The Water Cycle and Water Insecurity – Physical Geography Notes
Paper 1
The hydrological cycle:
• This is the movement of water and its transformation between the gaseous, liquid and
solid form
The Operation of the Hydrological Cycle at a Global Scale
3 concepts are key to understanding how water cycling operates:
• Stores – these are reservoirs where water is held, such as the oceans
• Fluxes – these measure the rate of flow between the stores
• Processes – these are the physical mechanisms which drive the fluxes of water
between the stores
The global hydrological cycle:
• The global hydrological cycle is an example of a closed system driven by solar energy
and gravitational potential energy
• In a closed system, there’s a fixed amount of water in the Earth-atmosphere system
• A closed system doesn’t have any external inputs or outputs, so this total volume of
water is constant and finite
• However, the water can exist in different states within the system
1
, • There are 4 major stores of water, of which the oceans are the largest – they contain
an estimated 96.5 to 97% of the world’s total water. The next largest stores occur in
the cryosphere, contains 1.9%, and then terrestrial surface ground water. The
atmosphere contains the smallest amount, 0.001%
Blue and green water:
• Blue water – is stored in rivers, streams, lakes and groundwater in liquid form (the
visible part of the hydrological cycle)
• Green water – is stored in the soil and vegetation (the invisible part of the hydrological
system)
2
,Where is the Earth’s water?
• Total global water and fresh water – this summarises where the Earth’s total global
water is stored, with about 96-97% stored in the oceans and 2.5% occurs as fresh
water
• Surface water and other fresh water – this includes all sources of surface water,
including ground ice and permafrost, which are difficult to access
The Operation of the Drainage Basin as an Open System
The drainage basin water cycle:
3
, • The drainage system is a subsystem within the global hydrological cycle
• It is an open system as it has inputs and outputs that cause the amount of water in
the basin to vary over time
• These variations can occur at different temporal scales, from short-term hourly
through to daily, seasonal and annual
• A drainage basin can be defined as the area of land drained by a river and its
tributaries, and is frequently referred to as a river catchment
• The boundary of a drainage basin is defined by the watershed, which is usually a ridge
of highland which divides and separates waters flowing to different rivers
Drainage basin system inputs:
Precipitation:
Conditions that are needed for precipitation for form:
• Air cooled to saturation point with a relative humidity of 100%
• Condensation nuclei such as dust particles to facilitate the growth of water droplets
• A temperature below dew point (the temperature at which dew forms, a measure of
atmospheric moisture)
6 influencing factors on the impacts of the drainage basin hydrological system:
• Amount of precipitation which can have a direct impact on the drainage discharge, in
general the higher the amount the less variation in discharge pattern
4
Paper 1
The hydrological cycle:
• This is the movement of water and its transformation between the gaseous, liquid and
solid form
The Operation of the Hydrological Cycle at a Global Scale
3 concepts are key to understanding how water cycling operates:
• Stores – these are reservoirs where water is held, such as the oceans
• Fluxes – these measure the rate of flow between the stores
• Processes – these are the physical mechanisms which drive the fluxes of water
between the stores
The global hydrological cycle:
• The global hydrological cycle is an example of a closed system driven by solar energy
and gravitational potential energy
• In a closed system, there’s a fixed amount of water in the Earth-atmosphere system
• A closed system doesn’t have any external inputs or outputs, so this total volume of
water is constant and finite
• However, the water can exist in different states within the system
1
, • There are 4 major stores of water, of which the oceans are the largest – they contain
an estimated 96.5 to 97% of the world’s total water. The next largest stores occur in
the cryosphere, contains 1.9%, and then terrestrial surface ground water. The
atmosphere contains the smallest amount, 0.001%
Blue and green water:
• Blue water – is stored in rivers, streams, lakes and groundwater in liquid form (the
visible part of the hydrological cycle)
• Green water – is stored in the soil and vegetation (the invisible part of the hydrological
system)
2
,Where is the Earth’s water?
• Total global water and fresh water – this summarises where the Earth’s total global
water is stored, with about 96-97% stored in the oceans and 2.5% occurs as fresh
water
• Surface water and other fresh water – this includes all sources of surface water,
including ground ice and permafrost, which are difficult to access
The Operation of the Drainage Basin as an Open System
The drainage basin water cycle:
3
, • The drainage system is a subsystem within the global hydrological cycle
• It is an open system as it has inputs and outputs that cause the amount of water in
the basin to vary over time
• These variations can occur at different temporal scales, from short-term hourly
through to daily, seasonal and annual
• A drainage basin can be defined as the area of land drained by a river and its
tributaries, and is frequently referred to as a river catchment
• The boundary of a drainage basin is defined by the watershed, which is usually a ridge
of highland which divides and separates waters flowing to different rivers
Drainage basin system inputs:
Precipitation:
Conditions that are needed for precipitation for form:
• Air cooled to saturation point with a relative humidity of 100%
• Condensation nuclei such as dust particles to facilitate the growth of water droplets
• A temperature below dew point (the temperature at which dew forms, a measure of
atmospheric moisture)
6 influencing factors on the impacts of the drainage basin hydrological system:
• Amount of precipitation which can have a direct impact on the drainage discharge, in
general the higher the amount the less variation in discharge pattern
4
