Where Earth's Water Is Found

Introduction

From space, Earth looks like a blue planet, and it holds roughly 1.4 billion cubic kilometres of water. Yet many places struggle to find enough water to drink. The reason is that almost all of this water is salty, frozen or hidden deep underground. Knowing where Earth's water is kept, and how much of it is available to us, is the starting point for understanding the water cycle, water supply and water pollution.

Core explanation

The main stores. Scientists divide Earth's water into several stores . Approximate shares of the total are:

Store Share of all water --- --- Oceans and seas about 97% Ice caps and glaciers about 2% Groundwater about 0.7% Lakes and rivers about 0.01% Soil moisture about 0.005% Atmosphere (vapour and clouds) about 0.001% Living things about 0.0001%

Salt water versus fresh water. The oceans contain on average about 35 g of dissolved salts per kilogram of seawater, mainly sodium chloride. This water cannot be drunk or used for most farming without treatment. Only about 3% of all water is fresh, and roughly two-thirds of that is locked up as ice in Antarctica, Greenland and mountain glaciers.

Groundwater. Most of the liquid fresh water is underground. Rain soaks into the soil and down into rocks such as sandstone and chalk, which have tiny spaces (pores) and cracks. A layer of porous rock that holds useful amounts of water is called an aquifer . Wells and boreholes draw water from aquifers, and in many countries they supply a large share of drinking water.

Surface water. Lakes and rivers hold a tiny fraction of the total but are the most accessible fresh water, so they are heavily used for drinking, farming and industry.

Water in the air. At any moment the atmosphere holds only about 0.001% of Earth's water as vapour and cloud droplets. Even so, this small store moves enormous amounts of water around the planet every day.

Residence time. Water moves between stores at very different rates. A water molecule may stay in the ocean for thousands of years, in deep groundwater for up to many thousands of years, in an ice sheet for hundreds of thousands of years, but in the atmosphere for only about nine days. Small, fast-moving stores are quickly renewed; large, slow ones are not.

Step-by-step reasoning

To estimate how much of Earth's water is liquid fresh water:

1. About 97% is salt water, so about 3% is fresh. 2. Roughly two-thirds of the fresh water is ice. 3. That leaves about one-third of 3%, or around 1%, as liquid fresh water. 4. Most of that is groundwater, so surface water is a very small fraction.

Visual explanation

Imagine all Earth's water poured into a 100-litre bath. About 97 litres would be salty. Around 2 litres would be ice cubes, less than a litre would be groundwater hidden in a sponge, and all the lakes and rivers together would fill only about a tablespoon.

Real-world analogy

Earth's water is like the money a family owns. Most is locked in a pension fund they cannot touch (the oceans and ice). A little is in a savings account that needs effort to reach (groundwater), and only a small amount is cash in the wallet, ready to spend (lakes and rivers).

Real-world example

In south-east England, much drinking water is pumped from the chalk aquifer beneath the ground. During long dry spells, groundwater levels fall and water companies may introduce hosepipe bans to protect this store.

Why?

Why can pumping groundwater too quickly cause lasting shortages? Deep groundwater has a long residence time and is refilled slowly by rain soaking in. If it is removed faster than it is replaced, the water table falls and may take decades or longer to recover.

Common misconception

"Because Earth is mostly covered with water, there is plenty of drinking water." About 97% is salty and most fresh water is frozen, so readily available fresh water is only a tiny fraction of the total.

Worked example

Question: Earth holds about 1.4 billion km³ of water. Estimate the volume of fresh water, using the fact that about 3% is fresh.

Reasoning: 3% of 1.4 billion = 0.03 × 1 400 000 000 = 42 000 000 km³.

Answer: About 42 million km³, of which roughly two-thirds is ice.

Quick check

1. Which store contains the largest amount of Earth's water? Answer: The oceans and seas, with about 97%.

Exam focus

Remember the headline figures: about 97% salt water, about 3% fresh water, and most fresh water frozen. Be able to define an aquifer and explain why surface water, although small in amount, is so important to people.

Advanced insight

As the climate warms, water is shifting between stores. Melting glaciers and ice sheets move water into the oceans, adding to sea-level rise, while warmer seawater also expands. Satellites that measure tiny changes in Earth's gravity can now track the loss of ice and groundwater from space.

Summary

Earth's water is stored in oceans (about 97%), ice (about 2%), groundwater, lakes and rivers, soil, the air and living things. Only about 3% is fresh, and most of that is ice or groundwater. Stores differ greatly in residence time, from days in the air to thousands of years in the oceans and deep aquifers.

Practice questions

1. Approximately what percentage of Earth's water is fresh water? Answer: About 3%. 2. Where is most of Earth's fresh water found? Answer: In ice caps and glaciers. 3. What is an aquifer? Answer: A layer of porous rock underground that holds and transmits groundwater. 4. Explain why the atmosphere is an important water store even though it holds very little water. Answer: Water passes through it very quickly (about nine days), so it transports huge amounts of water around the planet as vapour and returns it as precipitation. 5. Why can seawater not be used directly for drinking? Answer: It contains about 35 g of dissolved salts per kilogram, far too much for the human body to cope with.