An atmometer is an instrument that measures evaporation and simulates transpiration by plants. It is an aid to irrigation scheduling and can also be useful for studying microclimates.

It simulates plant transpiration by drawing water from a reservoir by capillary action then allowing the water to evaporate from a surface that is exposed to the the same environmental conditions as the plant(s).  These environmental conditions include temperature, humidity, wind, and exposure to sunlight. A change in the level of water in the reservoir indicates how much evaporation has taken place.

I first read about atmometers while researching irrigation methods and I thought it would be very useful to have one. A search of the internet turned up a manufacturer but their least expensive model costs around $400.00.

The description provided by Wikipedia makes it sound like a simple device but I couldn’t figure out how to relate the readings to plant irrigation requirements. Then I came across a website by an Australian gentleman with lots of interesting information. I found a description of how to use such an instrument which he calls an “evaporation gauge”. There is also a picture of one on his site. From what I learned there, I was able to come up with a simple design using recycled materials.

Here’s what you’ll need:

• Plastic one liter bottle with cap (1)
• Unwanted CD or DVD (1)
• Absorbent fabric ( I used an old pair of blue jeans)
• Paper clips (3)
• Glue (I used GOOP brand contact adhesive)

Additional items you will find useful for taking readings:

• Rubber band
• Ruler or tape measure.

My only expense was for the glue (less than $5.00). I had everything else on hand.

How to make it:

1. Drill a 1/2inch (~15mm) hole in the center of the bottle cap

2. Glue the disk to the top of the cap aligning the center hole of the CD/DVD over the hole in the cap

3. Cut a circular piece of cloth to just cover the disk
4. Cut three narrow (~1/2 in/~15mm) strips of cloth about 2.5” to 3” (6.5cm to 7.5cm) longer than the height of the bottle

5. When the glue is dry,  screw the cap with the attached disk onto the bottle
6. Feed the three cloth strips through the hole in the bottle cap until they just reach the bottom of the bottle
7. Lay the exposed portions of the strips out flat on the disk and trim them to the edge of the disk
8. Arrange the cloth strips so that they are evenly distributed on the disk

9. Place the cloth circle on the disk and fasten it and the strips in place using the paper clips

10. Carefully unscrew the cap from the bottle and fill the bottle with water until the water is near the top of the straight side of the bottle. It is a good idea to moisten the cloth on the top also
11. Replace the cap on the bottle and you’re done

How to use it:

To track change in the water level in the bottle you can mark the starting level of the water by placing a rubber band around the bottle at that level. The rubber band can be adjusted when you refill the bottle or start a new measurement. When you are ready to take a reading, measure the distance from the rubber band to the new water level.

For irrigation scheduling

When planning a watering schedule for your garden there are two questions that need to be answered:

1. How often should I irrigate?
2. How much water should I apply?

An atmometer can help answer the question of how often to irrigate. Measuring the amount of evaporation that occurs from the time you irrigate until the plants just start to show signs of stress from lack of water gives you a basis to work from. It will be time to irrigate when the amount of evaporation since the last irrigation has reached 50% to 80%  of the stress level.

For studying microclimates

Make two or more Atmometers using the same size bottles. Since the DC/DVD disks are all the same size, we have a modicum of standardization. Place these atmometers together in an area so that they all have the same exposure for a day or two and observe any differences in their performance.

Now place each Atmometer in a different microclimate. You will be able to compare evaporation rates, and therefor growth conditions, in each microclimate.

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