by Jeff Miller of Artisan Family of Wines (Seven Artisans, Sly Dog Cellars, Red Côte)
As I am sure most of you are aware, we are in the middle of one of the worst droughts that California has ever faced. What is the likely effect of this on the upcoming grape growing season?
To begin with, it’s important to have an understanding of how much water soils actually hold. There are three concepts that are relevant to understand in order to form any assessment of how much havoc the drought is likely to cause. The first concept is saturation. When a significant rain occurs, the soil quickly become saturated. The most important thing to understand about saturation is that it’s really not very relevant to anything at all.
The reason for that is that the soil cannot hold onto all the water it contains when it is saturated. The water in the soil will drain away until it reaches what is called field capacity.
Without getting into too much chemistry and electrons and things like that, water binds to soil particles. At saturation, the power of the soil particles to hold onto the water is less than is necessary. For that reason, water drains away until there is just enough of it that the soil particles can hold onto it. That point is field capacity. For all intents and purposes, field capacity is the maximum amount of water the soil can hold. Once the soil is at field capacity, more rain does not translate into more water in the soil for the upcoming season.
As water is used up in the soil, eventually the point is reached where the remaining water is so tightly bound to soil particles that it is no longer accessible to grapevines. That point is called the wilting point.
So after the rainy season, the amount of water that is available to the vine is the difference between field capacity and the wilting point.
So, for example, if the soil is 3 feet deep, then by determining for each foot of soil how many inches of water there are between field capacity and the wilting point, you can calculate how much water the vines will be able to retrieve from the soil. Using our example of a soil which is 3 feet deep, and assuming that for each foot there is 3 inches of available water, then there will be 9 inches of water available. If you factor in the area available to each vine, you get a pretty good approximation of the water that will be available to it.
If you factor in even a minimal amount of rain and add in some amount of water left over in the soil from the prior season, there will be enough to fill the soil to field capacity, or at the least fairly close. This season, as difficult as it is, is unlikely to change that.
In regions that experience summer rains, it is possible that rainfall will provide all of the water necessary to the vine for the entire growing season (since the summer rains replenish the water in the soil). But that is not our situation here in California, where, by and large, we need irrigation. And, without question, the drought is affecting the accumulation of water that can be used for irrigation.
In the Central Valley, where winter runoff is widely used for agricultural irrigation, the drought is something of a disaster since those supplies will be much reduced.
In many areas, well water is used for irrigation. While less affected by the drought than those farmers that rely upon outside water supplies, they are nonetheless dependent upon the water table. If the water table in their area remains adequate, they will be able to get by.
Other factors enter into the equation as well. For example, pretty much all grapevines are planted on rootstocks of American origin (which can survive phylloxera). Most of those rootstocks are crosses that use either a riparia or berlandiera vine. Berlandiera is fairly drought resistant, while riparia is not. In fact, the name riparia comes from the same linguistic source as the word riparian; the vine is native to relatively wet areas adjacent to rivers or other water sources.
So vines that are planted on a berlandiera cross will probably do better than vines planted on a riparia one.
Another all too obvious factor is how hot it is during the summer growing season. The more heat there is, the more the vine will use up what water there is in the soil. A relatively cool growing season, just as obviously, will help us all get by.
Grape vines, as a general rule, are not as thirsty as a lot of other crops that can be grown. I have a vineyard at my home that I irrigate with well water. I probably only use about 50 gallons per vine for the entire season. I am quite sure I could get by with less.
Since there are so many unknowns about what will happen between now and harvest, it is hard to know with any degree of certainty whether we will dodge the bullet, or not. I suspect that the overall impact is going to be less than many people might think, at least here in the coastal regions.
One final factor. As I’m writing this, I’m looking out my window as the rain is coming down, as it has for about the last 36 hours. I don’t think that even a relatively wet February, March, and April is going to overcome the total lack of rainfall we’ve had to date. But, obviously, every drop helps.