Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

behaviour from season to season. Atmospheric conditions and time of day should be considered when grapevine water potentials are measured. For optimum irrigation scheduling, soil and grapevine water status measurements should be combined, particularly to set refill lines. Growers should be aware of the limitations of instruments with respect to the soil water content range in which they can function. For instance, in the case of low frequency irrigation, tensiometers will be of little use, since the soil water matric potential is bound to fall below -80 kPa. Growers must insist on proof of instrument calibration against gravimetric soil water content or matric potential. Furthermore, they must confirm the availability of service, spares and/or backup instruments, as well as replacement parts, access tubes and installation costs before they purchase irrigation scheduling equipment. Growers must make sure that irrigation models, e.g. SAPWAT, SWB, PUTU and VINET, were validated against actual ET for accuracy. Likewise, ET values obtained from remote sensing systems should be validated by means of soil based ET measurements.

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