Build a low cost evaporation pan for irrigation scheduling






One of the more popular methods used for irrigation scheduling is estimating and summing the evapotranspiration (ET) of the crop. ET is the water that the crop uses. You simply add up the daily ETs, then apply that amount of water to the soil. This was a complicated process because so many variables, wind, temperature, humidity, canopy cover, etc., contribute to the ET of the crop. However, researchers have simplified this for us by publishing charts that estimate the ET rate from just pan evaporation data,

Use these charts like this. Measure the amount of water that evaporates from a pan of water each day. Lookup your crop in the left column of the chart, then look to the right until you find it's present growth stage, and read the ET coefficient. Here's how you use the coefficient. Say, for example, you're growing potatoes, and it's mid season. Lookup potatoes on the chart, then move to the right until you're under mid season. The number in that box is 1.15. That's the coefficient you're looking for. If the pan evaporation was 0.25 inch, you multiply this by the coefficient ,1.15, and the ET, or water the crop used, was 0.29 inches for that day. You'll find one of these chart below.

US Weather Service evaporation pans are expensive, costing well over $1000. Much too expensive for an individual grower to economically justify. Unfortunately, for best results you want the pan as close to your field as possible. So what can you do? Fortunately, the literature has many models of home built evaporation pans that are inexpensive and work great. We tested one in the Delta Junction, Alaska area in 2004. We modified the original design of our pan to make it easier to read the water level. Instead of a dip stick we used a level tube that made reading as easy as just looking. We didn't have to worry about ripples messing up our reading either.

The total cost of materials for our pan was less than $5, if you have to buy everything. You should be able to find a lot of the materials around the shop or home.

Bill of Materials

We used an automobile oil drain pan that is galvanized. It's just about the right size for field use.

Building the evaporation pan

Drill a hole just above the bottom of the pan with a diameter that is snug with the copper tubing. Drill another hole the size of the OD of the plastic tubing in the lip on top of the pan just above the lower hole

Bend the copper tubing at a 90 angle. If you don't have a tubing bender click here for a method of making the bend without kinking the tube. Solder, or glue with an appropriate epoxy, the tubing in the lower hole with one leg of the bend pointing to the upper hole. Keep the upright leg close to the pan, but with enough space for the clear tubing and clamp, see figure 1.



Figure 1 Copper tubing at bottom of pan.







Cut a piece of plastic tubing to fit from the just over the top of the pan to about a half inch over the copper tube at the bottom, see figure 2.


Figure 2 Top of clear tube protruding over top of pan.







Cut the measuring tape from the start of an even foot, or inch, on the tape so it fits along the side of the pan. Glue it to the side of the pan as close to the plastic tubing as possible. We used 5 minute epoxy for this because it sets up fast and works well.

To assemble the evaporation pan, place a hose clamp on the copper tubing and let to hang on the bend. Put the plastic tubing through the top hole, to just about to the copper tubing. Place a hose clamp on the plastic tubing and position it as far up as you can, to just below the lip. Push the plastic tubing over the copper tubing and spread the lower hose clamp and position in to seal the connection.

That's all there is to making the evaporation pan. Figure 3 will give you a picture of what you should have.


Figure 3 Completed evaporation pan showing 5mm, 0.2 inch evaporated.



Using the evaporation pan

For the evaporation pan to work right it should be level and placed on a light colored board, usually white. Take some wood shims with you when setting it up. Place it no less than 3 inches off the ground. Make sure it's not covered with foliage, the pan area should be completely exposed. After it's level fill it with water.

Don't fill it more than inch from the top of the pan. If it's windy keep the water level low enough to keep it from blowing out. Maybe to inch from the top.

When you have it filled move the top hose clamp to right at the water level in the tube. Move one ear of the hose clamp over the ruler as shown in figure 3. This indicates the start of the readings. You'll only need to move this clamp when you add water to the pan.

You'll have to protect the pan from animals that may upset it, or drink from it. We placed some post and chicken wire around it to protect it from moose. If you are using it in a garden area, that is already fenced, covering it with a piece of hardware cloth will protect it from pets.

Useful Information

Here are some of the pan coefficients for different crops, complied by the British Colombia Ministry of Agriculture. You can get the full list for many crops, and many other irrigation publications there.



Crop

Early in season

Mid season

Late season

Beans, green

0.5

1.05

0.90

Broccoli

0.7

1.05

0.95

Cabbage

0.7

1.05

0.95

Carrots

0.7

1.05

0.95

Cauliflower

0.7

1.05

0.95

Grains

0.3

1.15

0.25

Grass hay

0.4

1.00

0.85

Peas

0.5

1.15

1.10

Potatoes

0.5

1.15

0.75


Back to text

Example

Using this list is easy. Say you're growing potatoes and it's mid season. If you measure the pan evaporation for a few days and you get a 0.7 inch water loss. Multiply 0.7 inch by 1.15 from the above table. You get 0.7 x 1.15 = 0.805. You would have to apply about 0.8 inches of water to replace what the potato crop used from the soil.

More information

Feel free to visit our web site or come in to the Salcha-Delta Soil and Water Conservation District office to get more information on this and other irrigation subjects. For soil moisture measuring equipment and logging see AEC Systems .



Last changed: 02/20/2005, 12:24:03 Copyright0 2005 by Chuck Mancuso for AEC Systems