Crop yield response to water - Cra

Figure 15Relative yield as a function of relative evapotranspiration in wine grapes (closed symbols).The solid and dashed lines attempt to capture the upper limit for wine grapes. Table grapes(open symbols) and the function fitted for Thompson Seedless (dotted line) are shown forcomparison. Sources: Messaoudi and El-Fellah (2004), Intrigliolo and Castel (2008), Du et al.(2006) and Grimes and Williams (1990).Merlot, Morocco Tempranillo, Spain Thompson Seedless, USA Rizamat, China1.21.00.8Relative yield0.60.40.20.00.0 0.2 0.4 0.6 0.8 1.0 1.2Relative evapotranspirationgrapes. The variations in the scarce data in wine grapes of Figure 15 suggest that the productionfunction is probably variety and cropping system dependent.Qualitatively, the relationship between gross revenue and ET c in wine grapes is expected to featurean optimum resulting from the mismatch between the ET c required to maximize yield and the ET crequired to maximize quality and value per unit volume (Box 19 in Chapter 4). The actual parametersof this function, however, are likely to be specific for local production systems with their own waterand grape prices.Suggested RDI RegimesFor wine grapes, where the quality objective is critical to profit, the recommended irrigationregime must account for many more factors than in most other crops. To achieve the tripleaim of high yield, quality and irrigation water productivity, irrigation of red grape varieties intemperate environments needs to ensure vegetative growth and early reproductive growth inspring, and allow for progressive water deficit to develop towards maturity. For white grapes,water management should seek to avoid both severe water deficit and excess water supply.Management of irrigation after harvest accounts for the need to build up carbohydrate reservesin vines. Owing to the many factors involved, we selected five case studies to illustrate thisgeneral pattern, and the variations particular to the target production system. These examplesalso highlight the application of different monitoring methods (Box 2).478crop yield response to water