Handbook for Irrigation of Wine Grapes in South Africa

The mean precipitation rate of a micro-sprinkler (mm/h) is calculated from the discharge rate (  /h) and radius of the wetting pattern (m) as follows: Precipitation rate = Discharge rate ÷ A Eq. 4.3 where A is the wetted area which is calculated as follows: Eq. 4.4 where r is the radius (m) of the wetted area. For example, if a micro-sprinkler has a discharge rate of 32  /h, and the radius of the wetted area is 1.5 m, the precipitation rate will be 4.53 mm/h. However, due to the inherent, non-uniform distribution patterns, micro-sprinkler systems should be designed to allow overlapping of the wetting patterns to enable more uniform water distribution. If this is the case, the precipitation rate is a function of the discharge rate, number of micro-sprinklers and the area that is being irrigated. For one hectare, the precipitation rate is calculated as follows: Precipitation rate = (Discharge rate x (100 2 /(d R x d D )) ÷ 100 2 ) Eq. 4.5 is the spacing between the micro-sprinklers (m). For example, if the row spacing is 2.4 m and the 32  /h micro-sprinklers are spaced 2 m apart on the laterals, the precipitation rate of the system will be 6.67 mm/h. If the water pressure in the irrigation system is too high, micro-sprinklers will produce a fine mist spray (Fig. 4.15). This will increase evaporation losses as the fine droplets travel from the micro-sprinkler orifice to the ground. Warm, windy conditions will cause further evaporation losses. This is one of the major reasons why the system efficiency of micro-sprinklers is lower than that of drippers (Table 4.1). For optimum results, micro-sprinklers should be installed horizontally. However, in many cases the water is distributed at an angle which causes uneven water distribution. This usually happens if the lateral line warps around the wire to which it is attached. To ensure even water distribution, micro-sprinklers can be installed on plastic pegs. For optimum results, the micro-sprinkler pegs must be planted to the design depth as indicated on most pegs (Fig. 4.16). To ensure uninterrupted, horizontal water distribution, micro-sprinklers can also be suspended from a high lateral (Figs. 4.17 & 4.18). Insects nesting in micro-sprinklers may cause clogging or interfere with the water distribution pattern. The occurrence of this problem can be reduced by installing micro-sprinklers that are fitted with anti- bug caps (Fig. 4.19). A = (22 ÷ 7) x r 2 where d R is the row spacing (m) and d D

82 CHAPTER 4 – IRRIGATION SYSTEMS