Handbook for Irrigation of Wine Grapes in South Africa

9.4.3 OVERHEAD SPRINKLER IRRIGATION The application of water by means of overhead sprinklers is probably the most commonly used method for reducing the risk of cold damage in vineyards. The basic principle of this method is that heat lost from the grapevines during cold nights is replaced by the heat released as the applied water freezes (Perry, 1994). In fact, when 1 gram of water freezes, it releases 80 calories of energy. Therefore, this latent heat of fusion will provide heat as long as ice continues to form. When atmospheric conditions reach the temperature and wind speed thresholds (Table 9.3), the irrigation must start. Irrigation should continue until sunrise when melting water runs freely between the ice and the grapevine leaves and shoots. If windy conditions prevail during the morning, i.e. wind speeds higher than 8 km/h, continued irrigation will prevent excessive cooling due to evaporation from the grapevine surfaces. This means that the atmospheric conditions must conform to the thresholds in Table 9.3 before the irrigation can be stopped. Overhead sprinkler irrigation may also be effective against windborne freezes, i.e. if the wind speed remains below 8 km/h, to avoid over-cooling of the ice. If the latter happens, the temperature of the grapevines can fall way below the air temperature. When the volume of evaporating water exceeds the water that freezes, the grapevines will lose more heat than it can absorb. This means the temperature of an ice-covered grapevine will drop below that of a comparable “dry” grapevine as soon as freezing stops and evaporation continues. It must be noted that 600 calories of heat energy are absorbed from the environment when 1 gram of water evaporates (Perry, 1994). Therefore, 7.5 times more water must freeze to meet the calories used by evaporation in order to provide a net heat effect. Since wind increases evaporative cooling, wind speeds in excess of 8 km/h will reduce the efficacy of frost protection substantially (Perry, 1994). Under such conditions, it would be advisable not to apply overhead sprinkler irrigation. The overhead sprinkler irrigation should be applied at a precipitation rate of 2 to 3 mm/h. This means that the water flow rate through the nozzles should be between 250 and 350 mm/h for sprinklers with a 6 m radius. Sprinklers should rotate at least once a minute to avoid ice accumulation in, and around the nozzles. The above-mentioned precipitation rate would be adequate to protect vineyards if the air temperature is -5°C, or higher. If the precipitation rate is too low, not enough water might freeze to generate sufficient heat for protection against cold damage. If this happens, more cold damage could occur compared to no overhead irrigation (Perry, 1994). On the other hand, high precipitation rates will waste water and may cause excessive ice accumulation on the grapevines. Furthermore, shoots may break off if the ice load becomes too heavy.

296 CHAPTER 9 – PREVENTING COLD DAMAGE IN VINEYARDS