Global distribution and size of major stores of water in the water cycle

The Major Stores in Detail

Water is stored within four major physical systems:

• the lithosphere (land)
• the hydrosphere (liquid water)
• the cryosphere (frozen water, e.g., snow and ice)
• the atmosphere (air)

1. Hydrosphere

The hydrosphere includes all liquid water found on the Earth’s surface.

• The oceans store over 96% of all water on Earth, though it is saline and not directly usable for drinking or irrigation.
• Ocean currents transfer heat energy, influencing global climate systems.
• Surface water (rivers, lakes, wetlands) represents a tiny proportion of total water but is crucial for ecosystems, agriculture, and human use.

2. Cryosphere

The cryosphere includes all frozen water on Earth.

• Stores nearly three-quarters of the planet’s freshwater, mainly in the Antarctic and Greenland ice sheets.
• Seasonal snow and sea ice have shorter residence times, while continental ice sheets can hold water for millennia.
• Melting in the cryosphere affects sea levels, ocean salinity, and global albedo (reflectivity), influencing climate feedback loops.

3. Lithosphere

The lithosphere contains two key types of water stores:

• Groundwater – water stored in the pore spaces of permeable rocks. This is the largest accessible freshwater source for humans.
• Soil moisture – vital for vegetation and local hydrological processes.

Aquifers

Groundwater is stored within aquifers, which are layers of rock that can hold and transmit water.

• Unconfined aquifers are open to the surface and recharged directly by rainfall.
• Confined aquifers lie beneath impermeable rock layers and are recharged slowly from distant sources.
• Rocks such as sandstone and chalk are both porous (can store water) and permeable (allow water to flow through).
• Deep confined aquifers can hold water for thousands of years, while shallow aquifers respond more rapidly to rainfall and drought.

4. Atmosphere

The atmosphere holds the smallest share of Earth’s water, yet it plays a vital role in the global water cycle.

• Most of this water is in the form of water vapour, with a small amount as liquid droplets in clouds.
• The amount of atmospheric water varies with temperature — warmer air holds more moisture.
• The atmospheric store is responsible for precipitation, condensation, and latent heat transfer, driving global weather systems

Global Patterns

• Most water is stored in the oceans and is saline.
• Freshwater is unevenly distributed — concentrated in polar regions and a few large aquifers.
• Accessible freshwater (in rivers, lakes, and the atmosphere) represents less than 1% of total freshwater.
• Latitude and altitude affect storage: cryospheric stores dominate in high latitudes and mountains, while tropical regions have higher atmospheric moisture.

Residence Time and Accessibility

• Long residence times (e.g. deep groundwater, ice sheets, oceans) mean water moves slowly, and these stores act as long-term regulators of climate.
• Short residence times (e.g. atmosphere, rivers) indicate rapid transfer, making them more sensitive to short-term change such as drought or rainfall events.
• Only a small fraction of global freshwater is easily accessible for human use, emphasising the importance of sustainable management.

Summary

• The global water cycle operates as a closed system, but the major water stores function as open subsystems.
• The hydrosphere dominates in volume, while the cryosphere and groundwater hold most of the freshwater.
• Atmospheric water, although tiny in volume, plays a key role in driving climate and weather.
• The distribution and residence time of water across these stores determine how the planet’s hydrological system functions.