Handbook for Irrigation of Wine Grapes in South Africa

Winter rainfall stored in the soil, as well as rainfall during the growing season, are the only water sources of dryland (non-irrigated) grapevines. Therefore, the ET of non-irrigated vineyards can be considered as the baseline water requirements for wine grapes. In this regard, it was shown that the ET of dryland Cabernet Sauvignon near Stellenbosch amounted to ca. 300 mm from budbreak in September until harvest in February (Table 7.1). It must be noted that adequate rainfall occurred during winter, and that these vineyards had deep, well-developed root systems. Rainfall and subsequent evaporation losses contributed to the relatively high ET during the first part of the season. During berry ripening, the relatively low ET was primarily a function of transpiration. The rainfall plus soil water depletion of dryland Merlot near Wellington and Pinotage near Stellenbosch was also approximately 300 mm (Table 7.2). The foregoing indicates that vineyards will require irrigation if (i) winter rainfall does not saturate the root zone at budbreak, (ii) rainfall during the growing season is less than ca. 150 mm, (iii) deep root development is restricted due to poor soil preparation or (iv) the plant available soil water is limited due to high sand, gravel or stone contents. TABLE 7.1. Long term mean monthly rainfall, as well as mean ET of dryland Cabernet Sauvignon in four plots near Stellenbosch (after Laker, 2004). Sep Oct Nov Dec Jan Feb Total Rainfall (mm) 56.7 44.2 23.9 24.6 21.4 20.7 191.5 ET (mm) 64.1 66.7 53.1 65.5 26.0 18.0 293.4 TABLE 7.2. Rainfall and soil water depletion from September until February for dryland Merlot near Wellington (Myburgh, 2011b) and Pinotage near Stellenbosch (Moffat, 2017). Cultivar Locality Rainfall (mm) Soil water depletion (mm) Total (mm) Merlot Wellington 143 150 293 Pinotage Stellenbosch 153 165 318 IRRIGATION STRATEGIES Over the past decades, the focus of wine production in South Africa has shifted a number of times between either a production driven or a quality driven industry. The consequence of this is that irrigation strategies aimed at achieving maximum yields may not necessarily be applicable when high wine quality is the primary objective. Irrigation strategies for newly planted vineyards, during the post-harvest and dormancy periods, as well as irrigation during critical drought periods are also important considerations. Irrigation strategies for bearing vineyards can be divided into the following basic categories: (i) Low frequency irrigation, i.e. where six or less irrigations are applied during the growing season. This strategy is often referred to as “supplementary 7.2

180 CHAPTER 7 – IRRIGATION STRATEGIES