Chapter 13 -- Water - THE THIRD PLANET E. R. SWANSON spring...

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THE THIRD PLANET E. R. SWANSON – spring 2009 WATER The Water Planet The solar system’s eight major planets consist of four gas giant or Jovian planets and four small rocky terrestrial planets. Mercury, Venus and Mars are the terrestrial or Earth-like planets. There is no liquid water on Mercury and Venus. Mars likely has some water underground and as ice, but Earth is positively awash in water. Seawater covers most of the Earth’s surface to an average depth of nearly three miles, and there are lakes, rivers, swamps and glaciers in places on the continents. There is water beneath Earth’s surface and in Earth’s atmosphere. Earth is the water planet, but in spite of all that, almost all of it is too salty to drink or to grow crops. It is a case of “water, water everywhere but not a drop to drink”. 97.2% of the Earth’s water is in the oceans and too salty for consumption. Of the 2.8% of fresh water on Earth, 2.15% is found in ice caps like that covering most of Greenland and especially that found on the Antarctic continent. Only 0.65% of Earth’s fresh water is found in lakes, steams, the atmosphere and as groundwater. Of that, groundwater is the most abundant source of fresh water, yet even half of all groundwater is too deep below the surface to be economically extracted. This may be the water planet, but over 6 billion people are absolutely dependent on the less than one half of one percent of the Earth’s water that is fresh and accessible. Like other resources, there are the haves and have not when it comes to water. Minnesota is the land of 10,000 lakes and Michigan has more freshwater coastline than the entire salty eastern coast of the United States. On the other hand, in the Atacama desert in Peru (where the fertilizer and bomb nitrates are found) a typical year produces no rainfall at all and descent rain storms occur only about two to four times each century! The Atacama is one of the driest places on the planet, and yet it is not very far from the Pacific Ocean, the largest body of water on Earth. The importance of water to life and Earth’s uneven distribution has naturally led to some interesting water politics. Some say it is the ultimate politics. The Water Cycle Earth’s water moves from location to location in what is called the water cycle. Earth’s oceans, with 97.2% of all water, are naturally the most important source for water found in other parts of the water cycle. Water moves from the ocean by evaporation and then returns to the sea or land through precipitation. Water precipitated over land can either runoff down slope, infiltrate into the ground, evaporate back to the atmosphere, or be taken up by plants and animals. Plants return water to the atmosphere by transpiration. We will consider the runoff (floods) part of the water cycle in the last section of the course.
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Chapter 13 -- Water - THE THIRD PLANET E. R. SWANSON spring...

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