Our World of Water

The Hydrologic Cycle

When you turn on the faucet, where does your water come from? Why can a well supply you with water without running dry? Why does all water run to the oceans, and yet rivers continue flowing and lakes continue to exist after thousands and thousands of years? The answers to these questions can be found in something called the hydrologic, or water, cycle. This is a continuous exchange of water between the earth and the atmosphere. The ease with which water can participate in this cycle is the result of the physical properties of water, which allow it to change form so readily.

Most of the water is in liquid form and is found in the oceans. This amounts to 97.2 percent of all water. A little over 2 percent is found in solid form, as glaciers and icecaps. The remaining water, with the exception of a small amount of water vapor in the atmosphere, is found in lakes, streams, and underground. The following chart illustrates this distribution:

Most of the earth's water is saline, or salty, and contains relatively high concentrations of dissolved substances. The water that people use for drinking, industry, and irrigation is called fresh water. Fresh water also contains a certain amount of dissolved substances, but not as much as the amount contained in saline waters. Most visible fresh water is found in lakes, streams, and glaciers. Surprisingly, only 3 percent of all fresh liquid water is surface water. The rest of the fresh liquid water (97 percent) is found underground as ground water and is obtained from wells.

Why don't wells usually run dry? Are there actually underground lakes and oceans? Not exactly. Ground water is stored in rock or soils, called aquifers. Some of these aquifers are composed of tiny grains that resemble a bag of marbles. Ground water is found in the spaces between these grains and also in cracks and fractures that occur in solid, dense rock. If these spaces are connected, the water then can move through them. Sometimes the open spaces between grains or cracks are filled with air, and this reduces downward movement of water. Water also clings to the grains (in the same manner as water sticks to your fingers). Water from rain and snow seeps downward through this suspended zone until it reaches the water table. All open spaces below the water table are completely filled with water. Let's try an experiment to help us understand the water table.

How water occurs in the rocks.

EXPERIMENT #7

Fill a small, clear container (such as an old glass bowl) about halfway with sand. Pour a cup of water onto the sand. Where did the water go? Look at the bottom of your bowl. How did the water work its way to the bottom? The downward movement of water is called infiltration and happens whenever water has open spaces in the ground to move through. Notice the level at which the sand is completely filled or saturated with water. The top of the water-saturated sand is the water table. The wet sand above your water table is suspending some water between the grains, but not every space is filled with water. What happens when the water table reaches the land surface? Pour water onto the sand until all the sand is completely saturated with water. Scoop out a little sand. The small puddle of water that forms is the result of the water table reaching the land surface. When the water table is at the land surface lakes, ponds, and swamps can keep filled with water even during dry periods because they are continually resupplied with groundwater. You can make a river by digging a narrow trench. During times of drought, how do you think rivers still are able to flow? Carefully shove a straw to the bottom of the bowl and gently suck on the straw until water just enters the bottom of the straw. You have just made a water-well. Where does the water come from that enters your straw "well"? Continue to suck on the straw until water almost fills the straw. What happened to the water table level in your bowl? What do you think happens to a water table when a well takes more water out than infiltrates or moves into an underground water supply? Do you know how a well is constructed? Possibly you could check with a local water-well contractor to find out.

About 39 inches of water are evaporated from the oceans each year. About 21 inches of water are evaporated from the surface of fresh water lakes and rivers annually. Plants also transpire water each day. All of this water forms vapor (the gaseous phase) and enters the atmosphere. The amount of water vapor that the air can hold depends upon its temperature. Warm air can hold more water vapor. Why do you think this is so? As moist air cools and is less able to hold water as a vapor, condensation occurs and the water vapor changes back into a liquid. An example of this is what happens when a cool glass comes into contact with warm air on a hot, muggy day. The water droplets on the outside of the glass must have come from the air. This occurs when the warm water vapor in the air was cooled by coming into contact with your cold glass, causing the vapor to condense on the outside of the glass.

Water vapor in the atmosphere condenses on dust particles in the air in the same way it condenses on the outside of a cool glass. For example, have you ever noticed how dusty your family car can be after a rain sprinkle? The rain drops left their dust particles on the car when they evaporated.

If many tiny drops of water called droplets are concentrated in the atmosphere, clouds form. If the droplets are then frozen, ice or snow is formed. Precipitation occurs when the air becomes super-saturated—that is, when the air can hold no more water droplets. Precipitation can occur in the form of rain, snow, sleet, or hail, depending on the temperature and other conditions.

The amount of precipitation that occurs during the year affects every part of our lives. Because the world's food supplies depend on how much and what kind of precipitation occurs, a lot of money is spent on weather prediction. Some of these funds go toward weather satellites and other sophisticated weather prediction equipment. The weather forecast is an important part of the daily newscasts. Think of how many ways weather conditions affect our lives.

Once precipitation has reached the earth's surface, several things can happen to it. It can fall directly into a lake, river, or ocean and add to that water supply, or it can fall onto the land surface and seep into the ground. Water will slowly work its way downward, mostly due to gravity, through the air spaces in the soil until it reaches the water table.

If precipitation occurs in large amounts or the land surface is not porous, most of the water falling as rain or melting from ice and snow will not have time to seep into the ground. Instead, it will run over the surface of the land. This water is called runoff. How much runoff there is depends on four factors:

1. the amount and intensity of precipitation,
2. the slope of the land,
3. the vegetation present on the soil, and
4. the type of soil and how wet it already is.

EXPERIMENT #8

Construct a box such as the one pictured below. If you built a stream table for the 4-H Landforms manual, it also will work for this activity. STEP 1 —Fill the box level with wet sand. Tilt the box so that the end without the notch is an inch higher than the notched end. Place a shallow pan under the notched end (see diagram above). STEP 2 — Slowlypour a quart of water into the box at the end without the notch. Measure the time from when you first started pouring until no more water runs out of the box. Measure how much water was collected by the shallow pan. Is any water missing? Where did it go? STEP 3—Now quickly pour another quart of water into the box and measure the amount of time until no more water pours out of the notch. Measure the amount of water collected in the pan. Did more water run off the sand this time? Why? If the water you poured into the box is supposed to represent precipitation and the sand is supposed to represent the earth, what effect does the rate at which rain falls have on how much water will soak into the ground? What effect does the amount of water already in the soil have on the amount of runoff? STEP 4—Place more blocks under the end without the notch. Repeat, pouring a quart of water first slowly and then another quart quickly, measuring the runoff times and amounts of water collected in the pan. What effect does increasing the slope of the land have on increasing runoff? Was there any increase in the amount of sand washed out of the box when you increase the slope? What effect does the slope of the land have on the rate at which water will erode, or carry away, soil?

If the land is bare, and there is no vegetation, little water will be held back and most of the water will run off. Row crops encourage runoff because of the constant tilling of the soil and the scarcity of vegetation to retain the water. If the soil is sandy or contains much organic material, the amount of run-off will be reduced. Uncontrolled runoff can cause erosion and flooding. It is therefore important to preserve as much of the original vegetation as possible, or substitute suitable alternatives such as contour plowing, conservation tillage, terracing, or constructing ponds.

Runoff eventually reaches the lowest spot on the land surface, where it joins other water that has run off from previous storms or from storms in other areas. These low spots provide temporary storage for surface runoff and usually form lakes and streams. Water that seeps underground is stored in aquifers below the water table.

EXPERIMENT #9

Water does not seep or infiltrate into the ground at the same rate everywhere. Fill, but don't pack, a clear container like an empty pint jar level with sand and another identical container level with a clayey soil. You will probably have to break up the lumps of clay with a hammer until you can obtain enough clayey soil to pour into the container. Slowly pour water into both containers and measure the amount of water and the time to completely saturate the sand and the clay. Which container will absorb the most water? Why? What does this tell you about the amount of runoff coming from sandy soils compared to clayey soils? Which soil would allow the most rainwater to infiltrate? What effect does the "looseness" of the land surface have on the amount of precipitation that can infiltrate it? Repeat your experiment, but pack the sand and clay into the containers if you cannot answer the last question.

Storage is only temporary because water is always moving. For example, rivers are always flowing to the sea, but rivers may contribute to or take water from lakes. Also, ground water is always moving from high areas to lower areas. Along the way, it can be taken out of storage by being discharged into streams or pumped through a well.

Evaporation and plant usage (including transpiration) account for changes in storage. These processes remove large amounts of water that could be used for recreation or other uses. However, they are important parts of the hydrologic cycle because they recharge the atmosphere with water to keep the cycle going. Several factors influence the rates of evaporation and transpiration. The following experiment should demonstrate this.

Surface and ground water that is not transpired by plants or evaporated will flow until it eventually reaches the oceans. There the hydrologic cycle begins again when sea water evaporates and enters the atmosphere, only to fall on the land as precipitation. The journey of water to atmosphere and back to earth is a never-ending cycle powered by heat from the sun. The hydrologic cycle thus provides us with an everlasting resource.

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