GED Science Practice Test: Water Cycle

Water is a substance that is critical to the life of all living organisms.  Not only do plants and animals need water as a nutrient, but water also serves as a habitat for many aquatic organisms.  A habitat is the environment in which an organism lives.

In order to understand the water cycle, it is important to remember that water can exist in three states of matter:  solid, liquid, or gas.  We most commonly think of water in its liquid form in rivers, lakes, and oceans.  We think of water falling from the sky in its liquid form as rain.  However, water can also be present on earth in its solid form in glaciers, icecaps, and snow.  Also, water can be present in its gaseous form as water vapor that has evaporated from bodies of liquid water.  If you have ever experienced a humid environment, you have experienced water in its gaseous form.  In very humid environments, the air is holding onto a great deal of water vapor.

Additionally, in order to understand the water cycle, you need to understand how water can change among its solid, liquid, and gas states.  For example, water can melt to go from a solid to a liquid, or it can freeze to go from a liquid to a solid.  The following diagram shows all of the phase changes for water.

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All of these phase changes happen in the water cycle at various places.  The following diagram shows many of the complexities of the water cycle:

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While this diagram is very complex, two main processes are being represented here, and you can see those with the wide, partially transparent arrows on either side of the diagram.  In the water cycle, there are processes that allow water to escape from the earth and go to the sky, to condense into clouds.  These processes include evaporation from oceans and lakes, transpiration from plants, and sublimation from glaciers and snow.  There are also processes that allow water to return to earth.  These processes include precipitation of rain or snow and deposition of water vapor onto glaciers or snow. So, the water cycle can be seen as a cycle of water escaping from earth and water returning to earth’s surface.

One can also think of the water cycle in terms of where water is stored, and how water moves from one place to another while it is on the earth’s surface.   In the diagram above, water is stored in lakes, groundwater, oceans, snow, and glaciers.  Water can move from one place to another through rain and melting snow moving downhill, water infiltrating into the soil, surface flow of water in rivers, and movement of water from the soil into plants.

While it may seem like water is everywhere, ecologists are always concerned about the availability of water as a nutrient.  One reason for this is that for many living organisms, the only water available to them is freshwater;  the water in the oceans cannot be used as a nutrient by terrestrial organisms.  Only 3% of the available water on earth is freshwater:

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Ecologists are also concerned about the availability of water as a nutrient, because changes in ecosystem characteristics can affect any of the processes shown in the water cycle diagram above.  Some changes happen naturally, such as trees and plants increasing in size and number causing the rate of transpiration (water evaporating from plants) to increase.  Some changes are caused or accelerated by humans, such as decreasing the rate of transpiration by logging forests, or increasing the rate of evaporation by removing plants and introducing asphalt and other artificial building materials.

In addition to changes in the amount of water available to humans, another concern of ecologists is the pollution of available water.  You can read about events in which companies have dumped pollutants into a river, and then the organisms in that river suffer or die.  However, the concern about pollution is much larger, because water flows in a cycle.  Water that is in a river can end up in oceans, and precipitation can pick up pollutants from the air.  It is the very nature of water changing in a cycle that makes pollution an issue for all people, not just those people geographically close to the pollution.

One way to consider how far the effects of polluted water can reach is to consider how water flows in a watershed.  A watershed is an area in which all of the water flows to a single river or body of water.  Watersheds are typically defined by ridges or other geographic features that  separate water that flows to one body of water versus another.  A watershed can be thought of as a big bowl, allowing precipitation and other forms of water to flow downhill to eventually collect in bodies of water.  The following diagram shows a watershed:

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Use the diagram below to answer question 1:

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