Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

Irrigation water quality

5.1 INTRODUCTION Irrigation water is an essential, but limited, resource in most grape growing regions, particularly in the semi-arid parts. In many cases, quality of irrigation water is declining due to pollution caused by poor waste management. Over-irrigation and -fertilization also cause water pollution. Water quality is defined in terms of the physical, chemical, biological and aesthetic properties that determine its suitability for use, and for the protection of aquatic ecosystems (Van Zyl, 1981; DWAF, 1996). In agriculture, water quality is related to its effects on soils, and crops, as well as management inputs necessary to overcome quality related problems (Kirda, 1997). In addition to TDS and SAR, the toxicity thresholds of other elements and the pH also determine irrigation water quality for vineyard irrigation (Van Zyl, 1981). These threshold values are almost identical to the general water quality guidelines for agriculture in South Africa (DWAF, 1996). Irrigation of grapevines with poor quality water can result in the accumulation of elements, which will depend on factors such as soil texture, soil composition, drainage, rainfall and volume of irrigation water applied. Since poor quality water can reduce yields of commercial crops or damage irrigation equipment (DWAF, 1996), permissible levels of elements in water must be known. This will enable growers to apply available water judiciously to avoid negative effects and promote long term sustainable use. Therefore, irrigation water should be analysed at least twice a year, i.e. at the beginning and in the middle of the grape growing season. Two litres of water should be collected in a suitable glass or plastic bottle (Kirda, 1997), which has been rinsed with the same water which is being sampled. The water sample must be taken from where the water exits the dam. In the case of rivers, samples should be collected in the fastest flowing part (Kirda, 1997). In the case of lakes or ponds, the sample should be taken from the centre in such a way that it represents the variation over depth at that point. For this purpose, a simple water sampler can be made by attaching a plastic sample bottle to a length of PVC conduit (Fig. 5.1A). Samples can be taken deeper and further from the bank (Fig. 5.1B) if the handle is telescopic (Fig. 5.1C). The latter will also allow easier transport. It must be noted that the sample bottle should also first be rinsed with the same water before the actual sample is taken. Water samples should be taken to the laboratory as soon as possible to minimize the risk of biological changes.

102 CHAPTER 5 – IRRIGATION WATER QUALITY