Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

Figure 8.27

Vacuum gauge

Filling cap

Tube

Surface

Water in soil pores

Water inside tensiometer

Tube

W a t e r m o v e s o u t a s s o i l d r i e s o u t

Soil particles

Ceramic cup

W a t e r m o v e s i n a s s o i l b e c o m e s w e t t e r

FIGURE 8.27. Illustration showing the components of a Bourdon gauge tensiometer and the water dynamics as the soil water content changes.

Figure 8.28

The vacuum gauges, or so-called Bourdon gauges, usually indicate the matric potential in -kPa. However, in the literature matric potential is sometimes reported as -MPa. The vacuum gauge can be replaced with a glass mercury manometer (Fig. 8.28A). The latter allows more accurate readings than Bourdon gauges and is primarily used for research purposes. The glass tubes can break easily, thereby causing spills of highly toxic mercury. Consequently, these tensiometers are not recommended for on-farm use. The Bourdon gauges can also be replaced with electronic vacuum transducers (Fig. 8.28B). The transducers convert the suction to an electric current which can be recorded by means of a small data logger. Although this provides the opportunity for continuous measurements, the vacuum transducers need to be calibrated individually in order to calculate the soil matric potential from the millivolt output. If the data cannot be transmitted to a computer, downloading and processing will be time consuming. As in the case of mercury manometer tensiometers, the electronic ones are more suited for research rather than practical irrigation scheduling. Due to the physical properties of water, tensiometers can only measure up to -80 kPa. Therefore, the use of tensiometers is restricted to the range of readily available water, particularly in heavier soils. Unfortunately, there are no other liquids than can replace water in a practical way in order to extend the range of tensiometers used in the field.

IRRIGATION OF WINE GRAPES 261