Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

A

B

C

FIGURE 5.1. A 500 m  plastic bottle attached to a length of PVC conduit by means of cable ties (A) makes a simple, useful water sampler (B). The handle can be telescopic (C) to extend the reach of the sampler.

5.2 COMPARING RAIN, RIVER AND MUNICIPAL WATER QUALITY Rainfall and river water are regarded as natural water resources. However, rainfall is considered to be one of the purest natural water resources. This is illustrated by the fact that rain collected at Citrusdal and Cape Town contained considerably less basic cations, compared to water from the Holsloot River near Rawsonville (Fig. 5.2). In contrast, rainwater harvested at Kleinmond unexpectedly tended to contain more Na, K, and Ca than the water abstracted from the Holsloot River. Being so close to the ocean, the atmosphere at Kleinmond probably contained salts which dissolved in the rainwater. In most cases, municipalities first need to treat water obtained from natural resources before it is suitable for human consumption. Therefore, municipal water is not regarded as a true natural water resource. However, the cation levels in the water obtained from the Stellenbosch municipality were comparable to the levels in the Holsloot River (Fig. 5.2). It must be noted that this particular river runs through farmland where the pollution risk is low. It is also noteworthy that the levels of Na in all the waters were invariably higher, compared to the other basic cations, i.e. irrespective of the water source. Interestingly, air masses moving inland from the ocean are not necessarily the source of Na in rainwater (Vieira-Filho et al. , 2013). It was previously shown that Na concentrations in rainwater may be substantially more than K concentrations (Larson & Hettick, 1956; Vieira-Filho et al. , 2013). Higher levels of Ca than K is also common, but the actual amounts seem to depend on the type of cloud from which the rain precipitates (Khemani & Ramana Murty, 1968). The foregoing suggests that the relatively high levels of Na and Ca in water from the Holsloot River and Stellenbosch municipality (Fig. 5.2) were most likely contributed by the rainfall.

IRRIGATION OF WINE GRAPES 103