Earth’s Life Support Systems
The Distribution and Size of the Major Stores in the Water Cycle
Water is essential for life on Earth and exists in a variety of stores (or reservoirs) across the atmosphere, oceans, land, and cryosphere. The size and distribution of these stores vary, influencing global climate, weather systems, and the availability of freshwater.
1. Overview
The global water cycle operates as a closed system, meaning the total volume of water remains constant. However, at smaller (local or regional) scales, water moves between stores in open systems.
The distribution of water across different stores determines how much is available for ecosystems, human use, and climatic processes.
2. The Major Global Water Stores
| Store | Approximate size (as % of total global water) | Type | Key Features |
|---|---|---|---|
| Oceans | ~96.5% | Saline | The largest store of water on Earth. Saline water that regulates global temperature and climate through heat transfer. |
| Cryosphere (ice caps, glaciers, permafrost) | ~1.7% | Freshwater (solid) | Stores freshwater as ice, mainly in Antarctica and Greenland. Sensitive to climate change and a key regulator of sea level. |
| Groundwater | ~1.7% | Freshwater (liquid) | Includes deep groundwater and soil moisture. A vital source of freshwater for ecosystems and human use. |
| Surface water (rivers, lakes) | ~0.013% | Freshwater (liquid) | Small but dynamic stores that are crucial for human settlement, agriculture, and biodiversity. |
| Atmosphere | ~0.001% | Freshwater (vapour and liquid) | Water vapour and clouds that drive weather systems and transfer latent heat. Despite its small volume, this store is vital for energy balance. |
| Biosphere | Trace amount | Freshwater (within organisms) | Water contained within living organisms; essential for photosynthesis and metabolic processes. |
Key point:
Although atmospheric and surface stores contain only a tiny fraction of global water, they play a disproportionately large role in transferring energy and sustaining ecosystems.
3. The Lithosphere (Terrestrial Water Stores)
- The lithosphere includes all terrestrial stores of water, such as rivers, lakes, soil moisture, and groundwater.
- Although these stores account for only a small proportion of the Earth’s total water, they are vital for ecosystems, agriculture, and human use.
Key facts:
- Surface freshwater accounts for around 2.5% of all water on Earth.
- It is found in glaciers, lakes, reservoirs, wetlands, rivers, and streams.
20% of the world’s surface freshwater is stored in Lake Baikal (Russia), and another 20% in the Great Lakes of North America. - Only 0.006% of the world’s freshwater is held in rivers.
- Approximately 30% of global freshwater is stored underground in aquifers, forming groundwater reserves that are crucial for drinking water and irrigation.
Importance of terrestrial water stores:
- Rivers and lakes act as active, short-term stores linking land to the ocean.
- Soil moisture supports plant growth and regulates infiltration and runoff.
- Groundwater provides a stable, long-term freshwater source and buffers seasonal rainfall variability.
- Together, these terrestrial stores link the atmosphere, biosphere, and hydrosphere, maintaining local and regional hydrological balance.
4. The Cryosphere
The cryosphere plays a crucial role in regulating global temperatures and sea level.
- About 68.7% of the world’s freshwater is locked in ice caps, glaciers, and permafrost.
- Around 90% of this ice is found in Antarctica, and 10% in Greenland.
- Seasonal snow cover and permafrost also temporarily store significant amounts of water.
- Melting of cryospheric stores contributes to rising sea levels and alters global ocean currents, influencing the climate system.
5. The Atmosphere
- The atmosphere acts as a rapid-cycling store, containing around 12,900 km³ of water at any one time.
- Water vapour remains in the atmosphere for an average of 10 days before condensing and falling as precipitation.
- Despite its small volume, the atmospheric store controls weather patterns and transfers latent heat, influencing global energy balance.
6. The Distribution of Water Stores
The distribution of water varies spatially and temporally due to climate, latitude, and geology.
1. Latitudinal variation
- Tropical regions: High rainfall, dense vegetation, and rapid cycling of water between the land and atmosphere.
- Polar regions: Vast stores of frozen water in ice sheets and permafrost.
- Arid regions: Minimal surface water; groundwater is often the main freshwater source.
2. Climate influence
- Temperature affects the state (solid, liquid, gas) and movement of water between stores.
- Evaporation is greatest in hot, dry regions, while accumulation dominates in cold climates.
3. Geology
- Permeable rocks, such as limestone and sandstone, allow greater infiltration and groundwater storage.
- Impermeable rocks limit infiltration, increasing surface runoff and reducing subsurface storage.
4. Seasonal variation
- In temperate regions, precipitation, evapotranspiration, and river storage fluctuate through the year.
- In colder regions, snow and ice accumulation in winter is released as meltwater in spring and summer.
The distribution and size of the major water stores determine how water moves through the Earth system and how it supports life. While oceans dominate in volume, smaller and more dynamic stores — such as the cryosphere, atmosphere, and lithosphere are crucial for regulating climate and providing accessible freshwater. Human activity and climate change are now reshaping these stores, altering the balance of the global water cycle.