PRECIPITATION AND EVAPORATION

Transcription

1 PRECIPITATION AND EVAPORATION OBJECTIVES Use historical data to analyze relationships between precipitation, evaporation and stream flow in the San Antonio River Basin TOPICS Water cycle Precipitation Evaporation Stream flow TEKS ALIGNMENT Grade 4 Science: 2B, 2C, 2D, 2F, 3A, 6A, 8B Grade 5 Science: 2D, 2G, 3A, 5B, 6A BACKGROUND MATERIAL The amount of water flowing in a stream or river at a given time is tied most closely to the water cycle processes of precipitation and evaporation. Precipitation is the water water in the atmosphere falls back to Earth as rain, snow, sleet or hail due to the force of gravity. Evaporation is the water liquid water becomes water vapor by gaining heat energy. Stream flow is rate of water movement that is quantified as the amount of water passing through a given point in the stream channel over a certain period of time, such as gallons per minute. The relationships among these three water cycle processes can be seen by looking for patterns in real precipitation, evaporation and stream flow data. But it is important to realize that the evaporation data for the San Antonio River Basin represents potential evaporation and may not reflect actual evaporation, since water cannot evaporate if it is not there. The way in which the evaporation data is determined is that a standardized pan of water is exposed to the sun. It is checked every day to see how much water evaporated. Then it is refilled to its previous level. Since potential evaporation is greater than precipitation throughout most of the San Antonio River Basin, most rainfall that is not protected from evaporation by being stored as groundwater, will return to the atmosphere.

2 Human development of an area can significantly alter the relative amounts of water that are involved in precipitation, evaporation, and stream flow. Roads, sidewalks and houses create impervious surfaces on the ground, which inhibit water that falls as precipitation from infiltrating into the ground. The less water that enters the ground, the more water that remains on the surface to either evaporate back into the atmosphere or move off of the surface of the land and into a local water body, contributing to stream flow. KEY TERMS Basin a depression in the earth s surface. A body of water, such as a lake or river is often located in the bottom of the basin Condensation is the water water vapor becomes liquid water. Water vapor loses heat energy when it condenses. Evaporation is the water liquid water becomes water vapor. Liquid water gains heat energy when it evaporates. Gravitational Energy is stored energy that an object has because of its position. Gravitational energy is due to the force of gravity, and it is caused by the attraction of all other objects to the mass of the Earth. Heat Energy is the energy that an object has because of the motion of its atoms and molecules. The more an object s atoms and molecules move, the more heat energy it has. Impervious Cover is any type of material on the ground surface that prevents water from infiltrating. Examples are asphalt, concrete, brick or metal. Infiltration is the water liquid water enters the soil due to the force of gravity. Water loses gravitational energy when it infiltrates into the soil. Precipitation is the water water in the atmosphere falls back to Earth as rain, snow, sleet or hail due to the force of gravity. Water loses gravitational energy when it falls from the atmosphere. Runoff is the water cycle process by which water flows over the soil surface due to the force of gravity. Water loses gravitational energy when it runs off the soil surface.

3 PROCEDURES A. Start by asking your students to brainstorm what they think the words evaporation, precipitation and stream flow mean, and how they think these water cycle processes are related to each other. MATERIALS Student Sheets 1 and 2 Internet access B. Have the students study the column in the table on Student Sheet 1 that describes their county, and have them record their county s precipitation and evaporation data in the table on Student Sheet 2. C. Have the students research (or provide to them) data for average monthly stream flow measurements for the San Antonio River or one of its tributaries at a location near their home. See the Teacher Sheet 1 for more information on how to obtain these data. D. Once the data are obtained in cubic feet per second (CFS), have the students convert these numbers to gallons per day. Then have them record their calculations in the table on Student Sheet 2. E. Instruct the students graph the precipitation and evaporation data on one graph, and have them create a second graph next to it with the stream flow data. F. Ask the students to calculate simple statistics related to their data, including the range mean, percent of months above and below average, and frequency of months above and below the average for the variables of precipitation, evaporation and stream flow. GUIDING QUESTIONS In what months is precipitation the greatest? Least? In what months of the year is evaporation the greatest? Least? Is the difference between rainfall and evaporation the same in all months? How do you know this? In what months is stream flow the greatest? Least? Are the months of the greatest precipitation the same as the months of greatest stream flow? Are the months of least precipitation the same as the months of least stream flow? During dry months, from where do you think the water in the river comes? What do you think would happen to the river or stream if there were no ground water? EVALUATION Have each student research the average monthly precipitation, evaporation, and stream flow values for a different part of Texas. Have them graph this data and conduct simple statistical analyses. Then, encourage the students to compare their data with each other by creating a large map of Texas that displays each of their graphs placed at the location where the data were taken. The students should see that precipitation increases from west to east and that evaporation increases from east to west in Texas.

4 STUDENT SHEET 1 Table of average (mean) monthly precipitation and evaporation in Bexar, Wilson, Karnes and Goliad Counties, Texas

5 STUDENT SHEET 1

6 TEACHER SHEET 1 Stream flow data provided by the US Geological Survey can be found for the state of Texas by going to Select Surface Water Under Statistics, select Monthly Check the box next to County and hit the Submit button Select your county under the County box, select Stream under the Site Type box, and check the box next to Streamflow, ft 3 /s hit the Submit button Select the gauge site you want to explore the site most likely to be of use is the one for the San Antonio River itself Check the box next to the parameter code that says Discharge, cubic feet per second and type in the year from which you want the data, and hit the Submit button

7 REFERENCES Black, P. E Watershed Hydrology, Second Edition. Ann Arbor Press, Chelsea, MI. 449pp. Schlesinger, W. H Biogeochemistry: An Analysis of Global Change, Second Edition. Academic Press, San Diego, CA. 588pp. U.S. Geological Survey Groundwater Atlas of the United States: Oklahoma, Texas. U.S.G.S Publication HA 730-E. Wetzel, R. G Limnology, Second Edition. Saunders College Publishing, Orlando, FL. 857pp.