Where's the Water?
Part B: Water in Motion
Precipitation, evaporation, evapotranspiration, and runoff are the processes that move water through the water cycle. Values in the diagram show the volume of water that moves along each path in a year. Source: Igor A. Shiklomanov, State Hydrological Institute (SHI, St. Petersburg) and United Nations Educational, Scientific and Cultural Organisation (UNESCO, Paris) 1999; Max Planck, Institute for Meterology, Hamburg. 1994; Freeze, Allen, John, Cherry. Groundwater, Prentice-Hall: Englewood Cliffs NF. 1979.
Part A of this lab talks about where Earth's water is, but not about where it's going. Interactions between the sun's energy and Earth's gravity keep water moving through the water cycle at all times.
A diagram of the water cycle can be thought of as a "road map" for water moving through the water cycle. Check this diagram for the amount of water that travels along the various paths of the water cycle each year.
- Click the diagram to open a larger view in a new window. Compare the amount of water represented by the pairs of arrows over lakes, land, and the ocean. Note that more water evaporates from the ocean than falls back into it. Note also that more rain falls on land than rises into the atmosphere from it. The imbalance in these two pairs of arrows shows that there is a net movement of water from the oceans onto land. The result is a constant supply of fresh water falling on land. Water that falls on the land and flows downhill to the ocean is the most obvious part of the water cycle, but it only accounts for a small portion of the entire cycle.
- Water moves between locations in the water cycle at different rates. Click the table on the right to see a comparison of the average length of time that water remains in various locations.
- Use the residence times and your imagination to visualize the relative speed of water moving along the different arrows in the water cycle diagram above. Think about which portions of the cycle are available for human use.