Handbook for Irrigation of Wine Grapes in South Africa

In some cases, growers have their own automatic weather stations. These are usually installed and maintained by private operators, who also download and process the data. Weather data, including ETo values, can also be obtained from automatic weather station networks. Some of the networks are managed by industry organizations within specific regions. The ARC-Institute for Soil, Climate and Water in Pretoria also runs an automatic weather station network at the national level. However, the integrity of the data is crucial when it comes to irrigation scheduling, no matter how, where and by whom it is collected. Therefore, providers of weather data must make sure that the information is complete, tested and correct before it reaches the users. Crop coefficients (k c ) generally increase over the growing season as the weather becomes warmer and grapevine canopies expand. Initially, crop coefficients were determined for use with E p (Table 8.1). It is important to note that these crop coefficients are only applicable to full surface irrigation. Furthermore, the crop coefficients for low frequency irrigation are appreciably lower, compared to those for more frequent irrigation. In the case of fractional wetting, e.g. drip irrigation, evaporation losses will be less, compared to full surface wetted grapevines bearing the same leaf area as discussed in Chapter 2, Section 2.4.3. Due to the lower ET, crop coefficients for drip irrigation will also be lower, compared to full surface crop coefficients (Table 8.2). TABLE 8.1. Crop coefficients for estimating full surface irrigation requirements of wine grapes irrigated at a low frequency (Green, 1985) or medium to high frequency (Van Zyl & Van Huyssteen, 1988) using American Class A-pan data as the reference evaporation. Irrigation frequency Crop coefficient Sep Oct Nov Dec Jan Feb Mar Apr Low 0.20 0.15 0.25 0.25 0.25 0.25 0.20 0.20 Medium to high – 0.29 0.42 0.45 0.49 0.51 0.48 – TABLE 8.2. Crop coefficients for estimating drip irrigation requirements of wine grapes irrigated at low, medium and high frequencies (Lategan, 2011) using ETo as the reference evaporation. Irrigation frequency Crop coefficient Sep Oct Nov Dec Jan Feb Mar Apr Low 0.11 0.15 0.07 0.06 0.14 0.10 0.14 0.14 Medium 0.11 0.13 0.15 0.14 0.19 0.18 0.19 0.21 High 0.12 0.24 0.28 0.41 0.42 0.47 0.41 0.21 8.3.1 CROP COEFFICIENTS

240 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING