Irrigated tomato field

How long do I run my drip lines?

Rebecca Shortt, Engineer – Water Quantity, OMAFRA – Simcoe

Irrigated tomato fieldThis 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 (free but you must register) or
  • 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 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).

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)

Tomato example in Leamington

ET June 23-29 = 31mm/7days = 4.4mm

Kc = 1.0

(I’m using a higher Kc because I think we’re closer to full canopy than the chart value would suggest)

Area = ¾’ x 5’ = 3.75 ft2

Plant Water Demand = ET (mm) x Kc x Area (ft2) x conversion factor 0.0245

= 4.4 mm x 1.0 x 3.75 ft2 x 0.0245

= 0.4 Gallons/Plant/Day

Step 2

# Emitters/plant = plant spacing ÷ emitter spacing = ¾’ ft ÷ 1 ft = ¾

Flow rate of Emitter = 0.13 gph (gallons per hour)

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

= 0.4 Gallons/Plant/Day ÷ (¾ x 0.13 gph)

= 4.1 hours per Day

Crop Factors for Vegetables (Kc)

Bare soil 0.2
Cabbage, Cauliflower 0.4 From seedling or transplanting to start of heading
0.7 From start of heading to full row fill
1.0 Remainder of crop
Sweet corn 0.4 From seedling to 1st showing of tassel in whorl
0.7 1st tassel to silking
1.0 Remainder of crop
Potatoes, Tomatoes, Peppers 0.4 From seeding or transplanting to 1st flower
0.7 From 1st flower to maximum row fill (tomatoes) or tuber sizing (potaotes)
1.0 Remainder of crop


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