Freshwater

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Water covers nearly three-quarters of the earth's surface, mainly in oceans but also as rivers, lakes, snow and glaciers. Water is also found in the atmosphere and underground. Over 97% of the earth's water is salty with the remainder in various forms of freshwater.

Freshwater is defined as water that generally contains less than 1000 milligrams per litre of dissolved solids. Of the world's freshwater, about 99.9% is either found in glaciers and icefields, or is underground. However, the tiny percentage of freshwater we are most familiar with is that found either as surface water (lakes, rivers and wetlands), or which occurs in the atmosphere, or is found in the soil. According to some estimates, the quantity of groundwater in the earth would cover the entire surface of the globe to a depth of 120 metres. By contrast, the volume of surface water in lakes, rivers, reservoirs and swamps could be contained in a depth of about one quarter of a metre. Scientists estimate the world's freshwater at over one billion cubic kilometres.

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Freshwater is basic to life: it supports plants and animals in almost all ecosystems, even those in deserts. Aquatic ecosystems are perhaps the most obvious evidence of the importance of freshwater: lakes, ponds, rivers, streams and wetlands support a wide variety of species and provide important ecological functions.

Freshwater is also a basic component of human society. Not only does freshwater support resources such as agriculture, fish and forests upon which society depends for food, clothing and shelter, but freshwater also provides a medium for transportation and energy production, governs settlement patterns, provides a major recreational resource for society, and inspires artistic and cultural expression.

Water in the World

Figure 1 below demonstrates just how little of the world's total water supply is fresh water, and the tiny amount of the freshwater that is not in ice nor underground. ("Underground water" is defined as water at or beneath the water table; water found as soil moisture is considered to be surface water).

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Figure 1. The World's Water Supply

Canada is one of the most fortunate nations when it comes to available freshwater: Canada has only 0.5% of the world's population, but its landmass contains approximately 9% of the world's renewable water supply (i.e. water replenished by precipitation on a short-term basis). Tables 1 and 2 below show the earth’s salt water and freshwater stocks.

Table 1. Major Stocks of Salt Water on Earth (thousand cubic kilometres)

Table 2. Major Stocks of Fresh Water on Earth (thousand cubic kilometres)

* Marshes, wetlands and water incorporated in biota are often mixed salt and fresh water.
Note: Totals may not add due to rounding.

The Hydrologic Cycle

Water is unique among substances found in the earth's environment by occurring in all three states of matter: solid, liquid and gas. Both salt and fresh water occur as a liquid, but all forms of solid and gaseous water are freshwater. As a solid, water occurs commonly as glaciers, icefields and snow. As a liquid, freshwater occurs as surface water, soil moisture and groundwater (water in the ground, but found at the water table and below). Finally, as a gas, water occurs as water vapour. The water that drops to earth as precipitation can be in any of these forms, as rain, snow and fog demonstrate.

Each of these expressions of freshwater has a special place in ecology, and in uses by humans. The hydrologic cycle shows how types and locations of water link to one another.

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Figure 2. The Hydrologic Cycle

The hydrologic cycle (see figure 2 above) is an endless circulation of water from one state and place to another - from the atmosphere to the earth and its return to the atmosphere through condensation, precipitation, evaporation and transpiration.

• Evaporation / transpiration: On average, as much as 40% of precipitation in Canada is either evaporated or transpired. Transpiration is the process by which plants lose water to the atmosphere, generally through their leaves.
• Percolation into the ground: Water moves downward through cracks and pores in the soil and rocks to the water table. Water can move back up by capillary action or it can move either vertically or horizontally under the earth's surface until it re-enters a surface water system.
• Surface runoff: Water runs overland into nearby streams and lakes: the steeper the land and the less porous the soil, the greater the runoff. Overland flow is runoff before the water reaches a normal river or drainage channel. Overland flow is particularly visible in urban areas, due to the high proportion of area covered by asphalt and concrete. Ultimately all runoff goes into rivers and eventually into the ocean.

Although the hydrologic cycle balances what goes up with what comes down, one phase of the cycle is "frozen" in colder regions during the winter season. During the Canadian winter, most of the precipitation is simply stored as snow or ice on the ground. Later, during the spring melt, huge quantities of water are released quickly. This results in the familiar heavy spring runoff, and often results in localized flooding. Later sections of this issue discuss flooding problems.

Water, Ecosystems and Sustainable Development

In nature nothing exists alone. Living things, including humans, relate to each other and are supported by their non-living environment. These complex relationships are called ecosystems. Water is not only vital to life, but as it is in continuous interaction with the surrounding air and land and living things, it is also a vital component of healthy functioning of any ecosystem.

Understanding ecosystems is key to achieving a sustainable environment and economy. Sustainable development is a management philosophy whereby the use of resources and the environment today does not damage prospects for their use by future generations. To make the development truly sustainable, social, economic and environmental aspects must be integrated. Canada's water resources must be developed in harmony with natural ecosystems so that neither the water resource nor the plant and animal life dependent on it are depleted or destroyed for short-term gain and at the expense of their potential benefits for future generations of people. Long-term economic growth depends on a healthy environment.