Handbook for Irrigation of Wine Grapes in South Africa

Growers should dig soil pits to determine the root depth in order to prevent over- irrigation beyond the root zone in established vineyards. Irrigation must be applied where most of the roots occur. For example, if most roots occur to a depth of 60 cm, but a few roots penetrate to 120 cm, water will be wasted if the soil is irrigated to 120 cm. One of the worst mistakes that growers can make under drought conditions is to “pamper” the grapevines by applying small irrigations frequently. If only 5 to 10 mm irrigation is applied, most of it will be lost to the atmosphere during Phase I evaporation as discussed in Chapter 2. Tillage and mulching: Surface management practices can also affect evaporation losses from the soil. Shallow tillage does not seem to reduce evaporation losses, compared to bare, undisturbed soil (Fig. 7.31). In fact, it tends to increase evaporation losses, and is therefore not recommended as a water saving practice in vineyards. Unnecessary tillage will also contribute to destruction of the soil structure near the surface in the long run. Poor soil structure will impair water infiltration which will cause runoff losses. Furthermore, shallow tillage may destroy shallow roots that could have absorbed light rainfall in early spring and summer. A surface mulch, consisting of organic material, e.g. straw or wood chips, will reduce evaporation losses, particularly during the period of high evaporation following irrigation or rainfall (Fig. 7.31). Evaporation losses will decrease with an increase in the thickness of the mulch. In the beginning of the season, water saving will be limited to the surface layer, but as the season progresses mulching will also result in higher soil water contents in deeper layers, compared to bare, undisturbed soil or shallow tillage (Fig. 7.32).

35

4 mm

30

25

20

15

Bare soil Shallow tillage 4 t/ha Straw mulch 8 t/ha Straw mulch 12 t/ha Straw mulch

10

5

Cumulative water loss (mm)

0

0

3

6

9

12

15

18

Time (days)

FIGURE 7.31. Effect of tillage and thickness of wheat straw mulches on water losses from the 0-20 cm soil layer in a vineyard following irrigation on 19 January 1997 (Myburgh, 2013). Arrow indicates rainfall and vertical bars indicate LSD (p ≤ 0.05).

218 CHAPTER 7 – IRRIGATION STRATEGIES