Many specialty crop and market garden growers have small acreages in production and may not have full irrigation systems for their crops. These growers often rely on water tanks carted through the field, garden hoses with sprinklers or other watering methods to get plants through dry periods. However, my experience has been that most people greatly underestimate the amount of water it takes per week to adequately water a crop in a dry period.

Last week there was an article written by Rebecca Shortt, OMAFRA’s Water Quantity Engineer on estimating the water requirements for different crops based on canopy cover and crop type. Click here to review that article. Based on the article, at this time of year when plant canopies start to close, average crops require about 25 mm (1 in.) of water per week or slightly more. This includes crops with a small canopy, but grass or another cover crop between the rows that also compete with the crop for moisture (e.g. lavender with grass between rows and overhead irrigation). How much water does that equate to per hectare or per acre?

One square metre of ground to 25 mm depth is equal to 25 L of water. So 25 L of water (4 standard watering cans) is required for each square metre of crop every week. This equates to 250,000 L per hectare or roughly 100,000 L per acre.  The largest tanker trucks carry a volume of around 40,000 litres and most water supply companies have much smaller trucks than this maximum. Even with the largest trucks, it would still take multiple trucks per week to water one acre of a crop. If you are just hauling around a 500 L tank on the back of a truck or tractor, it would take 200 loads per week to water one acre of crop adequately. Put another way, it would take 17,000 standard watering cans to water an acre or 170 watering cans for each 1% of an acre (roughly 20 ft x 20 ft)!

Are you watering with a garden hose? At an average pressure with a 30 m (100 ft) hose,  a garden hose puts out about 15 L per minute. So to water an acre of crop with a garden hose you would need the hose running 110 hours or 4.6 days per week. That is also how long it would take to use that hose to fill up water tanks to haul out to the field. Is your hose longer than 30 m? If so, the output will be reduced the longer the hose gets.

If you are able to water right next to the plant, can you get away with much less? If you have a full canopy in your crop, then no, the plant still requires the same amount of water per week regardless of the proximity of the water to the plant. If however, you have large spaces between your rows (e.g. immature trees, lavender with large walkways between), then you can get away with less, but would still need enough water to feed the area covered by the bulk of the root zone and any cover crop between the rows will increase the water requirement of the crop.

So what is the bottom line? Unless you are willing to sacrifice crop productivity through dry periods to avoid the costs of an irrigation system, it is best to plan to put in an irrigation system capable of putting out the required volumes right from the beginning. There are simply not enough hours in a day to irrigate by hand or by tractor (it’s also not cost effective to pay for that amount of labour).  Bear in mind that some crops can survive with low water, but that doesn’t mean they will be productive in dry conditions. Growers have to make an economic decision on how much they can lose in productivity before the cost of an irrigation system becomes worthwhile.

Note that any person taking more than 50,000 L of water on any one day for irrigation of crops, must have a Permit To Take Water from the Ministry of the Environment and Climate Change.  Permits are required for all water sources including wells, streams, ponds on your own property, lakes, etc.  For more information contact MOECC at www.ontario.ca/page/permits-take-water?_ga=1.150045430.1941530580.1424959518