Technical Yearbook 2024

Procedure for the in-field fractional use (augmentation) of winery wastewater with raw water for vineyard irrigation Experience from a previous study showed that it would be impractical to augment winery wastewater to a pre determined level of chemical oxygen demand (COD) before each irrigation, i.e. specifically at the commercial level. This is mainly because it would be difficult to monitor the winery wastewater quality continuously in order to adjust the volumes of raw and wastewater to obtain a required level of augmentation. Therefore, a more practical approach to irrigating vineyards with winery wastewater was applied in this study. In this regard, the in-field fractional use (augmentation) of winery wastewater with raw water for the irrigation of the experimental plots was used. According to this approach, grapevines were irrigated as follows. For each irrigation, 50% of the irrigation requirement was applied as undiluted winery wastewater (Figure 4). Raw water was applied for the other 50% of the irrigation requirement. All vineyards in the study were irrigated with micro-sprinkler irrigation. This was to ensure that the full soil surface was wetted, as well as reduce the risk of clogging of the irrigation pipe. It should be noted that experimental grapevines were irrigated so that optimum wine quality would be obtained. Therefore, stem water potential ( Ψ S ) thresholds for optimum wine quality for the specific cultivars were used to set up the irrigation refill lines. Consequently, grapevines would therefore be under-irrigated rather than over-irrigated, because better wine quality is obtained when grapevines receive less water. Climatic conditions As expected, the rainfall in all the regions increased from summer to winter (Figure 5). Furthermore, the winter rainfall was higher in the Coastal region than in the other regions. Most of the rainfall occurred from May to August. The experimental plots in the Breede River region also received appreciable amounts of rainfall during some of the summer months. The rainfall at the experimental plots in the Lower Olifants River region was low. Given these prevailing conditions, as well as the sandier soils in this particular region with their lower water holding capacities, it was expected that the grapevines growing in this region would require more irrigation during the growing season than the other two regions. As expected, the winter (June to August) rainfall was substantially higher in the Coastal region than in the other regions (Figure 6). Given the extreme drought experienced in the Western Cape during the period of the study, the winter rainfall was generally lower than the long-term mean (LTM) in the first two years of the study all the regions. In the last year of the study, the winter rainfall was generally more or equal to the LTM.

FIGURE 2. The clay content in the 0 - 30 cm, 30 - 60 cm and 60 - 90 cm soil layers at the different experimental plots.

FIGURE 3. The sand content in the 0 - 30 cm, 30 - 60 cm and 60 - 90 cm soil layers at the different experimental plots.

FIGURE 4. Schematic illustration of in-field fractional use (augmentation) of undiluted winery wastewater with raw water when grapevines in experimental plots were irrigated with wastewater followed by raw water.

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TECHNICAL YEARBOOK 2024