Changes in Magnitude of Water Stores (AQA A Level Geography)

Residence Time of Stored Water

• Water is stored for varying amounts of time

Processes driving change

• 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

  • E.g. more precipitation increases the availability of water for storage

  • However, if the rainfall is too intense and surface flow increases, the opportunity for infiltration and water storage is less 

Table of Processes Responsible for Changes in Stored Water

Global Processes Driving Change

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

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 

Local Processes Driving Change

• A variety of processes affect change in the magnitude of stores on a local scale

• Consider a typical hillslope system, then under normal conditions:

• Transfers/flows move water through the system and enable inputs of water to be processed from one store to another

  • Transfers include throughflow, stem flow, infiltration, throughflow and groundwater flow

  • Groundwater fills the spaces between soil particles and fractured rock beneath the earth's surface

• Inputs include:

  • Energy from the sun for evaporation - too little and no evaporation takes place, too much and water is lost locally

  • Precipitation - storms of intense rainfall increase the magnitude of surface stores, snow and frozen ground interrupt water transfers and affect water stores

  • Inputs vary throughout the year (rain, sleet, snow, location etc) and intensities (flood, drought, temperature etc) and frequency (seasonal, monsoon etc), these all affect the magnitude of local water stores

• Outputs (losses of water) move moisture out of the locale and include:

  • Evaporation and transpiration from plants (collectively called evapotranspiration)

  • Run-off into the sea 

  • Water percolates deep into underground stores where it can be effectively lost from the system

Impacts and effects on natural conditions

• Farming practices impact stores: 

  • Ditches drain the land and increase water flow away from the land

  • The interception by vegetation can prevent as much as 40% of precipitation from reaching the surface

  • Over-abstraction of groundwater for irrigation removes stores of water

• Deforestation reduces interception and infiltration 

  • The interception by leaves stems and branches not only reduce the amount that reaches the ground but also slows the passage of water to the surface, allowing infiltration into the subsurface to occur

  • Deforestation affects surface stores as natural depression stores such as puddles, ponds and soil are removed

  • Roots help to break up the soil, increasing the rate of infiltration and groundwater recharge

• Urbanisation 

  • Development of the slope creates impermeable surfaces which effectively removes natural depression stores

  • Impermeable surfaces also reduce infiltration, impacting groundwater stores

  • Surface runoff is increased and river channel stores are increased - which may lead to flooding

• Industrialisation

  • Burning of fossil fuels generates CO2 in the atmosphere enhancing the greenhouse effect and global warming

  • This in turn affects precipitation rates and intensities of fall, affecting magnitude of stores

  • Industrial output can produce acid rain, which destroys vegetation and interception rates, affecting water stores