76 practical winery & vineyard May 2014

T E C H N I C A L R E S O U R C E F O R G R O W E R S & W I N E R I E S

78 Mount Eden Vineyards expands with Silicon Valley Bank loan By Cynthia Bournellis

85 Role of trace metals in wine ‘reduction’ By Marlize Viviers, Mark Smith, Eric Wilkes, Paul Smith and Dan Johnson

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California has not seen a drought year like 2014 in recent history. Many vineyards not only have a limited water supply for irriga-

tion, but the water in their root zone is well below the full water-holding capac-ity. With our rainfall future uncertain, we need to prepare for the worst. Here are four drought-management strategies for your consideration.

The first strategy is artificial rain. Flood, furrow or sprinkler irrigation arti-ficially refills root zone moisture. This recharging ought to occur about three or four weeks prior to bud break to ensure

roots emerge from dormancy normally, leading to normal bud break, shoot emer-gence and early season shoot growth. After that, schedule irrigations as in any other year.

The artificial rain strategy assumes a supply of water sufficient for both root zone recharge and normal irrigations through the growing season. This assumption, along with the surface irri-gation system requirement, is prohibitive for many vineyards.

All three of the next drought-manage-ment strategies need the following water-conservation methods for success. First, diligent control of weeds in the vine row to eliminate water loss through their tran-spiration. Second, disk and ring-roll the tractor rows to remove transpiring vegeta-tion and to make a barrier of loose soil that limits evaporation. Third, thin shoots early (≈ 3 to 6 inches long) and severely (less than six shoots per foot cordon) to reduce transpiration through vines. For the same reason, if canopies grow too large, hedge them to about 14 to 16 nodes per shoot after shoot growth stops.

These strategies also warrant maxi-mum value from applied water. To that end, ensure the best possible drip irriga-tion system performance with regard to uniformity in pressure, flow and distri-bution. Also, promote the highest possi-

ble infiltration rate into the soil through organic and mineral amendment addi-tions (Figure 1).

The second drought-management strat-egy familiar to many wine grape growers is regulated deficit irrigation (RDI). The only difference for 2014 is that RDI sched-ules should have begun three or four weeks before bud break instead of between May and July. This early onset of drip irri-gation is necessary to accommodate the drought-induced soil-moisture deficit.

Even with these early irrigations, the soil profile may only contain about 4% to 13% of the moisture that it could if fully recharged from rain. After initiating irri-gations, monitor vine moisture status, soil moisture status and evapotranspira-tion (ET) as you normally would, and irrigate accordingly. The longer RDI sea-son requires more irrigation water than in a normal year, but less than artificial rain due to greater irrigation efficiency and plant water use efficiency.

Stan Grant, viticulturist BY

Management strategies for drought conditions

Figure 2: Extensive defoliation of Chardonnay.

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Figure 1: Wetted soil with unimpaired water infiltration.

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practical winery & vineyard May 2014 77

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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.

ConclusionWe 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 Reading1. 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.