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Vine water requirements depend upon several factors, such as climate, vineyard soil characteristics, as well as grapevine variety, root depth, and grapevine growth stages. In order to schedule irrigation at different intervals (daily, weekly, 10-days, etc.), crop evapotranspiration has to be determined for that interval.

Vineyard evapotranspiration (ETc) can be calculated on the bases of potential evapotranspiration (ETo) which are estimated from the local weather data information, and crop coefficient (Kc) – in this case, grapevine Kc which changes with canopy width throughout the season. Therefore, vine water requirements can be calculated with the following equation:

ETc = ETo x Kc

Where:
ETc – crop evapotranspiration (mm or inch/day)
ETo – reference crop evapotranspiration (mm or inch/day)
Kc – crop coefficient

Growers can get evapotranspiration data (ETo) from a local weather service (CIMIS in California) 

Crop coefficient (Kc), on the other side, is dependent on the seasonal growth stage and trellis design and can be determined by measuring the canopy shade width at noon. Seasonal grapevine crop coefficient (Kc) is calculated on the basis of the percent shared area (PSA) by one of the several methods, the easiest one is to measure the average width of the shaded area beneath the vine row, and slope factor found for grape vines. Note that during the season, as the canopy grows larger, the values of estimated canopy shade change.

Example of Crop coefficient calculation:

A calculation example of Kc value during the growing year on the bases of the average width of the canopy-shaded area.

Row width = 9 feet
Vine spacing = 6 feet
Area per vine = row width x vine spacing = 54 sq. ft.
The average width of measured shaded area between two vines = 3 ft
Shaded area per vine = vine spacing x average width of measured shaded are between two vines = 18 sq. ft.

PSA = 0,33 = 33%

Therefore, the grapevine crop coefficient is calculated as following:

Kc = PSA x 0,017 (acording to research results)
Kc = 0,561

After calculating the Kc values, vine’s water requirements can be calculated, as well as water requirement for the calculated interval. The example below shows how to calculate vine water requirements on a daily bases in gallons.

Example of vine water requirements calculation

For an ETo of 0.25 inch and Kc of 0,375 water consumption of a grape vine in the vineyard is:

ETc = 0.25 x 0.375 = 0.0938, therefore full crop water use in inches is 0,0938.

To calculate daily water application amounts per vines, inches has to be first converted into the gallons (1 inch covering 1 acre = 27,500 gallons) or in case of ETo readings in mm into liters (1mm covering 1ha = 10,000 liters).

Daily use per acre = 0.0938 x 27,500 gallons per acre = 2,579 gallons

After that, simply divide the required gallons (or liters) per unit land area by the number of vines per acre (or hectare) as shown below.

At 9 x 6 feet spacing: 806 vines/acre = 3.2 gallons/vine/day

Therefore, each vine needs 3,2 gallons of water per day to supplement the loss of water.

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