# The Evapotranspiration Method for Irrigation Scheduling

Grapes May 01, 2012

Ed Hellman, Texas AgriLife Extension

One of the more challenging tasks of vineyard management is determining how much water to apply for irrigation. The Evapotranspiration (ET) Method is an excellent way to determine how much water to apply based on estimates of the amount of water lost from the vineyard. 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). Reference Evapotranspiration (ETo) is estimated for an area using 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). The grapevine crop coefficient changes during the growing season to reflect changes in the size of the canopy; large canopies use more water than small canopies so Kc increases as the vine grows.

## Water Use Estimate Formula

Water Use = (ETo X Kc) divided by the irrigation system efficiency

Consult a local agricultural weather station to obtain daily ETo rates. Daily ETo can be summed for a time period matching your irrigation schedule, or for the period since the last irrigation or significant rainfall. For the example calculation below, we will use daily ETo for the previous seven days.

Crop coefficients are a function of the size of the grapevine 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 Kc can be readily estimated for a vineyard by estimating the percentage of the vineyard area that is shaded during the solar noon hour. Table 1 provides Kc estimates from Williams (2001) for a vertical shoot-positioned (VSP) trellis with 8-foot row spacing. We assume a canopy approaching full size with Kc = 0.51 for the example calculation below.

Table 1. Example crop coefficients for grapevines with VSP training and 8 ft row spacing. (Source: Williams, 2001.)
Degree-Days (F) Kc (8 ft rows)
180 0.13
360 0.17
540 0.21
720 0.26
900 0.31
1080 0.37
1260 0.42
1440 0.46
1620 0.51
1800 0.54
1980 0.57
2160 0.59
2340 0.61
2520 0.62
2700 0.62

Irrigation system efficiency is estimated for the type of system used. Drip irrigation is the most efficient and values range from 0.85 to 0.95 (i.e., 85 percent to 95 percent efficient). Use a value between 0.60 and 0.75 for sprinkler irrigation and between 0.40 and 0.50 for furrow irrigation. For the example calculation below, we assume a drip irrigation efficiency of 90 percent.

## Example Calculation of Water Use

Assumptions
Cumulative 7-day ETo = 1.69 inches
Grape Crop coefficient Kc = 0.51
Drip irrigation system efficiency is 90 percent = 0.90

Calculation
Water Use = (1.69 X 0.51) / 0.90
Water Use = 0.96-inch

Our ET estimate has determined that 0.96 inch of water has been lost from the vineyard in the past seven days, but that doesn’t mean we necessarily want to replace all of the water lost. Maintaining abundant water availability to grapevines encourages continued vine growth, which is difficult to control, creates a shaded canopy, makes disease management more difficult, and reduces fruit quality. Vineyard managers often chose to limit water availability to mature grapevines as a tool to manage growth. Williams found that replacing only 75 percent of water used slowed growth of winegrapes with only a minor reduction in yield. Further limiting of water availability to grapevines is described as “deficit irrigation,” an advanced irrigation strategy used to manipulate vine growth and perhaps influence fruit quality. Vineyard managers are advised to consult the literature for a more complete discussion of deficit irrigation and consider experimentation on a small plot before utilizing deficit irrigation strategies in their vineyard.

## Example Calculation of How Much Water to Apply

For our example, we will assume we want to replace 85 percent of the water used.
0.96 inch X 0.85 = 0.82 inch of irrigation needed to replace 85 percent of the water used in the past seven days.
Inches must be converted to gallons for determining how long to run the irrigation system. This calculation uses the acre-inch conversion factor and the square footage occupied by a single vine in your vineyard. We will assume a vine X row spacing of 4 feet X 8 feet for this example, so the area occupied by a vine is 32 square feet.

Constants and Assumptions
Desired irrigation rate = 0.82 inch
1 acre-inch = 27,152 gallons
1 acre = 43,560 square feet
Vine spacing = 4 feet, Row spacing = 8 feet
Area per vine = (4 feet X 8 feet) = 32 square feet

Calculation
0.82 inch X 27,152 gal/acre-inch = 22,265 gal/acre
22,265 gal/acre X (32 square feet / 43,560 square feet)
22,265 gal/acre X 0.000735 acre/vine = 16.36 gal per vine to replace 85 percent of the water used in the past seven days.

## References and Recommended Resources

Williams, Larry E. 2001. Irrigation of Winegrapes in California. Practical Winery, Nov-Dec.

Hellman, E.W. No date. Irrigation Scheduling of Grapevines with Evapotranspiration Data, Texas AgriLife Extension.

Prichard, T.L. No date. Winegrape Irrigation Scheduling Using Deficit Irrigation Techniques, UC Davis.

Reviewed by Jim Wolpert, UC Davis and Eric Stafne, Mississippi State University

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## Resources

eViticulture and the Grape Community of Practice are funded by the NIFA-USDA Specialty Crops Research Initiative (SCRI) and eXtension.

This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.