By Rebecca Shortt, OMAFRA Water Quantity Engineer
The month of May and the first 2 weeks of June have been extremely dry (<40% of avg rainfall). The areas of Niagara and Peterborough are of particular concern because of lengthy dry conditions through the entire spring (since March 1st) and stretching back through 2015.
This is a critical period for most horticultural crops where the fruit or flesh development is occurring as well as rapid vegetative growth.
I know I need to irrigate; how much do I apply?
The websites www.vineandtreefruitinnovations.com/ (free but you must register) or www.onpotatoes.com provide evapotranspiration (ET) measurements on a daily basis for some of southern Ontario. Use the ET multiplied by a crop factor (Kc) to get the depth of water you need to apply. Remember that the crop factor reflects the “thirstiness” of the crop and the amount of canopy. Earlier in May the canopy was very small so the water use is much lower; we are now approaching canopy closure on most crops. Some of our soil moisture monitoring in fruit trees in Niagara has shown a big increase in water use since June 16. If you are not sure what crop factor to use, estimate the Kc by considering the % canopy coverage (full row closure is typically 100% or a Kc of 1.0).
Note that before full canopy/flowering/sizing the crop water demand may be lower but this doesn’t mean lack of water won’t affect yield/quality. Before full water demand, carefully calculate the crop needs and apply the small amounts regularly. For overhead irrigation the water demand should be applied once a week or, when the roots are small, split into 2 applications per week. For drip irrigation water should be applied daily or every other day (some deep rooted crops could be stretched to 2 times per week).
For overhead irrigation, use a rain gauge in the field to make sure you applied what you think you applied.
Crop Factors for Vegetables (Kc)
Crop Factors for Fruit Trees (Kc)
For specialty crops, there is less information available for Ontario growing conditions. The following table provides crop factors for some specialty crops from the FAO based on a sub-humid climate. This is provided only as a guide since it is unknown how these would compare for southern Ontario. Note that for perennial crops that have grass between the rows, the grass counts as part of the canopy, especially if you are doing overhead irrigation, since it will compete with the crop for moisture.
Crop Factors for Specialty Crops (Source FAO )
Potato example in Woodstock
ET June 15-21 = 30mm
Kc = 0.7
ET x Kc = 30mm x 0.7 = 21mm = 0.8 inches
Apple (mature with sod) example in Leamington
ET June 15-21 = 33mm
Kc = 0.8 (I’m using a higher Kc because I think we’re closer to full canopy than the chart value would suggest)
ET x Kc = 33mm x 0.8 = 26mm = 1.0 inches