Handbook for Irrigation of Wine Grapes in South Africa

Chapter 4

Visual observations revealed that the water flowed upwards to the soil surface by means of capillary rise, irrespective of installation depth. This suggested that surface evaporation occurred even where the dripper lines were 45 cm below the soil surface. It must be noted that the root systems were slightly lopsided since the subsurface dripper lines were installed ca. 20 cm from the grapevine rows. Low frequency, subsurface drip irrigation also did not have any positive effect on yield and water use efficiency, compared to conventional, single line above-ground drip (Table 4.5). The particular field trial was carried out with Merlot near Wellington (Myburgh, 2011a & b). A full surface straw mulch was applied for the duration of the field trial to avoid tillage that could cause damage to the dripper lines. Since the dripper lines were only 15 cm deep in the work rows, there was a risk that they could be damaged if a winter cover crop had to be established. Based on the foregoing, it can be assumed that subsurface drip will not necessarily prevent evaporation losses to the extent that it would improve irrigation water use efficiency appreciably. TABLE 4.5. Effect of low frequency above-ground and subsurface drip irrigation on irrigation volume applied, yield and water use efficiency (WUE) of Merlot near Wellington in the Coastal region during the 2006/07 season. Irrigation strategy Irrigation volume (m 3 ) Yield (t/ha) WUE (kg/m 3 )

Three irrigations on grapevine row

948

9.7 a*

10.2 a

Three irrigations subsurface in work row

948

9.9 a

10.4 a

Five irrigations on grapevine row

1 275

11.7 a

9.2 a

Five irrigations subsurface in work row 8.7 a * Values designated by the same letter within each column do not differ significantly (p ≤ 0.05). 1 275 11.0 a

The study in the Lower Orange River region revealed that clogging of the subsurface drippers was substantially higher, compared to above-ground drip. Laboratory tests carried out by NETAFIM SA indicated that ca. 12% of the subsurface drippers at 15 and 30 cm depths were clogged after a period of six years. There is no explanation why subsurface drip at 45 cm did not show any clogging. The primary reasons for the clogging was slime formation inside the drippers, as well as roots growing into the drippers via the orifices (Fig. 4.30). It should be noted that trifluralin was applied through the subsurface drip lines. By adsorbing to the soil around the drippers, this particular herbicide repels roots growing towards the drippers. Apparently this practice did not prevent root penetration completely, but if it was not done, the clogging might have been much worse.

IRRIGATION OF WINE GRAPES 93