PAK Diagram, Earth's Realms (Global Subsystems), Lithosphere, Atmosphere, Hydrosphere, and Biosphere

Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor


  1. Water plays a wide variety of roles in the global ecosystem. Water makes up the oceans and seas of the hydrosphere. Plants and animals in the biosphere consist of 65-90% water. Water vapor is the most important gas in the atmosphere for weather. Water has a high heat capacity and helps regulate the earth’s temperature. Water is also a universal solvent and helps to shape the lithosphere.

  3. The amount of water on earth is finite. However, water constantly moves from the hydrosphere to the atmosphere, the atmosphere to the lithosphere and biosphere, and then back to the atmosphere and hydrosphere. This is the hydrologic cycle. As water moves, it frequently changes state. Sometimes, water is a liquid, sometimes a solid (ice), and sometimes a gas (water vapor).

  5. The driving force for the hydrologic cycle is solar energy assisted by gravity. Solar energy moves water in a closed system where it is regularly recycled.

  7. Most of the world's supply of water is stored in the oceans (97.2%). Relatively small quantities of water are in the form of ice (2.15%) and groundwater (0.63%). Small but vital quantities of water are found in lakes and rivers (0.0001%), and in the atmosphere (0.001%).

  9. Every time water changes its state, heat is either absorbed or released by water molecules. This heat energy is called latent heat. Heat is absorbed as liquid water changes to water vapor during evaporation and transpiration. Condensation releases latent heat when water vapor changes to tiny water droplets. When ice freezes (sublimation) it releases latent heat and when it melts, it absorbs heat.

  11. These exchanges of energy take place constantly throughout the hydrologic cycle as water is evaporated over the oceans, transported over land areas in vaporized form, and then released by condensation as clouds and fog. Precipitation (rain, snow, hail, sleet, drizzle) is a by-product of condensation.

  13. Precipitation is the most important source of fresh water for the earth's surface. When water falls on land:


  1. What are some roles of water in the global ecosystem?
  2. Describe the three phases of water. Describe the pathways of moisture movement.
  3. Name the source of energy for the hydrologic cycle.
  4. Describe how much water is stored in the world's oceans, glaciers, lakes and rivers, ground, and the atmosphere.
  5. Describe the energy exchanges that take place as water changes state in the hydrologic cycle.
  6. Draw a diagram of the hydrologic cycle and define each of the processes that take place as water circulates in all its forms. Predict whether latent heat is absorbed or released during each process.
  7. Describe with the help of a diagram, the fate of precipitation once it has fallen on the earth's land areas.

Use the hydrologic cycle tutorial to help you name the processes that are taking place at each of the numbers shown in the diagram below. If the process involves water changing form, indicate with a minus sign (-) if heat is absorbed by a process, or a plus sign (+) if heat is released.

PAK Diagram, Identify the Hydrologic Cycle Processes

1. ________________________________________
2. ________________________________________
3. ________________________________________
4. ________________________________________
5. ________________________________________
6. ________________________________________

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This Geography Site Is Maintained By Patricia A. Kulda  Last Update July 14, 2010