Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

A single crop coefficient can be used to calculate vineyard ET simply as follows: ET = k c x E p Eq. 8.2 where E p is the reference evaporation, or ET = k c x ETo Eq. 8.3 where ETo is the reference evapotranspiration. As an example, the weekly irrigation requirement during December for a micro- sprinkler irrigated vineyard near Robertson can be calculated as follows. If the k c is 0.45 (Table 8.1) and the E p measured at the ARC Robertson Experiment farm is 8.3 mm/day in December, the vineyard ET = 0.45 x 8.3 = 3.74 mm/d. Therefore, the vineyard will require 3.74 x 7 days = 26.2 mm per week. In the case of drip irrigation applied at a medium frequency under the same conditions, k c will be 0.14 (Table 8.2). If ETo is used as the reference, and amounts to 6 mm/d, then the vineyard ET = 0.14 x 6 = 0.84 mm/d. Therefore, the vineyard will require 0.84 x 7 days = 5.9 mm per week. It must be noted that the irrigation applied on the wetted area will be 5.9 mm x 5 = 29.5 mm if the drip system wets only 20% of the soil volume. EVAPOTRANSPIRATION MODELS Various aspects can influence evapotranspiration of vineyards. For instance, evaporation losses under drip irrigation will be less compared to full surface wetting, e.g. micro-sprinklers. Water will also evaporate quicker from sandy soils than from heavier soils. Grapevines bearing more leaves on bigger canopies will transpire more water than smaller ones. In the case of grapevines on horizontal canopies, more leaves will be exposed to radiation over the course of the day, compared to grapevines on vertical canopies. Due to this variability, ET can be estimated more accurately by means of the dual crop coefficient approach. With the latter approach, k c is divided into two coefficients, i.e. k s for evaporation from the soil and k cb for basal crop transpiration. Each of the latter coefficients can be adjusted according to the various factors that can influence them (Fig. 8.5). 8.3.2

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