Handbook for Irrigation of Wine Grapes in South Africa

Chapter 4

As for any other irrigation system, it is essential to know how much water is applied with flood irrigation. In order to calculate irrigation applications, the stream flow must be known. The standard stream flow supplied by government irrigation schemes is usually ca. 150 m 3 /h. This means that 100 mm irrigation will be applied per hectare if the water flows for 6.7 hours. However, it is more accurate to calculate the irrigation application (mm) for a specific bed or furrow as follows: Application = (Q x (t ÷ 60)) ÷ (l x w) x 1 000 Eq. 4.6 where Q is the stream flow (m 3 /h), t is the time (minutes), l is the length (m) and w the width (m) of the bed or furrow. For example, if Q is 150 m 3 /h, and it takes 20 minutes to irrigate a 6 m wide and 100 m long bed, the irrigation application will be ca. 83 mm. If the irrigation needs to be applied in 1 m wide furrows along the grapevine rows, Q must be split in three to obtain the same application. This means that the stream flow will be 50 m 3 /h per furrow, and that three furrows can be irrigated at the same time. Furthermore, it must be noted that the time will be reduced to 10 minutes for this example. SUBSURFACE IRRIGATION SYSTEMS The rationale behind subsurface irrigation is primarily to reduce evaporation losses from the surface in order to improve irrigation WUE. A further advantage is that nutrients can be applied directly to the roots. However, subsurface irrigation systems are not commonly found in South African vineyards. A field trial in the Lower Orange River region showed that subsurface drip installed at 15, 30, and 45 cm depths had no advantage on the yield of Sultanina/143B Mgt over furrows or conventional above-ground drip irrigation (Table 4.4). Due to the silty nature of the soil, roots did not concentrate under the above-ground and 15 cm subsurface drip (Fig. 4.29A & B). In contrast, roots concentrated around the dripper lines at 30 and 45 cm depths (Fig. 4.29C & D). TABLE 4.4. Effect of irrigation system on irrigation volume, yield and WUE of Sultanina near Upington. Data are means for three seasons (Myburgh, 2007a & b). Irrigation system Irrigation applied (m 3 ) Yield (t/ha) WUE (kg/m 3 ) 4.3

Furrows

6 930

40.2 a*

5.8 a

Above-ground drippers

7 030

36.2 a

5.2 a

Subsurface drip, 15 cm deep

6 770

35.6 a

5.3 a

Subsurface drip, 30 cm deep

6 850

34.7 a

5.1 a

Subsurface drip, 45 cm deep

6 720

32.1 a

4.8 a

Subsurface drainage pipes, 15 cm deep 5.3 a * Values designated by the same letter within each column do not differ significantly (p ≤ 0.05). 6 740 35.6 a

IRRIGATION OF WINE GRAPES 91