Impact of Rain on the 2017 Vintage

The previous post discussed recent Napa Valley vintages. This post discusses one important impact on the 2017 vintage - RAIN!

Napa Valley average rainfall is 20" per year. The chart shows summary rainfall statistics over the course of the year. The y-axis shows rainfall as a percentage of average with 2015-16 being a typical year that matches the average. The x-axis shows the month of the year.  The chart runs from October thru September because that reflects the growing season.

2005-6 had 160% of normal and 2006-07 had 60% of normal rainfall, that is, 32" and 12" respectively. 

You can also see that 2016 rainfall through March is already over 160% of normal.

Another important point illustrated by this chart is that the majority of rainfall happens between November and April which includes the dormant season through bud break which is perfect. An important viticulture principle is to "stress" the vines because that encourages a vine to put energy into the grape not into growing more shoots and leaves in the canopy.  Think about it, you want just enough canopy to protect the grapes but not so much that it removes nutrients from the grapes themselves.

The following chart shows monthly rainfall statistics for the past six years.

Many observations can be made about these growing seasons. Notice the statistics show January to December, so they do not quite lineup with the chart above. The growing season statistics at the bottom reflect the growing season - Oct-Dec of the previous year followed by Jan-Sep for the current year.

1. Between 2011 and 2016 only 2013 and 2015 failed to get close to the 20" average rainfall.
2. The seasonal rainfall in 2012-13 and 2013-14 was on average half the average rainfall in  2011-12, 2012-13, 2015-16, and 2016-17.  One Napa Valley grower said that the first year of drought has only a small impact on the harvest the next year but two or more years of drought start to have a big impact on future harvests.  So, the drought in 2013 had no impact on 2012-13 and a small impact on 2013-14. But, the combined effect of the droughts in 2013 and 2014 had a big impact on the 2014-15 season. Thank goodness the drought was over by 2016.
3. Notice that the seasonal rainfall statistics for 2016-17 are already above 30", which suggests grapes will have plenty of water this year.  Now, cross your fingers and hope it doesn't rain between April and September!