At the beginning of this trial funding was sought for a three year project looking at sub-optimal levels of irrigation in vineyards. The reasoning behind this was that considerable research work had taken place to establish the optimal levels of irrigation for a grapevine but little seemed to have been done to establish what the consequences were of irrigating at lower levels. These lower levels of irrigation may come about by necessity where water is not available from say drought conditions or it could come about as a matter of choice by winemakers wanting a particular wine style.

A trial was set up on a commercial vineyard to establish the affects of irrigation reductions from around 110mm per year down to around 20mm per year. The results showed that while the vines adapted to the reduced irrigation and produced a crop there were serious consequences on the vines and fruit. Among the results was a reduction in yield of up to 45%. There were also carry over affects for the coming year with reduced cane available to tie down and reduced quality of cane. There was a belief that these negatives may be able to be compensated for by an improvement in wine quality. Small batches of wine were made (around 500 litres per treatment) to establish the affect on wine quality. A number of objective and subjective measures of wine quality were used. The results showed that there was certainly an effect on wine composition. Generally the very low yields, while producing wines of an acceptable standard, were not up to the top quality Sauvignon Blanc that can be grown in the district. Comments from winemakers were that they were of a standard that they would use in the blending process rather than as a selection in its own right.

The conclusion of the trial was that if water was a very scarce resource careful timing of application could result in the production of a useable wine at a yield that should ensure the grower covers enough costs to get through the season. This is an important result as some of the irrigation applications were very small in comparison to the district average and the vines survived the process although they certainly looked stressed for much of the season.

At the end of the first three years the question was asked as to how well the vines would recover from the water stress. A fourth year extension to the project was carried out, to effectively water the block at normal levels for a season and monitor the results.

The results showed that the vines recovered surprisingly easily. Shoot growth was back to normal, yields, bunch weights and berry weights evened out as did the juice composition. These results show the adaptability of grape vines and are encouraging for the grower. Should the grower be restricted in the amount of water they can use in a one year there are likely to be consequences, however the vines quickly adapt, reducing the impact and then when balance is restored they revert to their normal state.

The following are the key points and recommendations from this work:

• Vines are ‘water hogs’ and will consume the water you give them but there is clearly an efficient minimum to get the best result with the least amount of water.
• As irrigation decreases so does Crop Water Use indicating a certain degree of compensation by the vine to reduced water availability
• Irrigation below 40% ETc clearly will reduce yield, mainly because of reduced berry weight and size. This compares to our control at 70 % ETc which we believe is a typical and adequate requirement in Marlborough.
• If you have a limited water resource it is critical to keep soil moisture levels up close to full point during the flowering and fruit set period to obtain the best yields. I.e. timing of the use of what limited water is available can be more important than the total availability.
• Lower irrigation did not improve brix levels, but did tend to increase pH and reduce titrateable acidity.
• The experimentation with Partial Rootzone Drying indicated that it is not a tool for reducing irrigation requirements in Marlborough conditions.
• Vine performance variation increases as water stress is increased. It is unadvisable to reduce irrigation much below the Control level where high levels of soil variability are present within a block.
• Mulch is a useful tool to improve water retention but should not be expected to replace a significant amount of irrigation.
• Use of a Pressure Bomb to measure leaf water potential worked very well when used in conjunction with soil moisture readings. It is a good tool to help set Refill points and determine whether stress symptoms are actually water and not some other factor.
• Measurement of vine sap flow, stomatal conductance and leaf area and use of modelling techniques are all useful tools for researching water stress and the effects of irrigation treatments. A much faster (and subsequently cheaper) method of measuring leaf area is needed to commercially integrate canopy size with soil moisture and irrigation scheduling.

Download the PDF here: Maximising irrigation savings in grape vines and the effect on yield and wine quality