water, is now beginning to turn to sources other than rainfall and runoff to meet growing water demands. The Southwest has too little water to support rapid population expansion. California has embarked on multi-billion dollar projects to bring water nearly 800 miles to Los Angeles and southern California from the wetter northern parts of the state. Outside of the United States, expensive desalination plants have been in operation for many years in the Virgin Islands, many Persian Gulf states, the West Indies, Libya, South Africa, and Israel.
An obvious way to increase water availability is to recover fresh water from seawater or large underground stores of brackish water that are available in many arid regions, but that are generally not usable without significant treatment. Other options for increasing potable water supplies is to find ways to use the saline or brackish water for agriculture or other uses that can tolerate higher salinity levels, or to actively pursue water conservation measures to protect limited fresh water supplies that may be available.
This chapter introduces the reader to the general subject of global water supplies and water scarcity issues through a review of water availability and water sources, global water resources, global water resource issues, and the history of desalination. A series of illustrative examples are provided to highlight concepts presented throughout the chapter.
1.2 Water Availability and Water Sources
The water on the surface of the Earth can be found in eight distinct compartments which are shown in Table 1.1 along with how much of the Earth’s total water budget each makes up. These eight compartments are more commonly organized into three distinct groupings. First, there is surface water, which includes oceans, lakes, rivers, seas, etc. In short, surface water is all forms of liquid water that lie on the surface of the Earth. The second grouping is glacial and frozen water. These are resources that are located on the surface; however, these resources are in frozen, long-term storage. Long-term storage is defined as water that has historically taken thousands of years to move through the water cycle. The third and final grouping is groundwater. This is the water that is found below the surface of the Earth in conditions where the percent saturation in soil pore water is 100% (Mullen 2021). The process known as the hydrologic, or water cycle, has played a crucial role in the formation and distribution of all water resources on Earth today. The water cycle is used to describe the constant circulation of water through its three phases of solid, liquid, and gas as it moves within the eight compartments making up the water resource pool on the Earth.
Table 1.1 Breakdown of the total water budget on the Earth.
| Water compartment | % of Total Earth’s water budget |
|---|---|
| Oceans | 97.2 |
| Frozen glacier water & other ice | 2.15 |
| Ground or subsurface water | 0.61 |
| Freshwater lakes | 0.009 |
| Inland seas | 0.008 |
| Soil moisture | 0.005 |
| Atmospheric water | 0.001 |
| Rivers and streams | 0.001 |
1.3 Global Water Resources
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 |
