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p r a c t i c a l w i n e r y & v i n e ya r d J U ly 2 0 1 4 57 g r a p e g r o w i n g I n arid and semi-arid grapegrowing regions, irrigated viticulture is neces- sary to support consistent vineyard production. Moreover, well-managed irrigation is an essential part of vineyard practices and can potentially enhance quality. 5 Through controlling the amount of water supplied to grapevines, growers have the ability to control shoot growth and canopy size/density (which affect light exposure and air circulation in the fruiting zone), manipulate berry size and modify wine style in the vineyard. 18 However, ill-managed irrigation not only will not enhance grape quality, it could also be detrimental to the current growing season and even the following year's production. It is essential to understand how to manage vineyard irrigation. Moreover, under the global backdrop of increasing water shortages (due to climate change and/or urban development), producing better crops with less water is becoming especially important. Deficit irrigation in viticulture, when applied properly, can improve berry quality and also increase vineyard water use efficiency (WUE). 11 Fundamentals of irrigation scheduling Two fundamental questions related to irrigation scheduling are when and how much to irrigate. Approaches that can be used to aid irrigation decisions can be based on weather conditions, soil water status or vine water status. Evapotranspiration (ET) is the combi- nation of soil evaporation and plant tran- spiration. Reference ET (ET 0 ) is the ET from a hypothetical reference crop surface (i.e., grass) without any water stress. 1 The ET 0 can either be measured using a lysim- eter, estimated from pan evaporation or calculated using the Penman-Monteith equation based on meteorological data (humidity, solar radiation, temperature and wind speed). 1 As a standardized refer- ence, ET 0 from different locations or vari- ous years can be compared, and it offers accurate and real-time information of the changing weather. In order to adapt ET 0 to a specific crop (wine grapes), a crop coefficient (K c ) is multiplied with ET 0 to calculate crop-spe- cific evapotranspiration (ET c ) (ET c = ET 0 × K c ). 1 It has been reported that K c is cor- related with grapevine canopy features such as leaf area and canopy cover, 15 or growing degree days (GDD). 4 Moreover, K c is affected by climate, grape variety and layout of a particular vineyard. 12,15 Water stress would also alter the values of K c . 13 Therefore, local calibra- tion of K c is necessary, and the tracking of vine growth (or soil and/or vine water status) can improve the calibration. 12,16 For example, the changes in soil mois- ture in Figure 2 appeared as season-long water stress was imposed. However, it was not intended and irrigation was applied at 100% ET c (K c was calculated based on current season growing degree days. It demonstrates the importance of monitoring soil/vine water status other than simply applying irrigation based on a certain fraction of ET c . Soil water status can be measured as either soil water content (using neutron probe or time domain reflectometry, often expressed as percentage of soil vol- ume, v ) or soil water potential (using tensiometers or psychrometers, soil ). Using the soil water retention curve, v can be converted to soil , or vice versa. The advantage of measuring soil water status is that it directly indicates the amount of irrigation required to fill the soil profile to a target level. However, the soil water content or potential may not reflect the vine water status, which directly affects vine physiological pro- cesses. 19,20 Different types of soil have various water-holding capacity. There- fore, knowledge of the soil types in spe- cific vineyard blocks is essential for correctly interpreting the measurements of soil water status. Vine water status: A grapevine is the "middleman" between soil and atmo- sphere; its water status is affected by both soil water supply and atmospheric demand for transpiration. Therefore, measuring vine water status is the most direct way to assess the water stress level in a vine. Many parameters can be used to "sense" vine water status, such as simple visual indicators of vine growth, mea- surements of vine water potential ( vine ) and vine physiological responses to water availability (such as shoot growth rate, leaf gas exchange, sap flow rate and canopy temperature). • Visual indicators of grapevines: Observing and keeping a record of visual indicators of grapevines (the growth of shoot tips/tendrils and leaf orientation, see Figure 1) is the easiest way to know whether water is available to support active growth or not. For example: • Turgid and actively growing shoot tips/ tendrils indicate adequate water supply, while flaccid and wilting shoot tips/ tendrils are the first sign of water stress. • When water stress continues, shoot tips/tendrils will desiccate and fall off. • Leaves will orient away from direct sunshine under moderate water stress. • Any lateral shoot growth indicates no water stress. Observing visual symptoms serves well as a quick routine check-up. How- Yun Zhang, Washington State University BY Good, Bad and Deficit vINeyard IrrIgatIoN MaNageMeNt Figure 1. visual indicators of vine growth can be easily observed to assess water stress. Left: Water stress causes absence of tendrils and shoot tip, and changed leaf orientation on a grapevine shoot. right: actively growing tendrils and shoot tip are evident on a shoot without water stress.