Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

Figure 2.26

A

• 2  T ranspiration/ grapevine = 0.7 mm soil water depletion • Surface evaporation= 5 mm soil water depletion • Total soil water depletion = 0.7 + 5 = 5.7 mm/day

B B

• 2  T ranspiration/ grapevine = 2.8 mm soil water depletion • Surface evaporation= 5 mm soil water depletion • Total soil water depletion in wetted zone = 2.8 + 5 = 7.8 mm/day • Total soil water depletion based on full surface = 7.8  4 = 1.4 mm/ day

÷

In practice, the combined effects of evaporation and transpiration will determine the rate of soil water depletion by grapevines. Therefore, in drip irrigated vineyards the soil water in the smaller wetted volumes will be depleted much quicker, compared to micro-sprinkler irrigated vineyards (Fig. 2.27). However, the overall effect of evaporation losses dominate to such an extent that the ET of drip irrigated vineyards will be considerably less over the growing season, compared to micro- sprinkler irrigated vineyards under the same conditions (Fig. 2.28). It must be noted that the effect of fractional wetting on ET is more pronounced in summer than in winter. This trend is primarily due to the effect of warmer atmospheric conditions in summer on evaporation losses from the soil, compared to winter. FIGURE 2.26. Schematic illustration of evapotranspiration on the difference in soil water depletion where (A) 100% and (B) 25% of the soil volume is wetted. Calculations are based on a plant spacing of 2.5 m × 1.2 m.

IRRIGATION OF WINE GRAPES 51