UPSC Prelims 2021 Q62: Distribution of Earth's Water — Explained
With reference to the water on the planet Earth, consider the following statements:
1. The amount of water in the rivers and lakes is more than the amount of groundwater.
2. The amount of water in polar ice caps and glaciers is more than the amount of groundwater.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Answer: (b) 2 only
Statement-wise explanation
- 1: incorrect — Rivers and lakes hold very little of Earth's water. USGS data (Shiklomanov, in Gleick 1993) give fresh lakes about 91,000 km³ and rivers about 2,120 km³, together roughly 0.26% of all freshwater. Fresh groundwater alone is about 10.5 million km³, about 30.1% of freshwater, which is over 100 times the water in rivers and lakes.
- 2: correct — Ice caps, glaciers and permanent snow hold about 24.06 million km³. That is about 68.7% of Earth's freshwater, the largest freshwater store. Fresh groundwater is about 10.5 million km³ (30.1%). Even if saline groundwater is added, total groundwater is about 23.4 million km³, still less than the ice.
How to eliminate
You cannot get this one from wording tricks. Neither statement uses an absolute word, and all four options are possible. You need the share of freshwater in each store. Remember this order: ice caps and glaciers about 69%, groundwater about 30%, surface water (lakes, rivers, swamps) about 1%. Statement 1 tests a common mistake. Rivers and lakes are what we see, so they seem large, but they hold only a tiny part of freshwater. If you know only that surface water is the smallest store, you can drop Statement 1 and choose between '2 only' and 'Neither'. Knowing that glaciers are the largest freshwater store settles it.
Know more
Where Earth's water is stored
About 96.5% of Earth's water is in the oceans and is salty. Only about 2.5% is freshwater, and most of that cannot be used easily.
| Store | Volume (km³) | % of all water | % of freshwater |
|---|---|---|---|
| Oceans, seas, bays | ~1,338 million | ~96.5 | — |
| Ice caps, glaciers, permanent snow | 24.06 million | 1.74 | ~68.7 |
| Groundwater (fresh) | 10.53 million | 0.76 | ~30.1 |
| Groundwater (saline) | 12.87 million | 0.93 | — |
| Fresh lakes | 91,000 | 0.007 | 0.26 |
| Rivers | 2,120 | 0.0002 | 0.006 |
Source: USGS Water Science School, from Shiklomanov in Gleick (ed.), Water in Crisis, 1993.
Key points
- Antarctica and Greenland hold most of the ice. Freshwater here is locked away and not usable.
- Groundwater is the largest store of liquid freshwater, roughly 99% of it. This is why aquifers matter so much for drinking water and irrigation. In India, groundwater supplies most irrigation and rural drinking water.
- Rivers hold the least water of any store, but they move water the fastest. A drop stays in a river for days to weeks, in groundwater for centuries, and in ice sheets for thousands of years.
- Usable freshwater from rivers, lakes, wetlands, soil moisture and shallow groundwater is less than 1% of all freshwater.
- Do not mix area and volume. Water covers about 71% of Earth's surface (area). The 2.5% figure is a share of volume.
Possible UPSC angles: the order of stores by volume, residence time in each store, the share of the atmosphere (about 0.001% of all water), and India's share of world freshwater (about 4%, with about 17–18% of world population).
NCERT Concepts to Revise
- Distribution of the Earth's water: The question asks how Earth's water is split between stores such as rivers, lakes, groundwater and ice.
- Groundwater: Both statements compare another store against groundwater, so you need to know what groundwater is and where it sits.
- Hydrological cycle: The hydrological cycle links every store named in the question: rivers, lakes, ground and ice.
- Distribution of land and water: This entry gives water's share of Earth's surface area, which is easy to confuse with the volume figures tested here.