The hydrological cycle. Image credit BBC

The hydrological cycle. Image credit BBC

ZIMSEC O Level Geography Notes: The hydrological cycle.

  • Water does not leave or enter the planet earth.
  • It is instead continuously transferred between the atmosphere and the oceans.
  • The system of inputs, storages,flows and outputs is known as the water or hydrological cycle.


  • Inputs – refers to water coming into the system.
  • Precipitation – refers to all forms of moisture that reach the Earth’s surface e.g. rain, snow, sleet and hail.


  • Interception – this is when precipitation lands on buildings, vegetation and concrete before it reaches the soil.
  • Interception storage is only temporary as it is often quickly evaporated.
  • Vegetation storage – this is water taken up by vegetation. It is all the moisture in vegetation at any one time.
  • 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.


  • Flows and Processes – water moving from one place to another.
  • Baseflow – water that reaches the channel largely through slow throughflow and from permeable rock below the water table.
  • Channel flow – the movement of water within the river channel. This is also called a river’s discharge.
  • Groundwater flow – the deeper movement of water through underlying permeable rock strata below the water table. Limestone is highly permeable with lots of joints and can lead to faster groundwater flow.
  • Infiltration – the downward movement of water into the soil surface.
  • Interflow – water flowing downhill through permeable rock above the water table.
  • Percolation – the gravity flow of water within soil.
  • Stemflow – water running down a plant stem or tree trunk.
  • Surface Runoff – the movement of water over the surface of the land, usually when the ground is saturated or frozen or when precipitation is too intense from infiltration to occur.
  • Throughflow– the movement of water downslope within the soil layer. Throughflow is fast through pipes (cracks int he soil or animal burrows).


  • Outputs – water leaving the system.
  • Evaporation – the transformation of water droplets into water vapour by heating.
  • Evapotranspiration – the loss of water from a drainage basin into the atmosphere from the leaves of plants plus loss from evaporation.
  • Transpiration – evaporation from plant leaves.
  • River discharge – the amount of water that passes a given point, in a given amount of time.

To access more topics go to the Geography Notes page.