Hydrologic Cycle

What is the hydrologic cycle, and how does it affect climate?


 The hydrologic cycle is another term for the water cycle. The water cycle follows the path of water as it evaporates into the atmosphere to become water vapour and clouds and then returns to the surface as precipitation to fill the lakes, rivers, and oceans or to seep into the soil to be used by plants. The water in the lakes, rivers, and oceans then evaporates back into the atmosphere to continue the cycle. The water that is absorbed by plants is then released back into the atmosphere through transpiration.

The water cycle also follows the path of the water that has been frozen in glaciers and is now melting.


D4.10 The hydrologic (water) cycle



The water cycle plays a large part in moving water through the biosphere. To do this, water molecules undergo many changes in phase (state). Water changes from its liquid state in rivers, lakes, and oceans into water vapour when it enters the atmosphere. It then changes back to its liquid state to form clouds and rain or its solid state to form snow. When glaciers and snow are melting, the water is changing from its solid state to either its liquid or gas state.

  Did You Know?

D4.11 Fresh cob of ripe corn

A cornfield 2.5 acres in size can release as much as 37 000 L of water through transpiration each day. In comparison, an average adult human will exhale 0.35 L of water each day.

Every time water changes state, it is either releasing or absorbing thermal energy. When water is changing state, it is not changing temperature, instead the thermal energy is either being absorbed as bonds break (moving from a solid to a liquid or a gas) or is being released when bonds are formed (moving from a gas to a liquid or a solid). In this way, the hydrologic cycle is also transferring thermal energy throughout the biosphere. Since water is constantly changing phase in the hydrologic cycle and the water is not changing temperature while this is happening, it helps to absorb any excess thermal energy on Earth. This, in turn, helps to keep Earth’s temperature stable, since the thermal energy being used in the phase changes is not being used to raise the temperature of other parts of the biosphere.

The phase changes that occur in the hydrosphere play a huge role in the global transfer of thermal energy. When water evaporates in an area, it is absorbing 40 650 J of energy per mol. Often, it will then move to a different area and release thermal energy. In this way, it moves large amounts of thermal energy around Earth.
D4.12 Fog can occur when a large amount of water has evaporated

  Read This

Please read pages 382 to 383, and 389 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on what they hydrologic cycle is and the part it plays in transferring thermal energy around Earth. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Explain why 100.0 g of liquid water at 100 ˚C contains less thermal energy than 100.0 g of water vapour at 100 ˚C.

    While both have the same temperature, the water vapour would contain more thermal energy than the liquid water. This is because thermal energy is absorbed during the breaking of bonds that happens when a liquid changes state to a gas. The water vapour has more thermal energy because those bonds have been broken and the thermal energy has been absorbed. The energy has been stored in the bonds of the liquid water.

  2. How does the hydrologic cycle move thermal energy around the biosphere?

    The hydrologic cycle moves energy around the Earth because as the water evaporates into the atmosphere, it is breaking bonds and absorbing thermal energy. It then moves to a different area or part of the biosphere and changes back to a solid or liquid state in the form of precipitation and clouds. As water moves back into a solid or a liquid, bonds are being formed; this releases thermal energy into the surrounding area. In this way, thermal energy from one area or part of the biosphere was moved to a different area.