Earth’s Life Support Systems

The Characteristics of the Main Inputs and Outputs of the Water Cycle

The global water cycle represents the continuous circulation of water between the atmosphere, land, oceans, and cryosphere.
Each year, around 505,000 km³ of water is transferred between these stores through a range of inputs and outputs, which together form the basis of the water cycle budget.

Understanding these exchanges helps geographers analyse how the hydrological system operates at global, regional, and local scales.

Good to know

Although the global water cycle is a closed system (no overall gain or loss of water to space),  the terms “inputs” and “outputs” are used to describe the movement of water into and out of individual stores within this system, such as the atmosphere.

At the global scale, there are only energy inputs and outputs (mainly from the Sun and radiated heat). However, within the system, subsystems such as the atmosphere, cryosphere, and drainage basins operate as open systems with water entering and leaving through processes such as:

  • Inputs: Evaporation and transpiration (adding water vapour to the atmosphere)
  • Outputs: Precipitation and ablation (removing water from the atmosphere)

1. The Water Cycle Budget

The water cycle budget describes the total inputs, outputs, and storage of water within the global system.
Although the global cycle operates as a closed system (no water is lost or gained overall), each individual store acts as an open subsystem, with water constantly entering and leaving.

  • Inputs add water to a store.
  • Outputs remove water from a store.

For example:

  • Evaporation is an output from oceans and lakes, but an input to the atmosphere.
  • Precipitation is an output from the atmosphere but an input to the land and ocean stores.

2. Inputs to the Atmosphere

The atmosphere gains water mainly through evaporation and transpiration, which together form evapotranspiration.

Evaporation

  • Water changes from a liquid to a vapour due to solar heating.
  • Occurs over oceans, lakes, rivers, and soils.
  • Accounts for the majority of the water vapour entering the atmosphere — around 85% originates from the oceans.

Transpiration

  • Water is drawn up by plant roots and released through stomata in leaves.
  • Rates depend on vegetation cover, temperature, and soil moisture.
  • Links the biosphere and atmosphere, particularly in forested regions.

Evapotranspiration

  • Refers to the combined movement of water vapour from land and plants into the atmosphere.
  • Plays a crucial role in maintaining the atmospheric moisture supply and driving cloud formation.

3. Outputs from the Atmosphere

Moisture leaves the atmosphere mainly through precipitation and condensation.

Precipitation

  • Any form of water,  liquid or solid, that falls from the atmosphere to the Earth’s surface.
  • Includes rain, snow, hail, and sleet.
  • Adds water to both terrestrial and oceanic stores and replenishes soil and groundwater.

Condensation

  • Occurs when water vapour cools and changes into liquid droplets or ice crystals.
  • Produces fog, dew, and clouds, releasing latent heat that fuels atmospheric circulation.
  • Acts as an output from atmospheric vapour and a precursor to precipitation.

4. Outputs from the Cryosphere

Ablation

  • The loss of water from snow and ice stores through melting, sublimation, or calving.
  • Returns freshwater to rivers, lakes, and oceans.
  • Most significant during the spring and summer months and strongly influenced by global temperature increases.
  • Contributes to sea level rise and changes in oceanic circulation.

5. Outputs from the Land

Once precipitation reaches the land surface, it may either flow across the surface or enter the soil and rock beneath.

Run-off

  • Water moving over the land surface into streams and rivers, eventually reaching the ocean.
  • Transfers water from terrestrial to oceanic stores.
  • Global river discharge is estimated at around 40,000 km³ per year.
  • In some inland basins (e.g. Central Asia or the southwestern USA), run-off drains into closed depressions rather than the sea.

Groundwater Flow

  • Water infiltrates soils and percolates through permeable rock layers into aquifers.
  • This water may later re-emerge at the surface as springs or seepages, contributing to river flow and maintaining discharge during dry periods.

6. The Water Balance

At the drainage basin scale, the long-term relationship between inputs and outputs is known as the water balance (or water budget).
It can be expressed as:

P=Q+E±S

Where:

P = Precipitation
Q = Run-off (measured as river discharge)
E = Evapotranspiration
S = Change in storage

Interpreting the Water Balance

  • A positive water balance (surplus) occurs when precipitation exceeds evapotranspiration and run-off, often leading to wetter soils, higher groundwater levels, and increased river discharge.
  • A negative water balance (deficit) occurs when evapotranspiration and run-off exceed precipitation, causing soil moisture depletion and reduced flow.

Seasonal Variation in Water Balance

  • Winter: High precipitation and low evapotranspiration often lead to a surplus and increased run-off.
  • Summer: High temperatures and water use by vegetation result in deficits and declining soil moisture.
  • Autumn: Increased rainfall recharges soil and groundwater stores following summer depletion.

Understanding water balance helps hydrologists predict flood risk, drought periods, and the sustainability of water resources within a drainage basin.