Handbook for Irrigation of Wine Grapes in South Africa

3\03 5\03 9\03 11\03 15\03 17\03 19\03 23\03 25\03 29\03 31\03 2\04 7\04 Date Figure 8.33

0

-20

-40

-60 -80 -100 Soil water matric potential (kPa)

Tensiometer Watermark ® sensor

FIGURE 8.33. Comparison of soil water matric potential measured using Watermark ® sensors and mercury manometer tensiometers in the same soil over four drying cycles (after Van Dyk & Myburgh, 1996).

Preparation and installation: Similar to ceramic cup tensiometers, Watermark ® sensors should not be touched with bare hands to prevent clogging of the pores by skin oils. Always use tissue paper or a clean cloth to handle the sensor. 1. Soak the sensors for one hour in clean water before installation. 2. Holes for installation can be made using a tubular auger similar to those used for taking gravimetric samples. 3. Cut a length of PVC conduit 10 to 15 cm shorter than the lead wires of the sensor. 4. Mark the installation depth with masking tape around the PVC conduit. 5. Push the lead wires through the conduit. 6. Pull the lead wires tight so that the upper end of the sensor presses against the lower end of the conduit. 7. While holding the lead wires tight, use the conduit to push the sensor into the hole down to the desired depth as marked on the conduit.

8. Release the lead wires and remove the conduit from the hole. 9. Fill the hole with a slurry prepared from representative soil.

266 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING