Issue link: http://winesandvines.uberflip.com/i/297564
p r a c t i c a l w i n e r y & v i n e ya r d M ay 2 0 1 4 77 g r a p e g r o w i n g Stretch the limited irrigation water is the third strategy. Soil moisture status is the primary basis for irrigation sched- ules, using a specific level of depleted moisture or soil moisture tension thresh- old as the basis for applying water. This method requires soil moisture sensors within the root zone that are read at regu- lar intervals. Install at least two sensors at each monitoring location: one at about 10 to 12 inches deep for irrigation schedul- ing and another at 30 to 36 inches to mon- itor deep soil moisture depletion. The deep sensor indicates when vines become entirely dependent on irrigation water. It is insightful to have sensors at two locations within one block: one where vines are most prone to water stress and another where they are least prone. With stretch the limited irrigation water, stress-prone areas will become more pro- nounced than usual (Figure 3). For this reason, they ought to be carefully moni- tored and irrigations adjusted to mini- mize the severity of water stress there. If the water supply was sufficient, the early season soil moisture threshold should be higher than the later season threshold. Irrigating this way moderates water stress and assures complete canopy development. During this time, apply only enough water to maintain a moder- ate and steady rate of shoot elongation (3 to 4 inch internodes) until reaching full canopy (14 to 16 nodes or leaves per shoot). After that, greater water stress is not only beneficial for water conservation but also desirable for wine grape quality. For example, an early soil moisture threshold may be -60 to -70 cbars, while later it may -100 to -110 cbars measured with resistance blocks (Watermark sen- sors or gypsum blocks). Soil moisture content depletion levels may be similarly determined and set for capacitance sen- sors and neutron probes. Keep the vines alive is the last strategy and last resort. Under this strategy, apply water only when necessary to prevent complete defoliation. As such, irrigation scheduling depends almost entirely on visual observations of canopy condition. As with the stretch the limited irrigation water strategy, focus your observations on the areas prone to water stress. With keep the vines alive, however, there is greater tolerance for leaf damage and on the lower (basal) portions of shoots, leaf loss (Figure 2). This strategy also requires removing all crop to avoid crop stress and minimize vine decline. Conclusion We have four drought-management strat- egies that accommodate varying water supplies. Where there is an abundance of irrigation water, use the artificial rain strategy. Alternatively, make better use of your water with RDI or stretch the irriga- tion water using soil-moisture depletions or tension thresholds. Finally, where water is very scarce, irrigate to keep the vines alive until next year. PWV This article is edited from the Mid Valley Agricultural Services January 2014 Viticul- ture Newsletter. Further Reading 1. Burt, C.M., and S.W. Styles. 1994 Drip and microirrigation for trees vines and row crops (with special sections on buried drip). Irrigation Training and Research Center, Dept. of Ag. Engineering, California Polytechnic State University, San Luis Obispo, Calif. 2. Goldhammer, D.A., and R.L. Snyder. 1989 Irrigation scheduling: a guide for efficient on-farm water manage- ment. University of California, Div. of Agriculture & Natural Resources Publication 21454. 3. Grant, S. 2000 "Five-step irrigation schedule: promoting fruit quality and vine health." Practical Winery & Vineyard. 21(1): 46–52 and 75. May/June. 4. Hanson, B., S. Orloff, and B. Sanden. 2007 Monitoring soil moisture for irri- gation water management. University of California Agriculture & Natural Resources Publication 21635. 5. Prichard, T.L, B. Hanson L. Schwankl, P. Verdegaal, and R. Smith. 2004 Deficit irrigation of quality winegrapes using micro-irrigation techniques. University of California Cooperative Extension, Dept. of Land, Air, Water Resources, University of California, Davis. 6. Prichard, T., C.P. Storm, and C.P. Ohmart. 2008 Chapter 5, Water Management. In: Lodi Winegrower's Workbook, 2nd Ed. Ohmart, C.P., C.P. Storm, S.K. Matthiasson (Eds.). Lodi Winegrape Commission. pp. 142–186 7. Schwankl, L., B. Hanson, and T. Prichard. 1995 Micro-irrigation of trees and vines: a handbook for water managers. University of California Irrigation Program, University of California, Davis. Figure 3: Weaker grapevine growth areas appear red in this PurePixel vegetation map.