Water Cycle 2
  The drainage basin system
  • The drainage basin is a subset of the global 
   hydrological cycle
  • It is defined as a catchment area forming part of the 
   Earth's surface area which is drained by a particular 
   stream or river 
  • It is an open system which allows energy and matter 
   to be transferred across its boundary (the watershed 
   or drainage divide) from external areas
  Class activity
  Why is a river drainage basin an open system,   
  whereas the global water cycle is a closed system? 
        Water Cycle 2
        Drainage basin inputs
        • There is only 
             one physical 
             input into a 
             drainage basin: 
             precipitation
        • Precipitation  
             types are 
             varied: they 
             includes rain, 
             snow, sleet                     Class activity
             hail and frost Why do drainage basin inputs vary from 
  (c) Hodder Education [Edexcel       A-     place to place or from time to time?
  level Geography Book 2 (Dunn)] 
  Water Cycle 2
  Precipitation variations
  • The duration and intensity of precipitation both affect 
   how a drainage basin system responds 
  • Very high-intensity rainfall of 50-100 millimetres per 
   hour is rare; it can result in  flash flooding
  • In Boscastle, 185mm rainfall fell in just five hours in 
   2004 (equivalent to two months of normal rainfall) 
  • Low intensity but relatively long duration rainfall is far 
   more common in the UK 
  Class activity
  What processes could explain the link between high-
  intensity rainfall and flash flooding?
    Water Cycle 2
    Drainage basin flows (1)
                                                    Class 
                                                    activity
                                                    How many 
                                                    water flows 
                                                    are shown 
                                                    in this 
                                                    diagram?
                                                  (c) Hodder Education 
                                                  [Edexcel A-level Geography 
                                                  Book 2 (Dunn)] 
    Water Cycle 2
    Drainage basin flows (2)
     •  Rainwater dripping from leaves and branches 
        towards the ground is called throughfall 
     •  Water which flows to the ground via vegetation 
        stems and trunks is stemflow 
     •  Infiltration is the movement of water from the 
        ground surface into the soil
     •  Throughflow is the movement of water laterally 
        (sideways) through the soil, via (1) a matrix of pore 
        spaces, fissures and (2) pipes (animal burrows) 
     Class activity
     Why might drainage basin water flows vary from    
     place to place or from time to time?