By Rebecca Shortt, Water Quantity Engineer, OMAFRA

This blog post will show the 2 steps to roughly calculate how long to run a drip system.

First calculate the plant water demand.  This was shown in my post from last week.

Plant Water Demand (U.S. gal)

= ET (mm) x Kc x Area occupied by plant (ft²) x conversion factor 0.0245

• ET from vineandtreefruitinnovations.com/ (free but you must register) or www.onpotatoes.com
• Kc (Crop Factors) from the tables below and/or consider extent of canopy
• Area occupied by plant is the row spacing x the plant spacing
  • If there is significant space without crop canopy between rows (e.g. apple trees which have been hedged) then this area could be reduced. Keep in mind that the Kc (crop coefficient) may be partially accounting for this already (e.g. strawberries have Kc of 0.75 partly because we assume bare straw between rows).  Also keep in mind that living cover crops between rows will be competing with the crop for water.

Second, divide the Gallons/Plant/Day by the number of emitters per plant x emitter flow rate.

Daily Run Time = Daily Plant Water Demand ÷ (# Emitters/plant x Flow Rate)

Hops example in Prince Edward County

Step 1
ET July 1-7 = 24mm/7days = 3.4 mm
Kc = 1.05
Area = 0.9m x 3.7m = 3.3 m²          (3’ x 12’ = 36 ft²)

Plant Water Demand = ET (mm) x Kc x Area (m²)

= 3.4 mm x 1.05 x 3.3 m²
= 11.8 Litres/Plant/Day

Step 2
# Emitters/plant = plant spacing ÷ emitter spacing = 0.9 m ÷ 0.45 m = 2
Flow rate of Emitter = 1.6 lph (litres per hour)

Daily Run Time = Daily Plant Water Demand ÷ (# Emitters/plant x Flow Rate)
= 11.8 Litres/Plant/Day ÷ (2 x 1.6 lph)
= 3.7 hours per Day

Note: in Norfolk county where the ET has been 4.4mm per day the run time would be a ratio 3.7hrs x (4.4mm/3.4mm) = 4.8 hours per Day

Ginseng (3yr garden) with 2 drip lines per bed, example in Simcoe
Step 1

ET July 1-7 = 31mm/7days = 4.4mm
Kc = 0.8
Area = emitter spacing x bed spacing = 0.3m x 0.9m = 0.54 m²           (= 1’ x 6’ = 6 ft² )

Plant Water Demand = ET (mm) x Kc x Area (m²)

= 4.4 mm x 0.8 x 0.54 m²
= 1.9 Litres/Area/Day

Step 2

# Emitters/Area = 2 (because there are 2 drip lines, there are 2 drippers for every 0.3m)
Flow rate of Emitter = 1.6 lph (litres per hour)
Daily Run Time = Daily Plant Water Demand ÷ (# Emitters/Area x Flow Rate)

= 1.9 Litres/Area/Day ÷ (2 x 1.6 lph)
= 0.6 hours per Day

Estimated crop factors for ginseng (Kc)

Kc Crop Factors for other specialty crops see previous post https://onspecialtycrops.ca/2016/06/24/irrigation-update-june-24-2016/#more-3718