Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

FIGURE 8.25. A DFM ® probe used for recording soil water content and insert showing handheld console for downloading the data.

Figure 8.26 Figure 8.26 Field calibrations have shown that there is a good correlation (R 2 ) between soil water content measured by means of DFM ® probes and gravimetric soil water content (Fig. 8.26). As in the case of neutron probes, calibration lines for DFM ® probes may vary between soils, and within a soil profile. However, the DFM ® probes grossly overestimated the actual soil wat r content. This could be problematic if irrigation amounts, e.g. millimeters required to restore field capacity, are based on the DFM ® probe soil water content.

100 120 140

10 120 140

100 120 140

100 120 140

A

B

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0 20 40 60 80

0 20 40 60 80

0 20 40 60 80

0-20 cm 20-40 cm 40-60 cm 60-80 cm 0-20 cm: y = 0.5896x + 57.74 (R 2 = 0.8141) 20-40 cm: y = 0.9574x + 51.63 (R 2 = 0.9768) 40-60 cm: y = 0.9455x + 63.29 (R 2 = 0.9340) 60-80 cm: y = 0.9459x + 62.18 (R 2 = 0.9353) 0-20 cm 20-40 cm 40-60 cm 60-80 cm 0-20 cm: y = 0.5896x + 57.74 (R 2 = 0.814 ) 20-40 cm: y = 0.9574x + 51.63 (R 2 = 0.9768) 40-60 cm: y = 0.9455x + 63.29 (R 2 = 0.9340) 60-80 cm: y = 0.9459x + 62.18 (R 2 = 0.935 )

0-20 cm 20-40 cm 40-60 cm 60-80 cm 0-20 cm: y = 1.2851x + 59.15 (R 2 = 0.9633) 20-40 cm: y = 1.4321x + 52.32 (R 2 = 0.9628) 40-60 cm: y = 0.8585x + 77.06 (R 2 = 0.6151) 60-80 cm: y = 0.633x + 96.70 (R 2 = 0.8477) 0-2 cm 20-4 cm 40-6 cm 60-8 cm 0 c : y = 1.2851x + 59.15 (R 2 = 0.963 ) c : y = 1.4321x + 52.32 (R 2 = 0.9628) cm: y = 0.8585x + 7 .06 (R 2 = 0.6151) c : y = 0.63 x + 96.70 (R 2 = 0.847 )

Probe SWC (mm/20 cm) Probe SWC (mm/20 cm)

Probe SWC (mm/20 cm) Probe SWC (mm/20 cm)

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Gravimetric SWC (mm/20 cm) Gravimetric SWC (m /20 cm)

Gravimetric SWC (mm/20 cm) Gravimetric SWC (m /20 cm)

FIGURE 8.26. Comparison between SWC measured by means of DFM ® probes and gravimetrically determined SWC in (A) deep, red soil and (B) soil containing clay in the subsoil in two commercial vineyards near Stellenbosch.

IRRIGATION OF WINE GRAPES 259