Irrigation Scheduling Guidelines
Research Progress Report
Edward Hellman and Steven Shelby
Texas A&M University and Texas Tech University
Successful commercial production of winegrapes in Texas requires supplemental water application through irrigation. Water applied at the proper time and quantity can influence grape yields and fruit quality. In addition, water can be a scarce resource in many areas and its efficient use must be a high priority. Thus, methods for scheduling irrigation are an important aspect of good vineyard management. Irrigation scheduling is concerned with determining when to irrigate and how much to apply. When to irrigate can be determined by monitoring soil moisture and starting irrigation when soil moisture has been depleted to a predetermined amount. Alternatively, grapevine water status can be monitored to determine when the vine is beginning to experience a water deficit (about -10 bars). Grapevine water status is determined with the use of a pressure chamber (pressure bomb), a simple but somewhat expensive instrument that may only be appropriate for large vineyards.
One good method for determining how much water to apply is through the use of Reference Evapotranspiration (ETo), also known as Potential Evapotranspiration (PET). Evapotranspiration is the sum of water loss from evaporation from the soil surface and a plant’s loss of water through transpiration (water vapor moving out of leaf stomata). Evapotranspiration is estimated for an area with the use of local weather data and a reference crop - a standard plot of irrigated grass. Daily ETo rates of the reference crop are adjusted for grapevines by multiplying by a crop coefficient (Kc) for grapevines. A simple calculation provides an estimate of grapevine water use as a guideline for irrigation scheduling.
Estimated daily water use = (daily ETo) X (Kc)
A local weather station is consulted for the daily ETo rates and these can be summed over a one-week period or other duration that is appropriate for the vineyard’s irrigation schedule.
Estimated water use (one week) = (7-day sum of daily ETo) X (Kc)
Grapevine crop coefficients (Kc) are a function of the size of the grape canopy and how much of it is exposed to direct sunlight. The relationship of canopy size and sunlight exposure to grapevine water consumption has been extensively studied by Dr. Larry Williams in California. His research has shown that grapevine crop coefficients can be readily estimated for a vineyard by estimating the percentage of the vineyard area that is shaded. The percent shaded area (PSA) must be estimated during the solar noon hour (between 12:30 and 1:30 pm,) by one of several methods: 1.) measure the average width of shaded area (Figure 1) beneath the vine row, 2.) estimate the canopy shaded area from a 4X4 board with 6 in. gridlines (Figure 2) placed in the shade beneath the vine row, or 3.) determine the canopy shaded area from pixel analysis of a digital photograph of a plain white 4X4 board placed in the shade beneath the vine row.
Row and vine spacing must be known to determine the PSA. The example below shows the simple calculations used to estimate PSA by measuring the average width of the canopy shaded area (method 1).
Example
A = Row Width = 10 feet
B = Vine Spacing within row = 6 feet
C = Area per vine = A X B = 10 X 6 = 60 sq. ft.
D = Average width of measured shaded area between two vines = 3 ft.
E = Shaded area per vine = B X D
E = 6 X 3 = 18 sq. ft.
PSA = E/C
PSA = 18/60
PSA = 0.30
The grapevine crop coefficient is calculated with the following equation.
Kc = PSA X 0.017
Kc = 0.30 X 0.017 = 0.51
Continuing the example and assuming a one-week cumulative reference ETo value of 1.69 inches, use the equation below to calculate the estimated water use for the previous week:
Estimated water use (one week) = (7-day sum of daily ETo) X (Kc)
= (1.69 inches) X (0.51)
= 0.862 inches
Thus, to replace all of the water lost through evapotranspiration in the previous week, irrigation should be applied to deliver 0.862 inches of water. Be aware that vineyard managers may want to use water deficits to influence grapevine growth and perhaps influence fruit quality. The term deficit irrigation is used to describe irrigation strategies that replace less than 100% of the water lost through evapotranspiration. Deficit irrigation should be practiced with care, and only after considerable experience with regular irrigation strategies. The stage of vine and fruit development, and the extent of water deficit both influence whether the deficit is beneficial or detrimental to grapevines and fruit. Typically, deficit irrigation is imposed between fruit set and veraison. Irrigation amounts may range from 50-75% replacement of full water use. Again, caution is advised when first employing deficit irrigation strategies.
Grapevine crop coefficients were experimentally determined for three grape varieties in two locations in 2002. Method 1 (average width of canopy shade) was used to estimate PSA for the varieties Favorite and Cynthiana grown in the Gulf Coast region of Texas. Method 3 (digital photo analysis) was used to estimate PSA for Cabernet Sauvignon in the Texas South Plains. Tables 1 and 2 show the crop coefficients estimated from PSA calculations at the two locations. Note that crop coefficients increase during the season in response to increased canopy size. Crop coefficients in the early stages of shoot growth are small, corresponding to the lower water consumption rates of the smaller canopy.
It must be reemphasized that crop water use is a function of canopy size and sunlight exposure, so it can vary considerably from vineyard to vineyard. Therefore, the experimental crop coefficients published here should not be broadly used to schedule irrigations in other vineyards, even if the varieties are the same. Texas grape growers are encouraged to determine their own crop coefficients by estimating the percent shaded area of their grapevine canopy periodically through the season, and using the simple calculation described above. Probably the easiest method will be to measure the average width of the canopy shade (method 1 above) during the solar noon hour. An online worksheet to facilitate calculation of crop coefficients is under development and will be published on the Texas Winegrape Network website. The online calculator will also include links to weather stations that post daily Reference Evapotranspiration data.
Figure 1. Measuring the average width of canopy shade.
Figure 2. Estimating canopy shade with the use of a 4X4 board with 6 in. gridlines.
Table 1. Grapevine Crop Coefficients
South Plains Region, 2002
Cabernet Sauvignon
| Date | Crop Coefficient |
| April 1-15 | 0.0 |
| April 16-30 | 0.1 |
| May 1-15 | 0.28 |
| May 16-31 | 0.40 |
| June 1-June 15 | 0.46 |
| June 16-30 | 0.50 |
| July 1 -July 15 | 0.51 |
| July 16 - July 31 | 0.51 |
| Aug. 1- Aug.15 | 0.51 |
| Aug. 15 - Aug. 31 | 0.50 |
| Sept. 1 - Sept. 14 | 0.48 |
| Sept. 15 - Sept. 30 | 0.42 |
| Oct. 1 - Oct. 15 | 0.38 |
| Oct. 16 - Oct. 31 | 0.28 |
Table 2. Grapevine Crop Coefficients
Gulf Coast Region, 2002
| Date | Favorite | Cynthiana |
| June 1-June 15 | 0.49 | 0.37 |
| June 16-30 | 0.49 | 0.38 |
| July 1 -July 15 | 0.49 | 0.39 |
| July 16 - July 31 | 0.50 | 0.39 |
| Aug. 1- Aug.15 | 0.51 | 0.44 |
| Aug. 15 - Aug. 31 | 0.52 | 0.44 |
| Sept. 1 - Sept. 14 | 0.52 | 0.44 |
| Sept. 15 - Sept. 30 | 0.52 | 0.44 |
| Oct. 1 - Oct. 15 | 0.51 | 0.43 |
| Oct. 16 - Oct. 31 | 0.50 | 0.43 |
