Water Resources
(Michael Pidwirny and Tracy Gow)

1. Introduction

Humans live on a planet that is dominated by water. More than 70 % of the Earth's surface is covered with it. Scientists estimate that the hydrosphere contains about 1.36 billion cubic kilometers of this substance mostly in the form of a liquid that occupies topographic depressions on the Earth. Freshwater is a primary resource for all terrestrial life on this planet. Water is important for the facilitation of most biotic and abiotic environmental processes. Humans use water for basic survival and require water for use in industry, agriculture, transportation, and electrical power generation. As the world's human population and industrial activity increase, so does the need for water. In 1990, each human used approximately 710 cubic meters of water per year, creating a total use of about 2600 cubic kilometers per year. By the year 2000, expansions in population and economic activity will increase total use to approximately 6000 cubic kilometers per year.

Sources of water for use in the processes described above are becoming increasingly polluted from a variety of human controlled processes. Many humans are unable to gain access to clean water for consumption. Polluted water is causing outbreaks of disease in humans and is causing undue stress on natural ecosystems. Future increases in agricultural productivity, to feed a growing human population, will require water for irrigation. However, new sources of water are hard to find.

1.1 Chemical and Physical Properties of Water

Water has a very simple atomic structure. This structure consists of two hydrogen atoms bonded to one oxygen atom (Figure 4.1). The nature of the atomic structure of water causes its molecules to have unique electrochemical properties. The hydrogen side of the water molecule has a slight positive charge (see Figure 4.1). On the other side of the molecule a negative charge exists. This molecular polarity causes water to be a powerful solvent and is responsible for its strong surface tension (for more information on these two properties see the discussion below).

When water makes a physical phase change its molecules arrange themselves in distinctly different patterns (Figure 4.2). The pattern taken by water when its is frozen, causes its volume to expand and its density to decrease. Expansion of water at freezing allows ice to float on top of liquid water.

Water has several other unique physical properties. These properties are:

• Water has a high specific heat. Specific heat is the amount of energy required to change the temperature of a substance. Because water has a high specific heat, it can absorb large amounts of heat energy before it begins to get hot. It also means that water releases heat energy slowly when situations cause it to cool. Water's high specific heat allows allows for the moderation of the Earth's climate and helps organisms regulate their body temperature more effectively.
• Water in a pure state has a neutral pH. As a result, pure water is neither acidic nor basic. Water changes its pH when substances are dissolved in it. Rain has a naturally acidic pH of about 5.6 because it contains natural derived carbon dioxide and sulfur dioxide.
• Water conducts heat more easily than any liquid except mercury. This fact causes large bodies of liquid water like lakes and oceans to have essentially a uniform vertical temperature profile.
• Water exists as a liquid over an important range of temperature from 0 - 100° Celsius. This range allows water to remain as a liquid in most places on the Earth.
• Liquid water is a universal solvent. It is able to dissolve a large number of different chemical compounds. This feature also enables water to carry solvent nutrients in runoff, infiltration, groundwater flow, and living organisms.
• Water has a high surface tension (Figure 4.3). In other words, water is adhesive and elastic, and tends to aggregate in drops rather than spread out over a surface as a thin film. This phenomenon also causes water to stick to the sides of vertical structures despite gravity's downward pull. Water's high surface tension allows for the formation of water droplets and waves, allows plants to move water (and dissolved nutrients) from their roots to their leaves, and allows the movement of blood through tiny vessels in the bodies of some animals.

• Water is the only substance on Earth that exists in all three physical states of matter: solid, liquid, and gas. Incorporated in the changes of state are massive amounts of heat exchange. This feature plays an important role in the redistribution of heat energy in the Earth's atmosphere. In terms of heat being transferred into the atmosphere, approximately 3/4's of this process is accomplished by the evaporation and condensation of water.
• The freezing of water causes it to expand. When water freezes it expands rapidly adding about 9 % by volume. Fresh water has a maximum density at around 4° Celsius. Water is the only substance on this planet that does this.

1.2 Water Availability and Distribution

Precipitation is the primary source of freshwater for rivers, lakes, groundwater, and glaciers on the Earth's terrestrial surface. The average annual precipitation of the world is estimated to be 1050 millimeters per year or 2.9 millimeters per day. Figure 4.4 below illustrates the patterns of annual precipitation globally. The diagram indicates that the distribution of precipitation on our planet is not homogeneous. Areas of high precipitation are found near the equator, west coast of North America between latitudes of 35 to 60 degrees North, southeastern United States, coast of southeast Asia, and eastern Australia. Areas deficient of precipitation include the continental deserts at the subtropical high belts, central Eurasia and North America, and the polar regions above a latitude of 60 degrees.

The average annual precipitation of the world is estimated to be 1050 millimeters per year or 2.9 millimeters per day. However, Figure 4.4 indicates that actual values from vary from a minimum of 0 millimeters per day or to a maximum of 10 millimeters per day depending on location. The reasons for these patterns are as follows:

• The deserts in the subtropical regions occur because these areas do not contain any mechanism for lifting air masses. In fact, these areas are dominated by subsiding air that results from global circulation patterns.
• Continental areas tend to be dry because of their distance from moisture sources.
• Polar areas are dry because cold air cannot hold as much moisture as warm air.
• Areas near the equator achieve high rainfall amounts because constant solar heating encourages convection, and global circulation patterns cause northern and southern air masses to converge here causing frontal lifting.
• Mid-latitudes experience cyclonic activity and frontal lifting when polar and subtropical air masses meet at the polar front. Further, the air masses in this region generally move from West to East, causing levels of precipitation to decrease East of source regions.
• Mountain ranges near water sources can receive high rainfalls because of orographic uplift, if and only if the prevailing winds are in their favor. This can also result in a sharp reduction in rainfall in regions adjacent or on the leeward slopes of these areas. This phenomenon is commonly know as the rainshadow effect.

Table 4.1 below describes the major reservoirs of water found on the Earth. The table indicates that most of the water found on this planet is held within the oceans. The use of this sink of water by humans is limited because of the dissolved salts it contains. Icecaps and glaciers contain about 2 % of the world's total water, and about 60 % of the freshwater supply. The use of this water by humans is very restricted because of its form and location. Humans primarily use the freshwater found in groundwater, lakes, rivers, soil, and the atmosphere. This water makes up less than 1 % of the Earth's supply.

Table 4.1: Inventory of water at the Earth's surface.

(1) 750-4000 m below the surface; (2) 0-749 m below the surface.

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