some of this of the rock - depending on the size and number
throughfall may be intercepted by leaves and this of cracks, fault lines, joints and bedding planes
water may then flow off the leaves and drip to the in the rock. Chalk is a good exarnple of a porous
ground as dripflow - some plants have developed rock as it rnay actually absorb water in its many
drip tips and waxy shiny surfaces on their leaf pore spaces and lirnestone is a good example
surface to get water off the leaf surface quickly. of a permeable rock as it usually has many
• Stemflow - is the flow of water from precipitation joints and bedding planes within it, but does not
down the stems of plants. lf they reacr1 the norrnally absorb water.
trunks of larger trees the water will flow down the lnfiltration, therefore, is the actual entry of water into
trunks of trees, termed trunk flow, to reach the the surface of the soil, whereas percolation is the
land surface. The interception of precipitation by downward rnovement of infiltrateci water through the
trees and other vegetation means that the soil pores and spaces of soil once the water has actually
may be protected from rainwater irnpact and the entered the soil or surface.
water that is intercepted is then released slowly
• Throughflow is U1e lateral (sideways) movernent
to the land surface allowing it to infiltrate more
in soil of infiltrateci water. lt occurs when water
easily. In areas which experience high, intense
that r1as infiltrateci the surface is retaineci in
periods of rainfall this slow release of water will
the soil. The water then moves horizontally
prevent excess overland flow.
(parallel to surface) througr1 the soil, down a
• Overland flow when water flows over the land slope towards a river channel, usually along
surface. There are two types of overland flow - well-ciefined lines of seepage (called percolines)
channel flow and sheet flow. Channel flow is that have been formed in the soil or above an
when the water is flowing in small channels, rills, imperrneable layer (for exarnple, when there is a
which are less than 30 cm in width/depth, in a clay layer in the soil called a clay pan).
defined strearn or in a river channel. Sheet flow
• Baseflow (groundwater flow) is water that
is norrnally a relatively rare event and takes piace
has infiltrateci anci percolateci into the bed
when there is a layer / sheet of water on the ground
rock below the soil that then moves laterally
surface. lt rnay occur in two ways. Firstly, when
uncier gravity or hydrostatic pressure in a
there is either excess overland flow, when rainfall
downslope direction to feed springs and river
or water arrives too quickly on the land surface
channels. Baseflow will norrnally increase where
and does not have enough time to infiltrate the
conditions encourage infiltration anci percolation
soil, i.e. when rainfall intensity exceeds the
such as during periocis of steady rainfall or
infiltration capacity of the surface. Secondly, it
where the soils and/or underlying rocks/geology
may occur when water flows onto a relatively
2 are permeable and porous.
-
zone is called the lt may be variable in
Thrnughfall
height depending upon the nature of the rock and
the level of precipitation input and evapotranspiration
output. The water table will generally mirror/follow the
Stemflow
! shape of the surface topography and water will flow,
l - under the influence of gravity and by the hydraulic
....
gradient, to a point in the river basin where it will
Trunk flow Dripflow //,,.,,.
appear either as a spring or by contributing to river
discharge as baseflow. lt may also be abstracted by
Water --~---tPe,;~lation
humans in wells or boreholes.
-taG1e ___
~
The height of the water table will vary according to
Baseflow/groundwater flow the season - winter or summer, wet or dry, and the
Channel
amount of precipitation input and evapotranspiration
flow Aquifer
output. Within an aquifer there will be a zone of
Fig 1.2 Flows permanent saturation, called the phreatic zone.
Underground water
Recharge
Recharge of the groundwater takes piace when
Water tables water is added to the aquifer. Recharge takes piace
when precipitation on the lanci surface exceeds
When water infiltrates the lanci surface, it becomes
evapotranspiration and water then infiltrates the
groundwater. lt may then percolate, under the influence
ground and percolates down to the aquifer. How long
of gravity, through pores, cracks, joints and bedding
the groundwater takes to recharge will be controlled
planes and reach an area of saturation where all pores,
by the speed of infiltration and percolation.
joints, etc are full of water. The top of this saturateci
r
Exam-style questions
1. Define the term drainage basin systern as it 12. Briefly indicate how rates of infiltration might
applies to a river basin. [2] vary with the intensity of rainfall. [4]
2. Describe what is meant by the term 13. Explain how precipitation received by a river
interception storage. [2] basin rnay reach the river channel. [6]
3. What are the zones found in an aquifer and its 14. Explain how water reaches, is stored in and
water table? [2] removed frorn an aquifer. [6]
4. Define the terms interception and stemflow.[ 4] 15. What is rneant by inputs and outputs within a
5. Define the terrns throughfall and throughflow. [4] drainage basin system? [6]
6. Define the terms water table and springs. [4] 16. With the help of a labelled diagram, s~10w how
water makes its way through a drainage basin
7. ldentify and briefly describe two stores found in
system. [6]
a river basin. [4]
17. Explain how water from surface storage
8. Describe how groundwater recharge occurs. [3]
reaches groundwater storage. [6]
9. Describe how and when overland flow may
18. Define the terrns precipitation intensity and
occur. [3]
infiltrati on capa city. [ 4]
10. Explain how throughflow and groundwater flow
19. Describe how precipitation intensity might affect
(baseflow) occur. [ 4]
the surface flow of water in a river basin. [4]
11. Describe the difference between infiltration and
percolation. [4]
'-
3
1 Hydrology and fluvial geomorphology
drainage basins
The components of • Rising or ascending limb - the period when
discharge is rising from the start of a rainfall
event until it reaches peak discharge.
annual) • Falling or recession limb - the period when
discharge is falling.
There are two sorts of hydrographs - storm and annual.
The term river regime may be seen in some textbooks 50 Peak discharge
as an alternative to the term annua! hydrograph.
An annual hydrograph displays the pattern of seasonal
variation that takes piace to a river's discharge in a
typical year. lt is shown by graphs like the one in fig 1.3,
where the peak in the summer months is explained by
snow melt or a summer monsoon.
E
50
4500
E 40
u(C.l)ì) 4000 ~~ 3200
2E::i 3500 o~:: 10o
'Ce_l)D 3000 Time (hours)
CO
.r(:.:): 2500 Fig 1.4 A typical storm hydrograph
(/)
'6 2000 In addition to these key components, several other
;>s,
e 1500 pieces of information may be provided, such as:
o
E 1000 Approach segment - the period of time before
e
CO water from the rainfall event gets into the river
Cl) 500
~ channel.
o
•
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dee Bankfull discharge - when the discharge of the
river is at the top of the river banks - any further
Month
added discharge will cause the river to flood the
Fig 1.3 A typical annual hydrograph surrounding land and flow over its floodplain.
•
Storm hydrographs show how river discharge The different flows U1at make up the total
responds to a rainfall event. lt has a number of discharge. Each of the three flows will arrive, and
important components. The lag time between rainfall peak, at different times - the quickest to arrive
peak and flood discharge peak indicates how quickly and peak will be the overland flow/surface runoff,
the precipitation is reaching the river channel after a foliowed by the next fastest - throughflow and,
rainfall event. lastly, baseflow/groundwater flow.
Storm hydrographs allow an estimate to be made of
Influences on hydrographs
the relative importance of the quick flows (mainly
overland flow/surface runoff) and the baseflow/
The shape of a hydrograph may be influenced by
groundwater flow.
several clirnate factors, such as how the precipitation
The steepness of the rising and falling/recession amount, intensity and type, ternperatures and
limbs also indicates how quickly the precipitation evapotranspiration vary over the year.
input is reaching U1e river channel and being taken
away by it. A storm hydrograph plots two variables Climate
- the rainfall received during a rainfall event (in
millimetres.) and the river discharge (measured in
Precipitation type and intensity
cubie metres per second - shortened to cumecs).
Precipitation type is the form in which precipitation
A typical storm hydrograph (fig 1.4) will have severa!
is received by the drainage basin systern. Rain will
key components:
be available to the systern very quickly, whereas
• Rainfall peak - when the highest amount of
snow will delay the impact on the system; but it rnay
rainfall occurs during a rainfall event. lf it is the
then have a drarnatic effect on the hydrograph as it
same each hour the median time is taken - i.e.
may be released quickly as meltwater.
the middle of the event.
Precipitation intensity is the rate at which
• Peak discharge or peak flow - when the highest
precipitation is received at the ground surface - it is
amount of disc~1arge occurs.
the amount of precipitation in millimetres divided by
• Lag time - the time between the rainfall peak the time.
4 and peak discharge.
-
When the precipitation exceeds the
infiltration capacity of the soil or surface it lands on
characteristics
water will remain on the surface and overland flow/
surface runoff will result.
Precipitation intensity can lead to the infiltration Drainage basin size and shape
capacity of the soil or land surface being exceeded.
The larger the size of the drainage basin the greater
This leads to water building up on the surface
the amount of water is likely to be collected and
which, in turn, often leads to rapid surface runoff/
released as river discharge, although this might take
overland flow. This means that water will reach the
longer to reach the river channel and therefore have
river channel very rapidly, producing rapid rises
a longer lag time. lf a drainage basin is very large,
in river channel discharge and high flood peak
like the Mississippi or the Nile, a rainfall event may
discharges as water will reacr1 the river channel
affect only one part of the basin.
very quickly, possibly causing flooding. On a storm
An elongated basin, like the one on the left below,
hydrograph this will be indicateci by a short lag time
has relatively short lag times (i.e. they are said
and steep rising limb.
to have a flashy response), but peak discharges,
although fairly low, may be sustained fora long
Temperature, evaporation,
period of time. Round basins, like the one on the
transpiration, evapotranspiration right below, have longer lag times but a higher peak
discharge.
A river in an equatorial climate may have a
fairly constant annua! pattern as it may have
Drainage density
most of these factors constant throughout the
year. A river which experiences disti net wet and High drainage densities mean that water will reach
dry seasons will reflect the varying input of river and stream d1annels quickly, because water
precipitation. Drainage basins that experience will have a relatively short distance to flow to a river
freezing temperatures climates will have annua I channel, which will produce a rapid response - a
hydrographs that have very marked changes as flashy hydrograph - giving high, sharp flood peaks,
the seasons and the factors change. During the with short lag times.
winter there may be a lack of precipitation input as
Low drainage densities mean that water will reach
precipitation falls as snow and is not available to
river and stream channels more slowly, because
the river. In spring and early summer there may be
water will have a relatively long distance to flow to
a sudden input of meltwater as the snow is rnelted
a river channel, which will produce a much slower
by higher temperatures.
response - a delayed hydrograph - giving longer lag
times and lower flood peaks.
Antecedent moisture
Drainage density is found by measuring the total length
Antecedent moisture is the moisture retained in the of river and stream channels in a drainage basin and
soil before a rainfall event. This retained/residual water dividing it by the tota I area of the drainage basin.
moisture affects the soil's infiltration capacity. During
the next rainfall event, the infiltration capacity will cause Porosity and permeability of soils
the soil to be saturateci at a different rate, as the higher
The soil type will affect both infiltration rates
the level of antecedent soil moisture, the more quickly
and percolation rates. A clay soil has a very slow
the soil becomes saturateci. Once the soil is saturateci,
overland flow/surface runoff will occur.
.,.... ... ...
;
~
ro ro
.s:::. .s:::.
(.) (.)
o(/) o(/)
,,, ,/
-~"' .,..,,"'
'',,' ,,, .,..,,..,. .. "'"'.,. r r
Time Time
Fig 1.5 Drainage basin shape
5
1 Hydrology and fluvial geomorphology
infiltration capacity as it has very tiny pore spaces, that it will speed up the arrivai of water into the river
while a sandy soil has a fast infiltration capacity channel, producing higher peak discharges, steeper
as it has large pore spaces. The rate at which the rising and recession limbs and shorter lag time a
water percolates will depend on the porosity of the flashy hydrograph.
soil depending on the size and number of open pore
spaces in the soil and the permeability of the soil, Land-use
which will depend on the size and number of cracks
• The abstraction (removal) and storage of water by
and structures within the soil.
humans can have a major effect on the flows and
lnfiltration is the actual entry of water into the surface storages within a drainage basin. When water
of the soil, whereas percolation is the downward is taken from river channels it will affect river
movement of infiltrateci water through the pores and channel discharge, while groundwater abstraction
spaces of soil once the water has actually entered will lower water tables and reduce baseflow and
the soil or surface. A clay soil may have water quickly the amount released as ct1annel flow.
building up on the surface which will then start to flow
• The replacement of a natural woodland by
over the surface as overland flow (quick flow). This will
permanent pasture will lead to a reduction in
reach the river channel quickly and so a hydrograph will
interception and hence interception storage
have a high peak discharge, with short lag time and
(on leaves, stems, trunks, etc.). A natural
steep rising and fc:1lling limbs - a flashy hydrograph. In
woodland will slow the flow of water to the soil
contrast to a clay soil sandy or loamy soil has a high
surface and will reduce the amount of surface
infiltration capacity as it has large pore spaces. There
runoff/overland flow (quickflow). lnfiltration
will be less surface runoff/overland flow and more
rates are normally greater under woodland
throughflow and baseflow which are slower. As a result
and the water will make its way to the river
the hydrograph will have a lower peak discharge, longer
by throughflow and baseflow/groundwater
lag time and more gentle rising and falling limbs - a
flow rather than overland flow. lf permanent
delayed hydrograph. Also, a thin soil will not have the
pasture - grassland - is ploughed up and used
storage capacity of a deeper soil; this could mean that
for growing crops called arable farming - for
its storage capacity is reached quickly, possibly leading
a short time there will be no vegetation cover
to increased overland flow.
and even less interception until the crop grows
and interception increases.
Rock type
• The impermeable surfaces of concrete, roofs
Permeable and porous rocks (e.g. limestone and
and sealed roads found in urban areas will
chalk) store more precipitation and release it more
intercept most rainwater and prevent infiltration.
gradually/slowly as baseflow/groundwater flow,
This may lead to rapid surface runoff/overland
producing a delayed hydrograph with a lower flood
flow either on the surface or by storm drains.
peak discharge and long lag times. lmpermeable
Water may reach river channels in minutes rather
or non-porous rocks (e.g. sedimentary clay, igneous
than hours and days and in large amounts. This
granite and metamorphic schists) do not absorb as
will produce very steep rising and falling limbs
much water and so have more water running overland
on the hydrograph, a very short lag time and
as surface runoff called quick flow and so have a
more rapid response producing a flashy hydrograph
with high flood peak discharges, with steeper rising
Land use - Natural forestjwoodland
and recession limbs and short lag times.
Lag time
~
ro
Slopes ..e
(.)
(/)
'5
Steep slopes will encourage greater overland flow/ -o
e
surface runoff - producing flashy hydrographs as the ro
water will be moving quickly down the slope with little
time to infiltrate the soil, while on more gentle slopes
the water will have more time to infiltrate the soil Time
and there will be more areas of surface storage -
producing longer lag times and delayed hydrographs. Land use - Permanent pasture
Vegetation type
(1)
Dense vegetation will encourage both greater ~ro
..e
interception and infiltration which means that it will (.)
(/)
'5
slow down the arrivai of water into the river channel,
-o
e
producing lower peak discharges, flatter rising and ro
recession limbs and longer lag time - a delayed ~
e
hydrograph. ro
o:: L...J..J..J...J...J...L. _______________ _
Sparse/thin vegetation cover has opposite effects - Time
both less interception and infiltration which will mean Fig 1.6 The influence of land use on discharge
6
