The Earth’s renewable freshwater portion is about 40,000 km3 annually, corresponding to the difference between atmospheric precipitation and water evaporation on the surface of the continents. Not all of this volume, however, can be used by man. Almost two-thirds quickly return to waterways and oceans after heavy rains. The remainder is absorbed by the soil, permeating its surface layers and being stored in underground aquifers, which, in turn, will be the main sources of food for water courses during droughts.
The relatively stable portion of the water supply is therefore just under 14,000 km3 annually. This portion of fresh water accessible to humanity at the current technological stage and at costs compatible with its various uses is what is called “water resources”. Contrary to popular belief, the amount of water on Earth is constant and is neither decreasing nor increasing. The apparent scarcity of water in certain regions is due to factors such as climatic variations, excessive concentration of population and economic activities, with an equal concentration of water demand, pollution of water sources, change in the surface runoff regime and the feedback of underground aquifers, between others.
The countries most favored by nature in surface and underground water resources are countries with continental dimensions or located in the tropics, especially Brazil, Canada, China, Indonesia, the United States of America, India, Bangladesh, Myanmar, Colombia and Zaire. The least well-off are located in Saharan to Sub-Saharan Africa and the Middle East, or are insular countries and small territorial dimensions, such as Bahrain, Kuwait, Qatar, Malta, Barbados, Cape Verde, Djibouti, United Arab Emirates, Mauritania, Singapore, Libya and Cyprus. In global average terms, the water usable by humanity is very abundant, with about 6,800 cubic meters1 per individual per year, often the minimum necessary to guarantee a reasonable standard of living for all human beings, which is of the order of 1,000 cubic meters annually.
The uneven distribution of atmospheric precipitation over and within continents makes the availability of water vary greatly with geographic location and population concentrations. For comparative purposes, North America has 30 times more water resources per inhabitant than North Africa, and Canada 25 times more than Mexico. In this light, the countries with greater availability are no longer those with large territorial extension, to be small and sparsely populated countries, located close to the Arctic Circle or the Equator, such as Iceland, Suriname, Guyana, Papua New Guinea, Solomon Islands and Gabon.
The random variation of atmospheric precipitation over time aggravates the inequality in the spatial distribution of water. It makes it extremely sparse at certain time periods and very abundant at others. Both situations cause very serious problems, such as floods and droughts, which human beings have been learning to live with since the beginning of their existence. The modern realization that the availability of water is not infinite, varies in time and space, and is voluntarily and involuntarily affected by human actions, has led to the need for water resources to be managed. The purpose of managing water resources is, therefore, to guarantee the availability of water in quantity and quality for current and future generations.