Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

Evaporation

Evaporation

A

B

15 mm evaporated

45 mm remains

60 mm per root depth

100% Wetting Volume evaporated per ha = 100 m × 100 m × 0.015 m = 150 m 3 = 150 000 

25% Wetting Volume evaporated per ha = 100 m × 100 m × 0.015 m × 25%

= 37.5 m 3 = 37 500 

FIGURE 2.24. Schematic representation where the same depth of water is lost from the soil via surface evaporation from (A) full surface and (B) partially wetted soil volumes.

Figure 2.25

(Fig. 2.25). It must be noted that the volume of water lost by transpiration is usually considerably less than the volume of water lost through E s . However, the difference between E s and transpiration volumes will decrease as the rate of E s declines when the soil surface layers becomes drier following rainfall or irrigation as illustrated in Figure 2.6.

A

B

Transpiration

Transpiration

Volume transpired per ha = 3 333 grapevines × 2  = 6 667  /ha/day 100% Wetting

Volume transpired per ha = 3 333 grapevines × 2  = 6 667  /ha/day 25% Wetting

FIGURE 2.25. Schematic illustration of soil water depletion where the same volume of water is removed by transpiration from (A) full surface and (B) fractional wetted soil volumes. Calculations are based on a plant spacing of 1.2 m × 2.5 m, and that daily transpiration was 2  per grapevine.

IRRIGATION OF WINE GRAPES 49