Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

Figure 8.4 reference is the evapotranspiration of a short, well-watered grass surface or ETo. The Penman-Monteith equation is used to calculate ETo from atmospheric variables, as well as some additional coefficients and constants. The atmospheric variables include net radiation, temperature, humidity and wind speed, and are measured by means of automatic weather stations (Fig. 8.4B). For complete details of the Penman-Monteith equation, refer to Allen et al. (1998). The ETo represents a more realistic crop water use, compared to evaporation pans, and is currently the preferred reference. Since the water in evaporation pans accumulates latent heat during the day, evaporation could continue after sunset which is not a true crop related reference. Furthermore, if evaporation pans are not encamped, birds and animals can come to drink water, which will contribute to the inaccuracy of E p . value is high, vineyard ET will also be high, and vice versa when the reference value is low. The constant that relates ET to the reference value is called the crop coefficient, and is usually determined by research institutions. Crop coefficients are obtained by measuring ET and reference evaporation simultaneously under a given set of crop, soil and atmospheric conditions. The ET for a given period, e.g. day, month, week or year, is calculated by using the universal soil water balance equation as follows: ET = SWC b + I + P - D - R - SWC e Eq. 8.1 where SWC b and SWC e are the soil water content (mm) in the root zone at the beginning and end of the period, respectively. Where applicable, I is the irrigation applied (mm) and P is rainfall (mm) during the period, whereas D is drainage lost below the root zone (mm) and R is surface runoff (mm). For many years, American Class-A pans (Fig. 8.4A) were used to measure reference evaporation, i.e. the so-called pan evaporation (E p ). Another form of

A

B

FIGURE 8.4 . Examples of (A) an American Class-A evaporation pan and (B) an automatic weather station.

IRRIGATION OF WINE GRAPES 239