Geography Revision Notes:
Water
The Global Hydrological Cycle
The Global Hydrological Cycle
Introduction to the Global Hydrological Cycle
The global hydrological cycle is the cycling of water on Earth
It is studied using a systems approach
Systems theory helps us understand the main water stores and pathways
The system adjusts and changes due to physical and human factors
Types of Systems
Closed system: transfer of energy but not matter
Open system: receives inputs and transfers outputs of energy and matter
The Global Hydrological Cycle as a Closed System
All water is continually circulated through the stores
Driven by solar energy
Water evaporates into the atmosphere and returns to the land and oceans as
precipitation
Water moves through the system by plant interception, surface runoff,
infiltration, and throughflow
Some water is stored as soil moisture or groundwater
Water returns to the oceans via streams and rivers
The Global Water Budget
The annual balance of water fluxes and the size of water stores
Different water stores have different residence times
Water is generally considered a renewable resource
Fossil water is an exception and is non-renewable
Drainage Basins
Introduction to Drainage Basins
A drainage basin is an area of land drained by a river and its tributaries
, Separated from neighbouring basins by a watershed or divide
Drainage basins are open systems
Linked to other systems by inputs and outputs
Factors Affecting Distribution of Precipitation
Continentality affects precipitation distribution
Relief and prevailing winds complicate the pattern
High levels of precipitation occur where winds are forced to rise over higher
altitudes
Physical Factors Affecting Drainage Basin Flows
Interception: process by which raindrops are prevented from falling directly on
the ground
Infiltration and throughflow: movement of water through the soil
Direct runoff: water flowing over the surface of the ground
Percolation and groundwater flow: movement of water into the rock and
underground
Evaporation and transpiration: total amount of moisture removed from a basin
Channel flow: water collected to flow in a river
Human Disruptions to the Drainage Basin Cycle
Introduction to Human Disruptions
Human activities can disrupt the drainage basin cycle
Changing the speed of processes, creating new stores, or abstracting water
Hard engineering schemes, such as channelization, can disrupt the cycle
Case Study: Human Disruption in Amazonia
Deforestation in Amazonia reduces evapotranspiration and precipitation
Increased runoff and river discharge
Cloud seeding is an attempt to change precipitation
Urbanisation and dam construction disrupt the cycle
Groundwater abstraction affects groundwater flow and water table
Local-Scale Water Budgets and River Systems
Water Budgets
Water budget shows the balance between inputs and outputs
Precipitation = channel discharge + evapotranspiration ± change in storage
Useful for understanding water supply and demand
River Regimes
, River regime describes the annual variation in discharge
Physical factors and land use affect the regime
Human activities in the drainage basin can alter the regime
Storm Hydrographs
Storm hydrograph shows variations in discharge during a storm
Shape of the hydrograph changes due to physical and human factors
Useful for predicting flood risk and comparing basin responses
Factors influencing the hydrological system over short and
long-term timescales
Factors influencing the hydrological system over short and long-term
timescales
Deficits within the hydrological cycle (drought)
Drought is an 'insidious hazard of nature' that develops gradually with harmful
impacts that vary geographically
Different definitions of drought are used around the world
Four different types of drought: meteorological, hydrological, agricultural, and
socio-economic
Physical causes of drought can be explained by the global atmospheric
circulation system
Global Atmospheric circulation
Intense solar radiation at the Equator warms the air, which rises and starts
convection
The subtropical high-pressure zone is created where air that had risen at the
Equator has cooled and so sinks to form a belt of high air pressure and hot,
dry conditions
The air returns to ground level at the Equator, creating trade winds
The trade winds meet at the Intertropical Convergence Zone (ITCZ) where the
warmed air rises
The warm air moving from the subtropics to the mid-latitudes meets cold polar
air at the polar front, where the warm, less dense air rises, causing
condensation and rainfall
, The warmer air rises into the polar front jet stream and is transferred at high
altitude towards the poles, where it cools and sinks
El Niño– Southern Oscillation (ENSO) cycles
El Niño is a naturally occurring large mass of very warm seawater in the
equatorial Pacific Ocean
El Niño reduces precipitation in the western Pacific and causes drought in
affected countries
La Niña occurs when the warm mass of water is pushed even further west
than normal, causing drought in other regions
ENSO causes global variations in rainfall patterns, creating both drought and
floods in different areas of the world
Drought risk from human activities
Severe droughts are not purely natural hazards, but are influenced by human
activities
Human responses to water shortages influence water levels in reservoirs,
aquifers, and rivers
Human activities directly affect the development of droughts by abstracting
water and reducing downstream supply
Human activities indirectly affect the development of droughts by changing
land uses and altering hydrological processes
Anthropogenic climate change likely enhances the drought hazard in certain
regions
CASE STUDY: Drought in the Sahel, Africa
The Sahel experiences severe droughts due to a combination of physical and
human factors
Air pollution and higher sea-surface temperatures have been identified as
causes of Sahelian drought
Deforestation and over cultivation contribute to desertification and increase
vulnerability to drought
CASE STUDY: The Millennium Drought in south-eastern Australia, 1997– 2009
The Millennium Drought was the result of multiple physical and human causes
El Niño events and the strengthening of the subtropical ridge contributed to
the drought
Anthropogenic global warming may have intensified the drought
Water
The Global Hydrological Cycle
The Global Hydrological Cycle
Introduction to the Global Hydrological Cycle
The global hydrological cycle is the cycling of water on Earth
It is studied using a systems approach
Systems theory helps us understand the main water stores and pathways
The system adjusts and changes due to physical and human factors
Types of Systems
Closed system: transfer of energy but not matter
Open system: receives inputs and transfers outputs of energy and matter
The Global Hydrological Cycle as a Closed System
All water is continually circulated through the stores
Driven by solar energy
Water evaporates into the atmosphere and returns to the land and oceans as
precipitation
Water moves through the system by plant interception, surface runoff,
infiltration, and throughflow
Some water is stored as soil moisture or groundwater
Water returns to the oceans via streams and rivers
The Global Water Budget
The annual balance of water fluxes and the size of water stores
Different water stores have different residence times
Water is generally considered a renewable resource
Fossil water is an exception and is non-renewable
Drainage Basins
Introduction to Drainage Basins
A drainage basin is an area of land drained by a river and its tributaries
, Separated from neighbouring basins by a watershed or divide
Drainage basins are open systems
Linked to other systems by inputs and outputs
Factors Affecting Distribution of Precipitation
Continentality affects precipitation distribution
Relief and prevailing winds complicate the pattern
High levels of precipitation occur where winds are forced to rise over higher
altitudes
Physical Factors Affecting Drainage Basin Flows
Interception: process by which raindrops are prevented from falling directly on
the ground
Infiltration and throughflow: movement of water through the soil
Direct runoff: water flowing over the surface of the ground
Percolation and groundwater flow: movement of water into the rock and
underground
Evaporation and transpiration: total amount of moisture removed from a basin
Channel flow: water collected to flow in a river
Human Disruptions to the Drainage Basin Cycle
Introduction to Human Disruptions
Human activities can disrupt the drainage basin cycle
Changing the speed of processes, creating new stores, or abstracting water
Hard engineering schemes, such as channelization, can disrupt the cycle
Case Study: Human Disruption in Amazonia
Deforestation in Amazonia reduces evapotranspiration and precipitation
Increased runoff and river discharge
Cloud seeding is an attempt to change precipitation
Urbanisation and dam construction disrupt the cycle
Groundwater abstraction affects groundwater flow and water table
Local-Scale Water Budgets and River Systems
Water Budgets
Water budget shows the balance between inputs and outputs
Precipitation = channel discharge + evapotranspiration ± change in storage
Useful for understanding water supply and demand
River Regimes
, River regime describes the annual variation in discharge
Physical factors and land use affect the regime
Human activities in the drainage basin can alter the regime
Storm Hydrographs
Storm hydrograph shows variations in discharge during a storm
Shape of the hydrograph changes due to physical and human factors
Useful for predicting flood risk and comparing basin responses
Factors influencing the hydrological system over short and
long-term timescales
Factors influencing the hydrological system over short and long-term
timescales
Deficits within the hydrological cycle (drought)
Drought is an 'insidious hazard of nature' that develops gradually with harmful
impacts that vary geographically
Different definitions of drought are used around the world
Four different types of drought: meteorological, hydrological, agricultural, and
socio-economic
Physical causes of drought can be explained by the global atmospheric
circulation system
Global Atmospheric circulation
Intense solar radiation at the Equator warms the air, which rises and starts
convection
The subtropical high-pressure zone is created where air that had risen at the
Equator has cooled and so sinks to form a belt of high air pressure and hot,
dry conditions
The air returns to ground level at the Equator, creating trade winds
The trade winds meet at the Intertropical Convergence Zone (ITCZ) where the
warmed air rises
The warm air moving from the subtropics to the mid-latitudes meets cold polar
air at the polar front, where the warm, less dense air rises, causing
condensation and rainfall
, The warmer air rises into the polar front jet stream and is transferred at high
altitude towards the poles, where it cools and sinks
El Niño– Southern Oscillation (ENSO) cycles
El Niño is a naturally occurring large mass of very warm seawater in the
equatorial Pacific Ocean
El Niño reduces precipitation in the western Pacific and causes drought in
affected countries
La Niña occurs when the warm mass of water is pushed even further west
than normal, causing drought in other regions
ENSO causes global variations in rainfall patterns, creating both drought and
floods in different areas of the world
Drought risk from human activities
Severe droughts are not purely natural hazards, but are influenced by human
activities
Human responses to water shortages influence water levels in reservoirs,
aquifers, and rivers
Human activities directly affect the development of droughts by abstracting
water and reducing downstream supply
Human activities indirectly affect the development of droughts by changing
land uses and altering hydrological processes
Anthropogenic climate change likely enhances the drought hazard in certain
regions
CASE STUDY: Drought in the Sahel, Africa
The Sahel experiences severe droughts due to a combination of physical and
human factors
Air pollution and higher sea-surface temperatures have been identified as
causes of Sahelian drought
Deforestation and over cultivation contribute to desertification and increase
vulnerability to drought
CASE STUDY: The Millennium Drought in south-eastern Australia, 1997– 2009
The Millennium Drought was the result of multiple physical and human causes
El Niño events and the strengthening of the subtropical ridge contributed to
the drought
Anthropogenic global warming may have intensified the drought
