The hydrological cycle as a closed system

The hydrological cycle describes the continuous movement of water on, above, and below the Earth's surface. It operates as a closed system, meaning it has no external inputs or outputs of water – all water is recycled within the system.

Components of systems in the hydrological cycle

Systems like the hydrological cycle consist of several key elements that help explain how water moves and is stored.

Main components:

• Stores - Locations where water accumulates, such as oceans or the atmosphere.
• Flows (or transfers) - Movements of water between stores, for example, through evaporation or precipitation.
• Inputs - Additions of water or energy into the system, though in a closed system like this, there are no external water inputs.
• Outputs - Removals of water or energy from the system, but again, no water leaves the global cycle.

Water in the cycle can change states between solid (ice), liquid (water), and gas (water vapour), driven by energy gains or losses. For instance, energy is gained during evaporation (liquid to gas) and lost during condensation (gas to liquid).

Driving forces of the hydrological cycle

The hydrological cycle relies on natural energy sources to keep water moving between stores. These forces ensure water is evaporated, transported, and returned to the Earth.

Primary energy sources

• Solar energy - Provides heat that causes water to evaporate from surfaces like oceans, lakes, and rivers, turning it into water vapour in the atmosphere.
• Gravitational potential energy - Pulls water downhill, enabling flows such as surface runoff, throughflow in soil, and the descent of precipitation from the sky.

These energies work together to maintain the cycle's balance, with solar energy lifting water upwards and gravity bringing it back down.

Major water stores and their distribution

The hydrosphere encompasses all water on Earth, totalling about 1.4 sextillion litres (1.4 followed by 21 zeros). Most of this is saline (salty) water, with less than 3% being fresh water essential for most species, including humans.

Distribution of water in stores

Water is held in various stores, with quantities varying from global to local scales depending on flows between them.

Percentage of Earth's water in each store:

• Oceans - 96.5%, mostly saline and the largest store; around 430,000 km³ evaporates annually from here, contributing to 86% of global precipitation.
• Cryosphere (ice caps, glaciers, and snow) - 1.7%, storing frozen water.
• Groundwater (water held underground in rocks and soil) - 1.7%.
• Surface water (lakes, rivers, and other bodies) - 0.01%.
• Atmosphere (as water vapour and clouds) - 0.001%.
• Biosphere (in plants, animals, and soil moisture) - 0.0001%.

Over long timescales, store sizes change. For example, since the last glacial maximum (around 21,000 years ago), warming has melted glaciers, shrinking the cryosphere while enlarging oceans.

Availability of water for human use

Not all water in the hydrological cycle is accessible or suitable for human needs. The global water budget tracks how much water is stored, how it flows, and its residence time (the average time a water molecule stays in a store).

Factors limiting water availability

• Physical and economic access - Some water, like deep groundwater, is hard or costly to extract, making it impractical for use.
• Residence time and pollution - Shorter residence times often mean cleaner water with fewer pollutants, ideal for humans. Long residence times can lead to non-renewable water, which is not replenished quickly.
• Non-renewable water - Examples include ancient ice in the cryosphere or fossil water (old, trapped underground groundwater), which humans cannot rely on long-term.

Only a tiny fraction of Earth's water is both fresh and accessible, highlighting the need for careful management.

Key fluxes between water stores and their variations

Fluxes are the movements of water between stores, varying by location, season, and over longer periods. They include processes like evaporation and precipitation, influenced by factors such as temperature and solar radiation.

Main fluxes in the hydrological cycle

Evaporation:

Evaporation happens when liquid water turns into water vapour (gas) by gaining energy, usually from solar radiation, increasing atmospheric water storage. It varies with solar input, water availability, and air conditions – high in warm, dry areas with ample water, low in cool, saturated air.

Condensation:

Condensation occurs when water vapour cools to its dew point (the temperature at which it becomes liquid), losing energy. This can form dew or clouds, reducing atmospheric water. It depends on water vapour levels and temperature drops, such as at night when heat radiates to space.

Precipitation:

Precipitation is water falling from the atmosphere as rain, snow, or hail, and is the primary flow to the ground. Clouds form when condensed droplets gather around cloud condensation nuclei (tiny particles like dust). Precipitation varies seasonally (e.g., more in UK winters) and spatially (higher in the tropics than at the poles).

Cryospheric processes:

These involve accumulation (build-up of snow and ice) and ablation (melting). They alter cryosphere storage based on temperature – more accumulation in cold periods, more ablation in warm ones. Variations occur over millennia (glacial periods) or annually (winter snow vs. summer melt). Climate change is currently reducing cryosphere size.

Importance and scale of fluxes

The largest flux is ocean evaporation to the atmosphere, driving most of the cycle. Smaller fluxes include evaporation from vegetation and surfaces. Fluxes vary temporally (e.g., seasonal changes) and spatially (e.g., higher evaporation in sunny regions), affecting store sizes globally.