Handbook for Irrigation of Wine Grapes in South Africa

irrigation needs to be applied. The leaching requirement for this example will be 0.06 (3 mm ÷ 50 mm). It is important to note that leaching is primarily applicable for full surface irrigation. Where flood irrigation is applied by means of narrow furrows, leaching can occur (Van Zyl, 1981). In such cases, or where a wetted strip is obtained with micro-sprinklers, the leaching approach can also be followed. In the case of drip irrigation, salts do not leach readily from soils. Salts usually accumulate on the perimeter of the wetted volume. Under such conditions, salts can be washed back into root zones if rainfall occurs. Therefore, it is recommended that drip irrigation should be applied during rainfall to reduce negative backwash effects. This approach has practical limitations, since it is not always possible to irrigate all vineyards on a farm simultaneously while it rains. Reducing backwash will probably be more effective where wetted volumes have small diameters (< 30 cm), and salt concentrations around the root zone are high (EC > 0.75 dS/m). 5.4.3 DILUTION OF SALINE WATER Sometimes growers mix water that contains less salts with saline water, i.e. if the latter does not meet water requirements of their vineyards. In such cases, it is advisable to calculate the EC of the mixed water (EC m ) to determine if it will be suitable for irrigation. The EC m is calculated as follows: are electrical conductivities (dS/m) of the two sources, and 2 are volumes of water (m 3 ) from each source, respectively. Mixing saline borehole water with fresh dam water is illustrated by the following example. If the EC of 90 000 m 3 water in a dam is 0.1 dS/m and the EC of borehole water is 1.2 dS/m, EC m ( i.e. ECi w ) will be approximately 0.21 dS/m if 10 000 m 3 borehole water is pumped into the dam. The EC of the dam water will still be within the limits of the low salt hazard class (C1). If 30 000 m 3 borehole water is added to the dam, EC m will be approximately 0.38 dS/m. The dam water will now have a medium salt hazard (C2), but could still be used for moderately salt tolerant/sensitive crops such as grapevines, provided leaching is applied (Table 5.9). 5.4.4 TREATMENT OF SALINE WATER Reverse osmosis or electro-dialysis can be used to remove salts from saline water (DWAF, 1996). However, these technologies are expensive and therefore not economically viable to desalinize large volumes of water for irrigation. Other commercially available water treatment equipment includes an electro- mechanical catalytic conditioner (Fig. 5.9). According to the manufacturers, the latter equipment holds promise, since “un-useable borehole water containing elevated TDS levels can be used for irrigation” after the treatment. However, a field trial carried out for two seasons near Vanrhynsdorp showed that ca. 3.5 dS/m borehole water treated with this equipment did not improve grapevine growth or V 1 and V EC m ≈ [(EC 1 x V 1 ) + (EC 2 x V 2 )] ÷ (V 1 + V 2 ) Eq. 5.6 where EC 1 and EC 2

120 CHAPTER 5 – IRRIGATION WATER QUALITY