1
AQA AS Core section: Rivers
1- The drainage basin hydrological cycle TB page 2
Where slopes meet at the bottom of a valley, water is concentrated and forms a river channel.
A drainage basin is an area of land drained by a river and its tributaries. Drainage basins are separated
from each other by ridges of higher land called watershed.
What is a watershed? Flash animation of how rain falling on to the landscape will drain into a
river.
A drainage basin is an open system- a system where there are inputs and/or outputs of energy and
matter- with inputs (water and energy) and outputs of water. The water enters the system as
precipitation and leaves as stream flow or as evaporation or transpiration.
Inputs: energy from the sun/precipitation
Stores such as glaciers, rivers, lakes and vegetation
Transfers and flows move water through the system, such as infiltration.
Outputs: moisture lost by evapotranspiration + runoff into the sea
Key terms:
1- Precipitation (input): All forms of rainfall, snow, frost, hail and dew. It is the transfer of moisture in
the atmosphere to the land.
2- Evaporation + Transpiration=Evapotranspiration (output)
Evaporation: The change of state from liquid to gas from exposed water surfaces
Transpiration: The loss of water from the leaves of plants
Evapotranspiration: the combined losses
3- Surface stores such as glaciers and lakes
4- Interception (Storage): The process by which precipitation is trapped on the surface of vegetation.
Interception loss (output): The proportion of water which evaporates from the surface of vegetation.
After interception…transfers
5- Stemflow :Intercepted precipitation which runs down the stems of plants to reach the ground.
2
6- Throughfall: The precipitation which falls straight through the vegetation (or drips off leaves) and
reaches the ground.
7- Overland flow/ surface runoff (transfers): Water that moves over the ground surface.
Water moving as overland flow reaches the river channel much more quickly than water flowing
through soil and rocks. The more overland flow that takes place, the faster the river level rises and the
more likely flooding is to occur.
Overland flow happens from a combination of the following reasons:
- The ground surface is relatively impermeable and the water cannot infiltrate as a result of, foe
example, urban development.
- Vegetation cover has been removed, reducing the infiltration rates and the interception loss. The extra
water runs over the ground surface.
- When the soil store is already full of water, the term used is saturated overland flow.
- The rainfall comes down faster than it can infiltrate the soil. This can occur with intense summer
storms.
8- Infiltration (Transfer): process by which water soaks into or is absorbed by the soil.
It is a very important process, because water entering the soil will move slowly through the drainage
basin to the river channel, whereas water which does not infiltrate will flow relatively quickly over the
surface to a river as overland flow.
Infiltration rate: refers to how much water IS passing through in a certain time.
Infiltration capacity: is the maximum rate at which water CAN enter the soil in a particular case
[measured in cubic millimetres per hour].
9- Throughflow (Transfer): The downslope flow of water within the soil.
10- Percolation (transfer): As water reaches the underlying soil or rock layers, which tend to be more
compact, its progress is slowed. This slow movement , called percolation, creates groundwater storage.
11- Groundwater flow/baseflow (transfer): Groundwater refers to subsurface water. The upper layer
of the permanently saturated zone is known as the water table and it varies seasonally. Groundwater
flow is the slow movement of water through the underlying geology via cracks and joints below the
water table and it varies seasonally. The slow flow through rock produces a steady input of water which
varies relatively little in comparison with the discharge of the whole river. This explains why rivers can
continue to flow even if it has not rained for weeks.
3
4
Question catcher
(i) Identify flow X; (1)
Explain why the drainage basin hydrological cycle can be described as an open system. (3 marks)
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Distinguish between the inputs and outputs of the drainage basin hydrological cycle.
(2 marks)
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
5
Describe how water reaches a river channel in a drainage basin. (4 marks)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------The water balance TB page 4
The water balance or budget: balance between inputs and outputs of a drainage basin.
- Dynamic relationship between temperature, precipitation and evaporation rates over the year.
Water balance or budget P= Q+E = +/- change in storage
P= precipitation
Q= runoff
E= evapotranspiration
Shown using a water budget graph.
What is potential evapotranspiration?
For example, the actual evapotranspiration rate in Egypt is less than 250mm, because there is less
than 250mm of rain annually. However, given the high temperatures experienced in Egypt, if the
rainfall were as high as 2000mm, there would be sufficient heat to evaporate that water.
Therefore, the potential evapotranspiration rate there is 2000mm.
6
Soil moisture is the total amount of water in a soil.
Soil moisture recharge is the replenishment of stores of water used during warmer months when
evapotranspiration has exceeded precipitation, when there is a net balance in favour of precipitation,
water lost from the soil will be replaced.
Temperate climate e.g. the UK
Precipitation exceeds evapotranspiration
 Therefore positive budget over a year.
 During winter (high precipitation/low evapotranspiration) soil moisture surplus surface
runoff/rise in river levels
 In summer, rates of evapotranspiration> precipitation and soil water may be utilised, reducing
the amount remaining in the soil and river channels
 In autumn, the soil water will be replenished soil back to field capacity (the normal amount of
water that can be held by soil)
7
Extremely arid and Mediterranean climates
So much water can be
there can be a soil moisture
left in the soil to match the
Figure Soil moisture deficit
Question catcher
removed from the soil in summer that
deficit: that is, there is not enough water
potential evapotranspiration.
8
3- The storm hydrograph
Discharge: total volume of water in a river channel passing at any giving point every second.
To find the discharge of a river, we multiply the velocity of the river, measured in metres per second by
the cross-sectional area of the river measured in square metre. This gives the discharge in cumecs
passing any given point. (Cubic metres per second).
Figure How to find the discharge of a river
The storm hydrograph shows variations in a river’s discharge over a short period of time, usually a
storm.
Figure The storm hydrograph
9
Lag time: the time period between peak rainfall and peak discharge
Base flow: the constant part of a river’s discharge produced by groundwater and slow throughflow
seeping slowly into the river. It is the main contributor to river flow during dry weather.
The rising limb shows the increase of the discharge. The highest flow is shown by the peak discharge.
This occurs some time after the peak of the input because the water takes time to move through the
system to the measuring point of the basin. The falling or receding limb shows the fall in the discharge
back to the base level.
Runoff or storm flow: Combination of overland flow and rapid throughflow
Figure Type of flows and storm hydrograph
The processes operating in the drainage basin transfer water to the channel. However, they transfer
water at different speeds. Most hydrographs are controlled by the interaction of several variable
factors.
10
Factors affecting the shape of the storm hydrograph
Factor
Climatic factors
Precipitation (Intensity and
duration of the storm)
Snow (Water stored as snow)
Evapotranspiration
(temperature)
Soil characteristics
Soil moisture
(Antecedent /pre-existing
conditions)
Permeability
Drainage basin characteristics
Drainage density
Size of drainage basin
Slopes
Shape of drainage basin
Hydrograph dominated by
quickflow processes
Hydrograph dominated by
slowflow processes
High-intensity rainfall which
exceeds the infiltration capacity
of the soil.
Large amount of rainfall
Fast snow melt as temperatures
suddenly rise above zero
Low rates of evapotranspiration
outputs due to low
temperatures e.g. winter in
Britain
Low-intensity rainfall which is
less than the infiltration capacity
of the soil
Small amounts of rainfall
Slow snow melt
Basin already wet from previous
rain, water table high, soils
saturated and so low
infiltration/percolation
Impermeable soil
Dry soil-the soil store can hold
much more water. Low water
table. High
infiltration/percolation
Permeable soil
High drainage density (large
number of streams per km).The
higher the density the faster the
water reaches the main river
channel.
Low drainage density (small
number of streams per km)
means few streams and rivers
and so water is more likely to
enter the ground and move
slowly through the basin.
Small drainage basin tends to
respond more rapidly to a storm
than larger one, so the lag time
is shorter.
Steep slopes promote surface
runoff.
Rainfall reaches the river more
quickly from a round basin than
from an elongated basin.
High rates of evapotranspiration
outputs due to high
temperatures e.g. summer in
Britain
Gentle slopes allow infiltration
and percolation.
11
Rock type
Vegetation cover
Impermeable rocks- e.g. clay,
granite, restrict percolation and
encourage rapid runoff
Little vegetation cover
Lack of interception and root
development to open up the soil.
Rapid movement through the
system.
Deciduous trees in winter.
Soil depth
Thin soil- e.g. upland areas allow
little infiltration
Water stores
Lack of lakes and backwater
swamps
Human activity
Forests
Urban development
Agricultural practices
Question catcher
Deforestation
Urban development creates
impermeable concrete and
tarmac surfaces and water
quickly reaches the channel via
storms drains.
Low infiltration/percolation
Poor agricultural practices – poor
soil structure, trampling by
animals.
Permeable rocks-e.g. chalk and
limestone, allow infiltration and
percolation
Forest and woodland intercept
much rainfall, and root
development encourages
infiltration. More water lost to
evaporation from vegetation
surfaces.
Deciduous trees in summer.
Deeper soils provide a large soil
store –e.g. slope bottoms and
lowland areas.
Lakes and backwater swamps act
as water stores, and slow the
movement to the channel.
Reforestation
Rural land uses intercept more
precipitation and have more
permeable land surfaces. High
infiltration and percolation.
Good agricultural practices
which encourage soil aeration
and protect the soil surface.
12
(i) Draw sketch hydrographs on Figures 1a and 1b to show the contrasting effects of each of the two
rainfall periods on river discharge. (3 marks)
(ii) Suggest reasons for the differences in the shapes of the hydrographs that you have drawn in
1(b)(i). (3 marks)
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
13
Explain likely effects of the forest on river discharge. (5 marks)
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Extra space
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Study Figure , which shows a storm hydrograph.
14
(i)
determine the lag time for this event (1)
(ii)
estimate the contribution of surface run-off at the time of the peak discharge (1)
Explain why the rising limb is steeper than the falling llimb. (4)
How is the discharge of a river calculated? (2)
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------State and give reasons for the relationship between run-off and vegetation within a drainage basin.
(3 marks)
Infiltration is the movement of water from the ground surface into the soil. Explain why rates of
infiltration vary from place to place and from time to time. (6)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Extra space
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
15
How might human activity modify the drainage basin hydrological cycle? (7 marks)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Extra space
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------1- Graded assessment 1
Our first essay question: TB page X
Physical factors are more important than human factors in affecting river discharge. Discuss this view.
(15)
2- Answer ONE of the following essay questions:
Examine the factors that influence river discharge. (15 marks) (PAST AS)
Using a systems approach, explain how different components of the drainage basin hydrological cycle
are linked. (15 marks) (PAST AS)