Sprinkler Versus Drip Irrigation - Basic Effects on Vine Development and Physiology
By Pat Bowen, Pacific Agri-Food Research Centre

Probably the most widely researched subject in viticulture currently is the influence of water management on wine grape quality. Although it has been known for some time that excess water stress can be detrimental to vines and wine grape quality, researchers in recent years have demonstrated that mild water stress can lead to improved fruit quality. This finding has led to the development of deficit irrigation techniques such as regulated deficit irrigation (RDI) and partial root-zone drying (PRD) that cause and control vine water stress. However, the results of these techniques have been found to vary in effectiveness from region to region, likely due to variations in soil conditions, vine vigour and crop load, all of which interact with vine water status. To complicate matters, there are now several recommended methods for implementing deficit irrigation that differ in the seasonal timing, frequency, volume and spatial distribution of water applied, and there is little information available to guide growers on which methods will work best under the conditions of their vineyard. The bottom line is that nobody is really sure of what works best, as researchers have not yet developed a thorough understanding of how water management impacts quality.

Regardless of the desire to improve wine grape quality, there will likely be increasing pressure to conserve water in the Okanagan Valley. An effective way to reduce water use in vineyards is to use drip rather than a sprinkler irrigation. Although drip systems require close scrutiny to manage, they use many times less water than sprinkler systems and they enable effective control over water supply for imposing moisture deficits. In our research, we are investigating how vines respond physiologically to sprinkler and drip irrigation. In an initial study we determined how leaf function, including stomatal conductance and photosynthesis, is affected by the amount and spatial distribution of moisture in coarse and fine textured soils. Not surprisingly, leaves exhibited substantially reduced function sooner after drip than sprinkler irrigation, confirming that the root systems of drip-irrigated vines are less extensive and access smaller soil volumes than those of sprinkler-irrigated vines. However, we also found that regardless of the level of moisture in the soil, stomatal conductance was consistently lower under drip irrigation, which resulted in lower rates of photosynthesis (less sugar was produced per area of leaf). This reduction in leaf function was similar to that reported for vines under PRD which have their root systems divided between dry and moist soil. We also found that drip-irrigated vines had shorter shoots and canopies with lower leaf densities. These findings indicate that the amount of sugar produced by drip-irrigated canopies was much lower than that of sprinkler-irrigated vines.

In further studies of drip versus sprinkler irrigation, we found that despite having less productive canopies, drip-irrigated vines matured their fruit earlier than did sprinkler irrigated vines. This was likely due in part to the smaller berries (1.2 g vs 1.5 g) and lower yields (2.0 kg vs 3.4 kg) carried by the drip-irrigated vines. However, when drip- and sprinkler-irrigated vines carrying the same yield were compared, the fruit of the drip-irrigated vines ripened earlier and had lower acidity at harvest. The cause of this accelerated maturation was determined to be warmer temperatures in drip-irrigated vineyards resulting from smaller vine canopies and lower vegetation coverage on the vineyard floor.

Our results indicate that drip-irrigated vines differ fundamentally from sprinkler-irrigated vines in their development and physiological function. Effects of drip versus sprinkler irrigation on fruit development and maturation are due to not only to moisture deficits that create mild water stress but to alterations in vineyard climate.