Using ET Data to Make Irrigation Decisions

By Rebecca Shortt, Engineer, Water Quantity, OMAFRA

May 2021 has been very dry!  For most of South-Western Ontario we’ve received less than 40% of average rainfall.  Delhi and Vineland have recorded no effective precipitation in May (anything less than 5mm is not considered effective).

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

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 2021 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.  ET values can be used across an entire county, unlike rainfall which should be measured at every field.

Delhi ET (mm) May 1-26

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

For overhead irrigation a rough estimate of the irrigation required can be calculate 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 Delhi station for this past week.

May 19 – 25 ET = 35mm

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 35mm ÷ 0.65 = 22mm

Now we know that today we should apply 22mm or 0.8” of water to replace what was used by the peach trees this past week (May 19 – 25).

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 the Delhi station.

May 25 ET = 4.6mm

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 = 4.6mm

Kc = 0.6

Area = 2.8 m²

Kc x ET x A = 0.6 x 4.6mm x 2.8m² = 8 L

Now we know that today we should apply 8L/tree to replace what was used yesterday (May 25^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’

Modified from the B.C. Trickle Irrigation Manual by Ted Van der Gulik

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

From OMAFRA Irrigation Management BMP book

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) = 8L ÷ (1.5 x 1.6 L/hr) = 3.3 hrs

Now we know that to apply 8L of water to each apple tree in this example, we need to run the drip system 3.3 hrs today.

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

Friday (May 28th) is forecast to be much cooler with an ET of 2.2mm.  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 (future cool day)   =   Run Time (today)

      ET (future cool day)                       ET (today)

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

= 2.2mm x 3.3hrs ÷ 4.6mm = 1.6 hrs

BONUS Berries example:

For berries or veggies the calcuation is the same, just the spacing will be smaller and the Kc will be different.

Let’s try strawberries with 1’ x 4’ spacing with full canopy.  This example is in Norfolk so we’ll use ET from www.Farmwest.com at the Delhi station.

May 25 ET = 4.6mm

Because I’ve already got a full canopy, I’m going to use the full strawberry Kc of 0.75 (if the strawberries were not fully in leaf, then I’d reduce my Kc)

Convert the area (spacing) to metric 1’x4’ = 4 ft² ÷ 10.764 ft²/m² = 0.37 m²

ET = 4.6mm

Kc = 0.75

Area = 0.37 m²

Kc x ET x A = 0.6 x 4.6mm x 2.8m² = 1.3 L

Now we know that today we should apply 1.3L/plant to replace what was used yesterday (May 25^th).

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!

As I write this it’s pouring rain here in Waterford………..

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