Probably already the most frequent topic that I've written about during the last few years of writing this viticulture column, water management is such a critical component of good vineyard management that it bears repeating in one form or another at least once, if not multiple times during the year. Another year of drought and dry spring conditions have put winegrape growers, a relative water-thrifty group compared to most agriculture, into the position of cutting back further on their water applications.

The current season is particularly dire here in the California North Coast. After three years of below-average rainfall and water diversions to satisfy endangered species protection, reservoirs are currently well below levels of last year which was, even at that time, thought to be a dire year for water supply. One occurrence of a fish kill in one of the Russian River tributaries this spring was attributed to dewatering of the stream, allegedly created as vineyards withdrew its water for frost protection. This puts the industry in an uneasy situation, as there are few, if any, alternatives to frost control besides overhead sprinklers. As we move into the vineyard irrigation "season," we must be cognizant of the water resource limitations that we face.

Whether voluntary or mandatory, wise water management is simply good viticulture, regardless of the supply situation. Minimizing water inputs to vineyards is a key component of sustainable farming practices in the true sense of the word, not just as a buzzword used in sales and marketing or in green one-upsmanship.

Most growers that I talk to firmly believe that they are already applying minimal amounts of water to their vineyards and, in fact, some are pretty close to doing just that. Yet I can often make suggestions after viewing their situations that will allow them to more efficiently apply irrigation to their vineyards without suffering loss of yield or quality.

I think that the term "deficit irrigation" has taken on a negative connotation in the grower community, probably due to some poor experiences with growers attempting to adopt that kind of an approach. That makes sense. The idea of deficit irrigation is to give the vineyard less water than it is using, which means that any soil moisture reserves will be depleted over time and the vines will eventually run out of ample water supplies. When that occurs, the vines go into a stressed state, which means that some sacrifice will be made. I can't tell you how many stories I've heard from growers who practiced some sort of deficit irrigation only to have their vineyards drop leaves or shrivel up fruit before harvest.

For that reason, I shun the use of the word "deficit" irrigation for the most part. There has also been much discussion and writing about "regulated deficit irrigation (RDI)," which is a deficit irrigation management strategy that prescribes periods of vine water stress during a portion of the growing season, usually monitored and controlled through monitoring of the vines' water status and/or soil moisture content. I am much more amenable to the use of the RDI term, but I still avoid using the word "deficit" in my discussions about irrigation management because it almost always evokes scowls from some members of the audience.

Rather than thinking in terms of deficits or RDI, I would prefer to think of it as "regulated moisture management" (RMM, if you will, but I have no aspirations of creating a new acronym). I don't use evapotranspiration (ET) data any more to actively manage irrigation (ET data can still be useful for climatic evaluations and vineyard development planning, however). Rather, I find the proper irrigation application volume for the given site and use the interval between irrigation applications as the primary variable during the growing season (that is, after I begin irrigating at all). I use the variable interval to manipulate vine water status so that the vineyard is neither overly stressed nor provided with luxurious amounts of available water. There is not one single approach to that--and so many variables (including variety, wine style, climate, water availability)--that a general strategy cannot be presented here.

However, one aspect of the regulated moisture management practice that is essential is that of monitoring vine moisture status. Without this, there is no regulation and, by consequence, no management. Soil moisture may be a component of a moisture management scheme, but I prefer to use soil moisture measurements as a tool for calibration of an irrigation practice, not as primary feedback.

I firmly believe in vine water status measurement using the pressure chamber and porometer instruments. I've written about these before (WBM April 2006 and July 2007), so there is no need to again discuss details about what they are measuring, but I continue to be a believer in their use. And as I have said before, if I had only one instrument, it would be the porometer. Not just because I am a distributor of the Decagon Leaf Porometer (I became a distributor because I liked the instrument) but because it measures a physiological parameter of vine status--stomatal conductance. I think that its usefulness is enhanced when using it alongside a pressure chamber, which measures the suction or tension of the water column within the xylem vessels, but not a physiological response like stomatal conductance.

On the downside, I (and others) have found that the porometer is highly influenced by ambient weather conditions. This can make interpretation of the measurements challenging, since only a portion of the reading is reflective of water status or stress. This is not a surprise to me as I worked on remote sensing of stomatal conductance as my Ph.D. dissertation and have researched and developed models that account for the environmental influences on stomatal conductance.

Probably the most important variable is vapor-pressure deficit (VPD), which is loosely translated as how much water vapor the air can absorb and is a function of temperature and relative humidity. High VPD, which occurs under low relative humidity and high temperatures, causes stomata to close regardless of water availability. This is simply the vine's way of conservong water under dry, water-demanding, environmental conditions. I am currently working out a way to account for temperature and humidity factors on the stomatal conductance measurements and will be posting that information on my website (www.advancedvit.com).

Other Important Ideas and Practices

Don't want to buy a porometer, pressure chamber or soil moisture device? Here are a few things that anyone can do to improve their overall water efficiency of their vineyard:

Dig the Soil, Man. Dig some back­hoe pits in your vineyard and look at existing root patterns. How deep is the root zone? I'll bet it is shallower than you thought. Most root zones that I've seen in existing vineyards are less than 24 inches. That is not a bad thing--deeper root depths may be associated with high vigor vines. Do the roots cluster under the emitters or are they more evenly distributed away from the vine? Do roots run into the row middles? How about the wheel tracks? Is compaction preventing root exploration away from the vine row?

Knowing your root patterns will help you to determine how much you should be irrigating each time you run the system. After a root "survey," I like to install soil moisture devices at various depths in key locations, the deepest sensor being installed at or just below the root zone. Then, we can run the system for various durations, finding out how deeply the water penetrated for a given volume application. With that knowledge, we know that we cannot run the irrigation system for a longer time without losing water as it runs past the root zone. I generally irrigate at that same volume each time and vary the interval between irrigation events to manipulate vine moisture status during the season.

Shorter is Sweeter. Because most root systems are shallower than most growers think they are, it is frequently more efficient to irrigate in smaller volumes with shorter intervals between applications than in larger volumes with large gaps between them. Some people tell me that they irrigate long to "drive the roots deeper." That might be a valid concept in arid (say, less than 12 inches per year) or similar climates, but on the north coast of California, rainfall fills the soil profile even in so-called drought years. In such a climate, if roots are going to proliferate deeply, they will proliferate because of the soil moisture that is available to them in the spring. Deep drip irrigations will not cause roots to "chase" the water any more than will withholding water from the vines.

It's impossible to generalize, but we (myself, my clients and colleagues in the winegrowing consortium) usually irrigate vineyards with only about 2 to 4 gallons per vine for each irrigation application. There are some exceptions where we may go higher, but I've worked with many vineyards this way and growers are almost always amazed at how little we can get away with each time. That being said, a smaller application means that irrigation must be applied more frequently to deliver the same amount of water. But because larger irrigation volumes tend to be inefficient (with water lost below the root zone and/or to runoff), less water is almost always found to be needed when irrigating this way.

This is my "mantra" to growers. Take it from someone who has spent years doing it the "other" way. Shorter irrigations save water.

Two's a Company. If you have only one emitter per vine, consider adding a second one. Unless your vines are less than 4 feet apart, you should probably have two emitters per vine. Even better: if you are using 1-gallon-per-hour (gph) emitters, switch them out to one-half-gph emitters and double them up, spaced equally from the trunk of the vine. Drip irrigation creates a small wetted zone and roots tend to proliferate under the emitter. Creating two wetted zones means that you will extend your root volume, which will increase the time between irrigation events and decrease the inefficiencies associated with each irrigation application. It also means that there will be a greater volume of active root and soil later in the season, which means more access to nutrients by the vine.

One-gph emitters tend to be inefficient as the application rate exceeds the infiltration rate of many soils, resulting in runoff. On the opposite extreme, lighter soils with high infiltration rates may be able to accommodate the one-gph emitter rate, but the high application rate may prevent lateral water movement by the time the wetted front reaches the lower extent of the root system.

If your vineyard has one one-half gph emitter per vine and you're thinking of adding a second one per vine, make sure that your irrigation delivery system is designed so that it can accommodate the additional flow rate that a second emitter would bring.

Irrigate by Moonlight. For those who have the ability, consider irrigating at night or during early morning or evening hours. Irrigating during the daytime is the least efficient time. For one, direct evaporation is higher during the daytime. A small wetted surface means that evaporation from the soil is small, but not everyone considers the evaporative losses caused by splashing. It seems miniscule, but actually, the drip-drip-drip causes small splash-splash-splashes, which may be blown away. Even if not blown away, some droplets find their way to drier soil, from which they are easily evaporated.

Another aspect of night irrigation was pointed out in a discussion with my colleague Paul Anamosa of Vineyard Soil Technologies, who suggested that irrigating at night, at a time when vines are not actively transpiring, allows water to redistribute in the soil without being taken up by the vines. During an irrigation event, water moves downward and laterally, both as saturated and unsaturated flow conditions. Water is less tightly bound by matrix forces to the soil as it is being applied, which makes it more easily extracted by the vine. Night/evening or early morning irrigations will allow the applied water to redistribute in the soil, creating a larger wetted volume and preventing the vines from getting their "quick fix" of water, which would otherwise be used in a less efficient manner by the greedy vines. My thanks to Anamosa for this idea--I think it has merit.

Resist the Temptation to Sprinkle.

Most growers don't do this anymore, but in the hotter growing regions, some growers run their overhead sprinkler systems in the hottest times of day during heat waves. At the typical rate of 55-gallons per minute per acre, this represents a huge consumption of water, compared to an drip irrigation system running at 15 to 17 gallons per minute per acre. Many growers I speak to have moved away from overhead sprinkling during heat waves. Preemptive irrigation is a good idea--irrigate the vineyard a day in advance of an impending heat wave. Vines can cool themselves if not in a stressed state and early irrigation relieves the stresses on the vines much more than same-day irrigation does. Also, avoid fruit zone leaf removal on the afternoon sun side of canopies. Some growers are letting one side of their VSP canopies flop over without shoot positioning and/or are adding short cross-arms to better shade fruit during the hot afternoon. In hot regions, orient rows so that the sun is parallel with the vine rows a bit in advance of the hottest time of day (usually around 4 p.m. daylight savings time).

If water cooling is absolutely necessary, consider converting to a low-volume approach, such as the low-volume cooling system that I wrote about in the January 2009 Wine Business Monthly.

Don't Believe it? Come See for yourself!

For the third consecutive year, the Sonoma County Water Agency has asked me to work with them on grower communications regarding water use and conservation in vineyards. This season we are doing something we haven't tried before--an irrigation and water-use demonstration. We are working with a commercial vineyard in Alexander Valley to conduct live demonstrations of different irrigation approaches as well as the low-volume vineyard cooling system. Environ­mental and soil moisture data will be collected and made available on a real-time basis during the growing season via the Internet. Links will appear on my website (www.advancedvit.com) when they become "live." Many of the concepts discussed above will be showcased side-by-side, and vine water status measurements will be made throughout the season.

The purpose of this demonstration is to provide a venue for both education and two-way communication on the subject of vineyard water use. I urge Sonoma and Mendocino growers, especially those who lie within the Russian River basin, to attend one or both of the field days at the site. As of this writing, the actual days have not been set, but days, times and location will be publicized in the local media and through the WineBusiness.com website. Local media will be in attendance and the local public television station will be filming a short documentary on the project, which will be a good way for us to showcase the state of vineyard water efficiency to the general public. See you there! wbm

The Sonoma County Water Agency is offering complimentary on-site water management consulting from Mark Greenspan of Advanced Viticulture to growers in the Russian River basin of Sonoma and Mendocino counties. Consulting fees, for one hour of consulting, will be covered by the water agency and will be treated as confidential between the consultant and the grower. The complimentary consulting will be limited in scope to water-use advisory, and availability will be limited by available funding and scheduling constraints. To arrange for a complimentary consultation, contact Mark at Advanced Viticulture: mark@advancedvit.com or 707-838-3805.