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Is Irrigation Such a Bad Thing? An Australian Perspective.

©Richard Gawel

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Wine tasting vocabulary is full of buzz words and phrases. Two of the most emotive terms in recent times have been "old vine" and "unirrigated". Often these terms are seen on back labels together, or are uttered with some reverence by participants at wine tastings. This is not surprising as many of the existing old vineyards were planted on deep rich loamy soils. Why wouldn't they be? The old timers with their limited technology had to be practical souls, and therefore sought out sites that ensured trouble free grape growing and decent yields. Over time, these old vines have negated the need for irrigation by planting their roots firmly down a metre or three which have enabled them to access some moisture in even the driest of times. For us wine tasters, there is no doubt that these old unirrigated vines can make stunning wines with a denseness and richness that are unsurpassed.

However, in our hot Australian climate, not irrigating is definitely the exception rather than the rule. Nearly all of Australia's vineyards are plied with some form of supplementary irrigation. The stark reality is that even in the coolest and wettest winegrowing regions of Australia, evaporation during the grape growing season (Oct-Mar) easily outstrips the natural rainfall during that time. Even in regions such as the Lower Hunter where the summer rainfall is very high (getting more on average than even Bordeaux), irrigation is essential. This is because the high annual variation in rainfall experienced there results in the vines being considerably water stressed in some years. In short, for most places and in most years, if the vines were not irrigated they would either produce commercially unsustainable grape yields or they would shrivel and die.

There is a bit of an old chestnut that the use of irrigation must necessarily reduce wine quality. Perhaps this arises from the logical flip-side of the justifiable belief that unirrigated = high quality. The fact that irrigation is an illegal practice in many regions of France probably also contributes to the myth. However, in an Australian context, the belief that irrigating vines always results in lower quality wines is manifestly wrong. Granted, over-irrigating will produce wine that is low in alcohol and colour, and has a thin, weedy and dilute palate. You come across these regularly in wine tastings. They certainly stand out at least in their mediocrity. However, it is equally true that vines that are highly water stressed can also produce poor quality grapes with much lower colour, flavour and tannin than vines that have experienced low or moderate levels of water stress.

But why do moderately water stressed vines produce the best fruit? Is it a case of "that what doesn't kill us makes us stronger"? It's actually simpler than that. Moderately stressed vines produce less vegetative growth than their over-irrigated counterparts. This results in vines that are balanced in terms of the number of leaves required to ripen the fruit, and ensures that the vines canopy is not overly shady. Dense, shaded canopies are the cause of unattractive pale, thin, and acidic herbally flavoured wines.

The fact that balanced vines produce the best fruit has led to the development of some ingenious irrigation practices which have allowed viticulturists to balance quality with yield. The first of these was borrowed from the peach industry and modified for use with wine grapes. It is called Regulated Deficit Irrigation or RDI. As the name suggests, water is withheld from the vine during certain periods of the vine growing season. Typically, water is reduced during the period between when the grapevine first sets fruit (i.e. just after the vine drops the caps off its miniscule flowers and the first tiny hard green berries appear), and veraison (the time in which the berries begin to soften, and in the case of red grapes, get their first tinge of colour). If this is done, both vegetative growth is controlled, and the size of the grape berry is reduced. Now this is a great result. Remember that most of the flavour, colour and tannin of the grape are concentrated in or near the skin of the grape. So smaller berries mean more concentration in the finished wine, and the lower vegetative growth means a greater likelihood of ripe generous flavours. Less juice to more skin, a sure thing to make a red winemaker grin!

The other relatively new and very ingenious approach is called Partial Rootzone Drying or PRD. PRD which incidentally was developed by my old viticulture lecturer and former wine education colleague, Dr Peter R. Dry works as follows. A dripper line is run on either side of the grapevine trunk. This allows one side of the grapevine's root-ball to be watered whilst the other side is kept dry. This process is regularly switched from side to side in order to keep the entire root system healthy. But why bother? Watering one side of the vine keeps the entire vine well hydrated and yielding well. At the same time, the roots on the dry side send out a chemical signal to the vine's leaves telling them that it is suffering water stress. In effect, by using PRD, the vine is 'tricked' into thinking that it is experiencing water stress. In response, the vine slows it's lateral shoot growth and takes steps to minimises losses of water via leaf transpiration. The end result is that the process does not result in any great sacrifice in yield. However, the amount of irrigation water required is effectively halved as the vine has both fewer leaves and they are losing less water through 'sweating'. This doubling of water use efficiency is great news given the increasing demands on Australia's water resources particularly from the Murray Darling Basin. And who said trickery will get you nowhere?