Handbook for Irrigation of Wine Grapes in South Africa

Based on results of the field and pot experiments, irrigation of vineyards with winery wastewater diluted to a maximum COD of 3 000 mg/  will only be sustainable if: • there is adequate clean water available for dilution. • the EC in the diluted water is less than 0.75 dS/m. • the SAR in the diluted water is less than 5. • it is a sandy soil with low CEC. • the internal drainage is unrestricted. • the irrigation water does not percolate beyond the root depth. • the irrigation is applied with micro-sprinklers in such a way that the bunches are not wetted. • a summer interception crop and a winter cover crop are grown, slashed and removed. • the irrigation frequency and volumes (schedule) enhance, rather than negate, wine quality characteristics. The practical implication of these criteria is illustrated by the following example. If a winery crushes 30 000 tons, and uses ca. 2 m 3 raw water per ton, it would produce 60 000 m 3 wastewater annually. To dilute this wastewater to 3 000 mg/  COD for irrigation would require an additional 215 400 m 3 clean irrigation water. Six irrigations of 40 mm each applied with micro-sprinklers during the harvest period would require 2 400 m 3 water per hectare. This means that the diluted winery wastewater needs to be distributed over 90 ha sandy soil during the harvest period. Based on the foregoing, and logistical challenges of mixing large volumes of water, irrigation with diluted winery wastewater can most likely only be implemented by boutique or estate wineries where small volumes of wastewater are produced. CONCLUSIVE REMARKS Since low quality water is not suitable for irrigation, analyses are necessary to avoid irreversible damage to soils, grapevines and irrigation equipment. Furthermore, water quality may change substantially within a short period of time. Therefore, frequent analyses are essential to pick up rapid deterioration in water quality. Fortunately, there are internationally proven thresholds and norms for water quality variables. If irrigation water quality conforms to these criteria, growers can be assured that no negative effects will occur. Water resources are becoming limited, particularly during periods of droughts. In many cases, irrigation water quality decreases during droughts. In addition to this, irrigation water allocations are unlikely to increase, and may become more expensive where new resources need to be developed. Therefore, the use of treated wastewater for vineyard irrigation is an important consideration in situations where low water availability and high water costs are threatening the sustainability of growing grapes for wine production. Furthermore, it must be noted that wastewater generation, particularly municipal water, may decrease during droughts when authorities restrict the use of potable water. 5.7

144 CHAPTER 5 – IRRIGATION WATER QUALITY