Global water shortage does not consist in a reduction in the total supply of water. The amount of fresh water that lands on the earth as rain is constant at 12500 cubic kilometres per year. At present mankind is using less than a third of this.
The problem is that there is a mismatch between where the rain falls, and people live. In many densely populated areas, the renewable water resource is insufficient, leading to water being extracted from rivers and undergroun aquifers at an unsustainable rate. Increasing populations, expanding cities, and the increasing number of people that consume relatively much freshwater combine to cause critical localized water shortage.
Another major cause of water shortage is its heavy use in irrigated agriculture that are dry or where rainfall is concentrated in certain periods.
Map 1 shows that the total renewable water resources per capita are highest in the northern and southern areas of the world and lowest in the middle areas of the world.
Map 2 shows that the total water withdrawal tends to be highest in North America, the sourthern part of South America, Australia, the north eastern part of Africa, the south western part of Asia and the Arab Countries.
Map 3 shows that the proportion between total water withdrawal and resources per capita is highest in the middle part of the world. This is to be expected from a superposition of Map 1 and map 2. Water scarcity is thus shown to be greater in a belt around the middle of the world.
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