Cambridge IAS & IAL

Geography
1 – Hydrology and Fluvial Geomorphology
1-1 The drainage basin
1-2 Rainfall-discharge relationships within drainage basins
1-3 River channel processes and landforms
1-4 The human impact
 Colour Coding

Topic heading
Subheading
Key Term
1-1 The drainage basin
 Key Terminology

A drainage basin refers to the area drained by a river and its tributaries.
Hydrology is the study of water as it moves on, and under and through the Earth’s
surface.
The water cycle or hydrological cycle is the movement of water between air, land and
sea.
Precipitation (PPT) is the conversion and transfer of moisture in the atmosphere to the
land. It includes rainfall, snow, frost, hail and dew.
Interception is the precipitation that is collected and stored by vegetation.
Evaporation refers to the transformation of liquid water from the Earth’s surface into a
gas (water vapour).
Transpiration is water loss from vegetation to the atmosphere.
Evapotranspiration (EVT) is the combined loss of water to the atmosphere through
transpiration and evaporation.
Infiltration is the process by which water soaks into, or is absorbed by, the soil.
Runoff is water that flows over the land’s surface.
 The drainage basin system

A drainage basin is a natural system with inputs, flows and stores of water and
sediment.
Every drainage basin is unique, on account of its climate, geology, vegetation, soil
types, size, shape and human activities.
The drainage basin system is an open system as it allows the movement of energy and
matter across its boundaries.
The hydrological cycle refers to the cycle of water between atmosphere, lithosphere
and biosphere.

Water can be stored at a number of

stages or levels within the cycle.

These stores include vegetation,

surfaces, soil moisture, groundwater
and water channels.

Human modifications to these can be

made at every scale.
The drainage basin hydrological cycle
At a local scale - the drainage basin - the cycle has a single input, precipitation (PPT)
and two major losses (outputs):
• evapotranspiration (EVT)
• runoff.

A third output, leakage, may also

occur from the deeper
subsurface to other basins.
 Tip!

You may be asked to draw a diagram of

a drainage basin hydrological cycle.
A systems diagram – with inputs, stores,
flows and output is much better than one
that tries to show trees, clouds, rainfall,
glaciers, rivers, lakes and oceans.
 Outputs

Evaporation is the process by which a liquid is changed into a gas.

• The process by which a solid is changed into a gas is sublimation.
• These terms refer to the conversion of solid and liquid precipitation (snow, ice
and water) to water vapour in the atmosphere.
Evaporation is most important from oceans and seas.
• It increases under warm, dry conditions and
• decreases under cold, calm conditions.
Evaporation losses are greater in arid and semi-arid climates than in polar regions.
Factors affecting evaporation include meteorological factors such as:
• temperature
• humidity and
• wind speed.
Of these, temperature is the most important factor.

Other factors include:

• the amount of water available,
• vegetation cover and
• colour of the surface (albedo or reflectivity of the surface).
Transpiration is the process by which water vapour escapes from a living plant,
principally the leaves, and enters the atmosphere.
The combined effects of evaporation and transpiration are normally referred to as
evapotranspiration (EVT).

EVT represents the most important aspect of water loss, accounting for the loss of
nearly 100 per cent of the annual precipitation in arid areas and 75 per cent in humid
areas.
Only over ice and snow fields, bare rock slope , desert areas, water surfaces and bare
soil will purely evaporative losses occur.
The distinction between actual EVT and potential evapotranspiration (P. EVT) lies in
the concept of moisture availability.
Potential evapotranspiration is the water loss that would occur if there were an
unlimited supply of water in the soil for use by the vegetation.
• Eg: 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.

Hence the potential evapotranspiration rate there is 2000mm.

The factors affecting evapotranspiration include all those that affect evaporation.
In addition, some plants such as cacti, have adaptations to help them reduce moisture
loss.
River discharge refers to the movement of water in channels such as streams and
rivers.
The water may enter the river as direct channel precipitation (it falls on the channel)
or it may reach the channel by surface runoff, groundwater flow (baseflow), or
throughflow (water flowing through the soil).
 Inputs

Precipitation is the main input into the drainage basin system.

The main characteristics that affect local hydrology include:
• the total amount of precipitation;
• intensity (mm/hour);
• type of precipitation (snow, rain etc.);
• geographical distribution; and
• temporal variability (i.e. seasonality).
 Stores

Interception has three main components:

• interception loss
• water that is retained by plant surfaces and which is later evaporated away
or absorbed by the plant;
• throughfall
• water that either falls through gaps in the vegetation or which drops from
leaves, twigs or stems;
• stemflow
• water that trickles along twigs and branches and finally down the main
trunk.
Interception loss varies with different types of vegetation.
Interception is less from grasses than from deciduous woodland owing to the smaller
surface area of the grass shoots.
From agricultural crops, and from cereals in particular, interception increases with
crop density.
Coniferous trees intercept more than deciduous trees in winter, but this is reversed in
summer.
Soil moisture (soil water) refers to the subsurface water in the soil and subsurlace
layers above the water table.
From here water may be:
• absorbed
• held
• transmitted downwards towards the water table, or
• transmitted upwards towards the soil surface and the atmosphere.

In coarse-textured soils much of the water is held in fairly large pores at fairly low
suctions, while very little is held in small pores.
In the finer-textured clay soils the range of pore sizes is much greater and in particular
there is a higher proportion. of small pores in which the water is held at very high
suctions.
Field capacity refers to the amount of water held in the soil after excess water drains
away, i.e. saturation or near saturation.
Wilting points refer to the range of moisture content in which permanent wilting of
different plants occurs.
They define the approximate limits to plant growth.
There are a number of important seasonal variations in soil moisture budgets:
• Soil moisture deficit
• Soil moisture recharge
• Soil moisture surplus
• Soil moisture utilisation
Soil moisture deficit is the degree to which soil moisture falls below field capacity.
• In temperate areas, during late winter and early spring, soil moisture deficit is
very low due to high levels of precipitation and limited evapotranspiration.
Soil moisture recharge occurs when precipitation exceeds potential
evapotranspiration - there is some refilling of water in the dried-up pores of the soil.
Soil moisture surplus is the period when soil is saturated and water cannot enter, and
so flows over the surface.
Soil moisture utilisation is the process by which water is drawn to the surface through
capillary action .
A) Precipitation > potential evapotranspiration.
• Soil water store is full and there is a soil moisture surplus for plant use, runoff
and groundwater recharge.

B) Potential evapotranspiration > precipitation.

• Water store is being used up by
plants or lost by evaporation (soil
moisture utilisation).

C) Soil moisture store is now used up.

• Any precipitation is likely to be
absorbed by the soil rather than
produce runoff.
• River levels will fall or dry up
completely.
D) There is a deficiency of soil water as the store is used up and potential
evapotranspiration > precipitation.
• Plants must adapt to survive; crops must be irrigated.

E) Precipitation > potential evapotranspiration.

• Soil water store will start to fill
again (soil moisture recharge).

F) Soil water store is full.

• Field capacity has been reached.
• Additional rainfall will percolate
down to the water table and
groundwater stores will be
recharged.
There are a number of types of surface water, some of which are temporary, and some
are permanent.
Temporary sources include:
• small puddles following a rainstorm and
• turloughs (seasonal lakes in limestone in the west of Ireland),
while permanent stores include:
• lakes,
• wetlands,
• swamps,
• peat bogs and
• marshes.
Groundwater refers to subsurface water that is stored under the surface in rocks.
Groundwater accounts for 96.5 per cent of all freshwater on the Earth.
However, while some soil moisture may be recycled by evaporation into atmospheric
moisture within a matter of days or weeks, groundwater may not be recycled for as
long as 20 OOO years.
The permanently saturated zone within solid rocks and sediments is known as the
phreatic zone.
The upper layer of this is known as the water table.
• The water table varies seasonally.
• In temperate zones it is higher in winter following increased levels of
precipitation.
• The zone that is seasonally wetted and seasonally dries out is known as the
aeration zone.
Aquifers are rocks that contain significant quantities of water and are a great reservoir
of water.
They are permeable rocks such as sandstones and limestones.
The water in aquifers moves very slowly and acts as a natural regulator in the
hydrological cycle by absorbing rainfall that otherwise would reach streams rapidly.
In addition, aquifers maintain stream flow during long dry periods.
Where water flow reaches the surface springs (a natural flow of water from the Earth’s
surface) may be found.
These may be substantial enough to become the source of a stream or river.
It occurs when the water table (the upper surface of saturation within permeable
rocks) occurs at the surface.
In humid regions In semi-arid regions
Recharge refers to the refilling of water in pores where the water has dried up or been
extracted by human activity.
Hence in some places where recharge is not taking place, groundwater is considered a
non-renewable resource.
Groundwater recharge occurs as a result of:
• infiltration of part of the total precipitation at the ground surface
• seepage through the banks and bed of surface water bodies such as ditches,
rivers, lakes and oceans
• groundwater leakage and inflow from adjacent rocks and aquifers
• artificial recharge from irrigation, reservoirs, and so on.
Losses of groundwater result from:
• evapotranspiration, particularly in low-lying areas where the water table is close
to the ground surface
• natural discharge, by means of spring flow and seepage into surface water boilies
• groundwater leakage and outflow1 along aquicludes and into adjacent aquifers
• artificial abstraction example the water table near Lubbock on the High Plains of
Texas (USA) has declined by 30- 50m in just 50 years, and in Saudi Arabia the
groundwater reserve in 2010 was 42 per cent less than in 1985.
Channel storage refers to all water that is stored in rivers, streams and other drainage
channels.
Some rivers are seasonal, and some may disappear underground either naturally; such
as in areas of Carboniferous limestone, or in urban areas, where they may be covered
(culverted).
 Flows – Above Ground

Throughfall refers to water that either falls through gaps in vegetation or that drops
from leaves or twigs.
Stemflow refers to water that trickles along twigs and branches and finally down the
main trunk.
Overland flow (surface runoff) is water that flows over the land's surface.
• Surface runoff (or overland flow) occurs in two main ways:
• when precipitation exceeds the infiltration rate
• when the soil is saturated (all the pore spaces are filled with water).
In areas of high precipitation intensity and low infiltration capacity, overland runoffs
common.
This is clearly seen in semi-arid areas and in cultivated fields.
By contrast, where precipitation intensity is low and infiltration is high, most overland
flow occurs close to streams and river channels.

Channel flow or stream flow refers to the movement of water in channels such as
streams and rivers.
The water may have entered the stream as a result of direct precipitation, overland
flow, groundwater flow (baseflow) or throughflow (water flowing through the soil).
 Flows – Below Ground

Porosity is the capacity of a rock to hold water, for example sandstone has a porosity
(pore space) of 5-15 per cent, whereas clay may be up to 5O per cent.
Permeability is the ability to transmit water through a rock via joints and fissures.
Infiltration is the process by which water soaks into or is absorbed by the soil.
The infiltration capacity is the maximum rate at which rain can be absorbed by a soil in
a given condition.
Infiltration capacity decreases with time through a period of rainfall until a more or
less constant value is reached.
• Infiltration rates of 0-4.mm/hour are common on clays, whereas 3-12mm/hour
are common on sands.
Vegetation also increases infiltration.
This is because it intercepts some rainfall and slows down the speed at which it arrives
at the surface.
• Eg: on bare soils where rain splash impact occurs, infiltration rates may reach
10mm/hour.
• On similar soils covered by vegetation, rates of between 50 and 100mm/hour
have been recorded.
Infiltrated water is chemically rich as it picks up minerals and organic acids from
vegetation and soil.
Infiltration is inversely related to overland runoff and is influenced by a variety of
factors, such as:
• duration of rainfall,
• antecedent moisture (pre-existing levels of soil moisture),
• soil porosity,
• vegetation cover,
• raindrop size and
• slope angle.
In contrast, overland flow is water that flows over the land's surface.
Factors affecting infiltration and surface runoff.
Water moves slowly downwards from the soil into the bedrock - this is known as
percolation.
Depending on the permeability of the rock, this may be very slow or in some rocks,
such as Carboniferous limestone and chalk it may be quite fast.

Throughflow refers to water flowing through the soil in natural pipes and percolines
(lines of concentrated water flow between soil horizons).

Most groundwater is found within a few hundred metres of the surface but has been
found at depths of up to 4kilometres beneath the surface.
Baseflow refers to the part of a river's discharge that is provided by groundwater
seeping into the bed of a river.
• It is a relatively constant flow although it increases slightly following a wet
period