Water Cycle 1
   System components
   • Stores are natural reservoirs of water, such as the 
    ocean, lakes or ice caps
   • A water flow entering  a store is called an input: for 
    example a river entering the ocean
   • Flows leaving stores are called outputs: for example, 
    evaporation of ocean water
   • Flows are movements (or transfers or fluxes) 
    between stores in a system
   Class activity
   What is the difference between an open system and a closed 
   system? What examples are there of each?
    Water Cycle 1
    The global water cycle
                                    • The global water 
                                      cycle is shown as a 
                                      ‘black box’ model
                                    • What are the 
                                      strengths and 
                                      weaknesses of 
                                      viewing water 
                                      stores and flows in 
                                      this way?
    Class activity
    What systems have you studied in other Geography topics? What 
    are their similarities and differences?
    (c) Hodder Education [OCR A-level Geography (Raw)] 
     Water Cycle 1
     Another view of the water cycle
                                              Class activity
                                              Which illustration of the water 
                  (c) Hodder  Education       cycle do you prefer, and why?
                  [Edexcel A-level Geography Book 2 (Dunn)]
   Water Cycle 1
   The concept of mass balance
   • At a global scale, mass balance means that the total 
    amount of water is always conserved
   • At the global scale, there is a fixed amount of water in the 
    Earth-atmosphere system (about 1385 million cubic 
    kilometres of water) 
   • Transfers can occur; changes occur in where water is stored 
    without affecting the mass balance. For instance, water 
    vapour in the atmosphere is precipitated on to the land as 
    rain or snow after condensation occurs: although it has 
    changed state, all water still remains in the system
   Class activity
   What other water transfer processes are there? 
   Why don’t they affect the system’s mass balance? 
      Water Cycle 1
      Global water stores
     Store             Volume            Per cent of   Distribution and characteristics
                       (cubic km)        total water
     Oceans            1,335,000         97            Oceans cover two-thirds of the Earth
     Cryosphere        26,000            2             Ice and snow stores found at high 
                                                       latitudes (poles) and high altitudes
     Groundwater       15,000            1             Can remain stored for 10,000 years
     River and lakes 178                 0.01          Uneven distribution due to climate
     Soil moisture     122               0.01          Permafrost in Russia and North America
     Atmosphere        13                <0.01         There is sufficient moisture for ten days 
                                                       of rain in the atmosphere
     Biosphere         0.6               <0.01         Distributed unevenly because of climate
      Class activity
      (1) Compare the size of the different stores  (2) What local factors 
      affect water storage in different places?