From the beginning of time when water first appeared, it has been constant in quantity and continuously in motion. Little has been added or lost over the years. The same water molecules have been transferred time and time again from the oceans and the land surface into the atmosphere by evaporation, dropped on the land as precipitation, and transferred back to the sea by rivers and groundwater. This endless circulation is known as the "hydrologic cycle". Water is the the only natural substance which exists at the surface of the earth in three states: solid (ice), liquid (water), and gas (water vapor). In order to change the state of water from one form to another, heat is needed. In the natural environment, this heat energy comes from the sun.
Oceans represent 97.24 percent of all the Earth's water. All of the water in lakes and rivers will eventually end up in an ocean, and it is the oceans of the world which undoubtedly contribute most of the water vapor in the atmosphere. As a result of the oceans' impact on the water cycle, it is no wonder why oceans influence weather and climatic condition of the globes.
| Water source |
Water volume, in
cubic miles
|
Percent of
total water
|
|---|---|---|
| Oceans | 317,000,000 | 97.24% |
| Icecaps, Glaciers | 7,000,000 | 2.14% |
| Ground water | 2,000,000 | 0.61% |
| Fresh-water lakes | 30,000 | 0.009% |
| Inland seas | 25,000 | 0.008% |
| Soil moisture | 16,000 | 0.005% |
| Atmosphere | 3,100 | 0.001% |
| Rivers | 300 | 0.0001% |
| Total water volume | 326,000,000 | 100% |
Table 1: Global water distribution
Source: Nace, U.S. Geological Survey, 1967 and
The Hydrologic Cycle (Pamphlet), U.S. Geological Survey, 1984
The biggest determinant in water quality is what water runs over ex.: unpolluted soil, porous rocks, or concrete. Let's look at the three most common environments we live in...
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| The Rural Water Cycle includes fields, woodlands, and streams. Because of this, as long as humans don't pollute these areas, the water is cleaner than water coming off road ways. Their are other concerns for water in these areas though, like bacteria, animal pollutants, and stagnant water. Humans must never drink from a water source they are unsure of as bacteria in the water can make us sick. |
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| The Suburban Water Cycle includes small neighborhoods where houses using wells and city water add possible contaminants like pesticides, heavy metals, paints, oils, and gas. Some neighborhoods use septic systems where contaminated water from toilets and sinks is pumped into an underground storage tank and slowly allowed to peculate to the surface. |
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| Urban Environments need to use man-made waste treatment plants to clean the water of garbage, chemicals, and other dangerous materials before sending it back into the natural environment. Storm drains carry water from roads to Water Treatment plants while pipes from your bathroom carry dirty water to sewage plants where the water is cleaned and chemically sterilized before being sent by more pipes into streams or rivers. ------------------- |
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Vocabulary:
Students will review the What is an Aquifer? site to learn about ground water.
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Condensation
Water vapour is also emitted from plant leaves by a process called transpiration. Every day an actively growing plant transpires 5 to 10 times as much water as it can hold at once.
Sublimation
The changing of water from a solid to a gas.
Surface runoff
Excessive rain or snowmelt can produce overland flow to creeks and ditches. Runoff is visible flow of water in rivers, creeks and lakes as the water stored in the basin drains out.
Precipitation
Precipitation is made up of any type of water that falls to the earth like snow, hail, mist, or rain. Most of it (80 percent) evaporates or transpires through plants and never reaches lakes, streams, or ground water. The rest, about 6-10 inches of precipitation, runs off the land into lakes, streams, wetlands or rivers (also called "surface water"), or, it soaks right into the ground.
Percolation
Some of the precipitation and snow melt moves downwards, percolates or infiltrates through cracks, joints and pores in soil and rocks until it reaches the water table where it becomes groundwater.
Infiltration
Infiltration happens when water soaks into the soil from the ground level. It moves underground and moves between the soil and rocks. Some of the water will be soaked up by roots to help plants grow. The plant's leaves eventually release the water into the air through the plant's pores.
Some of the water keeps moving down into the soil to a level that is filled with water, called ground water. The very top of this layer filled with ground water is called the water table.
Ground Water
Ground water is simply water under the ground where the soil is completely filled or saturated with water. This water is also called an "aquifer." Ground water moves underground from areas where the elevation is high, like a hilltop, to places that are lowland areas. Water movement is slow and might move anywhere from less than a millimeter up to a mile in a day.
Where the water table meets the land surface, a spring might bubble up or seep from the ground and flow into a lake, stream woodland, or the ocean. Ground water that meets the land surface also helps keep rivers, streams, lakes and wetlands filled with water.
The Water Table
The Water Table is found underground where the rock and soil begin to be filled or "saturated" with water. It also marks the very top of the ground water layer.
Where the water table meets the land surface, a spring might bubble up or seep from the ground and flow into a lake, stream woodland, or the ocean. When ground water meets the land surface, it flows out and helps keep rivers, streams, lakes and wetlands filled with water.
Evaporation
Warmth from the sun causes water from lakes, streams, ice, and soils to turn into water vapor in the air. Almost all of the precipitated water (80 percent) goes right back into the air because of evaporation. The rest runs off the land or soaks into the ground to become ground water.
Water Vapor
Water vapor is water in a gas form that is held in the air until it changes back to water. You know, sometimes it #s sticky outside in the summer - that #s just water held in the air. The water can change into fine droplets by "condensing" in the air, and we get clouds. When the droplets get big enough, they are pulled to the earth by gravity as precipitation, better known as rain, sleet, snow, hail, dew, or frost.
Transpiration (evaporation of water from plants):
Plants give off water from their surfaces in a process called transpiration. This water leaves through stomata, which are tiny openings on the surface of a plant and are especially abundant on the undersides of leaves. Stomata open and close to let gases in and out. Water vapor is one of the released gases. Some leaves have more than a million stomata. In forests, trees need a large amount of water because about 98 percent of the water they take in is lost through the stomata. The leaves of tropical and temperate plants have stomata that are open longer periods of time and have larger openings than those of desert plants. In the desert, plants can survive with less water because the stomata of the plants do not open as wide and remain closed most of the time. The time stomata remain open as well as the size of their openings are largely controlled by the availability of water and of sunlight.
As the water evaporates from leaves during transpiration, more water is pulled into the plant at the roots. The water moves from the roots to the leaves through tubelike structures called xylem tubes. The water carries nutrients through plants.
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The Water-Climate Relationship
Water plays a basic role in the climate system through the hydrologic cycle, but water is intimately related to climate in other ways as well. It is obvious, from a water resource perspective, how the climate of a region to a large extent determines the water supply in that region based on the precipitation available and on the evaporation loss. Perhaps less obvious is the role of water in climate. Large water bodies, such as the oceans and the Great Lakes, have a moderating effect on the local climate because they act as a large source and sink for heat. Regions near these water bodies generally have milder winters and cooler summers than would be the case if the nearby water body did not exist.
The evaporation of water into the atmosphere requires an enormous amount of energy, which ultimately comes from the sun. The sun's heat is trapped in the earth's atmosphere by greenhouse gases, the most plentiful of which by far is water vapour. When water vapour in the atmosphere condenses to precipitation, this energy is released into the atmosphere. Fresh water can mediate climate change to some degree because it is stored on the landscape as lakes, snow covers, glaciers, wetlands and rivers, and is a store of latent energy. Thus water acts as an energy transfer and storage medium for the climate system.
The water cycle is also a key process upon which other cycles operate. For example one needs to properly understand the water cycle in order to address many of the chemical cycles in the atmosphere.
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Assessment Projects:
#1: Transpiration Activity:
Purpose
To determine how the number of stoma affects water loss.
Materials
4-by-8-inch (10-by-20-cm) piece of poster board
two 10-ounce (25-cm-diameter) plastic cups
pencil
scissors
paper hole punch
tap water
black marking pen
transparent tape
Procedure
- Fold the poster board piece in half by placing the short sides together.
- Stand one of the plastic cups upside down on the poster board.
- Use the pencil to trace around the mouth of the cup on the poster board.
- Cut out the circle tracing, cutting through both layers of the poster board.
- Use the paper hole punch to cut 2 holes in one of the paper circles, one hole across from the other.
- Randomly cut 20 holes in the other paper circle.
- Fill the cups with an equal amount of water so that they are about three-fourths full with water.
- Use the pen to mark a line on each cup that is even with the surface of the water.
- Use the tape to secure the edges of one paper circle over the opening of each cup. You want the holes in the circle to be the only openings.
- Set the cups near a window that gets direct sunlight.
- After 3 or more days, compare the level of the water in each cup to the black water mark on each cup.
Results
There is less water in the cup with many holes in its cover than in the cup with only two holes in its cover.
Why?
The fact that the water level goes down shows that water has left the cups. Water in the cups evaporates, forming water vapor, which escapes through the holes in the covering. The amount of vapor escaping increases with the number of holes in the covering. The holes represent stomata in the leaves of plants. The more stomata in the leaves, the greater the amount of water lost by transpiration. Desert plants have fewer stomata, which are closed most of the time, as well as stomata with small openings, so they lose a smaller amount of water than do other plants, which have leaves containing many large stomata that are open most of the time.
#2: Students will create a Mini Water Cycle Terrarium.
#3: Students will create a Water Cycle Disk.
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| Water Cycle 2 by misskprimary |
