The Global Water Cycle (AQA A Level Geography)

Global Distribution of Water Stores

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 

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 Water Cycle

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 

• 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