Water stress.

The Napa Valley has had its fair share of rain this winter (except for January) and, like most of Northern California, has accumulated more rain so far this month alone than on average for a typical March. As a consequence, there are many vineyards on the valley floor that are partially under water (as illustrated in the above photo of the Swanson Vineyard in Oakville). Thankfully, the vines, whilst in full dormancy are not adversely effected by being water-logged. Conversely, water-logging during the growing season, as a result of summer rains that saturate the soil, can create an anaerobic environment in which the roots cannot survive due to a lack of oxygen. As of Wednesday, State agencies began releasing water from California's reservoirs to make way for a series of storms (the meteorologists word, not mine) expected to last into next week.
Climate is usually listed, along with variety and soil type, as one of the most essential factors controlling the composition of grapes. Whilst sunlight might be the single most dominating climatic factor affecting the makeup of the grape, there is one other essential ingredient - water. The soil underneath a grapevine is a veritable reservoir of H2O, the volume of which is classified as; available, unavailable, gravitational, and superfluous. I don't know about the grapevines, but I would definitely categorise the current amount of water in Vinoland as superfluous...to my well-being!
The dry summers in California are a boon to the wine industry (if not in the least for the fact that arid climatic conditions mean that the vines are less likely to suffer injury from harmful pests and organisms). However, under extreme drought conditions a lack of rainfall may cause the failure of a crop to ripen. Vines, like any other plant, need sufficient water availability for photosynthesis, (photosynthesis begets sugar, which in turn begets alcohol). Soil moisture content is very important.
Napa Cabernet sauvignons, from 2007, are said to be some of the best in recent vintages. Preceded by two relatively dry winters (05/06 and 06/07) the berries at harvest time were notably smaller, meaning the skin to pulp ratio was up, a condition that boded well for a flavourful vintage. I have heard several people in the industry comment that if a winemaker had made a bad Cabernet sauvignon in 2007 then he/she really had no business making wine at all. That is perhaps a tad harsh, but reduced soil moisture content is generally accredited with the success of that particular vintage.
It is possible then that one could be forgiven for assuming that 2011 is already destined to be a difficult vintage. Truth is, studies have shown that winter rainfall in California makes little, or no, difference to the final grape crop as a result of an elevated soil moisture content at the time the growing season gets underway. So, when somebody brings up moisture stress in grapevines they are usually referring to a deficit water situation, not the other way around.
Maybe the only thing water-stressed around here is me!