The Global Water Cycle (AQA A Level Geography)

Earth's water 

• 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:
  • 68.6% 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 

Comparison of the world's freshwater stores

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

Hydrological cycle 

• The water cycle is known as the hydrological cycle is a closed system and defined as:

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

• It is a series of processes in which water is constantly recycled through the system
• The cycle also shapes landscapes, transports minerals and is essential to life on Earth
• These processes are:
  • 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
• This transfers the water on Earth from one store to another (river to ocean or ocean to atmosphere)
• 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 

The global hydrological cycle shows annual flows of water between global stores

• Stores are those places where water is held for a period of time. These include:
  • Water in the atmosphere in the form of water vapour or water droplets in clouds
  • Surface stores such as puddles, lakes, rivers and reservoirs
  • Interception is how precipitation is prevented from reaching the ground, usually by being caught on leaves or branches
  • Ice and snow
  • Seas and oceans 
  • Groundwater stores 
    • Not all rocks can store or transport water
    • An impermeable but porous rock such as clay, can store water but not transfer water
    • Un-weathered granite cannot store or transfer water as it has no spaces between rock particles 
    • Sandstone is both porous and permeable and is able to store and transport water
    • A rock which stores water is called an aquifer and is the most unpolluted source of reliable water when managed carefully

Aquifers

• Formed extremely slowly over a very long time
• Water infiltrates underground and collects in the pore spaces of porous rock
• Eventually, the interconnected pore spaces become saturated with water and form an aquifer
• Pore spaces can be openings between grains or fractures in the rock or even caverns
• Porosity is not enough to form an aquifer, pores must connect with each other to allow water to flow/transfer from one space to another - this is a rock permeability
• Compared to the transfer rate of water in a river, the flow through an aquifer is very slow
• There are two types of aquifers:
  • Unconfined - where porous rock is open to surface water and is directly recharged by precipitation
  • Confined - where there are thick layers/beds of rock over the aquifer, known as the confining beds; these contain the aquifer from the Earth's surface or other rocks

Image showing the two types of aquifers and residence time of water at varying depths