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Making Evaporation Real

Fifth Grade Water Unit
by Kenneth Nova


This activity is from Placing Geography in the Curriculum: Ideas from the Western PLACE Conference edited by A. David Hill and published by Center for Geographic Education, University of Colorado, 1988. This activity is by Steve Wanner and Nicholas Helburn.

BVSD Standard 4.3 Students know and understand major sources of water, its uses and importnace, and its cyclic patterns of movement through the environment.

Objective: After this activity students will be better able to define evapotranspiration, distinguish evaporation from transpiration, and describe the role of soil in keeping water available to plants between rains.

Background Information: Everyone is aware of precipitation. Getting wet in the rain or trudging through the snow are vivid experiences that we connect with precipitation.

However, in the water cycle, evaporation is just as important as precipitation. For growing plants and soil, precipitation is the water supply, and evaporation and transpiration (water given off from plants' pores) are the demand. When the demand exceeds the supply, plants dry up, die, or go dormant. For farms, when evaporation is greater than precipitation, crops shrivel and die. We call it drought. Evaporation is invisible, silent, tasteless, without color. About as close as we get to "experiencing" evaporation is when our mouths or nostrils sense the dryness of our western climate.

Equipment needed:

*Three identical cake pans or refrigerator storage containers

*Platform scale (and ordinary kitchen scale is OK)


From the lawn, dig out two very similar pieces of shallow sod to almost fill two of the cake pans. Water both well and pour off excess water so that they are exactly the same weight. Fill the third pan with water so it is the same weight as the other two. Soil is a little heavier than water so the first two pans can't be quite full.

Set all three pans on the window sill or on a table under the window. If possible keep the scales right there, too. The students should construct a chart something like this:

 Weight  Mon  Tues  Wednes  Thurs  Fri  Sat  Sun  Mon
 Pan #1                
 Water added                
 Weight after addition                
 Pan #2                
 Pan #3                

A note about sky conditions for each day could be included on the chart.

At the end of the experiment, students will make a line graph for each pan, with the days of the week (Monday through Monday) on the X-axis and the weight (either in standard measurement or metric, depending on your scales) on the Y-axis.

The first day, the three pans will weigh the same amount and no water will be added to pan #1. The following day, weigh all three pans and record the weights. Carefully pour enough water into pan #1 to bring it back to its original weight. Do not add water to pans #2 and #3 during the entire experiment. Record the weight of the water added to pan #1 each day. Repeat the procedure each time. Don't worry if you miss a day or two, as the evaporation will continue. Three days may be too long to miss as the reserve of moisture in the soil may be used up.

By the time a week is past the class will have a vivid example of evapotranspiration. Pan #1 will have healthy, bright green grass. Pan #2 will have dull shriveled grass, which may be turning brown. Pan #3 will still have open water, but the water level will be observably lower.

Discussion: Some of the discussion might include:

Why did pan #1 drink more than #2? (Pan #2 ran out of water. Pan #3 only had evaporation, with no transpiration)

How would the results have been different if the pans were much deeper so that they had twice as much soil? (The soil in pan #2 would have stored more water so the grass could have stayed green longer)

Would it be OK to think of the water added to pan #1 as precipitation? (Yes, the parallel is very close)

Would it be OK to think of the soil as a reservoir storing water between rains? (Yes)

What would happen if the precipitation was a lot more than the evapotranspiration? (Excess water would soak down into the ground water system--a part of the water cycle often ignored-- or run off into streams and lakes)

What do farmers call a long dry period when there is not enough precipitation to match the evapotranspiration? (Drought or a dry season)

What can they do to save their crops? (They can irrigate, the way we sprinkle our lawns, or use mulch, which slows evaporation)

If you got direct sunshine on the pans, did this affect evapotranspiration? (There is more evaporation and transpiration on sunny days)

What else affects evapotranspiration rates? (air temperature--more heat-- and higher winds increase evapotranspiration)


Assess the quality of the students' chart and graphs, and have them write a description in their own words of the experiments. Ask for definitions of key concepts, including evaporation and transpiration.

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