**2. Hydrologic cycle and water scarcity**

**1. Introduction**

90 Desalination Updates

ments [2].

and pressure over a long period of time [4].

opportunity to benefit from solar energy [3].

water scarcity is severe [5].

Freshwater and energy are two inseparable and essential commodities for sustaining human life on earth. Rapid population growth and industrialization, especially in developing countries in the recent past, have placed pressing demands for both freshwater and energy [1]. Both are to be conserved and preserved for the sustainable development of the world. On the other hand, there is an acute shortage of both energy and water, especially in the third world countries [2]. Only less than 1% of the water is available for the society for direct use, out of which the maximum fraction has been polluted due to non-manageable industrial develop‐

Due to the fast increase in the world population, the need for the energy increases rapidly. Various studies have been made to meet this extra energy demand. Most of these studies were focused on alternative energy sources. Solar energy is one of the most popular ones of these sources due to relatively less installation cost and long operation time without any need for maintenance [3]. Basically, all forms of the energy in the world, as we know it, are solar in origin. Oil, coal and natural gas is originally produced by photosynthetic processes, followed by complex chemical reactions, in which decaying vegetation was subjected to temperature

Solar energy is so powerful and abundant that 30 min of solar radiation falling on earth is equal to the world annual energy demand. It is also cheap, environment friendly and nearly inexhaustible. Most of the world receives powerful solar radiation and has a good chance and

All ecosystems and every field of human activity depend on clean water and it is one of the most precious resources in today's world. Water is a primary need for life, health and sanita‐ tion, which brings it into the international agenda as a very important issue [2]. The lack of potable water poses a big problem in arid regions of the world where freshwater is becoming very scarce and expensive. Clean drinking water is one of the most important international health issues today [5]. Remote and arid to semi-arid regions depend on underground water for drinking. Unfortunately, underground water is not always considered to be fresh drinking water. In some instances, the salinity is too high for being drinkable, and it is called brackish water. In such cases, fresh water has to be either transported for long distances or connected with an expensive distribution water network at extremely high cost for, usually, a small

Desalination is one of mankind's earliest forms of water treatment, and it is still a popular treatment solution throughout the world today [5]. Distillation is one of the oldest and most rustic desalination techniques. In fact, it reproduces the natural process of the industrial desalination. Solar distillation is a thermal process that represents a sustainable solution to water shortages in the world. The desalination technique by solar energy, based on the principle of greenhouse is not new. Solar energy coupled to desalination offers a promising prospect for covering the fundamental needs of power and water in remote regions, where connection to the public electric grid is either not cost effective or not feasible, and where the

population. Solar desalination can be a good solution for such a problem [6].

The earth has a global water amount of about 1*.*38×109 km3 , 97.5% of which is salty and only 2.5% is fresh water. Most of the fresh water is either frozen in the poles or remains in the soil as moisture or lies in aquifers beyond the reach of human being. A tiny fraction of global water resource (~1%) is available to be used by human beings, animals and plants. Being one of the most critical and vital needs of all plants, animals and human beings, fresh water is created mainly by a continuous evaporation and condensation process called "hydrological (water) cycle" [7].

The water cycle is caused by solar energy which heats the water in oceans, seas, lakes, rivers and lands containing moisture. Water evaporates and becomes water vapour while ice, hail and snow can transit from solid to vapour phase directly which is called sublimation. An average of 16 million m3 of water is evaporated every second and then the vapour is trans‐ ported in the atmosphere to where it comes across cooler temperatures which will eventually cause the vapour to condense and precipitate as rain, sleet, snow or hail. Nearly, four-fifth of the evaporated vapour falls back into the oceans and some other flows through the ground surface to the sea. Some water also goes deep into the ground layers and fills aquifers, which are the long-term water storages. Some groundwater comes out as springs through openings in the ground. Eventually, the water returns to the ocean and joins to the water cycle again. Figure 1 shows a schematic illustration of the water cycle (This Figure adapted from http:// www.srh.noaa.gov/jetstream/atmos/hydro.htm).

**Figure 1.** The global water cycle (the units are in 103 km3 . year−1) [9]

Due to wind regimes and atmospheric conditions, water vapour and rain are not evenly distributed throughout all the earth land. Therefore some parts of the earth take much more precipitation than the others. This situation causes a failure to provide enough usable water for everyone living in a certain region which can be caused by mainly two reasons. If the water in that region is physically less than the amount of water required in order to meet the demands of people in that region for drinking, home use and agriculture, this situation is called "physical water scarcity". On the other hand, if the water resources are present but they are not effectively used due to lack of investment or absence of qualified personnel or just administrations to process the water to provide people with the water they need, it is called "economic water scarcity" [8].

Although all parts of the earth is exposed to solar radiation throughout a year, the equator zone between 35°N and 35°S latitudes receives much more solar energy compared to the other parts of the earth and is called "Sunbelt region" [10]. About four-fifth of the world population lives in this region. Having abundant solar energy on one side, the Sunbelt region countries experience a serious water scarcity problem. This scarcity is not only physical water scarcity, indeed it is an economic water scarcity especially in Africa, which means lack of investment in water infrastructure or the lack of personnel capacity to adequately meet the drinking and irrigation water demand of the population. Figure 2 shows a schematic illustration of the Sunbelt region and global water scarcity in the world.

**Figure 2.** Global physical and economic water scarcity and Sunbelt region [10, 11]

One of the vital factors in combatting the economic water scarcity is a cheap, clean, affordable and environment friendly energy source which will be used to produce and distribute usable and drinkable water. Renewable energy sources such as; solar, wind, geothermal, biomass, etc. is suitable for this purpose.

Due to wind regimes and atmospheric conditions, water vapour and rain are not evenly distributed throughout all the earth land. Therefore some parts of the earth take much more precipitation than the others. This situation causes a failure to provide enough usable water for everyone living in a certain region which can be caused by mainly two reasons. If the water in that region is physically less than the amount of water required in order to meet the demands of people in that region for drinking, home use and agriculture, this situation is called "physical water scarcity". On the other hand, if the water resources are present but they are not effectively used due to lack of investment or absence of qualified personnel or just administrations to process the water to provide people with the water they need, it is called "economic water

Although all parts of the earth is exposed to solar radiation throughout a year, the equator zone between 35°N and 35°S latitudes receives much more solar energy compared to the other parts of the earth and is called "Sunbelt region" [10]. About four-fifth of the world population lives in this region. Having abundant solar energy on one side, the Sunbelt region countries experience a serious water scarcity problem. This scarcity is not only physical water scarcity, indeed it is an economic water scarcity especially in Africa, which means lack of investment in water infrastructure or the lack of personnel capacity to adequately meet the drinking and irrigation water demand of the population. Figure 2 shows a schematic illustration of the

Sunbelt region and global water scarcity in the world.

**Figure 2.** Global physical and economic water scarcity and Sunbelt region [10, 11]

One of the vital factors in combatting the economic water scarcity is a cheap, clean, affordable and environment friendly energy source which will be used to produce and distribute usable

scarcity" [8].

92 Desalination Updates

Sunbelt region is very suitable to use solar energy, which is a very popular and abundant energy source, to produce fresh water from saline water through a number of desalination methods, which will be discussed in detail later. 6 kWh/(m2 .d) of solar energy, which is easily reached in Sunbelt region, is equal to 0.6 l/m2 .d and 220 l/m2 .y of oil. Therefore it is practically possible to use solar energy instead of especially fossil fuels in this region [12].
