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Hydroelectric Energy: Classroom Activities

The Movement and Balance of Water in the Water Cycle


Goal

To develop a better understanding of the movement of water from one reservoir to another in the water cycle.

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Background Information

Water moves continuously from the oceans and continents to the atmosphere and from the atmosphere back to the oceans and continents. About 361,000 cubic kilometers of water are output each year from the oceans through evaporation. About 324,000 cubic kilometers are input into the oceans as precipitation. This means that over the oceans, output (evaporation) exceeds input (precipitation) by 37,000 cubic kilometers. This imbalance suggests that the level of the global ocean should be dropping, which is not the case. Balance is achieved by water being cycled via the continents to the oceans. Output from the continents through evaporation is about 62,000 cubic kilometers each year. About 99,000 cubic kilometers of precipitation is input onto the continents. This means that over the continents, precipitation exceeds evaporation by 37,000 cubic kilometers. Through various processes both at and below the surface, this water journeys to the coasts. There, it is input into the ocean, helping to maintain ocean levels and keep the Earth's water cycle in balance.

Several things can happen to the excess precipitation that falls onto the continents. Some can evaporate back into the atmosphere. Some precipitation seeps into the ground. The rest flows across the surface, under the influence of gravity, as runoff. This water flows down slopes into creeks or small streams, or eventually seeps into the ground. Streams flow downslope and coalesce to form larger streams. Larger streams converge with large rivers that flow to the oceans. It is the flow of water in rivers and streams that is harnessed to produce hydroelectricity.

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Activity Overview

Students examine a diagram of the rates at which water moves from one reservoir to another within the water cycle.

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Materials and Equipment

Water Cycle Rates and Reservoirs diagram (below) - color copies or projected

Water Cycle Rates and Reservoirs
Source: The American Geological Institute, EarthComm: Project-Based Space and Earth System Science, 2nd Edition, Armonk, New York: It's About Time, 2012, pg. 375.

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Activity

  1. Provide copies of the Water Cycle Rates and Reservoirs diagram to students. Alternatively, project the diagram onto a screen for the class to see.
    Water Cycle Rates and Reservoirs
    Source: The American Geological Institute, EarthComm: Project-Based Space and Earth System Science, 2nd Edition, Armonk, New York: It's About Time, 2012, pg. 375.
  2. Have students use the diagram that shows the rates at which water moves from one reservoir to another within the water cycle to answer the following:
    1. Rank the quantities of water within the reservoirs shown from highest to lowest.
      1. Volume of water in the oceans is 1350 x 1015m3.
      2. Volume of water on land is 33.6 x 1015m3.
      3. Volume of water in the atmosphere is 0.013 x 1015m3.
    2. b) Rank the rates at which water moves among the various reservoirs from highest to lowest.
      1. Evaporation from oceans to atmosphere is 361 x 1012m3/year.
      2. Precipitation from atmosphere to oceans is 324 x 1012m3/year.
      3. Precipitation from atmosphere to land is 99 x 1012m3/year.
      4. Evaporation/transpiration from land to atmosphere is 62 x 1012m3/year.
      5. Runoff/groundwater flow from land to oceans is 37 x 1012m3/year.
    3. What is the difference between the rates of evaporation and precipitation over the oceans?
      Over the oceans, evaporation exceeds precipitation by 37 x 1012m3/year.
    4. What is the difference between the rates of evaporation and precipitation over land?
      Over land, precipitation exceeds evaporation by 37 x 1012m3/year.
    5. How do the differences that you calculated in Steps 1.c) and 1.d) compare to the rate at which water is entering the oceans from the flow of runoff and groundwater?
      They are the same.

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