Handbook for Irrigation of Wine Grapes in South Africa

Chapter 2

requirements of vineyards. Refer to Chapter 8 for more details concerning the use of crop coefficients and a reference ET for irrigation scheduling.

2.4 EVAPOTRANSPIRATION Water use of a vineyard refers to the amount of water lost during a specific period of time. This is usually expressed in millimeters per day, week, month or year. Water use is primarily the combination of two processes viz . (i) transpiration from leaves and (ii) evaporation from the soil surface (Fig. 2.5). The term evapotranspiration is mostly used instead of water use. Consequently, the acronym “ET” is commonly used in literature and practice. The purpose of the following section is to explain the basic principles of evaporation and transpiration in order to understand water use differences between vineyards. 2.4.1 EVAPORATION FROM THE SOIL After rain or irrigation, evaporation from the wetted soil surface can be divided into two phases (Fig. 2.6). During the first phase, the rate of E s is mainly influenced by the prevailing weather conditions. In other words, the warmer and windier, the higher the rate of E s will be and vice versa . In Phase I, E s is relatively high and equates to evaporation from free water surfaces, e.g. dams or lakes. Phase I persists normally for a day or two only, depending on the weather conditions. During the second phase, however, the rate of E s is governed by the physical characteristics of the soil, e.g. the hydraulic conductivity. Therefore, E s in Phase II is considerably lower, compared to Phase I (Fig. 2.6).

50

Phase I

Phase II

Phase III

40

Atmospheric conditions

20 Cumulative E s (mm) 30

Soil properties

Vapour phase

Liquid phase

10

0

5

10

15

0

Time (days)

FIGURE 2.6. Cumulative E s

following an irrigation.

IRRIGATION OF WINE GRAPES 35