Determining When and How Much to Irrigate

In my previous blog on An Acre Inch of Water, I wrote that the number 27,154 is important to know because that represents the number of gallons of water it would take to cover one acre of land with one inch of water.
A really good article that explains when to irrigate can be found on line in the November/December 2001 issue of Practical Winery and Vineyard. It is written by Larry E. Williams, titled Irrigation of Winegrapes in California. This article encapsulates and explains what I learned in Lesson 7 of the U.C. Davis Online course, Viticulture for Winemakers. It answers two questions (1) when to irrigate and (2) how much water to apply.
When to irrigate can be determined by using:

• Soil-based tools can determine the actual or relative amounts of water in the rooting zone of grapevines
• Neutron probe
• Capacitance sensors
• Plant-based tools can be used to measure vine water status
• Pressure chamber

How much water to apply is determined by:

• Evaporative demand at the location of the vineyard
• Stage of vine development
• Percent ground cover by the vine’s canopy

To determine the vineyard's irrigation requirement, the following equation for full crop water use in inches is given by:

• ET_c is the crop ET (evapotranspiration) or vine water use
• ET_o is the potential evapotranspiration or the reference ET
• k_c is the seasonal crop coefficient, or the fraction of water a non- water-stressed crop uses in relation to the ET_o and is dependent on:
• Stage of vine growth
• Degree of ground cover (shading )
• Height or the canopy
• Canopy resistance (regulation by the vine or crop)
• k_c will vary throughout the growing season; it is not a constant fraction of ET_o

The above equation is used to calculate how much water we would need to irrigate in inches. Since this blog is getting a bit long, stay tuned for the next installment on how to convert inches to gallons for irrigating.
Reference:
1. Larry E. Williams, Irrigation of Winegrapes in California, Practical Winery and Vineyard, November/December 2001.