Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

During periodic droughts, natural water resources, e.g. rivers and boreholes, may become substantially more saline, compared to periods when higher annual precipitation occurs. There is some evidence that grapevines still seem to perform reasonably well if the EC does not exceed 2.5 dS/m (Table 5.10). Therefore, it might be possible to use saline water for one or two seasons if better quality water is not available. When the water quality returns to normal, it will be critical to leach any salts that could have accumulated in the soil via the high salinity water. Frequent soil sampling and analyses are essential to quantify the extent of salt accumulation where vineyards were irrigated using high salinity water. Any accumulated salts can be leached out as discussed below. It must be emphasized that using high salinity irrigation water is considered to be a temporary measure to manage survival of vineyards during critical droughts. TABLE 5.10. The effect of irrigation with saline water on relative yield of Colombar/99 R near Robertson (adapted from Moolman et al ., 1998). Year Irrigation water EC (dS/m) 0.25 0.75 1.5 2.5 3.5 5.0 1992 100 a* 83 ab 88 ab 97 ab 77 ab 73 b 1993 100 a 78 ab 75 abc 87 a 52 bc 48 bc 1994 100 a 70 ab 73 ab 96 a 45 b 34 b 1995 76 ab 71 ab 64 abc 100 a 41 bc 29 c * Values within each row followed by the same letter do not differ significantly (p ≤ 0.05). 5.4.2 LEACHING SALTS FROM THE ROOT ZONE When saline water is used for irrigation, the most common practice is to apply controlled over-irrigation. With this approach, it is assumed that salts in irrigation water which end up in the soil can be leached from the root zone. The salinity of the drainage water at the bottom of the root zone depends on EC iw , as well as the amount of leaching that occurs, and can be calculated as follows (DWAF, 1996): EC dw = EC iw ÷ LF Eq. 5.3 where LF is the fraction of the applied irrigation water that is leached below the root zone. The leaching fraction is derived as follows: LF = D dw ÷ D iw Eq. 5.4 where D iw and D dw are depths (mm) of irrigation and drainage water, respectively. The depth of additional water required to obtain sufficient leaching when the water requirement of a vineyard is replenished, is calculated as follows: D dw = [EC iw ÷ (EC dw – EC iw )] x ET Eq. 5.5 The permissible EC dw for vineyards is approximately 24 dS/m (Van Zyl, 1981). If the ET of a vineyard requires 50 mm irrigation, and the EC iw is 1.4 dS/m, 3 mm additional

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