How much water does a vineyard need?
A grapevine's water use, its crop evapotranspiration or ETc, depends on the weather and on how much leaf area is intercepting sunlight. The standard way to combine those two is the crop coefficient approach in FAO Irrigation and Drainage Paper 56: multiply a reference ET (ETo), which describes the day's evaporative demand, by a crop coefficient (Kc) that describes how this particular crop, at this particular canopy size, compares to that reference.
For vineyards, canopy size is the moving part. A dormant or newly leafed vine uses very little water; a vine at full canopy in midsummer uses several times more. Williams and Ayars (2005) showed that a vine's Kc rises in a straight line with the percent of ground shaded directly beneath its canopy at solar noon, measured over the ground area allotted to that vine. That gives a practical, field-measurable way to estimate Kc without needing degree-day models or trellis-specific curves.
| Canopy stage | Typical shaded area | Resulting Kc |
|---|---|---|
| Early season, partial leaf-out | about 15–25% | 0.25–0.42 |
| Approaching full canopy | about 40–55% | 0.67–0.93 |
| Full canopy, peak season | about 60–75% | 1.02–1.27 |
These Kc values follow directly from the shaded-area formula below; they are not separate published targets, so use them only as a sanity check against your own measured shade percentage, not as a substitute for it.
Not for newly planted vines. The shaded-area method was developed on established, fruit-bearing vines. A vine with little or no measurable canopy needs a different, establishment-stage watering approach; see when this calculator isn't the right tool.
How this vineyard irrigation calculator works
The calculator turns your canopy shade and local ETo into a crop coefficient and a water-use depth, applies your deficit irrigation percentage, then converts that depth into a volume for one vine using your row and vine spacing.
1. Crop coefficient from shaded area
Kc from percent shaded area
Kc = −0.007 + 0.017 × (percent shaded area)
Williams and Ayars (2005) developed this linear relationship from a weighing lysimeter on Thompson Seedless grapevines; the percent shaded area is entered as a whole number (40, not 0.40) and is measured beneath the canopy at solar noon, over the ground area allotted to one vine.
2. Crop water use (ETc)
ETc
ETc = ETo × Kc
Per FAO-56 (Allen et al., 1998), Equation 58. If you entered a weekly ETo, it's divided by 7 first to get a daily value.
3. Apply a deficit irrigation percentage
Applied water
Applied depth = ETc × RDI %
RDI % defaults to 100 (full replacement) if left blank. A lower value applies less than full ETc, following the sustained or regulated deficit irrigation approach described by Williams (UC ANR).
4. Convert depth to volume per vine
Multiplying a water depth by the ground area allotted to one vine gives a volume for that vine.
US units
Gallons per vine = Applied depth (in) × Vine area (sq ft) × 0.623
Vine area = row spacing × vine spacing. One inch of water over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches; 144 ÷ 231 ≈ 0.623.
Metric units
Liters per vine = Applied depth (mm) × Vine area (m²)
One millimeter of water over one square meter is exactly one liter.
5. Drip run time (optional)
Minutes per irrigation
Minutes = Volume per vine, per irrigation ÷ (emitters × flow per emitter) × 60
Only shown when you enter both emitters per vine and flow per emitter.
When this calculator isn't the right tool
- Newly planted or dormant vines without measurable canopy shade. These need frequent, small volumes tied to root establishment, not a canopy-based ETc.
- Row crops or orchards other than grapevines, which have their own crop coefficients. See the orchard irrigation calculator or the crop water requirement calculator.
- Detailed scheduling that also tracks soil moisture depletion between irrigations. See the soil water balance calculator.
- Sizing pipes, valves or a pump for the resulting flow. See the irrigation pipe size calculator.
Vineyard irrigation calculation example
A block is planted on 10 ft rows with vines spaced 5 ft apart, giving 50 sq ft per vine. The canopy shades about 40% of that ground area at solar noon, the local station reports ETo of 0.28 in/day, and the grower is applying 70% of full ETc (a moderate regulated deficit), irrigating 3 times a week.
- Vine area: 10 ft × 5 ft = 50 sq ft.
- Crop coefficient: −0.007 + 0.017 × 40 = 0.673.
- Crop water use: 0.28 × 0.673 = 0.18844 in/day, shown as 0.19 in/day.
- Applied depth: 0.18844 × 0.70 = 0.131908 in/day, shown as 0.13 in/day.
- Weekly volume per vine: 0.131908 × 7 × 50 × 0.623 ≈ 28.76 gal, shown as about 29 gallons.
- Per irrigation (3 a week): 28.76 ÷ 3 ≈ 9.59 gal, shown as about 9.6 gallons per vine.
- Drip run time (2 emitters at 1 gph each = 2 gph per vine): 9.59 ÷ 2 × 60 ≈ 287.6 minutes, shown as about 4 hr 48 min.
The same calculation in metric
A block on 3 m rows with 1.5 m vine spacing (4.5 m² per vine), 40% shaded area, ETo of 6 mm/day, the same 70% deficit and 3 irrigations a week:
- Crop coefficient: 0.673 (unchanged; the formula doesn't use unit-specific terms).
- Crop water use: 6 × 0.673 = 4.038 mm/day, shown as 4.0 mm/day.
- Applied depth: 4.038 × 0.70 = 2.8266 mm/day, shown as 2.8 mm/day.
- Weekly volume per vine: 2.8266 × 7 × 4.5 ≈ 89.0 L, shown as about 89 liters.
- Per irrigation: 89.0 ÷ 3 ≈ 29.7 L, shown as about 30 liters per vine.
- Drip run time (2 emitters at 2 L/hr each = 4 L/hr per vine): 29.7 ÷ 4 × 60 ≈ 445 minutes, shown as about 7 hr 25 min.
Because the crop coefficient is unitless, switching units changes only how the depths and areas are expressed, not the underlying Kc.
Practical guidance
Measuring shaded area in the field
Williams and Ayars (2005) originally measured shade with a grid laid on the ground beneath the vine at solar noon; later work by the same group used a digital camera and software to calculate the shaded fraction from a photo. For routine field use, a careful visual estimate, in 5% increments, of how much of the row-and-vine-spacing rectangle beneath one vine is in shadow is a reasonable substitute. Take the reading close to solar noon, since shade shifts through the day as the sun angle changes.
Choosing a deficit irrigation percentage
Regulated deficit irrigation (RDI) is normally timed to a phenological stage, commonly between berry set and veraison (color change) or between veraison and harvest, according to Williams (UC ANR, Deficit irrigation of wine grape vineyards). Published findings on how much deficit vines tolerate vary by cultivar and goal: Williams et al. (2010b) found that Thompson Seedless yield was maximized at 60–80% of full ETc, with the exact figure depending on the year, while Williams (2014b) found no significant yield difference for Chardonnay in a cooler growing region across a much wider range, 25–125% of estimated ETc. Neither finding is a universal rule for every vineyard; treat them as a starting range to test against your own vines' response, not a fixed target.
Sustained deficit irrigation (applying a reduced fraction all season) is a separate approach from RDI (targeting the deficit to a specific stage); this calculator's RDI percentage works the same way for either, since both are expressed as a fraction of full ETc.
Common mistakes
- Using an old shaded-area reading all season. Canopy shade changes as the vine leafs out and again after any hedging or leaf removal; re-measure through the season rather than using one spring reading all summer.
- Mixing an ETo period with the wrong multiplier. A daily ETo used as if it were weekly (or the reverse) changes the result sevenfold. Use the period selector next to the ETo field.
- Forgetting system efficiency and rainfall. This calculator estimates the vine's water need, not what to dial in at the controller; a drip system rarely applies 100% of what leaves the emitter to the root zone, and rainfall during the period should be subtracted before you compare it to a fixed schedule.
Factors that change the result
| Factor | Effect on water need | What to adjust |
|---|---|---|
| Canopy shaded area | Directly sets the crop coefficient; more shade means more transpiring leaf area. | Re-measure shaded area through the season rather than assuming one value. |
| Weather (ETo) | Hot, dry, windy days raise evaporative demand. | Use a current, local ETo value, not a seasonal average. |
| Row and vine spacing | Sets the ground area per vine, which scales the volume of water, not the depth. | Enter your actual planting spacing, not a nominal or rounded figure. |
| Deficit irrigation stage | Vines are commonly stressed only during a specific window, such as set to veraison. | Change the RDI percentage only during the intended stage; use 100% otherwise. |
| Trellis and row orientation | Already reflected in the measured shaded area, since a wider canopy casts more shade. | No separate adjustment needed if you measure shade directly. |
| Soil water-holding capacity | Deep, water-retentive soils buffer against short water deficits better than shallow or sandy soils. | See the available water capacity calculator before setting an aggressive RDI target. |
Frequently asked questions
How much water does a grapevine need?
It depends on canopy size and the weather, not a fixed number. A vine's water use (ETc) equals reference ET (ETo) times a crop coefficient (Kc) that grows with the canopy. Williams and Ayars (2005) found Kc rises roughly linearly with the percent of ground shaded beneath the canopy at solar noon, so a small, newly leafed-out vine uses far less water than the same vine at full canopy in July.
What is regulated deficit irrigation (RDI) in vineyards?
RDI is deliberately applying less than full crop water use (ETc) at specific growth stages, usually between berry set and veraison or between veraison and harvest, to manage vine vigor and berry size without cutting yield too far. It's calculated as a percentage of full ETc, so you still need an ETc estimate first, then apply a fraction of it.
How do I measure the percent shaded area beneath my vines?
Stand at solar noon and look straight down at the ground beneath one vine's canopy. Estimate what share of the ground area allotted to that vine (row spacing times vine spacing) is covered in shadow versus sunlit. Williams and Ayars (2005) measured this with a grid on the ground and, in later work, with photos analyzed in software; a careful visual estimate in 5 to 10 percent increments is a reasonable field substitute.
Does row spacing or vine spacing change the crop coefficient?
Not directly. The Williams and Ayars crop coefficient depends only on the percent of ground shaded, and that percentage already reflects how the canopy fills its allotted space. Spacing still matters for this calculator because it sets the ground area per vine, which converts a water depth into a volume of water for that one vine.
What reference ET (ETo) value should I use?
Use a local reference ET station where possible, such as a state ag-weather network or CIMIS in California, for the day or week you're scheduling. ETo changes with weather, so a value from a hot, dry week will be higher than one from a mild week. This calculator does not estimate ETo itself.
See the reference ET calculator to work it out from weather data.
Is this calculator accurate for young or newly planted vines?
No. The Williams and Ayars relationship was built on established, fruit-bearing vines with measurable canopy shade, and this calculator requires at least 5 percent shaded area. Newly planted vines with little or no canopy need a different approach, typically a small, frequent volume based on root establishment rather than canopy-driven ET.
Can I use this for table grapes or raisin grapes, not just wine grapes?
The shaded-area method itself is not specific to wine grapes; Williams and Ayars developed it on Thompson Seedless vines farmed for table and raisin production. The deficit irrigation guidance in this article, however, focuses on findings for wine grape quality and Thompson Seedless yield, so treat RDI targets for other end uses as a separate decision.
What if I don't use drip irrigation?
The gallons-per-vine result still applies; only the run-time conversion is drip-specific. For furrow, flood or micro-sprinkler systems, use the weekly or per-irrigation volume per vine and convert it with your system's own flow and wetted-area figures.
See the furrow irrigation calculator or the micro-spray calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Crop coefficient formula, unit conversions and both worked examples verified by hand.
- Kc formula and RDI findings checked against the Williams and Ayars and Williams sources below.
- Method follows FAO-56 crop coefficient practice; Kc source is peer-reviewed lysimeter research.
Sources
- Williams, L.E. and Ayars, J.E. 2005. Grapevine water use and the crop coefficient are linear functions of the shaded area measured beneath the canopy. Agricultural and Forest Meteorology 132(3-4): 201-211.
- Williams, L.E. Deficit irrigation of wine grape vineyards. University of California Cooperative Extension.
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. 1998. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56.
- Williams, L.E., Grimes, D.W. and Phene, C.J. 2010. Effects of applied water at various fractions of measured evapotranspiration on reproductive growth and water productivity of Thompson Seedless grapevines. Irrigation Science 28: 233-243. [VERIFY URL]