Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

TABLE 3.1. Water holding capacity according to soil textural class. Soil texture

Water holding capacity (mm/m)

Coarse sand

20-65 65-85

Fine sand

Loamy sand Sandy loam

90-100 105-115 125-165 165-210 150-165 125-140 100-140

Fine sandy loam

Silt loam

Silty clay loam

Silty clay

Clay

3.8 INFILTRATION RATE The movement of water into soil is called infiltration. Since water enters the soil via pores, it is related to the soil texture, particularly the clay content (Fig. 3.13). Water infiltration can be measured in the field by means of the double ring method (http://www.fao.org/docrep/s8684e/s8684e0a.htm). The equipment consists of two concentric stainless steel rings that are knocked into the soil (Fig. 3.14). Water is supplied to both rings. The purpose of the water in the outer ring is to minimize lateral flow from the inner ring. The water level in the inner ring is kept constant by means of a Mariotte tube in the container from which the water is siphoned. The infiltration rate is calculated from the volume of water that flows from the container over a certain time interval that will depend on the infiltration rate. Since the quality of the water, e.g. salt content, can affect the rate of infiltration, it is essential to use water from the same source as the irrigation water for infiltration measurements.

60

50

(i)

40

30

20

y = 27.245x -0.847 (R 2 = 0.991)

(ii)

Infiltration rate (m/h)

10

(iii)

(iv)

0

0

5

10

15

20

25

Clay (%)

FIGURE 3.13. Relationship between constant head water infiltration and clay content measured in (i) aeolian sand from Lutzville and (ii) alluvial sand from Rawsonville, as well as (iii) granitic and (iv) shale-derived soils from Stellenbosch (unpublished data).

IRRIGATION OF WINE GRAPES 65