Unit 4 Lesson 2 The Parts of the

Unit 4 Lesson 2 The Parts of the Hydrosphere and the Hydrologic Cycle

BELLRINGER: Question

IV. The Parts of the Hydrosphere and the Hydrologic Cycle n n Since the hydrosphere is actually the sum total of all water (gas, liquid, and solid) that exists on the planet, it is important to take a look at all the sources of water on the earth. Look at Table 5. 1

The Parts of the Hydrosphere and the Hydrologic Cycle n n The vast majority (97. 25%) of earth’s water is found in the oceans. Since the oceans contain saltwater, most of the earth’s water supply isn’t even drinkable. Where is the majority of the earth’s freshwater?

The Parts of the Hydrosphere and the Hydrologic Cycle n n Most people think it is contained in lakes, ponds, rivers, streams, and creeks on the planet, but they are wrong! It turns out ¾ of the freshwater on earth exists in the glaciers and icebergs of the planet.

The Parts of the Hydrosphere and the Hydrologic Cycle n n A large fraction of the remaining freshwater is underground. Scientist call this water groundwater.

The Parts of the Hydrosphere and the Hydrologic Cycle n n In the end, less than 0. 01% of the earth’s freshwater supply is in the lakes, ponds, rivers, streams, and creeks of the planet. This water, called surface freshwater, is what we typically think of when we think of earth’s freshwater supply. However, it represents only a tiny fraction of the freshwater the earth really holds.

The Parts of the Hydrosphere and the Hydrologic Cycle n 1. 2. 3. Although you need not memorize this table, there a few facts I would like you to remember: The vast majority of earth’s water supply is contained in the oceans as saltwater. The vast majority of earth’s freshwater supply is stored in icebergs and glaciers. The largest source of liquid freshwater is groundwater.

The Parts of the Hydrosphere and the Hydrologic Cycle n n n The interaction between these sources of water is described by the hydrologic cycle. Hydrologic Cycle – The process by which water is continuously exchanged between earth’s various water sources The hydrologic cycle is best illustrated by figure 5. 2

The Parts of the Hydrosphere and the Hydrologic Cycle n There are many ways of viewing and discussing the hydrologic cycle, but for the purposes of this course, Figure 5. 2 covers the major concepts.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Water gets into the atmosphere predominantly by evaporation and transpiration. Although you probably know what evaporation is, you might not be as familiar with the other term: Transpiration – Evaporation of water from plants.

The Parts of the Hydrosphere and the Hydrologic Cycle n n As part of a plant’s natural life processes, water vapor evaporates from its leaves. We call that process transpiration. If you think about it, evaporation takes water out of the oceans, lakes, rivers, and streams, while transpiration takes water from the soil. After all, plants absorb water from the soil, so any water they emit must have originally come from there.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Thus, transpiration depletes the soil moisture. Soil moisture can also be depleted when it soaks down into the groundwater sources and feeds into lakes, rivers, and streams by the process of groundwater flow.

The Parts of the Hydrosphere and the Hydrologic Cycle n n When evaporation and transpiration take place, water vapor goes into the atmosphere. It then forms clouds by a process we call condensation. Condensation – The process by which a gas turns into a liquid.

The Parts of the Hydrosphere and the Hydrologic Cycle n n I will discuss cloud forming in more detail a bit later, but for now, just realize it occurs when gaseous water (water vapor) turns into a liquid that stays suspended in the air. Eventually, the oceans, lakes, rivers, streams and soil moisture all get replenished when the water in the clouds falls out of the atmosphere as precipitation. Precipitation – Water falling from the atmosphere as rain, snow, sleet, or hail.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Some of the precipitation falls directly into the oceans, lakes, rivers, and streams, and some of it falls onto land. That water can replenish the soil moisture, or it can run along the surface of the land into an ocean, lake, river, or stream as surface runoff.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Even though water might start out as part of a saltwater source, through the process of evaporation, condensation, and precipitation, it can easily be transferred to a freshwater source. How can this happen?

The Parts of the Hydrosphere and the Hydrologic Cycle n Water evaporates but salt does not. So in a mixture of saltwater, when the water evaporates, the salt stays behind. This keeps the salt in the ocean but allows the water from the ocean to be exchanged with the many other water sources in the hydrosphere.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Distillation – Evaporation and condensation of a mixture to separate out the mixture’s individual components. When a chemist does a distillation, he or she typically has a mixture of two or more substances that need to be separated. When the mixture is boiled, the substances tend to evaporate one at a time, allowing the chemist to separate them.

The Parts of the Hydrosphere and the Hydrologic Cycle n n n You could do an experiment like this at home. If you boiled salt water in a pan and had an empty bowl next to the boiling water, you could hold lid over the boiling water and see evaporating water drip from the lid to the bowl. You could allow all the water to boil away and condense into the bowl. In the end, you would have had freshwater in the bowl and nothing but salt in the pan; thus, the saltwater mixture would have been separated into its components: salt and water.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Now that you understand that water can indeed, be exchanged between all water sources in the hydrosphere, There is one more concept we should talk about: Since water is continually being exchanged between the water sources of the hydrosphere, a given molecule of water can only stay in a given water source for a certain amount of time.

The Parts of the Hydrosphere and the Hydrologic Cycle n Ex. If a water molecule is in a river that eventually flows into the ocean, the water molecule will be in a river until it either evaporates away or follows the flow of the river and ends up in the ocean. Either way, the water molecule will eventually be transferred from the river to another water source in the hydrosphere.

The Parts of the Hydrosphere and the Hydrologic Cycle n n n This tells us that a given molecule of water will only stay in a given water source for a certain length of time. Scientists call that time the residence time of the water source. Residence time – The average time a given particle will stay in a given system. When discussing water molecules, the residence time is the time a given water molecule spends in a given part of the hydrosphere.

The Parts of the Hydrosphere and the Hydrologic Cycle n n n Now the residence time of a water molecule depends on the source in which it resides. For example, the oceans hold a lot of water, and there is really one way a molecule of water in the ocean can be transferred to another water source in the hydrosphere. It must evaporate from the ocean. As a result, the residence time for a molecule of water in the ocean is rather long. Most calculations of residence time indicate that the average water molecule spends as many as 4, 000 years in the ocean before being

The Parts of the Hydrosphere and the Hydrologic Cycle n n Water in rivers, however, tends to be transferred to other water sources rather quickly. After all, there isn’t nearly as much water in a river as compared to an ocean. Also rivers tend to flow and dump their water somewhere. As a result, the average residence time for water in a typical river is about two weeks. Look at Table 5. 2

The Parts of the Hydrosphere and the Hydrologic Cycle n n n The first thing you have to realize when looking at this table is that the numbers are approximations. After all, there is no direct way to measure residence time. These numbers are based on calculations that make a lot of assumptions and use current theories of how the processes in the hydrologic cycle work. Only if the assumptions and theories used in the calculation are good will these numbers be accurate. If not you cannot rely on them.

The Parts of the Hydrosphere and the Hydrologic Cycle n n Nevertheless, the numbers at least illustrate what I am trying to say. The nature of the water source determines the length of the residence time.

The Parts of the Hydrosphere and the Hydrologic Cycle n n The next thing you need to realize is that these numbers are averages. The residence time in a small lake will be much shorter than the residence time in a large lake. Averaged over all lake sizes, however, the typical residence time for water in a lake is 10 years.

The Parts of the Hydrosphere and the Hydrologic Cycle n Finally, you need to realize that I don’t want you to memorize this table. Instead, I want you to look at the numbers and understand why the residence times are so different.

The Parts of the Hydrosphere and the Hydrologic Cycle n Water in the atmosphere, for example, has a short residence time. Why? Well, the water in the atmosphere is constantly forming clouds and precipitating. Thus, a drop of water doesn’t spend much time there. In the same way, the residence time for a drop of water in a river is shorter than the residence time for a drop of water in a lake because rivers flow, transporting their contents to other sources in the hydrosphere. A lake does not flow, so it takes longer for water to move from a lake to some other source.