Concept of water resources

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.

Source: Aguaí State Biological Reserve plays a fundamental role in recharging the Guarani Aquifer, one of the largest underground water reservoirs in the world. Victor J. Zomer, 2021.

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.

Source: Groundwater is found beneath the solid surface. Notice that the water table roughly mirrors the slope of the land’s surface. A well penetrates the water table. Geoff Ruth, 2021.

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.

Source: Kilbourne Hole is a maar crater and one of Potrillo’s most distinctive features. It was created about 24,000 years ago by the explosive interaction of hot magma and an aquifer. Magma rising from deep within the Earth heated the underground water, producing steam. Pressure built up, culminating in an explosion that blew out an irregularly shaped hole more than a mile across, 1.7 miles long, and hundreds of feet deep. The blast scattered material far and wide and exposed layers of rock that preserved the history of Kilbourne’s bygone water. Among geologists, Kilbourne is also known for its xenoliths—rocks that harbor bits of crust or mantle material. Several kinds of xenoliths are found in and around Kilbourne, especially those containing olivine, the greenish mineral of the gemstone peridot. The presence of olivine can indicate material from the mantle, and olivine-rich rocks were among those brought back from the Moon by the Apollo missions. NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. Geological Survey

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.

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