Introduction to Desalination. Louis Theodore

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      Usable water is the key to sustainable life. However, out of all the water on Earth today, it is estimated that only 2.5 to 3.0% is usable. That still is quite a large amount of water, considering the Earth’s total global water supply is over 1.39 billion km³. However, when one delves deeper into this estimated usable supply, it is found that a little over 0.3% of the usable water on Earth is easily accessible (Gleick 1996). This usable water is found in rivers, streams, lakes, and swamps. With the ease of access to these forms of usable water being so high, it is estimated that over 50% of the world’s population lives within 3.0 km of these surface water sources (Kummu et al. 2011). The majority of the available water that is not considered easily accessible is found in ground or subsurface water in natural aquifers.

Natural aquifers are the most common forms of groundwater storage. Today, these natural aquifers are thought to hold roughly 34.2 million km³ of total water. Out of these 34.2 million km³ of water, it is estimated that close to 10.4 million km³ of this water is usable (Gleick 1996). Since these aquifers are below the surface of the Earth, they do require the construction of wells for water recovery. These wells can range from tens to hundreds of feet deep, depending on where the water table is located. It is also important to note that these wells will generally require the installation of a pumping system to bring the water to surface. Table 1.2 provides a comparison between the total water on Earth to the amount that is both groundwater and fresh-groundwater.

Table 1.2 Fresh groundwater, total groundwater, and total global water resources.

Water source	Water volume, mi3	Water volume, km3	% of Total water resource	% of Total fresh water resource
Fresh groundwater	2,526,000	10,530,000	0.80	30.1
Total groundwater	5,614,000	23,400,000	1.7
Total global water resource	332,500,000	1,386,000,000

As small as the ratio of total volume of usable water to unusable water is in the world, there are still considerable amounts of usable water resources present among the seven continents. The amount of usable water on each continent can be divided among the three water categories as just described earlier, i.e. groundwater; glacier/permanent ice caps; and surface water (i.e. wetlands, lakes, reservoirs, and rivers). These categories, along with the volumes of water in each for each continent across the globe are summarized in Table 1.3.

Table 1.3 Global water resources by continent (Modified from Rekacewicz 2006).

Continent	Glacier & permanent ice caps, km3	Groundwater, km3	Wetlands, large lakes, reservoirs & rivers, km3	Totals, km3	Total usable water resource, km3
North America	90,000	4,300,000	27,003	4,427,003	4,327,003
South America	900	3,000,000	N/A	3,00,900	3,000,000
Europe	18,216	1,60,000	2,239	1.620,745	1,602,529
Africa	0	5,500,000	31,776	5,531,776	5,531,776
Australia	180	1,200,000	221	1,200,401	1,200,221
Asia	60,984	7,800,000	30,622	7,891,606	7,830,622
Antarctica	30,109,800	N/A	N/A	30,109,800	0

On examining Table 1.3 it becomes immediately obvious that Antarctica has by far the largest amount of fresh water of any continent. However, this fresh water is in the form of glaciers and permeant ice caps, thus making it unusable. When comparing usable water resources, the top three continents are Asia, with roughly 7,830,622 km³ of water, Africa, with approximately 5,531,776 km³of water, and North America, with 4,327,003 km³ of total usable water resources. Furthermore, these three continents contain over 75% of the usable water on Earth. With this large amount of water being contained in three of the seven continents, mankind has adapted through the development of artificial reservoirs. These artificial reservoirs are most commonly constructed by the creation of a dam around an existing natural water source. Today, it is estimated that there are more than 515,000 of these man-made reservoirs of greater than 2.5 ac in size that cover roughly 258,000 km² of the Earth’s surface (Osborn 2019).

Table 1.4 summarizes the annual renewable fresh water resources for the top 25 countries in the world. Renewable fresh water resources are an estimate of the average precipitation, recharged groundwater, and fresh surface inflows from surrounding countries that each of these countries receives in any given year. What is interesting to note from these data is that despite a country having a great deal of renewable fresh water resources, water shortages and unequal distribution of resources can still take place. Consider Brazil, which tops the list with over 8,233 km³ of renewable fresh water each year. Despite what would appear to be an abundance of fresh water, Brazil is experiencing a long-term water crisis caused by poor water infrastructure and water resource management which has left over 5 million, or more than 2% of its citizens without access to safe potable water (Water.org 2019).

Table 1.4 Renewable annual fresh water resources for top 25 countries (Table 1.4 Advameg, Inc. 2010).

Rank	Country	Total fresh water renewable Скачать книгу В начало < 4 5 6 7 8 9 10 11 12 13 > В конец