Irrigation Scheduling Decisions

When to start irrigation
when mid-day leaf water potential @ -8 to -10 bars by pressure bomb
-10 for red, -8 for white or blocks prone to water stress or less vigorous blocks.
Other method(s): mark 10 marker shoots/block and msr weekly, plot when rate slows, make sure it doesn’t stop, need to get shoot to 4 ft (20-22 nodes). Or soil moisture readings. Best option is combine all three methods. Generally for North Coast, early June is too early. Most common mistake is pulling the irrigation “trigger” too early (the exception is Central Coast where irrigation might be needed to get thru bloom)
Bloom to Veraison
North Coast: (Maintain -10 bars through bloom. If no water stress during bloom and shoots are 4-5 ft wait until MLWP drops to -12 (whites) or -15 (reds), then irrigate at 60% of ETc. Change Km according to field observations.)
Central Coast: (If shoots are not 4-5 ft, irrigate at 80% ETc. Keep MLWP > -10 because vegetative growth stops around -10 bars. If shoots are 4-5 ft, reduce vegetative growth by allowing MLWP to drop to -12 (whites) or -15 (reds), then irrigate at 60% ETc. Change Km according to field observations.)

Veraison to Harvest
Shoot growth should be complete. Monitor MLWP weekly, aiming for -14 to -15 (Cabernet) until harvest and making sure it doesn’t fall below -15. (exception is Zinfandel). The berries should not shrivel from water deficit.

Post-Harvest
Use ETc model, through crop coefficient model should decline to reflect less transpiration due to loss of leafs and fruit. ETo values will be smaller with shorter days. Irrigate at 80% of ETc to prevent new shoot growth but encourage carbohydrate storage. This should continue until the first good rainfall or end of October for most regions.

Common mistakes:
Not irrigating under cool weather. In fact water use is more dependent on humidity, sunlight and wind than temperature. Water stress during bloom will cause all the flowers to fall off and complete loss of crop. Water storage (well, reservoir) might be an issue if there is no irrigation district, but suffice to say that enough must be stored for the critical time periods.

Further reading:
1. CM Burt and SW Styles. 1999. Drip and Micro Irrigation for Trees, Vines, and Row Crops. ITRC, Cal POly San Luis Obispo, CA.
2. Irrigation Scheduling: A Guide for Efficient On-Farm Water Management. DA Goldhamer and RL Snyder, Eds. 1989. University of California publication 21454.
3. Micro-irrigation of Trees and Vines. L. Schwankl, B. Hanson, and T. Prichard. Eds. 1996. University of California publication 93-03.
6. Willams, LE 2001. “Irrigation of Winegrapes in California” Practical Winery and Vineyard. pp. 42-45
Greenspan, Mark. “Vineyard Water Management Fallacies”, Wine Business Monthly (pdf)
7. Tom Shapland lecture

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