Handbook for Irrigation of Wine Grapes in South Africa
Chapter 8
Figure 8.21 To take actual measurements, the neutron probe is placed on the access tube (Fig. 8.21). The probe is then released and lowered by means of the cable to the first depth. Usually, stoppers are clamped around the cable for each depth where measurements need to be carried out. The stoppers are used to suspend the probe while a measurement is taken. A common problem is that the depth stopper clamps are incorrectly positioned on the cable. The position of the first clamp depends on the dimensions of the meter, the position of the neutron source within the probe, height of the access tube above the soil surface and the measuring depth. As an example, the setting of the first stopper clamp for measuring soil water content at 20 cm depth in an access tube which protrudes 10 cm above the soil surface is illustrated in Figure 8.21. For this example, the first clamp should be fixed at 34 cm, i.e. 35 + 10 + 20 - 31 cm, when the probe is inside the protective housing. If measurements are to be carried out at 40, 60 and 80 cm, the other clamps should be at 54, 74 and 94 cm, respectively.
Cable
Depth stopper
Meter
+35 cm
+10 cm
Surface
-31 cm
Probe
+20 cm
Neutron source
Access tube
FIGURE 8.21. An example of setting the depth clamps on the cable of a neutron probe for measuring soil water content at 20 cm increments.
Experience has shown that different soils, and even layers within a soil profile, have different calibration lines (Fig. 8.22). Therefore, the accuracy of the universal calibrations supplied by the manufacturers is questionable. Consequently, neutron probes will provide more accurate soil water content values if the probes are properly calibrated against gravimetric soil water content, as explained in Section 8.4.1.2.
IRRIGATION OF WINE GRAPES 255
Made with FlippingBook - Online catalogs