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The hydrological cycle and drainage basins

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The amount of water on Earth stays the same. It just keeps moving between stores in a closed system: the hydrological cycle.

Stores and transfers

Stores hold water: oceans (by far the largest), ice sheets and glaciers, groundwater, lakes, rivers, soil and the atmosphere.
Transfers move water: evaporation, transpiration from plants, condensation, precipitation, infiltration into the soil, percolation into rock, surface runoff, throughflow in the soil and groundwater flow.
It is a closed system: water is not gained or lost from the Earth.

The drainage basin

A drainage basin is the area of land drained by a river and its tributaries.
Watershed: the high ground forming the edge of a basin.
Source: where the river starts. Mouth: where it meets the sea or a lake.
Tributary: a smaller river joining the main one, at a confluence.
Together, the main river and its tributaries form the channel network.

River regimes

A river regime is the pattern of a river's discharge over a year.
It depends on precipitation (amount and timing; storm hydrographs show the response to one storm), temperature (snowmelt and evaporation), vegetation (interception), land use (urban areas increase runoff), water abstraction (taking water out), and dams, which store water and even out the flow.
Worked example

Explain why a river's discharge might be highest in spring in a mountainous region.

  1. Snow builds up on high ground over winter.
  2. As temperatures rise in spring, the snow melts.
  3. Meltwater runs off into the river, increasing discharge.

Answer: Snowmelt in spring adds large amounts of water.

Key idea

The hydrological cycle is a closed system of stores (oceans, ice, groundwater, lakes, soil, atmosphere) and transfers (evaporation, transpiration, condensation, precipitation, infiltration, percolation, runoff, throughflow). A drainage basin is bounded by a watershed. River regimes depend on precipitation, temperature, vegetation, land use, abstraction and dams.

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