The Water Cycle

• Outline the difference between open and closed systems.

  Open systems are systems that allow both energy and matter (e.g. solar energy and water/carbon) to enter and leave, whereas closed systems only allow energy to enter and leave, the amount of matter doesn’t change, it just moves between stores. On a global scale, the water and carbon cycles are closed systems whereas they are open on a smaller scale.
• Outline the concept of dynamic equilibrium.

  Dynamic equilibrium is the balance between inputs and outputs in a system, when inputs=outputs (e.g. evapotranspiration=precipitation), the system is said to be in dynamic equilibrium. If this equilibrium is upset, there is a knock-on effect called feedback which either amplifies this change (e.g. increased temp=melt permafrost=methane released=increased temp), or nullifies it (e.g. increased temp=more vegetation growth=more photosynthesis=reduction in temp).
• 5 subsystems

  • Hydrosphere
  • Lithosphere
  • Biosphere
  • Cryosphere
  • Atmosphere
• Hydrosphere


  Water including the oceans which contain 96.9% of all water and cover 72% of the Earth's surface
• Lithosphere


  The Earth's crust and uppermost mantle, it includes rivers, lakes and groundwater, and together with the biosphere (all living things) they hold 1.1% of global water (terrestrial store)
• Cryosphere


  All frozen water, including sea ice, ice shelves, ice sheets, ice caps and permafrost, it contains 1.9% of global water
• Atmosphere


  The gases that surround the planet and contain 0.001% of global water
• The subsystems


  Are cascading systems which means that outputs from one are inputs into another
• Precipitation driving change in the water cycle

  1. Atmospheric moisture is returned to the terrestrial system through precipitation

  2. Condensation is the direct cause of all forms of precipitation

  3. Condensation takes place when the temperature of air is reduced to dew point but the volume remains constant

  4. Condensation occurs when warm moist air passes over a cold surface (e.g. ocean)

  5. Condensation occurs when the volume of air increases but there is no addition of heat (adiabatic cooling – volume of air changes due to a difference in air pressure rather than the exchange of heat)

  6. Condensation happens when air rises and expands in the lower pressure of the upper atmosphere
• Causes of condensation and precipitation

  1. Air is forced to rise over hills (relief or orographic rainfall)

  2. Masses of air of different temperatures and densities meet. The less dense warm air rises over the denser cold air (frontal rainfall)

  3. Localised warm surfaces heat air above. This expands, becomes less dense and rises (convectional rainfall)
• Evapotranspiration drives change in the water cycle


  1. Evaporation

  2. Transpiration
• Evaporation


  Transfer of water from liquid state to gaseous state (water vapour)
• Transpiration


  Water is taken up by root systems and released through the stomata of the leaves
• Evapotranspiration


  Evaporation and transpiration taken together
• As water evaporates


  It uses energy in the form of latent heat and so cools its surroundings
• Factors affecting the rate of evaporation

  • Amount of solar energy
  • Availability of water
  • Humidity of the air
  • Temperature of the air
• Availability of water

  • More evaporation from a pond than from a grassy field
• Humidity of the air

  The closer the air is to saturation point, the slower the rate of evaporation
• Temperature of the air

  Warmer air can hold more water vapour than cold air
• Outline how cloud formation drives change in the water cycle

  Clouds form when evaporated water condenses onto a condensation nuclei. The more dust/smoke in the air, the more clouds there will be. High temperatures are needed for water to evaporate and rise, this leads to high cloud cover along the equator (0 degrees latitude). Global atmospheric circulation is also responsible for cloud formation, this leads to high cloud cover at 60 degrees latitude.
• Cryospheric processes drive change in the water cycle

  1. Large volumes of water moving between the cryosphere

  2. Accumulation higher than ablation, water removed from atmosphere and stored in cryosphere

  3. Ablation exceeding accumulation, allowing hydrological cycle to perform as it does today

  4. Water able to cycle between subsystems as it is not trapped in cryosphere
• There have been 5 major glacial periods in Earth's history
• During the last major glacial, sea level was approximately 120m lower than present and continental glaciers covered much of Europe and North America
• This caused an interruption in the hydrological cycle
• There were also interglacial periods when global ablation exceeded accumulation (more melting than freezing)
• These glacials and interglacials happen in cycles, there have been 8 cycles during the last 740,000 years
• Condensation driving change in the water cycle

  1. Air cools

  2. Air able to hold less water vapour

  3. Air reaches dew point temperature

  4. Excess water converted to liquid water through condensation
• Dew point temperature


  Temperature at which air becomes saturated
• Condensation


  • Water molecules need something to condense on (tiny particles or surfaces below dew point temperature)
• Deposition


  Water vapour changes directly from gas to solid (hoar frost) when surface is below freezing point
• List as many stores and flows of water in a drainage basin as you can.

  Precipitation, Condensation, Evapotranspiration, Interception, Vegetation storage, Stem flow, Throughfall, Infiltration, Surface runoff, Surface storage, Channels storage, Channel flow, Discharge, Soil moisture, Throughflow, Interflow, Percolation, Groundwater storage, Groundwater flow, Base flow
• Outline the components of a drainage basin.


  Drainage basins are the area drained by a river, any precipitation that falls outside the watershed (boundary of the system) would be drained by a different river. The river starts at the source, and flows downstream towards the mouth where is meets the sea. There are tributaries which as smaller rivers that join the main channel (if there is impermeable rock below there will be a higher drainage density which means more tributaries). Where the tributaries meet the main channel, it is called a confluence.
• Water balance


  Also known as water budget or soil moisture budget, the amount of water stored in the soil
• Calculating the water balance

  Precipitation = evapotranspiration + discharge (Q2) + change in stores
• Precipitation higher than evapotranspiration and discharge (Q2)


  Positive water balance, can lead to a flood
• Evapotranspiration and discharge (Q2) higher than precipitation


  Negative water balance, can lead to a drought
• Water balance in the UK

  1. Starts in surplus in January

  2. Moves to utilisation in spring

  3. Possibly enters deficit in summer

  4. Reaches field capacity in Autumn

  5. Then in surplus again
• What is the river regime? And what controls it?

  The river regime is the annual variation in river discharge and is controlled by human and natural factors. The natural factors are size, shape and slope of the drainage basin, Geology, vegetation type and density, antecedent and prolonged rainfall, snowmelt and climate. The human factors are deforestation and urbanisation.
• Components of a hydrograph

  1. Rising limb

  2. Peak discharge

  3. Falling limb

  4. Lag time
• Hydrograph


  Shows river discharge following a storm event