Teaching Notes

• understand the Earth system processes that influence the rate and timing of evaporation and surface runoff in a given area;
• compare annual precipitation patterns to annual evaporation and surface runoff patterns;
• gain skill and understanding in the use of the GIS analysis tools, specifically the FieldScope Web GIS;
• collect and record quantitative data in a data table and use it to support responses to questions; and
• analyze and interpret data displayed on maps.

Because precipitation and evaporation are components of the global water cycle, the total amount of water is constant over time with equal rates of precipitation and evaporation world-wide, but depending on location and time of year (i.e. seasons) these two may not balance with evaporation actually being greater than the precipitation. Students will explore precipitation and evaporation amounts across the U.S. As they collect data for study cities, they should recognize that there is not always a balance between the two components at a specific location. Some of the imbalance is the result of seasonal factors. For example, when precipitation occurs as snow during the winter, it doesn't contribute to surface runoff and evaporation as readily as when it melts during the spring. But, some of the imbalance may be due to the fact that there are regional or global factors involved.

Evaporation is a physical process by which a liquid or solid substance is transformed to the gaseous state. Evaporation represents the return of precipitation back into the atmosphere. Like precipitation, evaporation is not evenly distributed by location. The main sources of water vapor in the lower atmosphere are evaporation from Earth's surface and transpiration by plants.

Major factors influencing the rate of evaporation include atmospheric temperature, wind, atmospheric humidity, and the availability of water. Other contributing conditions and factors may include the following:

• The movement of water between Earth's surface and the atmosphere is partially controlled by atmospheric humidity.
• If there is little or no surface water, then the total amount of evaporation is low. In hot arid areas, such as the southwest, the total amount of evaporation is low because there is little water in the system to evaporate. However, the rate of evaporation (verses total amount of evaporation occurring over time) is high when water is available in hot arid areas. High evaporation is dependent on high precipitation, although there is sometimes a time lag in evaporation. For example, evaporation rates are high in the Pacific Northwest and Rocky Mountains in the spring and summer when there is little precipitation.
• The type of land surface and vegetation influences evaporation and evapotranspiration rates.
• The higher the wind speed, the greater the rate of evaporation.
• Evaporation rates increase with increasing temperature.
• During warmer months, plant evapotranspiration rates increase.

Surface runoff is the water that flows down creeks, streams and rivers. Precipitated water that has not evaporated back into the atmosphere or infiltrated into the ground runs off as surface water.

Major processes and factors influencing the timing and quantity of surface runoff include the amount of precipitation, rate of precipitation, form of precipitation (snow, rain, sleet, hail, etc), season the precipitation fell, infiltration rate, degree of ground saturation, land use and ground cover, and geology.

Before the students begin this investigation, ask them to develop a working hypothesis describing the relationship between total annual precipitation and total annual evaporation. Will areas with high precipitation have high evaporation rates? Will areas with low precipitation, such as the hot, arid west, also have high total annual evaporation? Write the various hypotheses down on the class whiteboard so that students can refer back to them as they work through the GIS analysis.

As part of this investigation, students fill out a table of annual precipitation, evaporation, and surface runoff, for four pre-selected cities as well as three of their choice. One extension of this activity is to graph the data using Excel or other graphing/charting program. The chart can help students visualize the relationships of these different variables across the country. One important relationship for them to observe is how little surface runoff occurs compared to evaporation.

An Excel spreadsheet has been created for this purpose. Click here to download: Water_Availability.xls (Excel 40kB Sep22 09) After downloading and opening, click on the Investigation III tab at the bottom of the sheet to open the appropriate table and chart. Blank cells are in the table for students to add their selected cities. Below is an example of the data from the four pre-selected cities in the Investigation.

Another option is to have students create the chart on graph paper.

Science as Inquiry

• 8ASI1.3 Use appropriate tools and techniques to gather, analyze, and interpret data.
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  The use of tools and techniques, including mathematics, will be guided by the question asked and the investigations students design. The use of computers for the collection, summary, and display of evidence is part of this standard. Students should be able to access, gather, store, retrieve, and organize data, using hardware and software designed for these purposes.
• 8ASI1.4 Develop descriptions, explanations, predictions, and models using evidence.
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  Students should base their explanation on what they observed, and as they develop cognitive skills, they should be able to differentiate explanation from description — providing causes for effects and establishing relationships based on evidence and logical argument. This standards requires a subject knowledge base so the students can effectively conduct investigations, because developing explanations establishes connections between the content of science and the contexts within which students develop new knowledge.

Earth and Space Science

• 8DESS1.6 Water, which covers the majority of Earth's surface, circulates through the crust, oceans, and atmosphere in what is known as the "water cycle."
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  Water evaporates from Earth's surface, rises and cools as it moves to higher elevations, condenses as rain or snow, and falls to the surface where it collects in lakes, oceans, soil, and in rocks underground.
• 8DESS1.10 Global patterns of atmospheric movement influence local weather.
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  Oceans have a major effect on climate, because water in the oceans holds a large amount of heat.

Science as Inquiry

• 12ASI1.3 Use technology and mathematics to improve investigations and communications.
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  A variety of technologies, such as hand tools, measuring instruments, and calculators, should be an integral component of scientific investigations. The use of computers for the collection, analysis, and display of data is also a part of this standard. Mathematics plays an essential role in all aspects of an inquiry. For example, measurement is used for posing questions, formulas are used for developing explanations, and charts and graphs are used for communicating results.

Earth and Space Science

• 12DESS2.2 Movement of matter between reservoirs is driven by Earth's internal and external sources of energy.
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  These movements are often accompanied by a change in the physical and chemical properties of the matter. Carbon, for example, occurs in carbonate rocks such as limestone, in the atmosphere as carbon dioxide gas, in water as dissolved carbon dioxide, and in all organisms as complex molecules that control the chemistry of life.

Science in Personal and Social Perspectives

• 12FSPSP3.1 Human populations use resources in the environment in order to maintain and improve their existence.
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  Natural resources have been and will continue to be used to maintain human populations.
• 12FSPSP4.1 Natural ecosystems provide an array of basic processes that affect humans.
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  Those processes include maintenance of the quality of the atmosphere, generation of soils, control of the hydrologic cycle, disposal of wastes, and recycling of nutrients. Humans are changing many of these basic processes, and the changes may be detrimental to humans.

The World in Spatial Terms

1. How to use maps and other geographic representations, tools, and technologies to acquire, process, and report information from a spatial perspective

Physical Systems

7. The physical processes that shape the patterns of Earth's surface

The Uses of Geography

18. How to apply geography to interpret the present and plan for the future

• Technology productivity tools. Students use technology tools to enhance learning, increase productivity, and promote creativity.
• Technology problem-solving and decision-making tools. Students use technology resources for solving problems and making informed decisions.