In many wine growing regions irrigation of vines is either necessary in order for the vines to thrive, or to fill-in the gap due to lack of water during the intensive vine growth period in the summer months. As water is a limited resource, many grape growers are forced to minimize water use while at the same time provide enough water in order for the vines to not experience heavy drought stress. Therefore, good water management is crucial. For precise irrigation management, accurate field data is necessary, which winegrowers can get from on-site weather stations, vineyard soil moisture sensors, pressure chambers, etc. Since not all vineyard managers can afford costly instruments, they can use data of Reference Evapotranspiration (ETo) in order to determine how much water to apply every day.

Evapotranspiration is the processes of water loss from the soil surface (evaporation) and from a plant (transpiration). The water is evaporating due to climatic circumstances, so the key influental parameters besides the plant parameters are the climate and environment.

When to start irrigating vineyard?

There are several ways to determine the start of irrigation timing. One of the options is by monitoring soil moisture at different levels of grapevine root zones, and starting to irrigate when the soil moisture level value, which indicates that vines need water, is reached. The value differs depending on the tools that are used for monitoring, type of the soil, as well as soil characteristics, cover crop, irrigation technique (RDI, full irrigation, etc.) and winegrowers’ goal for grape quality. There are several different types of soil moisture sensors available on the market. One of the most cost-effective and simple to use sensors are Vinduino plug-and-play sensors in combination with eVineyard irrigation control, which can measure soil moisture on 4 different depths and work plug-and-play with eVineyard.

Another option is to measure vine water status – at about 10 bars, vines begin to experience water deficit. Grapevine water status can be monitored with a pressure chamber, a simple yet relatively expensive instrument. You can read more about on “how to use a pressure chamber” here.

How much water to apply to each vine?

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, calculate it using multiple methods, or even measure it with instruments. eVineyard provides a global evapotranspiration data for any vineyard on the planet, based on satellite sensing which you can test it out for free for 15 days here.

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.

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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 = Shaded area per vine/are per vine

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.

eVineyard LiteExample 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 through the process of evapotranspiration.

Vineyard irrigation based on the efficiency of the irrigation system

The amount of water that needs to be added to the grapevine is affected also by the irrigation system used in the vineyard. Irrigation system efficiency can be estimated and differ on the type of irrigation system used in the vineyard.

Irrigation system Estimated efficiency (in %) Irrigation system efficiency coefficient
 Drip irrigation  85-95%  0,85 to 0,95
 Sprinkler irrigation  60-75% 0,60 to 0,75
 Furrow irrigation  40-50%  0,40 to 0,50

 

Vines water requirements in relation to irrigation system efficiency can be calculated as followed:

ETc = (ETo x Kc) / irrigation system efficiency

Calculation example – from the above, this time with the irrigation system efficiency:

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

ETc = 0.25 x 0.375 = 0.0938, which is the same as above.

Now let’s say we have a drip irrigation system installed in the vineyard with 90% efficiency, then ETc is 0,104. And therefore, daily use per acre = 0.104 x 27,500 gallons per acre = 2,860 gallons.

At 9 x 6 feet spacing: 806 vines/acre = 3.5 gallons/vine/day

Schedule vineyard irrigation

Since growers often do not irrigate on a daily bases, but rather on 3-days, weekly or 10-days intervals, ETc can be calculated also for those intervals. In this case, water use is estimated for a given time period, thus simply cumulate (sum) daily ETo for the given time period.

Calculating the amount of water applied is complex and time consuming, we recommend using the eVineyard LITE package which does everything completely automatic: from collecting the ET, to calculating everything all the way to daily irrigation time for your vineyard, based on your irrigation system properties and vineyard properties. While with the use of Vinduino sensor nodes and eVineyard irrigation control, the whole “when and how much water to apply” question is the past, just as well as “did we turn irrigation on or off”. Let eVineyard alert you on vine water requirements of your vineyard, and allow you to control your valves with your smartphone.

 

Sources:
Ed Hellman. Irrigation Scheduling of Grapevines with Evapotranspiration Data. Viticulture Extension Specialist.
Larry E. Williams. 2001. Irrigation of winegrapes in California. Practical Winery&Vineyard Journal. (online)
Evapotranspiration and grapevine water use. Washington State University. (online)