2.1.2

• drainage basin

  • Drainage basin is an area of land that is drained by a river and its tributaries. 
  • The edge of a river basin is marked by a boundary called the watershed. 
  • Drainage basins vary enormously in size, from small local basins to major river systems Mississippi and Amazon. 

  Drainage basins are open systems (energy and matter can be brought into the system).
• Evaporation – the transformation of water droplets into water vapour by heating.

  Transpiration is the transfer of water vapour from vegetation to the atmosphere.  

  • This occurs mostly via pores in leaves known as stomata 
  • Rates of transpiration vary between seasons
• Evapotranspiration – the loss of water from a drainage basin into the atmosphere from the leaves of plants + loss from evaporation.
• • Stemflow water trickles along twigs and branches and finally down the main trunk
• Throughfall is when water falls through gaps in the vegetation onto the ground, or from leaves or twigs
• Interception is where water is held on the leaves/ surface of the plant.

  Interception is a temporary store!
• Below-ground flows

  Water flows down through the soil due to gravity, then into the bedrock below.

  Infiltration - water soaks into/ is absorbed by the soil. 

  • The infiltration capacity is the max rate at which rain can be absorbed in a given condition. 

  It decreases with time through the period of rainfall, until more or less a constant value is reached.
• saturation excess overland flow

  If it rains for a long time

  • The soil becomes saturated (antecedent conditions) and throughflow (water moving through the soil) is deflected closer and closer to the surface. 
  • The upper and more permeable soil horizons become saturated as the water level in the soil rises. 
  • In time the entire soil becomes saturated right up to the surface.
• inflitration excess overland flow

  • When the rainfall intensity is so great that not all the water can infiltrate (irrespective of how dry or wet the ground is prior to the rainfall event). 
  • Common in semi-arid areas where rain encounters hard baked ground. IEOF can lead to flash flooding. Less common in the UK, however can occur during the summer. 

  In areas of high ppt intensity and low infiltration capacity overland runoff is common.
• Throughflow is the movement of the water sideways through the soil store, via pore spaces and pipes (wide gaps within the soil created by roots and animal burrows). 

  • These are relatively slow, apart from pipe flows. 
  • Throughflow is most effective in the top layer of the soil, where there are more pores, and the soil structure is open. 
  • Lower in the soil the layers are more compacted.
• percolation

  Water moves through the soil to the rock below. 

  • This depends on the permeability* of the rock. 
  • As it fills, this will generate movements of its own, this is called groundwater flow. 
  • This maintains the base flow of rivers. 
  • Below a certain depth, the bedrock of a drainage basin may be permanently saturated. This level below which the ground is saturated is called the water table.
  • Interflow is the water flowing downhill through permeable rock above the water table
• Drainage basin stores

  • Surface storage – the total volume of water held on the Earth’s surface in lakes, ponds and puddles.
  • Groundwater storage – the storage of water underground in permeable rock strata.
  • Channel storage -the water held in a river or stream channel.
• transpiration is the transfer of water vapour from vegetation to the atmosphere, occur via pores in leaves known as stomata 10% atmospheric water orginates from this process
• condensation involves water vapour returning to liquid state. when water vapour reaches dew point which is the temperature at which the air becomes saturated water droplets and clouds form
• precipitation is any form of water that falls from the atmosphere returning to land and oceans. in high latitudes usually falls as snow
• vegetation storage is water taken up by vegetation. it is all the moisture in the vegetation store at one time
• surface run off is the visible movement of water over a ground surface it can happen because there is an excess of rainfall and due to infilitration reaching capacity so does soil storage capacity
• channel storage is the water held in river stream or channel
• channel flow is the movement of water within river channel
• discharge river is the volume of water flowing in the river channel
• ground water storage is the volume of water store in the ground
• ground water flow is when water reaches an impermeable bedrock below ground and flows over the surface of the rock
• base flow is the water that reaches the channel through slow throughflow and permeable rock below the water table. storm water enters the drainage basin and the discharge rate increases
• interflow is water flowing downhill through permeable rock above the water table
• water table is the upper level of an underground surface in which the soil and rocks are permantely saturated with water table. seperates the ground water zone. water table fluctuates both seasons and yearly as affected by climatic variations by the amount of precipitation used by vegetation
• water surplus is the excess water available to the system occuring when precipitation exceeds evapotranspiration and excess not being used by vegetation
• soil moisture utilisation is the time which water is withdrawn from soil moisture storage happens when potential evapotranspiration exceeds precipitation
• soil mositure re
• soil moisture recharge when water is added to soil moisture storage. recharge period occurs when precipitation exceeds potential evapotranspiration soil reaches field capacity
• field capacity is the amount of water saturated in soil able to hold for 2-3 days
• potential evapotranspiration is the amount of potential evaporation from soil and transpiration from plants. only occurs when water from the process is not limiting