Earth System Science Module: Hydrosphere Introduction Water blesses our planet and makes it appear beautifully blue from space. The presence of liquid water clearly distinguishes Earth from all the planets and moons of the solar system. In fact, water covers almost three times as much of Earth's surface as land does. (Sussman, p.26) We do not just drink water; we are water. Water constitutes 50 to 90 percent of the weight of all living organisms. It is one of the most abundant and important substances on the Earth. Water sustains plant and animal life, plays a key role in the formation of weather, helps to shape the surface of the planet through erosion and other processes, and covers roughly 70% of the Earth's surface. Water continually circulates between the Earth's surface and its atmosphere in what is called the hydrologic cycle. The hydrologic or water cycle, is one of the basic processes in nature. Responding to heat from the sun and other influences, water from the oceans, rivers, lakes, soils and vegetation evaporates into the air and becomes water vapor. The water vapor rises into the atmosphere, cools, and turns into liquid water or ice, forming clouds. When the water droplets or ice crystals get large enough, they fall back to the surface as rain or snow. Once on the ground, water does one of three things; some of it filters into the soil and is either absorbed by plants or percolates downward to groundwater reservoirs. Some runs off into streams and rivers and eventually into the oceans. Some evaporates. The water in a lake, the snow on a mountain, the humid air or the drop of morning dew are all part of the same system. The total annual water loss from the surface of the planet equals the Earth's total annual precipitation. Changing any part of the system, such as the amount of vegetation in a region or land uses, affects the rest of the system. Despite its abundance, we cannot use most of Earth's water. If we represent the Earth's water as 100 liters, 97 of them would be seawater. Most of the remaining three would be ice. Only about 3 ml out of the whole 100 liters would be water that we can consume; that water is pumped from the ground or taken from fresh water, rivers and lakes. Water participates in many important chemical reactions, and most substances are soluble in water. Due to its effectiveness as a solvent, truly pure water rarely occurs in nature. Water carries many natural and human-introduced impurities as it travels through the hydrologic cycle. These impurities give each water its distinctive chemical makeup, or quality. Rain and snow capture small dust particles or aerosols from the air, and sunlight causes emissions from the burning of gasoline and other fossil fuels to react with water to form sulfuric and nitric acids. These pollutants return to Earth as acid rain or snow. The acids in the water slowly dissolve rocks, placing dissolved solids in water. Small but visible pieces of the rocks and soils also enter the water, resulting in suspended solids and making some waters turbid. When water percolates into the ground, it is in very close contact with rocks and more minerals dissolve into the water. These impurities dissolved or suspended in water determine its quality. [Continue]