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How much water is there?

By James Lombard   (2016-02-21)

When astrobiologists investigate any planet as a possible candidate for harbouring life, the first thing they look for is liquid surface-water. Water is not only essential for survival, but is what life was born in and is mostly made of. Prior to the recent discovery of liquid water on Mars, Earth was the only rocky planet known to contain this vital resource flowing freely on its surface; and not just in trickles: 71% of the Earth's surface is covered in water. But just how much water is there beneath the surface? And how much is suitable for our consumption?

Water availability: Usable vs. Unusable

Earth is aptly called the Blue Planet. Water can be found just about anywhere you choose to look: Above the Earth in the form of vapour; On Earth's surface in the form of rivers, ice caps, oceans and organisms and also inside the Earth, in the top few kilometers of the ground. It may therefore be somewhat of a shock to discover that water only makes up about 0.13% of the total volume of the Earth. This is visualized in the following image, obtained from the U.S. Geological Survey Water Science School website:

Depiction of Earth's water supply: Total amount, fresh water and amount contained in rivers and lakes (source)

Earth's water supply
The largest drop (covering the western half of the U.S.A), contains all the water on the planet, and is roughly 1385 km in diameter compared to the 12 742 km diameter of Earth. Unfortunately, 97.5 % of this water is saline and unsuited for general consumption.

The remaining 2.5 % is fresh water and is depicted by the second largest droplet on the image which is approximately 272 km in diameter. The vast majority of this water is caught in glaciers and ice-caps (68.7 %) and groundwater (30.1 %) and is currently inaccessible to us. The remaining 1.2 % of the fresh water supply is surface water, the majority of which is in the form of ground ice and permafrost (69 %), soil moisture (3.8 %) and vapour in the atmosphere (3.0 %).

If the reader were to squint their eyes, they may see a small blue dot beneath the second largest drop. On the scale of the image, this dot is 56.2 km in diameter and represents the fresh water contained in rivers and lakes, which is the primary source of the water people use daily. This means that currently only about 0.007 % of the total water supply on Earth is safe for consumption and needs to be shared between the entire world population.

Water demand

Apart from a relatively negligible net loss of water as hydrogen escapes into space from the outer atmosphere, the Earth's total water supply remains essentially constant. In contrast, the world's population has tripled within the last 100 years, which means the demand for water is drastically outgrowing the supply. 

New research shows that over 4 billion people currently live in areas of severe water scarcity for at least one month a year, making global water availability a much bigger problem than previously believed. Estimates predict that by 2030 the total demand for water will exceed the available global supply by 40 %.

Current situation in South Africa

South Africa is an arid country in the best of times. At an average of 450 mm rain per year, half of the world's average, South Africa is the 31st driest country on Earth. On top of this, a combination of natural climate variability, global warming and El Nino weather has seen South Africa face the driest season in the last century, with no immediate relief in sight. Apart from these external factors, a rapid increase in water demand and failing infrastructure further intensifies the challenge faced by the country in meeting its water needs.

What can be done?

Various things can be done to alleviate the water crisis, but it starts with realizing that we are a species on a water-scarce planet and citizens of a water-scarce country.

South Africa's water use is split largely between domestic (27 %) and agricultural (60 %) use. On an agricultural and industrial level, technology and policy should be aimed at making water use more responsible and efficient. On a domestic level, a lot can be done by simply being more mindful of the value of municipal water and taking steps to conserve this vital resource in your daily routine.