In the first part of this series I showed the benefits of deficit irrigation on grape yields, and quality. So for grape growers that have access to water, and are interested irrigating, where do they get started?

While it may be tempting to hook up the irrigation system and irrigate until there are a few inches of water on the soil it may not be the best option for the crop or for your pocket book. As mentioned in the first part of this series, over irrigating grapes can result in decreased wine quality and excessive vigour. So if possible it is best to monitor weather conditions and irrigate according to the crops needs. One method of adjusting irrigation application is regulated deficit irrigation (RDI) which involves withholding irrigation water during some periods of development and providing irrigation to maintain soil water level above the physiological wilting point at other times. RDI early in the season will reduce shoot growth and possibly reduce water demand later on due to a reduction in leaf area. RDI can be used most effectively when it is timed appropriately with the growth stages of the grape to optimize fruit quality while still saving water. Water is very important for cell division in grape berries right after flowering and fruit set. The vine is also simultaneously in a period of vegetative growth.

The most critical period for irrigation, therefore, is from berry set through veraison (the initiation of the ripening period indicated by berry softening and colour change). Deficit irrigation before veraison might decrease yield very slightly due to reduced cell number in berries, and will definitely reduce vegetative growth. RDI after veraison might also decrease yield slightly due to reduced berry size, but normally improves quality as a result of concentration.

So how do you determine how much water the grapes need?

The amount of moisture the grapes need will vary each year from site to site depending on the weather conditions (temperature, rainfall, snow, percolation in soil), and vine canopy. Crop water use can be calculated based on a measurement of evapotranspiration (ET) from a weather station. The crop's water use can be determined by multiplying the water loss through evaporation and transpiration from climatic conditions (reference ETo calculated from a weather station) by a crop coefficient (Kc) which is specific to the crop, and accounts for variation in vine canopy size.

  • ETc= ETo X Kc
  • ETc=crop evapotranspiration or the amount of water the grapes will use
  • ETo=reference ET which is the water loss through evaporation and transpiration based on climatic conditions (temperature, solar radiation, etc.). ETo assumes the crop cover is grass.
  • Kc= crop co-efficient

Start by determining the reference evapotranspiration (ETo), which is the water loss through evaporation and transpiration of a grassed field. ETo is affected by weather conditions and expresses the evaporating power of the atmosphere at a specific location and time of the year and does not consider the crop characteristics and soil factors. ETo is expressed in terms of mm over a specific time period (usually per day). ETo is at its maximum in midsummer where daylength (sunshine hours) is at is maximum, relative humidity is low and temperatures are highest. The ETo is calculated by inputting climatic data (e.g. mean temperature, relative humidity, etc.) into a weather based models (Priestly-Taylor equation, Penman-Monteith equation). Average ETo values for Ontario can be found in the OMAFRA Irrigation BMP on page 45. Alternatively, Weather Innovations Network Inc. (WIN) provides weekly maps of ETo based on the Priestly-Taylor equation. As is always the case the most accurate method of determining ETo is to use site specific or region-specific information.

So, for sake of argument, let's say you visited the WIN website one day and found that the peak ETo value for your region is 0.26 inches (6.6 mm) per day. We'll follow this value through our calculations and come up with how much water to apply to a vineyard.

Using a crop coefficient: Once you have determined the ETo you will need to calculate the crop co-efficient (Kc), which accounts for amount of soil covered by the crop. In the case of grapes, they are in rows that are ~2.5 m (8 feet) apart, and consequently much of the surface of the soil is not occupied by canopy. We need to account for the fact that grapes are trellised and may occupy up to 1.8 to 2 m (~6 feet) of trellis space, which if viewed from above would cover ~75 percent of the soil surface. This is the basis for use of a crop coefficient (Kc). If irrigation is being applied to a vineyard containing a full trellis, a Kc of 0.75 to 0.8 is usually appropriate. If irrigation is applied during canopy development, the volume of canopy would be less and must be estimated as accurately as possible. One method of doing so is to measure the length of the shadow cast by the vine canopy and divide this by row width (distance between rows).

So if the length of shadow cast by the vine canopy at solar noon was 1.88 and the row width was 2.5 m, the Kc would be as follows:

Kc=1.88 m / 2.5 m
= 0.75

This is the value that we use to calculate replacement water volume. If you use our original ETo value of 6.6 mm and a 0.75 Kc, then our ETc would be 6.6 X 0.75 or 4.95 mm of replacement water daily (0.19").

So in this case the ETc would be as follows:

ETc=ETo X Kc
Etc=6.6 mm X .75
=4.95 mm of replacement water daily (0.19").
= 0.0162 ft/day

So how much water (volume) do I send out to my vines?

Multiply the depth of water needed per day (0.0162 ft/day) by the plant area (4 ft x 8 ft = 32 ft2 for vinifera wine grapes).

Vd = ETc x Plant Area = 0.0162 ft/day x 32 ft2 = 0.5184 ft3
Vd = 3.9 US gal (1 ft3 = 7.481 US gal)

So on a hot summer day in Ontario water use in this example is 3.9 US gal/vine/day. That's 27.3 US gal/vine/week

The final step involves conversion of volume to time in hours for a drip irrigation system. In this example the irrigation system delivers 0.53 GPH (gallons per hour) from drippers spaced 2 ft apart. Since the vines are spaced 4 ft apart this means there are 2 drippers per vine (0.53GPH x 2) = 1.06 GPH/vine.

Each week the system needs to run 27.3 US gal/vine ÷ 1.06 GPH/vine = 25.8 hours

This irrigation system run time would be reduced if there was rain during the week.

With a drip system this should be applied in 2 to 3 applications per week.

So 8.6 hrs, three times per week.

Voila! You have just calculated how much to irrigate!