Posts Tagged: ETo

In a recent citrus meeting in San Diego County we asked how many growers were using soil moisture monitoring equipment to help schedule their irrigations.  Only 3 people out of 40 indicated that they were using some sort of equipment to monitor soil moisture.  We didn’t ask how many were using California Irrigation Management Information System (CIMIS) to figure out how much water their trees were using on a daily basis, but we imagine that we might have had even a worse response.

 

Quite frankly, in a county where water is costing $700 to $1000 per acre foot, we though this practice would have been a common practice. Added to this is the increasing pressure to reduce nitrate leaching into creeks and ground water, where there is a serious problem developing.  The natural response when water prices are high is to reduce water use, but we have seen groves where even a 10% reduction in water reduces the yield by 50%, and we have also seen quite a few growers irrigating too much with the belief that a couple of extra feet of water per acre will more than pay the cost of water in increased yield.  Clearly we need to apply enough water to make the trees produce a profitable yield,  How does a farmer accomplish this?

 

I believe every grower should be using tensiometers or some other kind of soil moisture monitoring equipment to determine when to water, and using CIMIS to determine how much to water.  There, just simply, is no an easier, or a better method.

 

Some growers said that tensiometers don’t work.  Well, they work just fine if they are installed correctly and serviced periodically.  If the soil gets too dry (the reading goes above 80 cb) the device breaks suction from the soil, and they don’t work until they are removed, filled, pumped and re-installed.  As for gypsum blocks, they work just fine also, but are not very accurate under wet conditions.  Both work a lot better than just guessing. There are newer electronic devices that work very well if calibrated with the soil moisture, but they don’t work very well in rocky soil (rocks don’t hold water).

 

Using CIMIS

 

This assignment is to help you figure out the water use in your grove.  The following is a step by step procedure that is not difficult.  Several of our grove managers use this on a weekly basis to calculate the water requirement in each of their groves.  We have one grower who has this task assigned to his child in the third grade…Really, this is not that difficult!

 

This assignment will demonstrate how to use CIMIS to calculate the irrigation requirement for an avocado grove in Escondido.  ETo is called the reference evapotranspiration (defined as the water use for eight inch tall grass), and all crops in California are related to this water use by adjusting ETo with a “crop coefficient”.  In this example you will see that the crop coefficient for avocado in November is 0.55.  ETo data is gathered from the automated weather stations that are part of the CIMIS network in California. The irrigation calculator you will be using multiplies the ETo number by the crop coefficient and gives you Etc, the water use by the crop in question.  This comes from the station in “inches” of water loss,  and the calculator changes this into gallons per tree per day.  The calculator then tells you how much water to apply to the avocados to replace the water they used during the last seven days.

 

Go the website www.avocado.org

Click on California Industry (on the top right side of the page)

Click on Growers

Click on Water

Click on Irrigation Calculator

 

Start with Evapotranspiration (ETo). 

Click on Go To CIMIS

Use the drop down box and Click on San Diego

Click on Submit

Choose Escondido

Click on Daily Data

1. “Select a Time Period”, in this example we will select the previous week;  select November 15 through November 21
2. In “Select Variables”, leave everything selected with the green checkmark.
3. Leave “English Units” selected.
4. Click “Retrieve Data”

 

Write down ETo for the last week.  In this case it will be: 0.12, 0.11, 0.11, 0.10, 0.12, 0.12 and 0.10.

 

Add these up, and you get 0.78 (this is your ETo for the past week). Minimize this window.

 

You are now back to the Irrigation Calculator on the Avocado website.

1. Evapotranspiration, delete the 0.22 and fill in your 0.78
2. Under “Crop Coefficient”, just click on November in the drop down box.
3. Leave “Distribution Uniformity” at 0.85.
4. Leave trees at 109 per acre.
5. Leave sprinkler output at 17 gal/hr. (of course, you can change this to match your sprinkler output, but for the sake of this example, leave this at 17).
6. Click on Calculate.

 

You should get 138 gallons (this is the amount of water used by one tree in the last seven days) and a watering run time of 8 hrs and 8 minutes.

 

As I mentioned earlier, you should have tensiometers (soil moisture meters) set at the 8 inch depth (avocado) or 12 inch depth (citrus) to tell you “when” to water.  In avocados, I like to irrigate when the shallow tensiometer reads 20-25 cb, and in citrus when the tensiometer reads 35 – 40 cb.  You cannot rely on irrigating every seven days because the tensiometer may tell you the soil is getting dry by the fourth day.  This often happens in the summer.

 

To review, CIMIS tells you how much to water, the tensiometer tells you when to water. Now, in actual use, you may find that, in a windy area or on the south side of a slope, your trees may need more water.  Merely add a 10% increase to the run time, and keep making minor adjustments until you get this right for your grove.  Or, if you have root rot, you may want to water 10% to 30% less water.

 

By the way, if you are using this calculator for citrus, merely put 0.65 into the crop coefficient for each month, and you can use the same calculator.  Some people believe the crop coefficient in the avocado calculator might be too low.  Both Ben Faber and I believe the coefficient should be 0.80, but we don’t exactly have good data to support this…just experience.  At any rate, the calculator will put you in the ballpark…and it is a lot better than “guessing”.

 

Give this a try, and Good Luck!