Pruning and irrigation are the cultural practices with the highest potential impact on yield and quality in wine grapes. In particular, irrigation start date, rates and frequency can be synchronized with crop development stages to control canopy growth and, in turn, positively influence light microclimate, berry size and fruit quality. In addition, canopy management practices can be implemented in vineyards with large canopies to ensure fruit zone microclimate is optimized for producing high quality fruit.

Spatial variability in soil properties such as water holding capacity causes variability in fruit yield and quality. Ideally, irrigation should be applied differentially throughout the vineyard in order to compensate for soil variation and optimize both fruit yield and quality. We report on the first-season effect of a variable rate irrigation (VRI) prototype on canopy development and yield. The prototype system was implemented in early 2013 in a 4.05-hectare quadrant inside a drip-irrigated mature Cabernet Sauvignon vineyard measuring 12.5 total hectares. The VRI quadrant contained the full range of lowest to highest yields present in the vineyard (14.4 to 28.1 tons/ha), based on the 2012 yield map. The VRI quadrant was split into one hundred and forty 15 x 15-meter irrigation zones which were watered independently by drip irrigation with weekly schedules calculated using an energy balance approach based on the Mapping Evapotranspiration at High Resolution with Internalized Calibration (METRIC) model. The 2012 yield map was used to split all irrigation zones among low, medium and high yield classes. Irrigation during the 2013 growing season was initiated in both the low and medium 2012 yield classes at a time when the soil was still saturated (i.e. earlier than standard practice), while watering in the high 2012 yield class was withheld until vines had used a significant amount of the soil water holding capacity (i.e. standard practice). Low yielding zones received up to 17% more irrigation water than the high yielding ones during the 2013 growing season. For both 2012 and 2013, normalized difference vegetation index (NDVI) was calculated and mapped from airborne images captured in mid-August and yield was mapped from yield monitor data collected at harvest.