The Global Hydrological Cycle (Edexcel A Level Geography)

• The global hydrological cycle is a system.  
• There are open and closed systems:
  • Open systems have external inputs and outputs of energy and matter exchange at its boundaries
  • Closed systems only have energy as its input and output, matter is contained within the system boundary
  • Energy is from the sun which is irradiated back from Earth to space

• Within global systems there are usually numbers of smaller subsystems e.g. drainage basin of a river
• The global hydrological system is a closed system meaning there are no external inputs or outputs, water is not lost or gained from space
• It is defined as :

'the continuous movement of water on, above and below the Earth's surface'

• The cycle is a series of processes in which water is constantly recycled through the system
  • Evaporation - the sun evaporates surface water into vapour
  • Condensation - water vapour condenses and precipitates
  • Flows - water runs off the surface into streams and reservoirs or beneath the surface as ground flow
• The hydrological cycle is powered by the sun.
  • The sun heats water in the oceans or on land and evaporation occurs
  • Water vapour then rises into the atmosphere and condenses to form clouds and this drives the atmospheric circulation
  • Cloud droplets then fall back to earth as precipitation

• The hydrological cycle involves energy exchange, leading to local temperature fluctuations
  • As water evaporates, it uses energy from its surrounding to perform this process
  • This effectively cools the environment
  • The reverse happens when water condenses (heat is released)
  • This heat exchange influences the local climate 
• When a water droplet falls on a mountain or slope, high above sea level, it has a high amount of gravitational potential energy (GPE)

Gravitational potential energy of a raindrop

• As the water droplet is in motion, the GPE turns to kinetic energy

Kinetic energy of a raindrop on a hill slope

• This allows the water to move over the surface

 

• Water is defined as:

A colourless, tasteless, transparent, odourless liquid that forms the seas, rivers and precipitation

• Freshwater makes up just 2.5% of all Earth's water
• Approximately 1.6% of the freshwater is locked away:
  • 68.7% as ice within the cryosphere
  • The remaining 30.1% is groundwater 
• In total, just 0.9% of the Earth's total freshwater, is accessible to humans 

The main stores of water in the global hydrological cycle

Major stores of water

• Stores or reservoirs
  • The term refers to a body of water that acts as a holding point – not just a man-made lake
• Water is stored within the major systems
• Most water is stored as saline water in oceans and freshwater as ice or within aquifers (groundwater stores)
• Frozen water in the cryosphere = 68.7%
• Liquid water in the hydrosphere = 1%
• Water vapour in the atmosphere = 0.2%
• Groundwater in the lithosphere = 30.1%
• Water is stored unevenly around the globe because of the uneven spread of land to sea and permeable or porous rock which enable aquifers to form

• The hydrological cycle transfers water, the flows which enable this are known as fluxes

• Annual flux are the variations in flows due to temperature, seasons and location
  • Flows such as evaporation will be greatest in warmer areas due to increased heating from the sun e.g. at the equator
  • This then leads to high rates of precipitation at the equator too
• Stores such as ice caps will be getting smaller as a result of climate change whereas ocean stores will be increasing for the same reason

• The global water budget is the difference between the inputs and outputs from the different stores
• Water is stored for varying amounts of time

Residence times

• The time water is held in a store is called the residence time
• The size of the stores of water along with water residence time is dictated by:
  • Flows/transfers such as evaporation
  • Global factors such as climate change
  • Local factors such as human activity on a hillslope

Flows/transfers

• Any change in the flow/transfer of water impacts the size of the water store and residence time of water
  • More precipitation generally increases the availability of water for storage
  • If the rainfall is too intense and surface flow increases, the opportunity for infiltration and water storage less 

Climate change

• During the last Ice Age (approx. 18,000 yrs ago), roughly a third of Earth's surface was covered in ice sheets and glaciers
• This increased the magnitude (size) of the cryosphere's stores
• But, it lowered the hydrosphere's store (no flow of liquid water) and sea levels were over 100 m lower than present day

Clouds and Precipitation

• The global atmospheric circulation drives the formation of clouds and types of precipitation
• The Equator receives more insolation, resulting in higher temperatures which cause high rates of evaporation
• As the warm, moist air rises, it cools, condenses and forms banks of towering clouds with heavy rainfall 
• This area is a low-pressure zone known as the Inter-Tropical Convergence Zone (ITCZ)
• The ITCZ (also called the 'thermal equator') tracks with the seasonal movement of the Sun, north and south of the equator bringing intense low-pressure rain/monsoon conditions with the movement
• This shows temporal and spatial changes in stores and transfers on a global basis

Global atmospheric circulation determines cloud and precipitation on a global scale

Cryospheric processes

• The second largest store of water is ice and 95% is locked as the ice sheets of Antarctica and Greenland
• Any changes to the size (magnitude) of these sheets impact globally
• The total melting of the ice sheets could result in a 60 m sea level rise, which is a lot of stored water
• Melting of the ice sheets adds water to the hydrosphere store of the oceans
• Ice shelves are further destabilised, which triggers ice calving, these icebergs subsequently melt; adding to the hydrosphere store and rising sea levels
• This is a positive feedback loop