by Rebecca Shortt, Engineer, Water Quantity, OMAFRA , 30 June 2016
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)
Strawberry example in Woodstock
Step 1
ET June 23-29 = 30mm/7days = 4.3 mm
Kc = 0.75
Area = 1’ x 4’ = 4 ft2
Plant Water Demand = ET (mm) x Kc x Area (ft2) x conversion factor 0.0245
= 4.3 mm x 0.75 x 4 ft2 x 0.0245
= 0.32 Gallons/Plant/Day
Step 2
# Emitters/plant = plant spacing ÷ emitter spacing = 1 ft ÷ 1ft = 1
Flow rate of Emitter = 0.42 gph (gallons per hour)
Daily Run Time = Daily Plant Water Demand ÷ (# Emitters/plant x Flow Rate)
= 0.32 Gallons/Plant/Day ÷ (1 x 0.42 gph)
= 0.76 hours per Day
Apple 3’ x 12’ (mature with sod) example in Leamington
Step 1
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 = 3’ x 12’ = 36 ft2
Plant Water Demand = ET (mm) x Kc x Area (ft2) x conversion factor 0.0245
= 4.4 mm x 1.0 x 36 ft2 x 0.0245
= 3.9 Gallons/Plant/Day
Step 2
# Emitters/plant = plant spacing ÷ emitter spacing = 3 ft ÷ 2 ft = 1.5
Flow rate of Emitter = 0.63 gph (gallons per hour)
Daily Run Time = Daily Plant Water Demand ÷ (# Emitters/plant x Flow Rate)
= 3.9 Gallons/Plant/Day ÷ (1.5 x 0.63 gph)
= 4.1 hours per Day
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 |