Handbook for Irrigation of Wine Grapes in South Africa

2014b). To obtain ET, the soil water balance was determined at 27 positions in the vineyard block. In the beginning of the growing season, FruitLook over-estimated ET compared to soil water balance ET (Fig. 8.47). In the second half of the season, the opposite trend occurred. It must be noted that this trend repeated itself over two seasons. FruitLook ET was also compared to soil water balance ET in a drip irrigated Shiraz vineyard in a sandy loam soil near Robertson. In this case, the soil water balance was determined at 30 positions in the vineyard block (Lategan & Howell, 2016). Although FruitLook correlated reasonably well with the soil water balance ET during the first part of the season, it also underestimated ET in the second half of the season (Fig. 8.48). Unfortunately, there was no FruitLook data available to allow comparisons in the post-harvest and dormant periods. In regions where winter rainfall is low, e.g. the Lower Orange River and Lower Olifants River regions, vineyards also need to be irrigated during these periods. Therefore, it is essential that satellite imagery should provide estimations of ET throughout the year in order to avoid unnecessary water deficits during these periods.

Figure 8.47

Figure 8.47 u 7

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FIGURE 8.47. Cumulative ET of micro-sprinkler irrigated Cabernet Sauvignon near Rawsonvillle as determined by means of the soil water balance approach, compared to FruitLook estimations during (A) the 2011/12 and (B) the 2012/13 seasons, respectively.

Figure 8.48

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FIGURE 8.48. Cumulative ET of drip irrigated Shiraz near Robertson as determined by means of the soil water balance approach, compared to FruitLook estimations during the 2014/15 growing season. 100 Cumulative ET (mm)

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278 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING