With California facing drought conditions in 2021 following a 2020 growing season that was one of the driest and warmest in recent history, University of California (UC) crop water management and irrigation specialists provided advice to Lodi winegrape growers during two March workshops to help manage irrigation this season. Growers are advised to begin irrigation early for proper canopy growth and fruit set, use available technologies to monitor water status and efficiently apply irrigation, and monitor soils and irrigation water for salinity levels.
The workshops resulted through the efforts of John Shinn, a LODI RULES certified winegrape grower and partner in Shinn & Son Vineyard Management, and Lodi Winegrape Commission (LWC) research, education and sustainable winegrowing director Dr. Stephanie Bolton who worked with UC experts to organize the program. Shinn explained, “There is an overall need to manage our water better. This is an important topic that’s only going to get more important.” He added, “If anybody in our area hasn’t started irrigating yet, I think they will have to fire-up their pumps soon.”
Vines Need Early Season Water
UC Cooperative Extension (UCCE) viticulture advisor for San Luis Obispo and Santa Barbara Counties Mark Battany has studied vineyard water factors worldwide. He discussed water needs and desired stress levels during the three main stages of vine growth.
–Bud Break Through Fruit Set—Strong early growth is needed to develop canopy and set fruit. Water is needed for rapid growth with no to low stress.
–Post Fruit Set Through Veraison—Once developed, slow down growth to achieve a balanced canopy and balance vegetative growth with fruit growth. This is the period for higher water stress.
–Veraison Through Harvest—Maintain functional foliage to ripen fruit and maintain canopy to the end of growing season with moderate water stress.
With insufficient rainfall this year, Battany emphasized the need for early season irrigation. He explained: “After bud break, we want maximum growth to achieve canopy size. If the vine does not receive enough water for growth it’s very hard to make up for it later. We want to make sure early vine growth has no stress so the plant can develop flowers and fruit.”
Challenges to achieving this are that rainfall varies from year to year, stored soil moisture is not consistent, and these factors are compounded by year-over-year effects on vines. Shinn said Lodi growers saw reduced vine growth and vigor last season because of lower rainfall, and post-harvest vine pruning weights were 60 to 70 percent of normal.
Battany observed, “A grower who uses the same irrigation program without changing it from year to year is one who will have problems.” He advised checking soil moisture levels and vine growth early to be ready to begin irrigation earlier than normal. “The take home message in years like this one is that we will have to irrigate to make up for less rainfall received,” Battany summarized. Based on his experience, it is more challenging to measure water and make calculations in imperial units, such as converting rainfall in inches to gallons of water. “Metric units are much easier to work with, and it’s something I encourage you to explore using to make life simpler,” he advised.
Monitoring Water Status and Irrigation Scheduling
UC Davis agricultural water management specialist Dr. Daniele Zaccaria said to properly schedule irrigation, a grower must answer three questions: When to apply water? How much water to apply? and How to best apply the necessary amount? Three primary methods, described below, are used to measure and monitor vine water status for irrigation scheduling. Zaccaria said, “All irrigation scheduling methods require skilled on-farm personnel and the capacity for quick trouble-shooting.” He suggested using a combination of the three methods for best results.
–Weather-Based: Uses weather data to estimate crop evapotranspiration (ET) with data from weather stations in the California Irrigation Management Information System (CIMIS), or grower-owned weather stations. Commercial vendors are available who measure and calculate ET as a service to growers such as Tule Technologies, and Arable Mark. ET based methods are common but they require good data and calculations.
–Soil-Based: Uses soil moisture monitoring sensors placed at different depths in the vine root zone. “Soil moisture monitoring helps answer when to start irrigating at the beginning of the season, and to determine if enough water has infiltrated the root zone after each irrigation,” Zaccaria said.
–Plant-Based: Includes measures of leaf or stem water potential, commonly using a pressure chamber. Other technologies include sap flow meters, leaf porometers, and canopy temperature sensors. These methods can be labor intensive.
Dr. Mae Culumber, UCCE Fresno County farm advisor, provided detail on using a pressure chamber to take samples in the vineyard to measure midday leaf water potential (LWP). She first explained, “As water potential decreases, we see decreases in plant functions. Leaves, shoots and tendrils respond differently to water status, and tendrils will dry more quickly.” LWP or water status is measured as water pressure in “bars” and is always a negative amount, which gets lower as the vine has less water and less water pressure.
Culumber advised, “You can use LWP to fine tune your deficit irrigation to make sure you’re in the right target range to avoid excess stress.” Moderate water deficits control expansive vegetative growth while allowing photosynthesis to continue. “Threshold” LWP values can vary by cultivar, rootstock, soil water holding capacity and trellis system. The following are general guidelines for vine water status based on measured LWP: Less than -10 bars—no stress, -10 to -12 bars—mild stress, -12 to -14 bars—moderate stress, -14 to -16 bars—high stress, More than -16 bars—severe stress.
Growers also visually use vine growth characteristics as indicators of water status. Shinn said when tendril growth slows down it’s a sign to start irrigating. Tendril growth begins slowing at around -11 to -12 bars, and tendril dieback occurs about -14 bars.
UCCE irrigation specialist Dr. Khaled Bali advised adjusting water application rates and irrigation frequencies based on soil type. Sandy soils have lower water holding capacity and high infiltration rates, so more frequent irrigations for shorter time periods are more efficient. He also advised, “Install a flow meter to make sure the irrigation system is running correctly and delivering the desired amount of water.”
Deficit Irrigation, ET Replacement
UCD viticulture extension specialist Dr. Kaan Kurtural provided results from a 2020 deficit irrigation trial with Cabernet Sauvignon at the UCD Oakville Research Station. The trial compared three different ET replacement levels: 100 percent, 50 percent and 25 percent, and compared data for yields and grape chemistry quality factors for each treatment. The 50 percent treatment improved berry quality without significant impact on yield.
Kurtural said recommended irrigation deficits for red Bordeaux winegrape varieties in warmer California regions are 50 to 60 percent ET replacement. For white winegrape varieties planted in the Central Valley and managed mechanically, he recommended 75 to 80 percent ET replacement. He cautioned that not all cultivars respond as desired to deficit irrigation, such as Sangiovese.
Zaccaria and Kurtural are part of a research group, the California Crop Coefficient (3C) Science Collaborative, working to improve and update the crop coefficient ratio for grapes and other crops to enable growers to more accurately calculate ET values. The group has two ET measurement sites in Fresno County vineyards, and Zaccaria offered to install two sites in the Lodi appellation with assistance from growers to identify appropriate locations.
Soil salinity problems–excess concentrations of dissolved salts that create toxicity hindering vine growth and health–are site specific and vary by region, but these issues can be exacerbated by drought. Without sufficient winter rainfall, salt accumulation in surface soils may not be leached below the vine root zone. In addition, drip and micro-irrigation systems along with deficit irrigation practices do not provide adequate leaching and lead to salt buildup over time.
In drought years, supplies and allocations of surface water are lower or may be unavailable from irrigation districts, and growers must rely more on groundwater for irrigation. Poor quality groundwater in some locations can have unwanted levels of dissolved salts that contribute to toxic concentration levels. Growers are advised to monitor vines for indicators such as vine necrosis and leaf senescence and soil drainage problems, and to test soils and irrigation water. Zaccaria suggested testing irrigation water in the middle of the irrigation season (June and July) for salt and mineral content when it is more likely to show higher concentrations. However, salinity mitigation is more commonly done during the dormant season with soil amendments, or by applying water if available.
Videos from the two LWC March water workshops and information on the next workshop May 13 are available at www.lodigrowers.com.