Water is a substance that is the source of life. For example, water is needed in all functions in the human body (such as digestion, circulation, respiration, excretion, reproduction). For this reason, the existence of liquid water is being investigated in the search for life outside the Earth.
When viewed from space, our planet appears as a blue sphere. This is because approximately 71% of its surface is covered with water. Liquid water is the most important substance that separates the Earth from other planets in the Solar system. Geological evidence shows that liquid water has existed on Earth for 3.8 billion years.
There are often large differences between the melting and boiling temperatures of a substance. However, water can exist as solid, liquid and gas within a relatively narrow temperature range.
So, is water, the source of life, as abundant as we think on our planet? There is 1,386,000,000 km3 of water on Earth, approximately equal to the volume of a sphere with a diameter of 1,385 kilometers. 96.5% of the water on Earth is in the oceans, and the rest is in the form of water vapor in the atmosphere, oceans, lakes, rivers and glaciers. However, fresh water is needed for the life of living things. Only 2.5% (10,633,450 km3) of the water in the world consists of fresh water.
Water consumption on a global scale is increasing day by day. In addition, changes in climate due to global warming, increased frequency of extreme weather events and pollution of clean water resources lead to the gradual depletion of clean fresh water resources.
One-third of the world’s countries have no or very limited access to water. 18% of the world’s population cannot access clean water. The decrease in water resources leads to negative effects such as migration and epidemics.
The supply of clean fresh water in the world is limited. The reason why this resource is not depleted despite its constant use is that water is recycled on a global scale through a process called the water cycle. Every year, 577,000 km3 of water (more than the water mass in the Black Sea) is circulated in this cycle.
502,800 km3 of this water evaporates from the ocean surface, and 74,200 km3 evaporates from the land. As the evaporated water moves upward in the atmosphere, it cools and condenses into precipitation. Thus, the same amount of water that evaporates reaches the oceans (458,000 km3) and the land (119,000 km3) as precipitation.
The difference between the amount of water moving away from the land surface by evaporation and returning to the land by condensation is 44,800 km3 per year. This figure is equal to the amount of water carried from the land to the oceans by rivers (42,700 km3) and groundwater (2100 km3). This is the source of fresh water necessary to sustain people’s vital needs and economic activities.
However, fresh water is not evenly distributed around the world. While some regions are richer in terms of water resources, in some regions water resources are quite limited. Approximately 32% of our annual 42,700 km3 fresh water resources are in Asia, 28% in South America, 7% in Europe and 6% in Australia.
When the amount of fresh water resources in a region is less than the amount of water needed, it is called water stress. While water stress is high around the world, especially in Africa, water stress is low in North America.
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