Water Cycles Study Guide (page 2)
Life as we know it cannot exist without water. Though other crucial molecules are needed for living things, one might say that water is the most important molecule of all. Where is water found? How does the water cycle work?
Forms off Water
The chemical formula of water is well known: H2O. Two atoms of hydrogen are bonded via the sharing of electrons to a single atom of oxygen. The two hydrogen atoms are also slightly attracted to each other, which makes the water molecule slightly bent at an angle. The angle causes the water molecule to be slightly polarized into negatively and positively charged sides, which is why water is so excellent at dissolving a huge number of different substances.
For any substance that cycles around and around among various forms on the earth, it is useful to ask where that substance is found. This, of course, is true for water. The locations of the substance are its compartments, pools, or reservoirs (all words can be used, and they mean the same thing). For water, one reservoir is obviously the salty ocean. Another reservoir is the atmosphere, with its water vapor and water droplets in clouds. Obviously, the ocean has a lot more water than does the atmosphere, but how much more?
We will get to that answer in just a bit (or perhaps that would be a good calculation to do for one of the practice questions). But first, let's briefly describe the reservoirs of water and then look at their volumes in Table 11.1.
The oceans contain salt water (we will talk more about the ocean's chemistry in an upcoming lesson). The next largest reservoir is water frozen in glaciers and ice caps. Glaciers refer to mountain glaciers, such as those found in Alaska and the Himalayas. The ice caps are the great sheets of ice that cover Antarctica and Greenland.
Groundwater is another reservoir, which can be divided into shallow and deep groundwater. Groundwater is contained and flowing through the pores in rock, moving very slowly (in some cases taking up to thousands of years to percolate across large geographical regions through rock such as sandstone). Most places on Earth have groundwater, if you dig down far enough. It is groundwater that is tapped when people with homes in the country dig wells for their water supply. The groundwater they seek is obviously fresh water, but in many places, the deep ground water is somewhat salty.
Lakes and rivers are a reservoir of freshwater. The soil itself also contains water, which is necessary for the growth of plants that sink their roots into soil for its supply of water. As noted previously, the atmosphere contains water in the form of vapor and small droplets.
Finally, our own bodies contain water. In fact, we are mostly water by weight. All life forms contain significant amounts of water as fractions of their bodies. What happens if we add up all the water in all creatures on land and in the seas? It turns out, that of all the reservoirs of water described so far, the water in the sum of all life is the smallest amount.
Now look at Table 11.1 for the numbers.
NOTES: The total of the rounded off percents equals a bit more than 100%. The total volume has also been slightly rounded off.
To consider the global water cycle, we must connect some of the major reservoirs with flows between them, because no reservoir stays constant. Consider lakes, for example. Inflowing rivers supply lakes with water and outlet rivers take water away from lakes. In addition, evaporation takes water away from lakes. A lake, therefore, is a dynamic reservoir. The water in it is not permanent. If you went on visits to a lake in two year intervals, it is possible (depending on the size of the lake), although it looks the same to you, that all the water in the lake would be different.
Consider your own body as a reservoir of water. You are constantly drinking fluids and excreting water in your wastes. The water in your body, like that of a lake, is continually being renewed. You are a dynamic, not a permanent, reservoir of water.
You drink water from a glass; lakes are supplied with water by rivers. In these particular transfers from one reservoir to another, water stays as water (as a liquid). But some of the changes between reservoirs involve a change in the state of water. Water can occur in three states, like any substance: liquid, solid, and gas. Reservoirs such as the oceans and rivers contain water in its liquid state. The atmosphere contains water in its gas state—water vapor. Finally, glaciers and ice caps contain water as its solid form—ice.
When water changes state, say between ice and liquid or between liquid and gas, energy is either required or is released. Basically, water in its liquid form contains more energy than water in its solid form at the same temperature. In other words, at the freezing point of water, 32° F (0° C), the liquid water contains more energy than does the solid form of water (ice). This extra energy is what makes liquid water move around so easily—its molecules are not locked together like they are in ice.
The energy that is transferred between the states of water is called latent heat. Let us consider the latent heat that is transferred when water goes from liquid water to ice (the latent heat of freezing) and when water goes from ice to liquid (the latent heat of melting). When liquid freezes, energy must be removed. That's why you put an ice cube tray of liquid water into the freezer of your refrigerator. The latent heat of freezing is 79 calories per gram of water. Let's put that into practical terms. First, remember that a calorie is the amount of energy it takes to change the temperature of one gram of liquid water by 1° C. Therefore, the latent heat of freezing is enough energy to lower 79 grams of water by 1° C. The latent heat of melting is also 79 calories per gram of water, which is the energy that must go into a gram of ice to change it into liquid water.
The change of state between liquid water and water vapor requires an even greater latent heat. When liquid water evaporates, an amount of energy equal to 540 calories per gram must be supplied. That's why it takes so much energy on your stove to boil water beyond the boiling point until it is boiled away. In other terms, the energy it takes to boil one gram of water would be able to raise 540 grams of water (more than 1 pound) by 1° C. The latent heat of condensation—going from water vapor to liquid water—is also 540 calories per gram, but in the opposite direction compared to the latent heat of evaporation. The latent heat of condensation is the energy that is released into the air when water vapor condenses into cloud droplets.
A crucial part of the water cycle takes place when water vapor in the atmosphere forms into water droplets in clouds, which eventually leads to rain, returning liquid water back to the surface of the land or ocean. So how do clouds work?
The first step in changing water vapor into water droplets in the sky takes place when the temperature drops far enough so that the saturation vapor pressure of the air (the maximum water that can be held by that air) falls below the amount of water in the air. The water then begins condensing on what are known as cloud condensation nuclei. Cloud condensation nuclei are tiny particles that can serve as centers for the beginning of the condensation process, which eventually leads to raindrops.
Substances that can serve as cloud condensation nuclei include the following: salt in the air over the ocean, dust in the air over continents, sulfate aerosols (tiny droplets of sulfuric acid, which can be natural or caused by humans) over both oceans and land, and other substances. The condensation nuclei are invisible to our eyes. They are very small, as are the initial water droplets. The droplets are so small, in fact, that they are kept aloft by the currents inside the clouds. That is why clouds stay in the sky.
As the droplets accumulate more and more water, they increase in diameter and eventually fall as raindrops.
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).
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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