Hydrological Cycle

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Drainage Basins

The global hydrological cycle is a closed system. Drainage basins are open systems.

  • Drainage basin (river's catchment) = the area surrounding the river where the rain falling on to the land flows into that river.
  • Watershed = boundary of a drainage basin. Any precipitation falling beyond the watershed enters a different drainage basin.
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Inputs

  • Precipitation: moisture comes out of the atmosphere
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Storage

  • Interception: precipitation lands on vegetation or other structures before it reaches the soil. Interception storage is only temporary because the collected water evaporates quickly.
  • Vegetation storage: water that has been taken up by plants. It is all the water contained in plants at any one time.
  • Surface storage: water in puddles (depression storage), ponds and lakes
  • Groundwater storage: water is stored in the ground, either in the soil (soil moisture) or rocks. The water table is the top surface of the zone of saturattion - the zone of soil or rock where all the pores in the soil are rock are full of water. Porous rocks (lots of holes) that hold water are called aquifiers.
  • Channel storage: water held in a river or stream channel.
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Transfers

  • Surface runoff (overland flow): water flowing over the land. It is common in arid areas where torrential rain falls on hard baked land.
  • Throughfall: water dripping from one leaf (or other plant part) to another.
  • Stemflow: water running down a plant stem or tree trunk.
  • Throughflow: water mooving slowly downhill through the soil. It is faster through 'pipes' - little cracks in the soil or animal burrows.
  • Infiltration: water soaking into the soil. It is influenced by soil type, soil structure and how much water is already in the soil.
  • Percolation: water seeping down through soil in the water table.
  • Groundwater flow: water flowing slowing below the water table through permeable rock. Rocks that are highly permeable with lots of joints (gaps that water can get through) have faster groundwater flow, e.g. limestone.
  • Baseflow: groundwater flow that feeds into river through river banks and river beds.
  • Interflow: water flowing downhill through permeable rock above the water table.
  • Channel flow (river discharge): water flowing in the river or stream.
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Outputs

  • Evaporation: water turning into water vapour (liquid to gas)
  • Transpirarion: evaporation from plant leaves
  • Evapotranspiration: process of evaporation and transpiration together.

Potential evapotranspiration (PET) is the amount of water that could be lost by evapotranspiration. Actual evapotranspiration is what actually happens.

Example: in a desert, potential evapotranspiration is high because heat increases evaporation, but actual evapotranspiration is low because there isn't that much moisture in the first place.

  • River discharge
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Water Balance

Water balance is worked out from inputs and outputs. The water balance affects how much water is stored in the basin.

The general water balance in the UK shows seasonal patterns:

  • In wet seasons, precipitation exceeds evapotranspiration. This creates a water surplus. The ground stores fill with water so there's more surface runoff and higher discharge, so river levels rise.
  • In drier seasons, precipitation is lower than evapotranspiration. Ground stores are depleted as some water is used (e.g. by plants) and some flows into the river channel, but isn't replaced by precipitation.
  • At the end of a dry season, there's a deficit (shortage) of water in the ground. The ground stores are recharged in the next wet season.
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