Handbook for Irrigation of Wine Grapes in South Africa

100 120 140

0 20 40 60 80

SWC (mm/120 cm)

y = -15.88lnx +133.18 (R 2 = 0.9985)

0 2 4 6 8 10 12 14 16 18

Mulch thickness (cm)

FIGURE 7.33. Effect of the thickness of compost mulches on soil water content (SWC) on the grapevine row following a series of small rainfall events (Moffat, 2017).

Adapt irrigation systems: Since the water use efficiency of conventional full surface flood irrigation is lowest, compared to most other systems, it can be temporarily adapted to save water during drought periods. Irrigation water can be applied in narrow furrows along the grapevine rows or in alternating work rows as discussed in Chapter 4. In both cases, the fractional wetting of the soil surface will reduce the evaporation losses, compared to full surface flood irrigation, but the grapevines will still receive enough water to maintain comparable yields (Table 7.18). At the same time, WUE will be higher. However, in the case of fractional wetting, it is critical to avoid excessively dry conditions in the post-harvest and dormancy periods, particularly in the summer rainfall regions (Myburgh, 2003a). In the case of extreme drought, short pieces of poly pipe can be slit open and clamped over micro- sprinklers. This will reduce the distribution pattern to a tricle of water under the micro-sprinklers. These “super drippers” will allow the water to penetrate deeper into the soil, compared to a small, full surface irrigation, and reduce evaporation losses. If only used for a short period, the limited soil wetting is unlikely to cause any changes to the root system in established vineyards.

220 CHAPTER 7 – IRRIGATION STRATEGIES