Lesson 5 - Water and Water Resources
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 5 Quiz
By the end of this lesson you should be able to:
- Review the distribution of water on earth
- Discuss at least three reasons why water molecules are polar
- Relate temperature to relative humidity in the atmosphere
- Evaluate how insolation drives the water cycle
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Distribution of Water on Earth
Water is the second major earth system that we are discussing in depth. You will recall that geography is comprised of the ‘earth systems’ concept, in which the earth is made up of four systems: the atmosphere (which we discussed in week 3 and 4), the hydrosphere, the lithosphere and the biosphere. The hydrosphere is defined in your text as ‘an abiotic open system which includes all of the earth’s water’. Recall the definition of an ‘open system.’ It is a system with inputs and outputs which interact with surrounding systems. An abiotic system is a system which does not involve living organisms (in fact all earth systems except the biosphere are abiotic). Water is made up of two hydrogen and one oxygen atoms, and it has the familiar chemical formula H2O.
The hydrosphere contains about 1.36 billion cubic kilometers of water, and 71% of the earth’s surface is covered with water. Conduct some research and fill in the appropriate distribution of the earth’s hydrosphere:
Ocean________
All fresh water________
o Groundwater________
o Deep groundwater________
o Soil moisture________
o Surface________
§ Ice and glaciers________
§ Freshwater lakes________
§ Rivers and streams________
§ Saline lakes________
§ Atmosphere________
The water cycle is an open system. Water is lost from the system when gaseous water in the atmosphere disassociates (breaks apart) when it is hit by high energy waves from the sun (insolation). Because hydrogen is the lightest element, it often will escape the earth’s gravitational pull and be lost to space. Water is gained by the system by ‘outgassing’ from the earth’s crust. When the liquid rock in the interior of the earth’s crust rises to the surface and cools, certain compounds precipitate out, including water. This water is very hot, so it is in its gaseous form (steam), and it can often be seen rising from geothermal (a term that means hot earth) areas.
Water is a polar molecule (see the model of it above), meaning that one side of the molecule has a positive charge, and the other side of the molecule has a negative charge. This polarity is the reason behind the many unique properties of water, because the positive and negative ends of water stick to each other in a loose form of bonding called ‘hydrogen bonding’. Hydrogen bonding is the reason that liquid water is ‘wet’ because it sticks to many things and dissolves many substances. Hydrogen bonding also is the cause of capillary action of ‘wicking’ where water will climb up a narrow tube or space (such as between the fibers of a towel). Capillary action is the way that most plants are able to get water from their roots to their leaves. Hydrogen bonding is also the reason that solid water (ice) arranges itself into ring-shaped structures which actually take up more space than liquid. This is why water is the only substance that expands as it freezes, making it a powerful and destructive force on our planet.
Forms of Water on Earth
Water comes in three forms, or phases: solid, liquid and gas. Solid water is commonly known as ‘ice’. Ice forms when water cools to 0ºC. As stated above, when water becomes solid, it takes up a greater volume than when it was liquid. Most substances tend to take up a smaller volume, or become more compact when they change from liquid to solid. Solid water can expand up to 9% from its liquid volume. This can be a very powerful and destructive weathering force. Liquid water that gets into very small cracks in rocks or roads, then freezes, forcing the cracks to expand. Another property of solid water is that it floats in liquid water. When solid water expands, it becomes less dense than its liquid counterpart. This is why ice in a glass of water floats at the surface, and why icebergs form. Without this property of water, lakes would freeze from the bottom up in winter, making it impossible for fish and water plants to survive.
It takes 80 calories (a unit of energy) to melt one gram of ice. This energy is called ‘latent heat’ (recall this term from lecture 3). Liquid water also exhibits the unique quality of hydrogen bonding discussed above which makes it such an important molecule to all living things. To raise one gram of liquid water from 0ºC to 100ºC, 100 calories of energy must be added. To change one gram of liquid water at 100ºC to its gaseous phase, 540 calories of energy must be added. This is known as the latent heat of vaporization. When gaseous water changes to liquid water (such as when raindrops form from a cloud), 540 calories are released for each gram of water. This is known as the latent heat of condensation.
Does this mean that all water that is evaporated (a term which simply means ‘change from liquid to solid’) is boiled? No. It takes 540 calories to change water from liquid to vapor at 100ºC, but water can be changed from liquid to vapor at a lower temperature by simply adding more energy. For example, at 20ºC, it takes 585 calories to evaporate 1 gram of water. This latent heat of evaporation is the dominant cooling process in the earth’s energy budget. Water is constantly evaporating from the surface of the ocean, lakes and streams. Solid water (in glaciers) also changes from solid directly to gas. It takes approximately 680 calories to transform one gram of ice into water vapor. This is known as the latent heat of sublimation.
Water in the Atmosphere
Gaseous water in the atmosphere is known as humidity. The amount of water that the atmosphere can hold is a function of the temperature. Hot air can hold a lot more water than cold air. I like to think of this as the air being a giant sponge. When the air is cold, the sponge is smaller, so it cannot hold as much water as when it is hot, and the sponge is larger. So for example, San Francisco and Chicago in the summer may both have 90% humidity... but Chicago will feel a lot more icky (a good technical term to describe high humidity!). This is because San Francisco is 60 degrees F, while Chicago is 95 degrees F -- so there is a lot more water in the air in Chicago. The temperature at which air is saturated with water is the dew-point. The dew-point in warm air is much higher than the dew-point in cold air.
One way of monitoring atmospheric moisture is with satellites. The GOES satellite allows you to view the current water vapor in the atmosphere. The GOES satellite captured a set of time-lapse images which documented a plume of water vapor from the great lakes.
Clouds and fog are different from humidity. Clouds are formed when air becomes saturated with water vapor and moisture droplets form. Moisture droplets are very small (about 0.002 cm) and they can either be liquid or solid (ice). Fog is simply a cloud which forms at the ground. Fog forms when the air temperature and the dew point are nearly identical. Because the atmosphere can hold so much water at high temperatures, we usually only see fog in cool, moist environments.
San Francisco is an excellent example of a prime environment for fog. Warm air blowing off of land collides with cool air from the ocean. The air is quickly cooled and brought to its dew-point, causing a type of fog called advection fog. The warmer the air blowing off the land is, the more likely, and more severe, the advection fog is likely to be in San Francisco. That is why the fog in San Francisco is usually the worst in the summer when the inland areas heat up.
If you have ever driven through the Great Central Valley on your way to go skiing in the winter, you may have encountered the severe valley ground fog known locally as Tule fog. This is more generally known as radiation fog. Radiation fog forms when radiative cooling of the ground chills the air directly over it, brining it to the dew point. The Great Central Valley is prone to this type of fog because moist air is blown into the valley through the San Francisco Bay delta and then chilled by the ground overnight.
The Water Cycle
Go to the NOAA (National Oceanic and Atmospheric Administration) description of the water cycle and read it carefully. The elements of the groundwater cycle are:
- Evaporation
- Condensation
- Precipitation
- Interception
- Infiltration
- Percolation
- Transpiration
- Runoff
- Storage
Use your text and the NOAA site to write a definition for each term above.
Our Water Supply
Water is essential to life. Humans require fresh water to live and to irrigate our crops -- yet, only about 2.5% of the water on earth is freshwater. In California the problem is even more extreme. If you look at the Average Annual Precipitation maps on page 72 of Goodes World Atlas, you will note that the western half of the United States receives far less precipitation than the eastern half. Note also in the ‘Moisture Regions’ figure that most of the west is listed as being a ‘dry climate, with the majority of southern California considered semiarid or arid. This becomes a major issue when you recognize that California is the most populous state in the union. Where are we getting our fresh water?
Unlike the eastern United States, California does not have a large underground aquifer. The small aquifer that was under the Great Central Valley was mostly depleted within the first 100 years of farming the region. So where do we get our water? Look again at the figure of Precipitation from November 1 to April 30 in Goodes World Atlas. Note the band of dark green and blue (20-40 in, and 40+ in) in the northeastern part of California. This represents the Sierra snow pack.
California harvests the melt water from the Sierra snow pack through an elaborate network of dams, reservoirs and canals (recall the map of dams from the ICE maps in week 2?). These direct the water from where it is precipitated (in the Sierra Nevada mountains) to where there is a high demand for it: in agriculture in the Central Valley, and in urban areas such as the San Francisco Bay Area and Los Angeles. This is officially called as the State Water Project, and it has been called by someone of the world’s most ambitious public works projects, and by others an enormous environmental disaster.
Review learning outcomes.
Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.
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