The Earth is often referred to as the “Blue Planet” because water covers approximately 71% of its surface. However, the distribution of this water is highly skewed, with the vast majority being saline and held in oceanic basins.
Primary Categories of Water Distribution
Water on Earth is categorized into two primary types: Saline water and Freshwater.
| Water Type | Percentage of Total Water | Key Reservoirs |
| Saline Water | 97.2% | Oceans, Seas, and Saline Lakes |
| Freshwater | 2.8% | Glaciers, Groundwater, Surface Water, Atmosphere |
Breakdown of Freshwater Resources
Although 2.8% of Earth’s water is fresh, only a tiny fraction of this is easily accessible for human consumption. Most is locked in ice or buried deep underground.
Glaciers and Ice Caps
- Percentage of Freshwater: ~68.7%
- Significance: This is the largest reservoir of freshwater on Earth. Most of this is located in the Antarctic and Greenland ice sheets. Due to their solid state and remote locations, these resources are largely inaccessible for direct use.
Groundwater
- Percentage of Freshwater: ~30.1%
- Significance: Groundwater represents the largest source of liquid freshwater. It is the primary source for irrigation and drinking water in many parts of the world, including India. Over-extraction in regions like the Indo-Gangetic plain is a major environmental concern.
Surface Water and Other Freshwater
- Percentage of Freshwater: ~1.2%
- Composition: This category includes water in lakes, rivers, swamps, and the atmosphere.
Detailed Distribution of Surface Freshwater
The 1.2% of surface freshwater is further divided into various sub-reservoirs. This distribution is critical for biological life and human civilization.
| Reservoir | Percentage of Surface Freshwater |
| Ground Ice and Permafrost | 69.0% |
| Lakes (Fresh) | 20.9% |
| Soil Moisture | 3.8% |
| Atmosphere | 3.0% |
| Swamps and Marshes | 2.6% |
| Rivers | 0.49% |
| Biological Water | 0.26% |
Spatial Distribution and Variability
Water distribution is not only uneven by “reservoir” but also geographically and temporally.
Latitudinal Variation
- Equatorial Regions: High precipitation leads to a surplus of surface water (e.g., Amazon and Congo basins).
- Subtropical High-Pressure Belts: Areas around 20° to 30° latitude experience high evaporation and low rainfall, resulting in major deserts and water scarcity.
Temporal Variation
- In monsoonal climates like India, approximately 75% to 80% of annual precipitation occurs within four months (June to September). This creates a cycle of seasonal abundance and scarcity, necessitating large-scale storage solutions like dams and reservoirs.
Critical Facts for Prelims
- River Volume: Despite being the most used source for human civilization, rivers hold only 0.0001% of the Earth’s total water and about 0.49% of its surface freshwater.
- Atmospheric Water: The atmosphere contains more water than all the rivers on Earth combined, yet it only accounts for 0.001% of total global water.
- Saline Lakes: Some inland lakes, such as the Caspian Sea or the Great Salt Lake, contain more water than all the freshwater lakes combined, but their salinity renders them unusable for most human needs.
- The “Water Tower” of Asia: The Hindu Kush Himalayan (HKH) region is often called the “Third Pole” because it contains the largest volume of ice outside the polar regions, feeding ten major river systems in Asia.