2.1.1

• components of water cycle

  • A water flow entering a store is called an input
  • Flows leaving stores are called outputs
  • Water that is being held in a location is a store
  • If water is moving between stores it is called a flow/ transfer
• Global water cycle:

  • The water cycle is a closed system (water is not lost or gained)

  At the global scale, there is a fixed amount of water in the earth-atmosphere system, amounting to about 1,385 million cubic kilometres in volume.
• Mass balance means that the total amount of water stays the same, but where it is stored changes. 

  This water is either held in one of several stores or is being transferred between them via a series of flows operating over varying timescales.
• local scale, such as a drainage basin, it is an open system. Precipitation is an input and runoff to the oceans is an output. 

  There ar
• Positive feedback occurs within a system where a change causes a further, or snowball, effect, continuing or even accelerating the original change, e.g. sea level change due to human induced climate change.
• Negative feedback is when a system acts by lessening the effect of the original change and ultimately reversing it, e.g. eroded sediment after a storm is removed off the beach through longshore drift. 
• Open systems tend to balance inputs and output through flows to have a steady state system, known as dynamic equilibrium. 

  This kind of adjustment is called self-regulation, and much of physical geography can be understood in part as the study of self-regulating systems, e.g beaches have when their summer and winter profiles balance each other out.
• The 4 main stores in the water cycle

  Water is constantly being recycled, stored and transferred between the lithosphere (soil and rocks), hydrosphere (rivers, seas, oceans, lakes), cryosphere (ice sheets, glaciers, ice caps and sea ice) and the atmosphere (water vapour and clouds).
• Distribution, size and characteristics of major stores of water 

  • Most of the earth’s water is saline water in the oceans. 
  • Only 2.5% is freshwater, stored as snow and ice sheets and groundwater
  • Surface and other freshwater comprises only 1.2% (of the 2.5%), most freshwater is held in ice and groundwater.
  • Atmospheric water vapour contains only around 0.001% of the Earth’s total water volume!
• Water flows (moves) between these stores at different rates, and remains in storage for different periods of time.  

  The residence time is the time taken for a water molecule to enter and leave a part of the system. These are crude averages, and are affected by a number of factors, such as climate and weather, and differences in drainage basins.
• Change in size of stores over space and time including , sea level change and cryospheric processes (ice accumulation and ablation):

  • The distribution of freshwater around the planet is changing. 
  • Researchers used a pair of satellites called the Gravity Recovery and Climate Experiment, or GRACE, to track freshwater movement over the last fifteen years. 
  • The twin GRACE satellites were launched in 2002 as a joint mission between NASA and the German Aerospace Center (DLR).
• Change in size of stores over space and time including , sea level change and cryospheric processes (ice accumulation and ablation):

  • They found that some regions' water supplies are relatively stable, others experienced increases or decreases. 
  • Climate change has driven freshwater loss from the ice sheets at the poles, which has implications for sea level rise. 
  • Other areas saw groundwater depletions because of humans using water for irrigating crops or increases due to higher amounts of rainfall because of natural variability.
• Arctic sea pack ice

  • Reduced by about 50% during the summer in the northern hemisphere, before reforming each winter
• Recent evidence suggests that these and other cryosphere water stores may actually be experiencing a permanent reduction in size because of anthropogenic (human-induced) climate change
• seasonal changes in the cyrosphere

  Seasonal variations ice accumulation and ablation bring cyclical fluctuations in water flows and storage in mountainous glaciated regions such as the Alps.

  • Ablation and accumulation is steady-state equilibrium (the system maintains a balance) when viewed over the longer term
• long term changes in the cyrosphere storage

  Significant and far longer-lasting changes in the water storage include glacial and interglacial epochs. 

  -These changes occurred because of long-term natural processes, including 3 cycles affecting the Earth’s orbit around the sun, which bring warming and cooling over long periods of time.
• long term changes in the cyrosphere

  First, every 100,000 years, the Earth’s orbit changes from spherical to elliptical, changing the solar input. Second, the Earth's axis is currently tilted at 23.5°, but this changes over a 41,000-year cycle between 22° and 24.5°, also affecting solar input.
• long term changes in the cyrosphere

  Third, the Earth’s axis wobbles, changing over 22,000 years, bringing further climate change. These orbital cycles are called the Milankovitch cycles.
• Seasonal changes in rainfall

  Significant transfers take place over relatively short periods of time on account of meteorological processes associated with the onset of the annual monsoon in east Asia.

  • Example: In Mangalore in southwest India, more than ⅔ of the year’s rainfall falls in just 3 months each year. This means that water transfers from the atmosphere to the land, and from the land to the sea, are seasonally uneven.
• What is El Niño?

  The name 'El Niño' is the warming of sea surface temperature that occurs every few years, typically concentrated in the central-east equatorial Pacific. An El Niño is declared when sea temperatures in the tropical eastern Pacific rise 0.5 °C above the long-term average. El Niño is felt strongly in the tropical eastern Pacific with warmer and wetter than average weather. 
• What is La Niña?

  'La Niña' or "the girl" is the opposite side of the fluctuation, which sees episodes of cooler than average sea surface temperature in the equatorial Pacific. The conditions for declaring 'La Niña' differ between different agencies, but during an event sea temperatures can often fall 3-5 °C below average. Cooler, drier than average weather is experienced in the tropical eastern Pacific.
• Year-to-year climatic rainfall variability

  • The El Nino southern oscillation (ENSO) is a natural climatic phenomenon in the Pacific Basin which recurs every 3-7 years and usually lasts for 18 months.
• Spatial patterns across the globe

  Latitudes close to the poles and tropical oceans, and monsoon regions will see an increase in overall rainfall. 

  However, large parts of subtropical regions will likely see a decrease in rainfall, leading to drier conditions.
• Long-term (future) changes in rainfall and aridity

  Climate change will affect water transfer processes around the world. 

  Global warming is expected to result in changes in precipitation

  • Areas that already have a lot of rain can be expected to experience more
  • Areas that are already arid may suffer lower rainfall. 
  • More heavy, episodic downpours, perhaps punctuated by longer periods of drought. 

  Areas at high risk of increasing aridity include desert fringes, such as the Sahel region. Rainfall has been lower in recent decades