Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.22 Fig e .22

When using a neutron probe, the count ratios are calculated as discussed above, and then used in appropriate calibration equations as indicated in Figure 8.22.

70

70

60

60

A

B

60

60

50

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50

40

40

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40

30 ( /20 c ) 30

30 ( /20 c ) 30

20

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20

SWC (mm/20 cm)

SWC (mm/20 cm)

SWC = 86.18CR - 54.73 (R 2 = 0.9694) SWC = 87.89CR - 61.76 (R 2 = 0.9745) SWC = 198.67CR - 215.60 (R 2 = 0.8020) SWC = 167.29CR - 185.50 (R 2 = 0.8480) S C = 86.18CR - 54.73 (R 2 = 0.9694) S C = 87.89CR - 61.76 (R 2 = 0.9745) S C = 198.67CR - 215.60 (R 2 = 0.8020) S C = 167.29CR - 185.50 (R 2 = 0.8480)

0-20 cm 20-40 cm 40-60 cm 60-80 cm 0-20 c 20-40 c 40-60 c 60-80 c

SWC = 114.78CR - 57.71 (R 2 = 0.8749) SWC = 89.03CR - 38.76 (R 2 = 0.9768) SWC = 86.55CR - 38.38 (R 2 = 0.9948) SWC = 75.02CR - 40.16 (R 2 = 0.9466) S C = 1 4.78CR - 57.71 (R 2 = 0.8749) S C = 89.03CR - 38.76 (R 2 = 0.9768) S C = 86.5 CR - 38.38 (R 2 = 0.9 48) S C = 75.02CR - 40.16 (R 2 = 0.946 )

0-20 cm 20-40 cm 40-60 cm 60-80 cm 0-20 c 20-40 c 40-60 c 60-80 c

10

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0

0

0

0

0 4 0 6 0 8 1 0 1 2 1 4 0 4 0 6 0 8 1 0 1 2 1 4

0 4 0 6 0 8 1 0 1 2 1 4 1 6 0 4 0 6 0 8 1 0 1 2 1 4 1 6

CR

CR

FIGURE 8.22. Relationships between SWC and neutron probe CR for (A) deep, red soil and (B) soil containing clay in the subsoil in two commercial vineyards near Stellenbosch.

Figure 8.23 Fig .2 A disadvantage of the neutron scattering technique is that the registration, handling and management of any radio-active sources must comply with the strict legislation administered by Radiation Control at the Department of Health. This applies to all radio-active sources, even if the radiation is not harmful to the human body, as in the case of commercial neutron probes. Therefore, neutron probes must be annually subjected to leak tests to ensure the safety of users. For the latter reason, each neutron probe is inside a protective housing which also contains the metering equipment and batteries. Furthermore, neutron probes must be stored in a safe place to prevent theft, and a log must be kept to record its usage. 8.4.1.4 ENVIROSCAN® SENSORS This instrument measures electrical capacitance in soil, which is converted to soil water content in millimeters. A set of cylindrical sensors consisting of two bands each, are mounted on a PVC shaft (Fig. 8.23A). The sensors fit closely inside a PVC tube installed in the soil. The position of the sensors can be adjusted on the shaft to the desired measuring depths. The instrument is then pushed into the PVC tube so that only the cover cap remains above the soil (Fig. 8.23B). The set of sensors is then connected to a data logger which allows continuous monitoring. This provides an almost complete picture of soil water changes over time and soil depth, i.e. depending on the number of sensors per shaft and the depth between them. Similar to neutron probes, EnviroScan ® sensors can measure over the full range of plant available water content. The cable length between the sensors and the data logger is limited to 500 m. This means that any measurements beyond a radius of

256 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING