Handbook for Irrigation of Wine Grapes in South Africa

TABLE 3.2. Classification of downward water permeability through soils. Permeability class

Permeability rate (mm/h)

Very rapid

>254

Rapid

125-255

Moderately rapid

65-125

Moderate

20-65

Moderately slow

5-20

Slow

1-5

Very slow

<1

Figure 3.17 be determined by means of the constant head or falling head methods. It should be noted that these methods require special equipment and must be carried out by skilled persons. 3.10 HYDRAULIC CONDUCTIVITY Hydraulic conductivity designates the soil’s ability to conduct water through its pore network. In contrast to permeability where only downward flow is considered, K is applicable to soil water in more dimensions, i.e. downward, lateral and upward flow. Saturation hydraulic conductivity usually occurs following heavy rainfall or irrigation. Similar to infiltration and permeability, K s is high in coarse textured materials, e.g. gravel or sand, and appreciably lower in fine textured materials, e.g. silt and clay (Fig. 3.17). For most soil textures occurring in the wine growing regions, K s can

HYDRAULIC CONDUCTIVITY (cm/s) Water at 20°C

10 1

10 0

10 -1

10 -2

10 -3

10 -4

10 -5

10 -6

10 -7

10 -8

10 -9

Practically impermeable

CLASS

High

Medium

Low

Very low

Fine sand

Loess Loam Structured clay

Homogenous clay

MATERIAL

Gravel

Sand

Sandy clay

Silt Mixtures of sand, silt & clay

Tri-axial cell

Constant head

Falling head

METHOD

Transducer response

10 -4

10 -5

10 -6

10 -7

10 -8

10 -9

10 -10

10 -11

10 -12

10 -13

10 -14

FIGURE 3.17. Saturation hydraulic conductivity ( K s

) of various soil materials, as well as

methods proposed for determining K s

(redrawn from Klute & Dirksen, 1986).

68 CHAPTER 3 – WATER RELATED SOIL PROPERTIES