Water Cycles Study Guide (page 2)

Updated on Sep 26, 2011

The Global Water Cycle

Now we will look at how all these reservoirs of water are connected into a global water cycle. We'll examine some numbers in detail shortly, but let's first get a sense of how the cycle works by following a single molecule of water on a hypothetical but possible path through the water cycle. This water cycle is also called the hydrological cycle.


Our water molecule is at the surface of the ocean. Solar energy warms the water and the water molecule obtains a high enough energy to evaporate as water vapor into the atmosphere. In the air, it floats up and up, carried by ascending air, until the temperature drops and the molecule joins others as liquid water around a cloud condensation nucleus—say a micro scopic bit of salt—within a cloud. The cloud thickens as the day progresses, and though the molecule might return to the ocean as rain, today the winds blow the pack of clouds over the land.

In the next several days, in fact, the molecule probably does fall back to Earth in a raindrop over land. It hits the ground and runs into the soil, becoming part of soil moisture. Now the droplet has four main potential paths it might take. First, it might sink deeper and deeper, past the soil, into porous rock and become part of groundwater. It might even then be pumped up by a farmer's well and consumed by the farmer in a glass of water for lunch.

Second, the molecule might flow within the soil to enter a stream, which flows into a river and eventually returns to the ocean in what is called runoff.

Third, the molecule in the soil moisture might evaporate again, as it did from the ocean's surface, pulled into the atmosphere by the air's dryness and given enough energy by the sun warming the soil. Once in the air, it again will go into a cloud and rain back upon land or even be carried out over the ocean.

The fourth possible path for the molecule is to be pulled into the root of a plant, because the plant needs water to live. The molecule travels up into the stem and then into the leaves, through the plant's network of veins. It could then even be split into an oxygen and two hydrogens, during the process of photosynthesis. The oxygen (as O2) would be released by the plant as a waste gas. The hydrogen would go into a carbohydrate of the plant. But in the case of our particular molecule, it serves only as a carrier of nutrient ions from the soil up into the plant. Once in a leaf, the water exits the plant through a tiny pore in the leaf called a stomate. This exit is called transpiration, the process by which the plant converts liquid to water vapor. That process does take energy, the heat of vaporization, and so acts to cool the leaf. You can notice this cooling when you touch a leaf on a hot day and feel that it is quite a bit cooler than the air.

  • Evaporation from the ocean: 500 ×
  • Rain on to the ocean: 450 ×

More water leaves the ocean than returns as rain. What happens to the excess 50 thousand cubic kilometers per year? It is transported by the winds to the land. The ocean is a source of water to the land. Let's now look at this flow (the amount of water carried by wind from ocean to land) and add the known flux of rain to land.

  • Wind transport from ocean air to land air: 50 ×
  • Rain on to the land: 120 ×

Considering the air over the land, we can see that 120 units rain out, but this land air gets only 50 units from the winds that come from the ocean. Therefore, how much more water does the land need to receive from some other source in order to be able to rain at 120 units per year? Clearly, the land needs 120 50 = 70 more units from somewhere else. Those 70 units come rain to land. from two sources: (1) evaporation from soils and lakes and (2) transpiration from plants. Those two sources are approximately equal. Therefore, we can write the following numbers.

  • Evaporation from land's soil and lake: 35 ×
  • Transpiration from land plants: 35 ×

We need one more flux to complete the cycle. (For simplicity, we are leaving out groundwater and ice.) The land receives 120 units from rain and loses 70 from evaporation and transpiration. We can now calculate how much the land must lose as runoff in rivers that goes back to the ocean. That number is 120 70 = 50. So 50 units must flow in rivers back to the ocean. In fact, those 50 units exactly balance off the amount that is transported by winds from the ocean's air to the land's air to rain over the land. We have now completed the cycle. Note the important point that all the reservoirs are balanced. The air over the land must receive as much water as it loses. The ocean must receive as much water as it loses. All the fluxes can now be put into a figure (see Figure 11.1).

Figure 11.1

One more concept should be discussed before we close the subject of the global water cycle. This is the concept of residence time for a reservoir within the cycle. The residence time is how long the water stays in a particular reservoir, on average. Let's work through an example of residence time, using the atmosphere as the reservoir.

The water in the atmosphere as water vapor leaves the atmosphere as rain and is replaced from the surface of ocean and land by evaporation and transpiration. As you can see from Table 11.1, there are 13 thousand cubic kilometers of water in Earth's atmosphere as water vapor. What are the water fluxes into and out from the atmosphere? From Figure 11.1, it is clear that the total rain from Earth's atmosphere is 120 + 450 = 570 thousand cubic kilometers per year. Verify for yourself that this same amount enters the atmosphere, as the total sum of the evaporation fluxes from both ocean and land plus the transpiration flux from land plants. We can define the residence time as follows:

Residence time = (Mass in reservoir)/(Sum of entering fluxes)


Residence time = (Mass in reservoir)/(Sum of exiting fluxes)

We can use either definition, because the entering fluxes equal the exiting fluxes, so the calculations will be the same. In the case of the atmosphere's water vapor,

Residence time = (13 x 103 km3)/(570 × ) = 0.023 yr

How long is 0.023 years? Put in terms of days, that's about eight days. That means the water stays as vapor in the air only about eight days, on average, before it rains out and is replaced by new water vapor coming from Earth's surface. This calculation shows the power of the concept of residence time, because it gives us clues about how the cycle works.

Practice problems of this concept can be found at: Water Cycles Practice Questions

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