Handbook for Irrigation of Wine Grapes in South Africa

Sandy loam Coarse sand and Ψ m

Figure 3.1

simultaneously (Fig. 3.10). Due to the limited Ψ m range that tensiometers can measure, the in situ soil water characteristic curves are limited to ca. -70 kPa. However, computer software can be used to fit a regression model to the data. The regression model can be used to calculate the soil water content at -100 kPa, or even lower Ψ m .

10 15 20 25 30 35 40 45 50

A

30 40 50 60 70 80 90 100 110 Ȍ m (-kPa)

Figure 3.10

Soil water content (mm/30 cm)

0 5

0

10 15 20 25 30 35 40 45 50

B

y = 43 77x -0 368 R² = 0 8204

Soil water content (mm/30 cm)

0 5

0

10

20

30

40

50

60

70

80

Ȍ m

(-kPa)

FIGURE 3.10. Due to the gravelly nature of the weathered gneiss soil (A), the soil water characteristic curve (B) had to be determined in situ .

62 CHAPTER 3 – WATER RELATED SOIL PROPERTIES