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
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
*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:
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
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
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.