Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

8.4.1.8 WATERMARK® SENSORS The electrical conductivity between two electrodes embedded in gypsum blocks depend on the water content of the gypsum, i.e. the conductivity decreases as the water content decreases. This principle is used to measure soil water content, since the water content in the gypsum comes in equilibrium with the water in the surrounding soil. In the case of Watermark ® sensors, the electrodes are embedded in a “gypsum-like” substance in a perforated, cylindrical housing (Fig. 8.32). Unlike porous ceramic cup tensiometers, the measuring range of Watermark ® sensors is not limited to -80 kPa. Since the measurement depends on electrical conductivity, the sensors are affected by salts in the soil water. The higher the salt concentration, the higher the electrical conductivity. In fact, it was shown that the addition of fertilizers to enhance grapevine growth and functioning will affect the accuracy of the Watermark ® sensors (Van Dyk & Myburgh, 1996). A thermal correction is also required, which means that the soil temperature needs to be measured. A portable, hand held meter (Fig. 8.32) is used to convert electrical resistance to soil water matric potential (kPa). Each watermark probe must first be connected to the meter to obtain a reading. This feature could be time consuming if a large number of sensors need to be read. Furthermore, practical experience has shown that Watermark ® sensors over-estimated the soil water matric potential when compared to porous cup tensiometers in the same soil (Fig. 8.33). Due to this, it is recommended that Watermark ® sensors should be calibrated against tensiometers to ensure sure that the soil water status is not under-estimated.

FIGURE 8.32. Watermark ® meter and sensor used for measuring soil water matric potential.

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