Using ET Data to Make Irrigation Decisions

May 2023 has been dry in some parts of Southwestern Ontario.  The orange areas (on the map below) have received 40% to 60% of the average rainfall and the red areas, have less than 40%.  Delhi and Harrow have recorded less than 10 mm of effective precipitation in May (anything less than 5mm is not considered effective).  Vineland and Collingwood recorded about 20mm of effective precipitation in May.

Keep in mind the area from Guelph and Waterloo down to Brantford and Hamilton (see yellow circle on map below) had very low precipitation in 2022.  Some shallow aquifers in this area have not recovered from the dry conditions last year.

But what about plant water use? Isn’t it low in the spring?

The plant water use is related to the amount of canopy or leaf area.  So, in the early spring, when leaves are just emerging, the plant water use will be low.  Plant water use is also related to the type of plant; for example, grapes vines use less water than apple trees.  These factors can be represented by a crop factor (Kc) which ranges from 0.2 for bare soil to 1.0 for high-density fruit trees in full canopy.  See tables at the end of this article. In addition, water use is impacted by the age of the tree, density of the planting and rootstock.

The plant water use is also related to the temperature, relative humidity, wind speed, and solar radiation.  These weather parameters can be used to calculate the potential Evapotranspiration or ET for short.  May 2023 started cooler than average but has recently been warmer than average.  Daily ET data can be found at www.farmwest.com where it is calculated from Environment Canada weather station data.  There is also a panel showing the forecasted ET for the next 5 days.  The current ET forecast is for 6mm per day, which is high for Ontario.  ET values can be used across an entire county, unlike rainfall which should be measured at every field.

Delhi ET (mm) April 30 – May 29

TIP: check the box “Not cumulative in graphical display” to see the data like I’m showing above.

While it might seem like extra work to calculate the amount of irrigation required, during a busy growing season, there are significant benefits. Calculating irrigation needs can help growers increase the timely applications of their irrigation applications, resulting in improved benefits to the crop. Having a better understanding of irrigation requirements also can result in reduced water use and reduced costs (less fuel use for irrigation guns etc.)

For overhead irrigation a rough estimate of the irrigation required can be calculated as follows:

Depth of water to apply = Kc x ET ÷ Efficiency of traveling gun

Let’s work through an example with mature peach trees and permanent sod.  We’ll use ET from www.Farmwest.com at the Vineland station which has been a bit cool this past week.

May 23-29 ET = 26mm

But the forecasted ET is much higher at 5.3 mm or 37 mm/week

So, we’ll use the forecasted ET

Deciding on the Kc is tricky in the spring because we’re something less than a full canopy.  I choose Kc of 0.4 (half of the Kc for a full canopy – see tables at the end of this article).

The application efficiency of a traveling gun is 65% to 75% depending on the operating conditions.  I choose 0.65 for this example.

Kc x ET ÷ Efficiency = 0.4 x 37mm ÷ 0.65 = 23mm

Now we know that today we should apply 23mm or 0.9” of water to replace what was used by the peach trees this past week (May 23-29) and what we anticipate is coming this week.

For Drip irrigation, calculate the individual plant water use.  A rough estimate can be calculated by multiplying the crop coefficient Kc with ET and the area that the plant occupies.

Litres per Plant per Day = Kc x ET (mm) x A (m)

Let’s work through an example with mature high-density apple trees, with 3’x10’ spacing and permanent sod.   We’ll use ET from www.Farmwest.com at Collingwood.

May 30 ET = 6.1 mm

Deciding on the Kc is the trickiest part.  My charts indicate a Kc of 0.3 for May but I feel the canopy is becoming full (in July Kc for high-density apples would be 1.0).  I choose Kc 0.6

Convert the area (spacing) to metric 3’x10’ = 30 ft² ÷ 10.764 ft²/m² = 2.8 m²

ET = 6.1 mm

Kc = 0.6

Area = 2.8 m²

Kc x ET x A = 0.6 x 6.1mm x 2.8m² = 10 L

In this example,  we know that  we should apply 10L/tree to replace what was used on May 30^th.

Crop Factors for Fruit (Kc)

	Crop Coefficient	Approximate Spacing
Bare soil	0.2	N/A
Early season	0.4	N/A
Strawberries	0.75	1’ x 4’
Raspberries	0.70	2.5’x10’
Blueberries	0.80	5’x10’
Grapes	0.75	5’x12’
Tree Fruit High Density	1.0	3’x10’ to 5’x12’
Apples	0.9	7’x12’ to 20’x20’
Peaches	0.8	7’x12’ to 20’x20’
Pears	0.8	7’x12’ to 20’x20’
Cherries	0.9	7’x12’ to 20’x20’

Early Spring Crop Factors for Fruit Trees (Kc)

Month	Permanent sod with herbicide strip		Clean cultivation plus cover crop
	Non-bearing	Mature	Non-bearing	Mature
April	0.2	0.2	0.2	0.2
May	0.3	0.3	0.3	0.3
June 1-15	0.3	0.4	0.3	0.4
June 16-30	0.5	0.6	0.4	0.5
July	0.6	1.0	0.5	0.65
August	0.6	1.0	0.5	0.65
September	0.5	0.95	0.3	0.5

How long should I run my drip system?

In this example we’ll use the high-density apples from the example above (3’x10’), a drip system with 2 ft emitter spacing and an emitter flow rate of 0.42 gal/hr (your emitter flow rate can be found on the packaging of your drip line)

First convert to metric 0.42 gal/hr x 3.785 L/gal = 1.6 L/hr

Next, calculate how many emitters are there per tree?

Tree spacing ÷ Emitter spacing = 3ft ÷ 2ft = 1.5 emitters per tree

Run time = Volume per tree ÷ (# emitters per tree x emitter flow rate) = 10L ÷ (1.5 x 1.6 L/hr) = 4.2 hrs

Now we know that to apply 10L of water to each apple tree in this example, we need to run the drip system 4.2 hrs today.

How long should I run my drip system on cooler, cloudier days?

For example, May 24^th was much cooler with an ET of 2.8mm.  The run time doesn’t have to be recalculated.  Just use a ratio of the ET values against the ratio of the Run Times.

Run Time (cool day)   =   Run Time (today)

      ET (cool day)                      ET (today)

Run Time (future cool day) = ET (future cool day) x Run Time (today) ÷ ET (today)

= 2.8mm x 4.2hrs ÷ 6.1mm = 1.9 hrs

Taking some time to run through the calculations can give you a much better picture of the water demand of your crop and HOW LONG TO RUN YOUR DRIP LINES!

And when it eventually rains…………………….

• Check rain gauge in the field to know depth of rainfall
• Use soil moisture instruments to see if the rain infiltrated
• Keep watching ET data from www.farmwest.com to calculate irrigation required