Water resources are generally limited in most grape-growing regions, and inconsistent rainfall causes periodic droughts. By Carolyn Howell and Philip Myburgh (ARC Infruitec-Nietvoorbij)
The shortage of water in many grape growing areas may worsen if climate change reduces rainfall, and increases air temperature. Therefore, growers must use irrigation water more efficiently by means of sound irrigation scheduling practices. The table grape industry also needs to reduce its “water footprint” to convince consumers that scarce water resources are being used responsibly.
Calibration of instruments used for scheduling is not necessarily correct or accurate, because calibrations can differ between soils and/or different soil layers. Refill points, i.e. when irrigation is required, are often selected haphazardly. Consequently, table grape vineyards are over-irrigated in many cases. Instruments can be calibrated against soil water content or plant water status.Read More
However, soil calibrations are tedious and require specialised skills and equipment. On the other hand, it is fairly simple to measure grapevine water status by means of the pressure chamber technique to measure stem water potential (ψs). In this regard, it has been proven that instruments currently used for irrigation scheduling of commercial vineyards can easily be calibrated against grapevine water status.
The objective of this study was to develop guidelines to use this approach for table grape irrigation by setting irrigation refill lines according to midday ψs thresholds and determining how different table grape cultivars responded to midday ψs thresholds.
The project was carried out in commercial table grape vineyards in the Noorder-Paarl area of the Berg River Valley region, for three seasons (2018/19, 2019/20 and 2020/21). Ten of the more popular white and red cultivars, based on SATI statistics of the past few years, were included (Table 1). The vineyards were located within 5 km of each other.
For each cultivar, there were two experiment plots adjacent to each other. The irrigation systems were adapted to allow separate irrigation of the first experiment plot. The second plot, i.e. a reference plot, was irrigated with the rest of the block according to the growers’ schedules. After bud break, soil water content (SWC) and midday ψs were measured concurrently in both plots to determine the relationship between ψs and SWC (Fig. 2). The soil in the experiment plot was allowed to dry out until midday. ψs in the grapevines reached -0.8 MPa in the pre-harvest period and -1.2 MPa in the postharvest period. These thresholds were based on previous research data that indicated that at this level of ψs, berry mass and yield of table grapes are not negatively affected.
Once the refill points were established for each cultivar, grapevines in the experiment plots were irrigated when the SWC was depleted to the refill point. All other vineyard management practices and bunch manipulation were carried out according to the growers’ standard methods. Berry yield and its components, as well as juice characteristics at harvest were determined. Grapes were packed and evaluated after six weeks in cold storage.