VNO EMBER/DECEMBER 2001 42

WINEGR OWING

Irrigation of wi

in Cal Lorry E. WIIIIams y.t

Department of Viticulture Enology University of California-Qavis, and Kearney Agricultural Center

SYNOPSIS: How much irrigation water is required to grow quality winegrapes depends upon site, the stage of vine growth, row spacing, size of the vine's canopy, and amount of rainfall occur-ring during the growing season. Below, growers are presented with means to determine when irrigations should commence and to calculate full vine water use based on the results of 10 years of field trials in California wine-grape growing regions. Implications for such information to assist in vine-yard irrigation management are included.

c oastal, winegrape production areas in California are character-ized by warm days and cool nights, although high tempera-

tures (104Q to 1162F) may occur for a few days each growing season. Some areas may have fog lasting late into the morning.

Rainfall is greater in northem,coastal valleys and diminishes as one travels south. In coastal valleys, evaporative demand can range from 35 to 50 inches of water throughout the growing sea-son (between budbreak and the end of October).

Many of the soils in the coastal production areas are clay loam to clay-type soils, which at field capac-ity, generally hold more water than sandy - type soils. Since the majority of rainfall occurs during the dormant

portion of the growing season in these areas and vineyard water use can be greater than the soil's water reservoir after the winter rainfall, supplemental irrigation of vineyards may be required at some point dur-ing summer months.

IRRIGATION MANAGEMENT No matter where grapevines are

grown, two major questions concern-ing vineyard irrigation management must be answered: 1) When to start? and 2) How much water to apply?

When to start irrigating .1;7' Deciding when to begin irrigating

can be determined several ways. Soil-based tools such as a neutron probe and capacitance sensors can determine the actual or relative amounts of water in the rooting zone of grapevines. Plant-based tools, such as a pressure chamber, can be used to measure vine water status.

Regardless of the method, a "value" is determined which indicates that the vines may need water. Once this value is reached, an irrigation event should occur.

Using a pressure chamber Water has free energy, a capacity to

do work. In plants, water's free energy (or chemical potential) is usually referred to as "water potential." Pure water will have a water potential of 0 bars (bar is the unit of measurement).

Any solute (such as sugars, mineral ions, and amino acids) added to water will lower its water potential, i.e. the water potential will become more neg-

ative. The same can be said of a plant's water potential.

For example, when more water is lost from a leaf via transpiration than moves into the leaf from the vascular tissue, its water potential will become more negative due to a relative increase in its solute concentration. This is important as water in plants and soils moves from regions where water potential is relatively high to regions where water potential is rela-tively low. Such differences in water potential will result in movement of water from cell to cell within a plant or from regions within the soil profile that contain more moisture to those with less.

One way to measure the water potential of a plant organ in the field (such as a leaf) is by using a pressure chamber.

The leaf's petiole is cut and the leaf quickly placed into the chamber with the cut end of the petiole protruding out of the chamber. Once the leaf is

Randell Johnson (Hess Collection Winery, Napa, CA) uses pressure chamber. (Photos by Richard Camera.)

rapes

201

NOVEMBER/DECEMBER 2001 .i41,3%:4; rFW-0.1 254 mew- WINEGROWING

77,71 aSe

The petiole should be carefully observed in order to capture the measurement when the sap just exudes from the cut end.

removed from the plant, the tension in the petiole's xylem is released and the sap withdraws from the cut surface and moves into the blade.

As the chamber is pressurized, the water potential of the leaf is raised by the amount of pressure applied so that at the balance pressure (the pressure required to force the sap to the surface of the cut end of the petiole), the water potential is zero. The original leaf water potential plus the balance pres-sure equals zero. Therefore, the nega-tive of the balance pressure equals the original leaf water potential.

A more complete explanation of the pressure chamber technique, theory,

.-possible errors, and problems can be found in "Measurements of plant water status," Hsiao. 2

The water potential of a plant leaf will be greatest at pre-dawn, then decline (become more negative) during the day to reach a daily minimum, then increase as the sun sets. This type of pattern will occur regardless of the availability of water in the soil profile.

However, pre-dawn and midday minimum values will be more negative for plants experiencing soil moisture

deficits than those given greater amounts of water.' Thus, leaf water potential can be used to estimate the water status of a plant. Units of water potential are expressed in bars, as men-tioned above, or megapascals (MPa) (1.0 MPa =10 bars).

In all of my irrigation trials, I have measured leaf water potential to assess vine water status. I usually measure midday leaf water potential between 12:30 and 1:30 PM. I select mature, fully expanded leaves exposed to direct sunlight (no shading on the leaf). I have found that any fully expanded leaf on the outside of the canopy will be appropriate as long as it is not senescent (starting to turn yellow), diseased, or suffering from insect damage.

The leaf blade is enclosed inside a plastic bag (plastic sandwich bags are satisfactory) and then the petiole is cut with a sharp razor blade. The plastic bag enclosing the leaf blade is to mini-mize transpiration between petiole excision and pressurization within the chamber. I have found a difference in leaf water potential of 2 to 3 bars between bagged grape leaves and

those that were not bagged (the latter being more negative).

The bagged, leaf is. placed inside the chamber with the petiole sticking out (see photo)..-Theiiime from enclosing the leaf inside the plastic bag to placing it inside the chamber should be 10 sec-onds or less-:.The chamber is pressur-ized with compressed nitrogen until the sap just exudes from the cut end of the petiole. If the sap forms into a lens or hemisphere, then the sample, has been over-pressurized. •

The recommended rate , of pressur-ization is leSS - than 1 bar 1>er -second initially, then slowed to less than 0.2 bar per second as"the balanCing pres-sure is approached.' - The end point should be observed with a magnifying lens and adequate light.

While the above description of the pressure chamber involves use of -com-pressed nitrogen, a new chamber has been developed that doesn't require compressed gas cylinders. This cham-ber is pressurized via a manual pump and is very portable.

Water potential values obtained by using this technique can be dependent upon ambient vapor pressure deficit (VPD), which increases as relative humidity decreases; temperature and light, because all of these contribute to evaporative demand; time of day the measurement is made; and the amount of water in the soil profile.

Since, time of day is very important, and the evaporative demand will vary considerably throughout the day, I limit taking leaf water potential mea-surements to one-half hour on either side of solar noon. That is when a grapevine uses the greatest amount of water on a daily basis.'

I have found that midday leaf water potential values of fully irrigated vines on a day of low evaporative demand (ambient temperature at the time of measurement 85 2F) will be approxi-mately 1 bar higher (less negative) than on a day when ambient temperature is 982F at the time of measurement. This is also true for vines that are deficit-irrigated.

One can also assess vine water sta-tus by taking water potential readings prior to sunrise (pre-dawn leaf water

ZoS

potential) or by measuring stem water potential at midday.

Stem water potential is deter-mined by enclosing a leaf in a plas-tic bag surrounded by aluminum foil, at least 90 minutes prior to when readings are to be made. This procedure eliminates transpiration and the leaf water potential will come into equilibrium with the water potential of the stem (i.e. stem water potential).

A recent study on almond trees has shown that water potential values measured on shaded leaves (covered with a damp cloth just before leaf exci-sion) are very similar to values of stem water potential.'

I have compared leaf water poten-tial of leaves under naturally occurring shade with stem water potential values of grapevines this past summer and found the two are very similar in some instances. In other instances they were not.

I believe the 'discrepancy is due to the fact that on some trellis systems (such as the VSP), it is difficult to find leaves that are completely shaded (no sunflecks present on individual leaves) or in deep enough shade that transpi-ration truly is minimal.

Some researchers feel that stem water potential is a better %measure of vine water status than leaf water potential since it somewhat minimizes the effects of the environment on an exposed leaf as outlined above.

However, Dr. Merilark Padgett-Johnson and I have demonstrated (unpublished data) that pre-dawn leaf, midday leaf, and midday stem water potential of Vitis vinifera culti-vars and different Vitis spp. are all highly correlated with one another and with other measures of vine water status. Based on these findings, mid-day leaf water potential is an appro-priate and convenient means of esti-mating vine water status, however, one must follow precisely the tech-niques outlined above.

It is interesting to note that D.A. Goldhamer and E. Fereres found that a major source of variation in determining tree water status (stem and shaded leaf water potential) is due to operator error.' Thus, whoever is taking your vine water status measurements, whether midday leaf or stem water potentials, should be cognizant of possible errors associated with their technique.

My research indicates that the mid-day leaf water potential of vines that are irrigated at 100% of water use is gener-ally never more negative than -10 bars (equivalent to a stem water potential value of -7.5 bars).' Measurements are

made one-half hour on either side of solar noon, that is 1 FM PDT.

In my current irrigation experi-ments, I generally do not initiate the application of water until midday leaf water potential is at or more negative than -10 bars.

At present, many growers and vine-yard consultants do not begin irrigation of white wine cultivars until a midday leaf water potential value of -10 bars has been reached or a -12 bar value for red wine cultivars. The date during the grow-ing season these values are obtained is dependent upon rooting depth of the vines, soil texture, soil moisture content, vine canopy size, row spacing, trellis type, and evaporative demand.

For example, in 2001, leaf water potential of Thompson Seedless grape-vines at the Kearney Ag Center did not reach -10 bars until bloom (the first week of May), at which time irrigations began.

In a Cabernet Sauvignon trial near Oakville in 2001, irrigation was not ini- tiated in a trellis and irrigation study until a midday leaf water potential value of -11 bars was measured. Accordingly, irrigation was initiated on June 3, June 11, and July 10 for a VSP trellis (1m X lm planting), a Wye (or lyre) trellis with 9-ft. row spacing, and a VSP trellis with 9-ft. row spacing, respectively.

Since soil type and rooting depth were the same for all trellises in the trial near Oakville, the differences in the date irrigations began (i.e. when leaf water potential reached -11.0 bars at midday) were due to the differing amounts of water used by each. Hence the rate water was depleted from the soil profile.

How much water to apply I have spent the last 10 years deter-

mining irrigation requirements for raisin, table grape, and wine grape vineyards in all major grape-growing regions of California.

Regardless of grape type, in my opinion, once the decision to begin irrigations is made, vine water requirements are dependent upon evaporative demand at the location of the vineyard, stage of vine develop-ment, and percent ground cover by the vine's canopy. This is because the amount of water depleted from the soil profile has been significantly reduced by that time (especially if no water had been applied from the time of budbreak to that point) and the majority of the water a vine subse-quently will use is dependent upon what is applied.

The information needed to sched-ule irrigations at daily, weekly, or other intervals throughout the grow-ing season includes potential evapo-transpiration (ET0 ) and reliable crop coefficients (k c). Potential ET (also known as reference ET) is the water used per unit time by a short green crop completely shading the ground.

Ideally, the crop is of uniform height and never water-stressed.

Potential ET (ET0) ET, is a measure of the evaporative

demand of a particular geographic region throughout the year. Current (or near-real time) ET° data are available from the California Irrigation Manage-ment Information System (CIMIS) which is operated by the California Department of Water Resources.

There are more than 90 weather sta-tions located around the state where environmental data are collected to cal-culate ETo.

Environmental variables measured to calculate ET, are mean, hourly solar radiation, air temperature, vapor pressure, and wind speed. These variables are then used to cal-culate other variables such as net radiation and vapor pressure deficit which are then inserted into an equa-tion to calculate ET 0 . 4

Potential ET may also be obtained from weather stations operated by other entities (such as stations oper-ated by the Paso Robles Vintners & Growers Association in the Paso Robles region).

Potential ET will vary seasonally and is low at the beginning of the season, highest in mid-summer, and then decreasing thereafter. Between budbreak and the end of October, ET, can range from 35 to 50 inches of water in the coastal valleys of California.

For example, ET, from March through October in 1997 for the Cameros region of Napa Valley, Green-

' field in the Salinas Valley, Paso Robles, and Fresno were 44, 44, 51, and 48 inches, respectively.

Historical ET 0 in the Santa Maria region for the above-mentioned months is estimated to be 36 inches. Therefore, if identical vineyards (same cultivar, trellis system, row spacing,

canopy size, etc.) were growing at all five locations, then seasonal vine water use would be lowest in Santa Maria and highest in Paso Robles. The differ-ences would be due to varying evapo-rative demand at those locations.

Crop coefficients The next piece of information

needed to determine vineyard water use is seasonal crop coefficients (k,). The k, is the fraction of water a non-water-stressed crop uses in relation to that of ET,:

k, = ET, + ET, where ET, is crop ET. The k, is dependent upon the stage of vine growth, degree of ground cover (shading), height, and canopy resis-tance (regulation by the vine or crop). The k, will vary throughout the growing season; it is not a constant fraction of ET° . It is low early on and then as the canopy develops, it will increase (Table I).

In the past, seasonal crop coeffi-cients have been developed for vine-yards in the San Joaquin Valley. Unfortunately, when these seasonal crop coefficients were utilized in coastal valley vineyards, they did not work very well.

Various means of adapting these crop coefficients to different trellises and row spacings have included the use of another coefficient (canopy coeffi-cient) that is a function of canopy size.

In order to develop crop coeffi-cients, one must be able to measure or estimate grapevine water use through-out the growing season. With the aid of a weighing lysimeter, I have deter-mined seasonal crop coefficients for Thompson Seedless grapevines grown at the University of California's Kearney Agricultural Center.''

A weighing lysimeter is a sensitive piece of equipment that is able to mea-sure ET of plants on an hourly, daily, and seasonal basis. The lysimeter at the Kearney Ag

Center is comprised of a large soil container (2m wide, 4m long, and 2m deep) that sits upon a scale. The soil surface of the container is at the same level as the soil level of the vineyard surrounding it. Therefore, the soil container and scale are below ground.

Two grapevines were planted in the lysimeter in 1986. Vines were also planted around the lysimeter with vine and row spacings of approximately 7V2 and 111/4 ft, respectively. The trellis sys-tem used for the vines is a two-foot crossarm approximately five feet above the soil surface. The scale has a resolution of 0.2 mm of water, which is less than 0.5 lbs.

As water is lost from the tank, either by evaporation from the soil or transpi-ration from the vines, a datalogger records the loss in weight (i.e. it measures ETC) hourly. The vines within the lysime-ter are automatically irrigated whenever

they use 2 mm of water (slightly more than two gallons) and are therefore assumed not to be under water stress.

Water use of the vines in the lysimeter, between budbreak and the end of October, from four years after planting until the present, has ranged from 29 to 34 inches (approximately 1,400 to 1,700 gallons per vine). 6 Potential ET at the same location over the same years has ranged from 42 to 47 inches. Daily water use of vines growing within the lysimeter will average 10 to 12 gallons at maximum canopy, mid-summer.

During the 1998 and 1999 growing seasons, a study was conducted to determine the relationship between leaf area of the vines in the lysimeter, shaded area cast on the ground at solar noon, and grapevine water use. Thus, leaf area was estimated and shaded area on the ground was determined at various times throughout the growing season.

The study found that, at full canopy, shaded area on the ground comprised approximately 50% of the total land area allocated per vine within the vine-yard.

In 1999, the shoots of the two vines growing within the lysimeter were allowed to grow across the row mid-dles on either side of the lysimeter. The shaded ground area just prior to that time was about 60%. A support system was then constructed to raise the shoots (simulating an overhead trellis system) and the percent ground cover increased to approximately 75%.

Actual vine water use prior to raising the canopy was 12.7 gallons per day, and after raising the canopy, it increased to 16.7 gallons per day. This indicates that it was the orientation of the canopy (determined by the trellis system) and not the total leaf area per vine that dic-tated how much water the vine used, if the vine was not water-stressed.

It was also found that vine water use (ET,) and the crop coefficient were linear functions of the amount of shade measured beneath the vines at solar noon. The equation to describe the relationship between the crop coefficient (1< c) and percent shaded area was: lc, = 0.002 + 0.017x, where x is percent shaded area.

A linear relationship between the crop coefficient and light interception (or shaded area), with a slope similar to that of the grapevines, has also been found by Dr. Scott Johnson for peach trees growing in a weighing lysimeter at the Kearney Ag Center.

There are several reasons why grapevine water use and the crop coefficients may be related to the percentage of shaded area when mea-sured at midday: 1) The driving force of ET, net radia-tion, is greatest between 11 Am and 2 Pm. 2) Approximately 75% of the daily water use by vines growing in the lysimeter occurs between 10 AM and 2 PM. 3) The shade beneath a vine is an indi-rect measure of how much solar radia-tion the vine is intercepting. 4) The shade beneath the vines varies only slightly between 9 AM and 3 PM for east/west rows (row direction in the lysimeter vineyard). 5) As the season progresses, the vine's canopy gets larger, resulting in more light being intercepted (more shaded area on the ground) and greater water use.

Impact of trellis and row spacing There are numerous trellis systems

used for winegrape production in California today. There are systems in which little management is used (sprawl systems) and those, which are highly manipulated. The latter systems include the VSP (vertical shoot positioned trellis) and vertically

, divided canopies such as the Scott Henry or Smart/Dyson trellises. Horizontally divided canopy trellis systems include the lyre, U and Wye trellises, and the GDC (Geneva Double Curtain).

Any of the above winegrape trellis systems that increase the percent

ground cover should also increase vineyard irrigation requirements based upon observations using Thompson Seedless grapevines in the lysimeter.

In addition, as the tractor-row width decreases, the percent ground cover or shaded area will increase. One would therefore assume that vineyard water use would increase as the distance between rows decreased.

I have independently developed crop coefficients for two different train-ing/trellis systems: a VSP trellis with unilateral cordons and a modified GDC trellis with quadrilateral cordons trained to a four-foot crossarm. In order to develop k cs for both systems, water was applied at various fractions of my initial estimate of ET, for each trellis system. This was replicated in trials in two commercial vineyards.

The fractions of ET, used were 0.25 to 1.25 ET, in increments of 0.25. The VSP-trellised vineyard was in Cameros

1 0

and had a seven-foot row spacing and the quad-cordon vineyard in Temecula had a 12-foot row spacing.

A vine water balance was then determined for each irrigation treat-ment. Water inputs from rainfall and irrigation and the soil water content in each irrigation plot were mea-sured.

Based upon my work at the Kearney Ag Center, if vines are irri-gated at full ET, soil water content will remain fairly constant. If vines are being deficit-irrigated, then soil water content will decrease. In addi-tion, my previous work with Thompson Seedless indicated that midday leaf water potential gener-ally would not be more negative than —10 bars if vines were irrigated at 100% of ETc .

These were the two main criteria used to determine if vines were receiv-ing applied water amounts that replaced water used by the vines on a weekly basis. Qnce it was determined which irrigation treatment (applied water amount) was equivalent to vine ETC , I was able to calculate how much water the vines used at regular inter-vals throughout the growing season and then to calculate seasonal crop coefficients.

The maximum k for the vines on a VSP trellis system, in a vineyard with a seven-foot wide tractor row, was slightly greater than 0.7 (Table I). The maximum k for the modified GDC trellis (12-foot wide tractor row) was approximately 0.75 (Table II). Both of the above values are less than that found for Thompson Seedless grape-vines in the lysimeter where the maxi-mum k can be close to 1.0. 3

The VSP trellis seasonal crop coeffi-cients initially developed on the seven-foot wide row spacing have been tested in other commercial vineyards with different row spacings, row direc-tions, cultivars and rootstocks, and at four different locations. At all loca-tions, irrigation treatments at various amounts of estimated full ET, were included in a replicated trial.

The seasonal VSP trellis crop coeffi-cients were adjusted either upward or downward for narrower or wider row spacings, respectively, from those developed in Cameros.

For example, the seasonal k cs for a six-foot row spacing were increased by 1.17, relative to those developed in Cameros on a seven-foot row spacing (7 ft 6 ft = 1.17) while those for a 10-foot row spacing were decreased by 0.7 (7 ft + 10 ft = 0.7) relative to those developed in Carneros (Table I).

Therefore, the maximum k for VSP vineyards with tractor row widths of six feet, eight feet, nine feet, and 10 feet, were calculated to be 0.82, 0.62, 0.55, and 0.49, respectively (Table I).

Crop coefficients developed for the VSP trellis will increase or decrease water use per unit land area as row spacing decreases or increases. However, if vine spacing within the row is the same among those vineyards, then water use per vine will remain the same. An exam-ple of this can be found in Table II. The water use per unit land area (ET c in inches column), comparing a VSP trellis at six- and nine-foot row spac-ings is greater for the six-foot row vineyard but water use per vine (last column) is the same for both.

In order to validate my assumptions about row spacings and crop coeffi-cients used, the crop coefficient was considered appropriate (i.e. applied water amounts replaced what the vines used) if midday leaf water potential remained above -10 bars all season and if berry weight was maximized at applied water amounts at 75% of esti-mated full ETc (i.e., applied water amounts at greater values did not result in a further significant increase in berry size.) These parameters were based upon data collected on Thompson Seedless in San Joaquin Valley.

At various points during the 1998 and 1999 growing seasons, shaded areas under vines at all trial sites with VSP trellises, were measured at solar noon.

This was done by placing a four-foot by four-foot piece of plywood, on which a grid (six-inch by six-inch squares) had been drawn on the ground beneath the vines and either visually estimating the pprcent shade in each square of the grid or by record-

, ing an image of the plywood with a digital camera. The image was down-loaded to a computer and the shaded area was determined with appropriate software.

Crop coefficients used to schedule irrigations that week in each vineyard were compared with the crop coeffi-cient calculated as a function of shaded area measured that particular week. The equation used was: k - 0.017 x percent shaded area, where 0.017 is the slope of the equation describing the relationship between the percent shaded area and the crop coefficient of Thompson Seedless vines growing in the weighing lysimeter.

0

The data indicated that the k, calcu-lated from percent shaded area and the k being used that week to schedule irri-gations were linearly correlated with one another (coefficient of determina-tion Efq was 0.86). This also implies that the use of the shaded area technique to calculate a crop coefficient could be a viable tool in vineyard irrigation man-agement to approximate the value of potential vine water use at full ET,.

In the 2000 growing season, crop coefficients were developed for vine-yards using a lyre trellis and a high-den-sity VSP-trellised vineyard (the latter planted lm x lm; 4,049 vines per acre), using the shaded area technique.

The calculated k c at maximum canopy development (i.e. late in the summer) was 0.83 for the lyre (planted to nine-foot wide tractor rows) and 0.91 for the lm x lm planting (Table II).

Results indicate that trellises such as the lyre, which spread the canopy, will have higher per vine and per unit land area water requirements than trellises that don't (such as the VSP), when both are planted to with same tractor row width.

However, when there is a VSP trel-lis in a vineyard with a closer row spacing (such as a six-foot vine row spacing or lm x 1m planting), water use per unit land area of those vine-yards may be comparable to the lyre planted on nine-foot or wider row spacing. This illustrates that both trel-lis (or canopy type) and row spacing will determine percent ground cover in the vineyard and ultimately poten-tial vineyard water use.

Also during the 2000 growing season, the shaded area was measured beneath vines on vertically split canopies (such as the Scott Henry and Smart/Dyson systems) to derive their crop coefficients. Vineyards in which these systems were used included row directions east /west and north/south.

Shaded area measurements taken at solar noon on a vertically-split trellis planted to north/south rows were not similar to those planted to east/west rows.

I subsequently determined that the shaded area of a vertically split canopy planted to north/south rows measured an hour before or an hour after solar noon was very similar to the shaded area of rows oriented east/west when measured at solar noon.

I also have found that crop coeffi-cients developed on east/west rows were appropriate for north / south rows for both VSP trellises and vertically-split canopies.

The water requirement of a verti-cally-split canopy is approximately

25°)0 greater than that of a VSP canopy, . when planted on the same row spacing when the calculations are made after veraison (Data was generated during the 2001 growing season).

Practical use of measuring shaded area in your vineyard

As shown above, the shaded area beneath vines in your vineyard can be determined by several methods. One can use a grid and then estimate the amount of shade within each grid, then calculate the total shaded area of the vine. The same grid can be used to pro-duce images of the shade on plywood with a digital camera.

There are software packages that allow calculation of areas of different colors on the image. However, the dig-ital camera cannot be used if the vine's canopy extends to the soil surface. Lastly, one can use a tape measure to determine the average width of the shade beneath the grapevine and then calculate the total shaded area. This can be done where the canopy within the vineyard is highly uniform.

It should be noted that determining the shaded area of canopies early in the growing season or in newly planted vineyards is difficult with a measuring tape since such canopies are very dis-continuous. It would be hard to get an average width of the shaded area.

However, if one can obtain a good approximation of the amount of shade under the vine, then estimation of a crop coefficient would prove useful in determining the amount of water a vine would use at 100% of vineyard ET.

Below is an example of how poten-tial vineyard water use could be derived using percent shaded area to estimate the crop coefficient: A. Tractor row width = 10 feet B. Vine spacing = 6 feet C. Area per vine = 60 ft' D. Average width of measured shaded area between two vines = 3 feet E. Shaded area per vine = B x D = 6 ft. x 3 ft. = 18 ft' F. Percent shaded area = E C = 18 ft' ÷ 60 ft' = 0.3 or 30% G. Crop coefficient (k c) = F x 0.017 (slope of relationship between k c and percent shaded area of Thompson Seedless) = 30 x 0.017 = 0.51 H. Vine water use = ET, (obtained from CIMIS) x kc.

Irrigation scheduling To determine a vineyard's irrigation

requirement, the equation in H above can be used (ETc = ET, x kc). Remember this equation is for calculat-

ing applied water volumes that replace amounts of water the vineyard uses daily or at other intervals. That is, the soil's water reservoir would not be depleted. The above equation will give full crop water use in mm or inches.

To determine water application amounts per vine or to calculate pump run times, one must convert mm or inches into liters or gallons. The metric conversion is one mm covering one hectare equals 10,000 liters. The English conversion is one inch cover-ing one acre equals approximately 27,500 gallons.

Once the amounts have been deter-mined, divide the liters or gallons required per unit land area by number

of vines per hectare or acre (Tables II and HI). The pump run time is also dependent upon the number and sizes of emitters per vine and the irrigation system's emission uniformity.

The seasonal progression of vine water use for a VSP trellis, planted with 10 foot-wide tractor rows is shown in Table III. The data illustrate how vine water use is lower early in the season due to a smaller canopy.

Once the canopy is established and fills the trellis (beginning the middle of July), differences in ET,, from week to week, will result in differences in vine water requirements when this technique to irrigate vines is used. Thus, the benefits of calculating vine water use with this method is that both evaporative demand (ET 0) and the seasonal progression of a canopy's development or trellis type and trac-tor row spacing dictate how much water to apply.

The following illustrates how sea-sonal vineyard water use of one trellis system may vary as functions of row spacing and of different trellis systems at the same location.

The calculated vineyard water use between budbreak (April 3) and har-vest (September 21) in 2000 for a Cabernet Sauvignon vineyard, using a VSP trellis and a six-foot tractor row near Oakville in Napa Valley, was 17.7 inches and applied water was 89% of that value. Potential ET for the same timeframe was 34.4 inches.

The calculated water use for a sim-ilar vineyard but with tractor row widths of 7, 8, 9, and 10 feet would have been 15.2, 13.3, 11.8, and 10.6 inches, respectively. Calculated water use for a lyre trellis, with 9-foot row spacing, and a 1m x 1m planting at the same location would have been approximately 20 inches and 2.5 inches of water, respectively.

Effect of water amounts on growth and yield

Research conducted in my labora-tory and elsewhere, indicates that deficit-irrigation will have minimal effects on berry growth or yield and with possible advancement in date of harvest and an increase in fruit qual-ity.

In each of my irrigation trials, irri-gation amounts that are fractions of estimated full ET have been included. These amounts are applied from the first irrigation of the season up until at least harvest or beyond. This allows determination of the effects of both under- and over-irrigation on vine growth, berry characteristics, and yield of raisin and table grapes or wine qual-ity of wine grape cultivars.

I have found,that irrigation amounts, at approximately 80% of full ET, maxi-mize berry size for raisin and table grapes." Yield of Thompson Seedless vines grown at the Kearney Ag Center is maximized at applied water amounts that are 80% of lysimeter water use whether used for raisin" or table grape production (unpublished data).

It has been found that the yield of Thompson Seedless grapevines used for raisin production actually de-creases when applied water is greater than 100% of ET, determined with the weighing lysimeter. This is due to reduced bud fruitfulness and lower cluster numbers per vine.

Vegetative growth of Thompson Seedless vines generally increases as applied water increases from no irriga-tion to 120% of ET.'''

My studies in various table grape vineyards, in both the Coachella and San Joaquin Valleys (over a four-year period), have demonstrated that pack-able yields may be only slightly reduced at applied water amounts equal to 50% of full ET.

It should be noted that the above results were obtained in vineyards that were irrigated daily. Therefore, the irri-gation frequency used in those studies may have mitigated possible negative effects of deficit irrigation.

My studies in Chardonnay and Cabernet Sauvignon vineyards in Napa Valley and along the central coast of California have somewhat mimicked results found with Thompson Seedless vines. In these studies, irrigation frequency is only once or twice per week. Berry size is usually maximized with applied water amounts at 75% of estimated full ET (Table IV). Yields are slightly dimin-ished at applied water amounts at 75% of full ET (Table V).

Yields have been maximized, in some instances, at applied water amounts equal to 50% of full ET, but this is dependent upon weather (years in which rainfall may have occurred late into the spring), location, soil type, and rooting depth. Pruning weights, a measure of vegetative growth, gener-ally increased as applied water amounts increased (Table VI).

My wine grape studies have also addressed the effect of rootstock on vine water use. At least three rootstocks were used at each site (five in Paso Robles) and these included 5C, 110R, 3309C, Freedom, 140R, and 1103P.

I have found no differences in vine water use or vine water relations among individual rootstocks at a specific loca-tion. This may be due to the fact that all vineyards utitind the VSP trellis sys-tem. Shoot hedging and positioning, as performed for the VSP trellis, does not allow possible differences in vine vigor (increased canopy size) among root-stocks to be expressed. In addition, drip irrigation may limit exploration of the soil profile by each rootstock.

While, I have rarely found significant interactions between rootstocks and irri-gation amounts among the different fruit and yield parameters measured at each site, I have found significant differences

^ in those parameters among rootstocks. Small wine lots have been made of

Cabernet Sauvignon grown at Oakville and Paso Robles and Chardonnay from Cameros with sensory analysis per-formed on each. While deficit irriga-tion will increase color of wine made from Cabernet Sauvignon, there has been no consensus as to preferences among the irrigation treatments.

One possible reason is that irriga-tion amounts that I calculate as full ETc for the VSP trellis are much less than previously thought. For example, cal-culated ETc for the Paso Robles Cabernet Sauvignon vineyard (with 10-foot wide tractor rows) from bud-break to the end of September has only been between 10 to 12 inches per grow-ing season. This is much less water than many growers presently apply.

Conclusions Information regarding the use of a

pressure chamber and estimation of crop coefficients presented above can provide growers an objective means in determining when irrigations should commence, while also helping them estimate how much water a vine potentially may use at full ETc .

I have used both the pressure chamber and measured soil moisture content with a neutron probe in studies to determine when irrigations should commence. Both have worked extremely well.

trellis, and row spacing. It is also reflec-tive of changes in evaporative demand from day to day during the growing sea-son and from year to year.

For example, if evaporative demand is high one week, irrigation require-ments will be high, if it is lower the next week, then irrigation require-ments will be lower.

Initiating vineyard irrigation later in the growing season or the use of deficit irrigation may restrict excess vegetative growth, whether for grapevines grown in the interior valley of California or along the coast. This would minimize the cost of canopy management for vines that, in the past, became too vig-orous due to excessive irrigation.

Grapevines can be deficit-irrigated at various fractions of estimated full ET c with minimal impacts on yield but with a potential to increase fruit quality. Thus, in most cases, one may not have to apply water amounts that meet or exceed estimated vineyard water use requirements presented in this article.•

Table I - Crop coefficients for a VSP trellis as a function of degree-days (DDs) from budbreak and row spacing.

Degree-days (DDs) are expressed in Centigrade (C) and Fahrenheit (F). Base temperatures used in calculating DDs are 10°C and 50°F.

DDs (C) DDs (F) k, 6 ft•

k, 7 ft

k, 8 ft

k, 9 ft

k, 10 ft

100 180 0.17 0.15 0.13 0.12 0.10 200 360 0.22 0.19 0.17 0.15 0.13 300 540 0.28 0.24 0.21 0.19 0.17 400 720 0.35 0.30 0.26 0.23 0.21 500 900 0.42 0.36 0.31 0.28 0.25 600 1080 0.49 0.42 0.37 0.33 0.29 700 1260 0.56 0.48 0.42 0.37 0.33 800 1440 0.62 0.53 0.46 0.41 0.37 900 1620 0.67 0.58 0.51 0.45 0.40 1000 1800 0.72 0.62 0.54 0.48 0.43 1100 1980 0.76 0.65 0.57 0.50 0.45 1200 2160 0.79 0.67 0.59 0.52 0.47 1300 2340 0.81 0.69 0.61 0.54 0.48 1400 2520 0.82 0.71 0.62 0.55 0.49 1500 2700 0.82 0.71 0.62 0.55 0.49

T able II - The effect of trellis type and row spacing on estimated vine water use of grapevines assuming an ET, of 1.5 inches for the

time frame used HD stands for a high density planting, 1 m x 1 m, using a smaller version of a VSP trellis.

Trellis Type

Row Spacing

ET. (inches) k,

ET, (inches)

ET, (gal/ acre)

ET, (gal/ vine)

VSP 6 ft. 1.5 0.81 1.22 33,550 27.8 VSP 9 ft. 1.5 0.54 0.81 22,275 27.6 Lyre 9 ft. 1.5 0.83 1.25 34,375 42.5 I-I1D 1 m 1.5 0.91 1.37 37,675 9.3

GDC 12 ft. 1.5 0.75 1.13 31,075 51.4

Assumptions: Vine spacing for all trellises is 6 ft., except in the HD vineyard. Therefore vine density is 1,208, 808, 808, 604, and 4,049 vines per acre for the VSP 6 ft. row, VSP 9 ft. row, Lyre, and GDC trellis and HD planting, respectively. The k,s used for the VSP 6 and 9 ft. rows is from DDs (C) 1300 row in Table I. Other Ice used are from L.E. Williams,

assumed that there is 27,500 gallons per acre-inch of water.

However, the measurement of a vine's leaf (or stem) water potential may be preferable, since it integrates the amount of water the vine's roots have access to throughout the soil's profile. In addition,

measurements of leaf water potential can be made at more than one site within the vineyard with relative ease.

Many times, determination of a vineyard's soil water status with a neu-tron probe in a commercial situation is based upon just one access tube per vineyard. Placement of this single assess tube in the vine row, including distance from an emitter (if drip irriga-tion is used), may not accurately deter-mine soil water status of an individual vine nor account for variability within the vineyard due to different soil types and / or rooting depths.

Calculation of irrigation requirements using crop coefficients and ET0 , provides an objective means to determine how much water should be applied based upon the vine's growth characteristics,

T-:

unpublished data. ET, in inches was obtained by multiplying ETo by the k,. It was

Table III - Calculated water use of Cabernet Sauvignon grapevines in Paso Robles during the 2000-growing season.

Potential ET (ET0) was obtained from the Paso Robles Vintners and Growers Association's PR1 weather station.

Row spacing was 10 ft. and vine spacing was 6 ft. (726 vines per acre). It was assumed that there were 27,500 gallons per acre-inch.

ETc ET Month Week ET. (in.) lc ETc (in.) (gal./acre) (gal. /vine) Rain (in.)

May 1 . 8 15 22 29

June 5

12 19 26

July 3 10 17 24 31

August 7 14 21 28

*, **, and *** denote dates of the initiation of irrigation, approximate date of bloom, and approximate date of veraison, respectively. Vines were harvested September 27, 2000.

Table IV - Relative berry weight as a function of applied irrigation amounts (fraction of estimated full ET) at four locations and two

cultivars, Cabernet Sauvignon and Chardonnay. All vineyards used a VSP trellis. Values at each location are the mean of three different

rootstocks except at Paso Robles, which had five rootstocks, with data collected for a minimum of three growing seasons.

Table V - Relative yield as a function of applied irrigation amounts (fraction of estimated full ET) at four locations and two cultivars,

Cabernet Sauvignon and Chardonnay. All vineyards used a VSP trellis. Values at each location are the mean of three different rootstocks except

at Paso Robles, which had five rootstocks, with data collected for a minimum of three growing seasons.

Location Cultivar Irrigation Treatment (fraction of estimated ET C )

0.25 0.5 0.75 1.0 1.25

maximum weight)* - (percent of -

Oakville Cabernet 77 96 100 99 99

Paso Robles Cabernet 61 70 81 91 100

Gonzales Chardonnay 65 81 87 89 100

Edna Valley Chardonnay 92 90 92 98 100

" The weights of each treatment were divided by the treatment with the greatest weight. The treatment with the greatest weight was set to 100%.

Table VI - Relative pruning weight as a function of applied irrigation amounts (fraction of estimated full ED at four locations and two

cultivars, Cabernet Sauvignon and Chardonnay. All vineyards used a VSP trellis. Values at each location are the mean of three different

rootstocks except at Paso Robles, which had five rootstocks, with data collected for a minimum of three growing seasons.

Location Cultivar Irrigation Treatment (fraction of estimated ET,)

0.25 0.5 0.75 1.0 1.25

maximum weight)* - - (percent of

Oakville Cabernet 85 89 100 96 99

Paso Robles Cabernet 61 67 79 88 100

Gonzales Chardonnay 75 81 87 96 100

Edna Valley Chardonnay 87 87 92 98 100

*4*

**

1.38

0.14

0.19

5,225

7.2

0

1.38

0.17

0.23

6,325

8.7

0

1.50

0.18

0.27

7,425

10.2

0

1.69

0.22

0.37

10,175

14.0

0

1.89

0.25

0.47

12,925

17.8

0

1.61

0.28

0.45

12,375

17.0

0.2

1.73

0.32

0.55

15,125

20.8

0

1.69

0.36

0.61

16,775

23.1

0

1.97

0.39

0.77

21,175

29.0

0

1.57

0.41

0.64

17,600

24.2

0

1.61

0.43

0.69

18,975

26.1

0

1.97

0.44

0.87

21,450

29.5

0

2.05

0.44

0.90

24,750

34.1

0

2.05

0.45

0.92

25,300

34.8

0

1.89

0.46

0.87

23,925

33.0

0

2.05

0.47

0.97

26,675

36.7

0

1.73

0.48

0.83

22,825

31.4

0

1.26

0.49

0.62

17,050

23.5

0

Location

Oakville Paso Robles

Gonzales Edna Valley

- (percent of maximum Cabernet 83 93 98

Cabernet 78 88 95

Chardonnay 77 89 96

Chardonnay 82 89 97

Irrigation Treatment (fraction of estimated ETc ) Cultivar

0.25 0.5 0.75 1.0 1.25

weight)* - 100

98

98

100 98

100

99

100

* The weights of each treatment were divided by the treatment with the greatest weight. The treatment with the greatest weight was set to 100%.

V * The weights of each treatment were divided by the treatment with the greatest weight. The treatment with the greatest weight was set to 100%.

OWINI°

4111.6

Irrigation Management

Scheduling Monitoring

Strategy

I II MARCH/APRIL 2005 21

Abscission (yellowing and dropping) of

basal leaves is an indication of excessive

water stress and should generally be

avoided.

given that irrigation is such a funda-mental aspect of viticulture, it is more appropriately discussed in its entirety, which I will endeavor to do here.

Integrated irrigation management may be thought of as an interaction of three elements: A. Irrigation scheduling, B. Water status monitoring and, C. Irrigation strategy.

All three components are required for proper wine grape irrigation, and all will be discussed in this article. MONITORING AND SCHEDULING

Integrated irrigation of California winegrapes III Mark Greenspan, Ph.D.,

Viticulture research manager Gallo of Sonoma, Healdsburg, CA

poes dry farming produce superior wine? Not always. We are fortu-nate in California to enjoy condi-tions that are undoubtedly

among the best in the world for growing wine grapes. Water resources are abun-dant to maintain vineyards throughout the season, even with a nearly complete lack of summer rainfall.

Despite living in a climate of seasonal aridity, we all too often attempt to mimic the Old World style of grape growing without supplemental irrigation.

However, it is critical to remember that California's rainfall pattern differs tremendously from Europe's. For instance, Bordeaux receives almost 33 inches of annual rainfall, distributed almost uniformly throughout the year (Figure I). In contrast, St. Helena in Napa Valley totals almost 35 inches —most all of it falling over a six- to seven-month period!'s

St. Helena, like much of California, receives essentially no rainfall from June through September. While the

rainfall in Bordeaux is often perfect for maintaining vineyards without sup-plemental irrigation, most California vineyards require irrigation to carry them through the growing season. Fortunately, California is blessed with enough rain during the wet season to allow capture and storage for irrigation during the thy season.

In many ways, that is a distinct bene-fit to California, as we are not beholden to weather patterns that influence the growth of vine canopies or development or quality of our fruit crop. While irriga-tion is not required in some vineyard sites, due to a high water table or extremely deep and fertile soils, dry farming is essentially a misnomer, since non-irrigated vineyards that have an ample natural water supply are any-thing but dry!

I have found, through experimenta-tion and trial-and-error, that judi-ciously irrigated grapevines can pro-duce intensely flavored and rich wines that are sought after by those operating "old-style" dry-farmed vineyards.

Irrigation articles often focus on one or two aspects of the topic, such as using ET or pressure "bombs." But

A. IRRIGATION SCHEDUUNG Irrigation scheduling is simply the

practice of deciding how much irriga-tion to apply, how often. Blunders in irrigation scheduling can be made in several fundamental ways, including irrigating only on specific days of the week, cutting off irrigation during cool or cloudy weather, or by waiting for the vineyard to show signs of stress before pouring on a lake of water.

Scheduling of irrigation should be performed consistently and, to some extent, independently of vineyard moisture stress symptoms (see Section B). Stress symptoms should be used to modify irrigation schedules, not to determine actual irrigation applications.

When to start irrigation Irrigation scheduling cannot begin

until a specific starting date has been determined each season. Generally speaking, for regions that receive a suf-ficient amount (roughly 8 to 10 inches)

212

Mo

nth

ly P

rec

ipit

ati

on (

inch

es)

A 0

■

1 iS2

CAI •

Ik to

cr

, ...4

co

--*--Bordeaux - 32.7 annual inches

-111-St. Helena - 34.9 annual inches

MI110°1 •

, <,..r. 40, 44.06

1... 4 0 0 . 5t ‘,

06 s es..

.0

Figure I: Rainfall distribution comparison between Bordeaux and St. Helena. Data are 30-year averages from each region.

of dormant season precipitation to I the soil profile to field capacity by ear spring, we do not wish to begin sche tiling irrigation until a sufficie amount of water has been extract from the profile. In more arid regions large irrigation application is cor monly applied prior to the time of bu break to "fill" the soil profile to son extent, allowing for rapid vine grow early on.

However, it is not advisable to beg irrigation scheduling in the spring un the vines have extracted sufficient wat from the soil so that their growth can 1 controlled. Exactly when that state reached will depend on numerous fa tors, including whether or not the sr profile is filled to field capacity by wint precipitation; soil water-holding capa ity; and presence or absence of cov crop. The latter factor will have a treme dous influence on water depletion rate the spring, especially if a tall perenni grass crop is maintained.

There are several ways to determine the trigger for irrigation initiation. They include: • Monitoring soil moisture, • Monitoring shoot elongation rate, and • Measuring mid-day leaf water potential with a pressure chamber (see Section B [page 311).

I have found that the pressure chamber is the most useful tool for this purpose.

A good rule of thumb is to begin the irrigation season when mid-day leaf water potential reaches -8 to -10 bars. The -8 bar end is used for white grapes or blocks that are more prone to water stress during the season. For most red varieties, -10 bars is 'seri The -8 bar end might also be chosen in less vigorous blocks, for which some additional shoot elongation is desired.

Some growers do not wish to invest in a pressure chamber. Fortunately, it is not absolutely necessary to have a pressure chamber in order to determine the irriga-tion trigger Shoot elongation is highly cor-related to mid-day leaf water potential

(Figure II). Elongation ceases at a mid-day leaf water potential of about -9 to -10 bar This provides an opportunity to determine the point of initiation at very low cost.

Mark out about 10 shoots throughout each vineyard block and measure the shoot lengths weekly. By using a spread-sheet to graph the shoot lengths, it will become apparent when shoot elongation slows down and, eventually, stops. This method will take a bit more trial and error than using the pressure chamber, but it is an acceptable second choice.

Be careful to look not only at shoot elongation rate, but also at the actual shoot length. If shoot elongation slows down before the shoots have attained sufficient length (about four-foot-long shoots or 20 to 22 nodes), it is imperative that irrigation be initiated to allow for continued growth. Once shoots stop growing, they will rarely restart unless excessive irrigation is applied.

While I suggest using the pressure chamber as a primary tool, the most

intelligent method is to use soil moisture, shoot length measurement, and pressure chamber data together to arrive at this critical decision point

How much water to apply? Irrigation scheduling is like having

a checking account. Viticulturists need to monitor how much is being with-drawn (by the vines) against the amount that is being deposited (by irri-gation). The bank is actually the soil matrix, and we want to manage the "cash" flow such that we don't keep too much in the (low interest) account, while also being careful not to over-draw it.

The analogy to grapevines is that water availability in luxurious amounts results in excessive vine growth, and too little water in the soil creates excessive water stress in the vines. Mind you, a little water stress can be a good thing (regulated deficit irrigation, or RDI). This will be dis-

cussed in Section C (May /June 2005 PWV). But first, let's discuss the deposits and withdrawals into our soil reserve.

Measuring water application We can account for the amount of

irrigation applied to a vineyard in sev-eral different ways. The important thing is to have the application units equivalent to withdrawal units. Tradi-tionally, irrigation volume is expressed in inches of depth or in acre-inches of volume. It is more convenient to express drip irrigation amounts in terms of gallons per acre or hours of application. Since consumptive values are usually expressed in depth units, we need to convert them to more use-ful units: 1 acre-inch = 27,154 gallons (1)

To convert from inches to hours: hours = (2)

inches x 27,154

emitter rate (gph) x emitters per vine x vines per acre

or, from hours to inches: inches = (3)

hours x emitter rate (gph) x emitters per vine x vines per acre

27,154

Measurement of gallons applied, using an inline flow-meter, is probably the best method of irrigation application tracking. In that case, we would need to use the above equations to convert back and forth from inches. However, it is gen-erally easiest to express irrigation appli-cation in terms of hours, since that is how

21'S

we operate in the field. Using hours to track irrigation means that we need to translate consumption values from inches into hours using equation #2.

Estimating water consumption Tracking applied irrigation is easy,

since we have full control over it. On the "debit" side of the ledger, though, is water consumption. The best way to determine water consumption is the "ET" approach ET is a friendly abbrevia-tion for "evapotranspiration," which is a long word that simply means: water that evaporates either from the soil (evapora-tion) or from the plant (transpiration). With drip irrigation, almost all ET comes from the vine (because most of the soil surface is dry), but we still call it ET.

ET is highly weather-dependent and is increased by solar energy, dry air, and windy conditions. Solar energy heats up the vine's leaves and evapora-tion of water from leaves serves as a cooling mechanism. Wind increases ET by stirring up the humid layer of air that surrounds each leaf and mixing it into the dry surrounding air.

Contrary to common belief, cool weather does not automatically imply low ET, since it is the capacity of the air to absorb additional water that most closely affects ET and not the air tem-perature itself. However, cool air holds less water than warm air, so there is generally a reduction of ET during cool periods, although cool temperatures at low relative humidity will still present a significant driving force for ET.

ET can be determined by many methods, but the most commonly used method is called "reference ET" or ET,. Reference ET uses weather-measuring equipment to estimate a value of ET for a "reference crop" consisting of well-watered, mowed grass. For more infor-mation on ET,, consult Reference #4.

Fortunately ET0 is readily available on a daily basis from many sources. In California, the CIMIS (California Irrigation Management Information System) is available at no cost (aside from tax dollars). Daily values can be obtained from the CIMIS and the UC Davis 1PM websites. Use these resources to locate the nearest CIMIS sta-tion to your vineyard, and then stick with the same site consistently.

Other sources of ET° include private weather stations (in your vineyard, if you'd like); weather station networks run by consultants; newspapers in some agricultural communities; and maybe even the winery that buys your grapes. ET, is reported in inches or millimeters, which must be translated into gallons per acre or equivalent irrigation hours (equations #1 and #2 above) to match our accounting for applied irrigation.

But obviously we're not growing grass. We need to convert ET of the grass crop to grapevine ET, referred to generically as crop ET, or ETc. To get ETc values from ET0 values, we multi-ply ET0 by a "fudge factor" called the "crop coefficient," Kc.

ETc = ETo x Kc (4) '

As grapevines are planted in rows and are more water-thrifty than grasses, the crop coefficient is always less than 1.0 (some fraction of full ET 0). Unfortunately, the crop coefficient is not a fixed number, and it is dependent mostly on vine size (or leaf area index). Since the leaf area index of a vineyard changes with time of year and trellis type, site vigor, row width, etc., K, is not a very easy term to pin down.

The good news is that, after years of research into obtaining accurate crop coefficients, I've concluded that consis-tency is more important than accuracy when determining Era. The reason for this is that we can think of ETc as an

index, like the Dow Jones Industrial Average and S&P500 are to the stock market. We can raise or lower a second fudge factor (the management coeffi-cient) in reaction to -observations of vineyard water status (Section B) to compensate for errors and for vineyard management strategy.

Whatever model you use for K,, stick with it year after year. Previously-pub-lished crop coefficients for grapes were determined for rather bushy California sprawl vineyards in the Central Valley and are, therefore, too high for the thriftier winegrape canopies that cur-rently exist throughout the state.'

I have done some experimentation using direct methods of measuring vineyard ET and have found that K, values of our vineyards are roughly 75% of earlier published values. Our K, estimates are very similar to those determined by Larry Williams (University of California), so I recommend using his model for Kr .'

The discrepancy in crop coefficient values has, in my opinion, generated some confusion regarding desirable deficit irrigation levels for winegrape vineyards. While published results stating that deficit irrigation treatments of 60% of full ET, have proven to pro-duce desirable wine quality, we must be diligent in determining what crop coefficient model they were using.

If they were using "traditional" crop coefficient values, their ET, was likely overestimated. That means that their 60% of full ET, could actually translate into about 80%, using the new crop coefficient model. A grower who makes the error of applying 60% of full ET, using the new crop coefficients might overstress the vineyard.

Some growers who have been using the old Kc model do not wish to switch to the new, lower values since that would change their management factors. That is fine, as long as they remember they're on the old model when contemplating modification of their irrigation strategy based on more recently published research. Again, consistency is more important than accuracy, and what has been working for a grower in the past should continue to work in the future.

To aid in implementing an irrigation strategy, another factor, the manage-ment factor (Km), is used in addition to Kc. Our ET equation now becomes:

ET° x Kc x Km (5)

la is irrigation to be applied in units of inches (or whatever units ET, is in). The Km value is chosen to reflect the percentage of ETc that is to be applied (80% of ETc equals a Km of 0.8). Km can be changed at veraison for pre- or ost-veraison deficit strategy.

Some indicators of excessive water stress are: leaf sun avoidance by folding and tighter angle with the petiole (Left), and shrivel of ripening berries (Right). These conditions are to be avoided.

1 '1

. • •

Shoo

t Elo

nga

tion

Rat

e (i

nc h

e 6 0 0

p

0 N

A

CT

C ....

.•

. . .

• •

a . . R 2 = 0.77

. .

. .. .

• • : . .

. .

—5.0 —6.0 —7.0 —8.0 —9.0 —10.0 —11.0 —12.0

Midday Leaf Water Potential (bars)

—13.0

Figure Shoot tip elongation rate plotted against midday leaf water potential. Measurements were taken in 2003 in Sonoma County vineyards for three varieties (Merlot, Cabernet Sauvignon, and Pinot Gris). Regression line indicates a strong relationship between shoot elongation and midday leaf water potential, with elongation ceasing at about -10 to -11 bars.

Any errors in ET estimates can be absorbed into the Km since it is set arbitrarily and experimentally. It is very important that records be kept from year to year so that the Km value can generate a consistent ET-based schedule.

To make scheduling easier, crop coefficient and management coefficient values can be embedded in a spread-sheet or database irrigation scheduling program. Then, the only inputs needed on a daily basis are ET, and hours irri-gated. Irrigation requirements are

determined by subtracting hours irri-gated from the hours of ET,.

One does not need to go back to the beginning of the irrigation season to determine how much to irrigate the next day or week. It is sufficient to use between one and two weeks of irriga-tion and ET, history (using a sum of ET, and a sum of irrigation hours over that same time period). For those who do not wish to create their own computer-based scheduling system, there are sev-eral references for irrigation scheduling software on the CIM1S web site.

How often to irrigate While the most critical (and difficult)

part of irrigation scheduling is determin-ing the irrigation quantity required, it is also important to know how often irriga-tion should be applied (irrigation fre-quency). In other words, if a ET, model indicates that 18 hours are needed in one week, do we apply three six-hour sets, two nine-hour sets, or a single 18-hour set?

While water use (ET,) is controlled by the vine and weather conditions, irrigation application is limited by soil properties. Selection of irrigation dura-tion and frequency is highly dependent on soil water-holding capacity.

Soil texture plays a large part in soil water-holding capacity, as does soil (and rooting) depth. Generally speak-ing, sandy soils have low water-hold-ing capacity per foot of depth, while clay and loam soils have high water-

holding capacity. Given any require-ment for irrigation, soils with low water-holding capacity will have to be irrigated with lower volumes more frequently than soils with high water-holding capacity (which can be irrigated with greater volumes less frequently).

Irrigating with a volume beyond the water-holding capacity of a soil will waste water through percolation below the root zone or through run-off. Approximate values of water-holding capacity can be found in references,' but with drip irrigation, it is difficult to deter-mine how much soil volume is wetted,

since water is applied from a point source • and not uniformly across the surface.

Some irrigation scheduling systems use a soil-water content approach to trigger each irrigation event, but with drip irrigated vineyards at Gallo of Sonoma, we have found this approach to be difficult and unnecessary with drip-irrigated vineyards. However, the soil-water content approach more directly addresses the issue of irriga-tion frequency, as irrigation events are triggered by soil-moisture content levels.

A better method is to monitor mid-day leaf water potential with a pres-sure chamber daily, following an irri-gation event. Examine both weak and stronger sections of a vineyard to see if vines are becoming temporarily stressed between irrigation events in weaker areas. If so, reduce the time between irrigations a day or two while cutting back on the volume accord-ingly

Figure 111-2

Figure III: Shoot tip ratings of vine water status. Fig. 111-1: Active shoot growth; tendrils reach past growing tip; basal tendrils turgid; Leaf Water Potential (LWP) > -8 bars. Fig. 111-2: Slowing shoot growth; tendrils even with growing tip; basal tendrils turgid; LWP -9 to -11 bars. Fig. III-3: Ceased shoot growth; leaves extend past growing tip; basal tendrils turgid to slightly droopy; LWP -12 to -13 bars. Fig. 111-4: Dead or missin shoot tip; basal tendrils droopy or falling off; leaf-petiole angle becomes smaller; LWP -14 to -15 bars.

It becomes less important to monitor soil-moisture profiles and/or day-to-day vine water status once a good feeling for the irrigation constraints is obtained.

Irrigation efficiency and uniformity Irrigation duration and frequency

directly influence application effi-ciency and, in the case of hillside vine-yards, application uniformity. There is frequently confusion over the two terms: uniformity and efficiency. They are not synonymous.

Uniformity refers to the equal distribu-tion of irrigation volume throughout a vineyard beingyrigated. In other words, if application uniformity is perfect, all vines would receive the same volume of water.

On the other hand, efficiency refers to the amount of applied water that is avail-able for uptake by vines. If efficiency is perfect, all water applied during each irri-gation event will be available for uptake.

In reality, it is not possible to achieve perfect uniformity or perfect efficiency. But we need to be aware of these factors so that we can maximize them.

Application nonuniformity is com-mon in hillside vineyards due to delays in system filling at startup and system drainage at shutdown. It can also arise from variation in emitter dis-charge rates due to pressure differ-ences and/or clogging, although good system design and maintenance should minimize that problem. (Refer to Reference #1 and #3 for more infor-mation on system design influence on irrigation uniformity).

In order to maximize uniformity, irri-gation should be held off until the required application is of sufficient dura-tion (six to eight hours), that the fill and drain times of the system are only a small percentage of the total irrigation duration

Application efficiency is defined as the percentage of applied water that is avail-able for vine uptake. While efficiency is reduced by evaporation from the soil sur-face, this loss is minor with drip irriga-tion. The primary cause for reduced effi-ciency is loss due to runoff and/or deep percolation below the root zone.

Soil can be thought of as a sponge Applying water faster than it can be

absorbed results in surface runoff. As the soil becomes saturated at deeper and deeper levels during an irrigation application, its infiltration rate decreases, potentially resulting in water runoff from the target location. Additionally, applying more water than the soil column can hold in its pores results in deep percolation like seepage out of the bottom of a satu-rated sponge.

Water that percolates below the root zone can move laterally below the ground (especially on hillsides), result-ing in areas deficient in water and areas with surplus water. This wastes water and can result in groundwater contamination, especially with regard to nitrates that move readily down the soil profile. Furthermore, the differen-tial quantities of water at the top ver-sus the bottom of the hills will increase nonuniformity of vine growth.

As for irrigation efficiency, vineyard and soil spatial variability must be con-sidered. Weak areas in vineyards are often caused by gravel or sand streaks, or by shallow soils on hilltops, or other changes in soil texture or depth such as those that occur during land develop-ment. The weakest or shallowest soils need to be focused on when determin-ing the maximum irrigation duration, since any irrigation beyond what can be held in the weak zone may result in vineyard nonuniformity.

A common mistake is to apply more water to weak areas by adding extra emitters. However, weaker areas of a vineyard actually consume less water than stronger areas due to less foliage being present, so extra watering does not make sense.

Our goal is to produce and maintain vineyards of uniform vine size, so we need to apply the same amount of water throughout the vineyard, regard-less of soil water-holding capacity.

The the best way to manage around variability is to irrigate so that the regions of low water-holding capacity are not irrigated beyond what they can hold. This can be accomplished by irri-gating with shorter, more frequent irri-gation events that maintain more con-sistent moisture availability in the weak soil areas while providing the same quantity of irrigation to the entire field.

In general, longer, less frequent irri-gation events will reduce irrigation efficiency, while increasing application uniformity. This contrasts with shorter, more frequent irrigation events that increase irrigation efficiency while decreasing application uniformity.

When using drip irrigation, it is N, important not to extend the duration

between irrigation events too long, no matter what the goals are for deficit irri-gation (see Section C). After winter rain-fall is depleted and the remaining soil water comes from irrigation, there is only a small water reservoir available to each vine, and ready water-availability during periods of hot weather is crucial to pro-tect vines against heat damage.

It is difficult to state, as a rule of thumb, how long is "too long" between irrigation events, as it is highly depen-dent on soil characteristics. "Too long" could range from a couple of days to a couple of weeks. Use soil moisture probes or, as discussed earlier, pressure chamber measurements to evaluate the drying cycle between irrigation events and learn about your vineyard.

Changing weather patterns are common in coastal grapegrowing regions, and cool weather should not be seen as an excuse to cut back on water. Remember that water consump-tion is determined less by temperature than by humidity, sunlight, and wind. Keep to your ETc model even during cool weather or you will be caught short during the heat wave that follows!

B. MONITORNG VINEYARD WATER STATUS Irrigating without monitoring vine-

yard water status is like driving a car blindfolded. That analogy may be tired, but it's very fitting. In irrigation scheduling, our best efforts at deter-mining water consumption through ETc are still an approximation. We need to watch the vineyard to deter-mine its response so we can make cor-rections, if necessary, during the season and in seasons to follow.

While many years of experience will allow a vineyard manager to expend less effort on vineyard water status monitoring, some monitoring will always benefit vineyard health and sup-port an irrigation strategy (see Section C, May/June 2005 PWV).

Soil moisture monitoring is a tradi-tional way to measure water status of agricultural systems. But in most agri-cultural applications, the soil moisture is maintained close to field capacity. By contrast, in winegrape growing we do not always want to maintain a high water status of vines (see Section C).

Furthermore, in drip-irrigated vine-yards, the wetted soil volume is usu-ally discontinuous and, as a result, there is great potential for measure-ment uncertainty, depending on loca-tion of the sensor with respect to emit-ter position, not to mention the variability in soil properties through-out any vineyard block. Consequently, it is generally advantageous to monitor the vines rather than the soil.

Hence, this discussion will focus primarily on vine monitoring using instruments and visual assessments. But first, a few points about soil mois-ture methodology.

Greater than —10 Bars —10 to —12 Bars

—12 to —14 Bars —14 to —16 Bars Less than —16 Bars

No stress Mild stress

Moderate stress

High stress

Severe stress

Soil moisture measurement does have its place in drip irrigation sys-tems. It can be used early in the season after winter rainfall has uniformly wet-ted the soil profile. That is probably the single most useful time of the season for soil moisture monitoring. Irrigation initiation decisions can be based on waiting for soil moisture levels (either water content or water potential) to reach a specified threshold after being depleted from winter rainfall storage.

In regions where the soil profile is not completely filled by winter rainfall, as on the north coast of California, soil mois-ture monitoring is useful in gauging when to commence irrigation or how much irrigation is needed to refill the soil profile by budbreak But remember that once drip irrigation has begun, the read-out of the instrument no longer repre-sents the entire horizontal extent of the root zone of the vineyard.

Instruments that measure soil water content include the Neutron Probe, Time-Domain Reflectometry (TDR), and Capacitance (dielectric) Probes. For mea-suring soil water potential (suction pres-sure), soil-moisture blocks and tensiome-ters are available. All of these instruments have some associated error. It is best to consider these measurements as site-specific — each site should be "calibrated" independently. This is yet another strike against soil-moisture mea-surement. For more discussion about soil-moisture monitoring methods, con-sult references #2 and #4.

Vine water status monitoring There is no better indicator of how

your vineyard is doing than the vines themselves. I've found that the pres-sure chamber is, by far, the preferable method for monitoring vine water status.

A pressure chamber (also called "pres-sure bomb") should be available to most winegrape growers, either owned or through a service. The pressure chamber is a portable, relatively low-tech and low-cost instrument that measures water potential of the vine at the location it is sampled (usually the mid-shoot leaves).

What is water potential? To under-stand it, you must first understand that water is not pushed through the vine, but is sucked through the vine. Suction arises in the leaves, where liquid phase water evaporates. Recalling the discus-sion about ET, the dryer the air sur-rounding the leaves, the greater the evaporation rate is. The water suction force is transferred throughout the vine that, in effect, has a continuous column of water from roots to the leaves through the xylem, or water-conduct-ing vessels.

Soil holds onto water like a sponge and, as happens with a sponge, it is more difficult to extract water as the soil becomes dryer. Hence, there is a tug-of-war between the soil and the atmosphere, with the vine stuck in between like the rope. So, the dryer the soil becomes and/or greater the "evaporative demand" of the air becomes, the stronger the suction on the water column will be. ruction can be expressed in units of

pregiture. Actually, negative values are used becauSe suction is synonymous with nega-tive pressure. Commonly, water potential is expressed in units of bars (1 bar is approxi-mately equal to one atmosphere).

In scientific journals, Megapascals (MPa) are most commonly used. Don't let that throw you off; to get bars from Megapascals, just multiply by 10. It is even more common for people to leave off the negative sign from the bars in casual conversation. That is technically incorrect but since everyone does it, it is acceptable. Larry Williams provided a good overview of the pressure cham-ber in a previous PWV text.6

The beauty of the pressure chamber is that it is easy to learn, requires no power source, and is portable. Just mount it to an ATV and take it where you want to go. Measurement is made by sampling —cutting off — an exposed, fully expanded leaf. Before sampling, though, it is impor-tant to endose the leaf blade in a plastic sandwich bag immediately before it is removed. This crucial step is often disre-garded, and can create substantial mea-surement error if not done.

A leaf is cut off with a razor blade and placed in the chamber, bag and all, with the cut end exposed. The chamber is slowly pressurized until the petiole sap just reaches the cut surface. If done correctly, the pressure in the chamber that is read off the gauge is equal to the suction that was in the xylem vessels before the leaf was removed. It takes some practice, but it is a skill that can be learned by almost anyone.

Measurements should ideally be taken about mid-day. I have found, how-ever, that the hours of noon to 3:00 work well for reliable readings. The main thing is to do it — and do it regularly.

It is difficult to make general specifica-tions about interpretation of water poten-tial measurements, although strategies will be discussed in Section C. As a general guideline, the following may be used:

Remember that since these are neg-ative numbers, "greater than" means "less negative than" and "less than" means "more negative than."

While the pressure chamber is an easy-to-use instrument, there are a few caveats required for accurate measure-ment, not the least of which is making sure to bag the leaf on the vine before removal. Vine water status is likely to vary considerably throughout a vineyard block. Be sure to sample numerous loca-tions within a vineyard to get either a composite value for the block or to map out zones of similar water status.

As a rule of thumb, sample at least one location per acre for very small blocks (less than five acres) down to one location per five acres for larger blocks (about 100 acres). At each location, take measure-ments on two leaves. If they are within 1 bar of each other, log the average and move on to the next site. If not, take a third measurement. If there seems to be an out-lier that differs more than 2 bars from the other two, discard that measurement.

Refrain from making measurements on abnormally hot and dry (or windy) days, when even well-irrigated vines will produce measurements that sug-gest they are stressed. Conversely, measurements taken on cloudy or foggy days will give meaningless values.

At Gallo of Sonoma, we generally wait at least one hour after the fog lifts to begin pressure chamber measure-ments and do not take readings when it is even partly cloudy.

Finally, realize that water potential is a measurement of the suction force in the vine. Vines have built-in drought resistance whereby they close their stomatal pores in the leaves when water stress is imminent. Water flow decreases when stomatal closure occurs and that tends to decrease the suction force. This situation could be misread as a non-stressed condition.

Measuring stomatal conductance requires the use of an instrument called a porometer, which is a fantastic tool but hardly practical for a grower. I sug-gest, to avoid misreading a highly stressed condition, that frequent (at least weekly) pressure chamber mea-surements be made in addition to care-ful attention to visual symptoms.

Visual water status assessment The second vine-monitoring instru-

ment is located between and in front of your ears. Visual symptoms of water status are reliable and cost nothing

MARCH/APRIL 2005 79

GR APEGR OWING

Integrated irrigation Continued from page 34

beyond your time. It is time well spent to look for signs of both excess water stress and, on the opposite extreme, signs of excess vigor brought on by too much easily extractable water.

By far, shoot tips are the best indica-tors of water status, especially (and pri-marily) between bloom and veraison (when we are trying to restrict shoot growth). Shoot tips are easy to observe just by driving past, although more thorough examinations are best.

Shoot growth ceases at about -11 bars of water potential, but will show signs of sloWing at -9 or -10 bars (Figure II). Beyond about -14 bars, shoot tips will dry up. This is not a bad thing if the shoots are long enough and have enough leaves to ripen the crop. Like the pressure chamber, shoot tips may be used as a "speedometer" for irrigation management, although they are not useful after growth ceases.

At Gallo of Sonoma, a rating system is used to describe and log shoot tip condition (Figure III). There are other systems in use, but they are all very similar in concept.

As discussed in Section A, a very good method for assessing vine water stress early in the season is to monitor shoot elongation rates using numerous marker shoots within a vineyard block. Shoot growth will slow down and then stop — usually before the shoot tip symptoms appear.

Other visual indicators include ten-drils, which droop when the vine is water-stressed. Tendrils are useful only before veraison, however, as they become tough and woody afterwards. Basal leaves (leaves dosest to the base of the shoots) will abscise (turn yellow, dry up, and fall off) when a vine is under water stress. This is undesirable, as leaves are needed to protect the fruit from the sun and for production of sug-ars for ripening. Hence, yellowing leaves are usually a sign of over-stressed vines, although a small amount of leaf abscis-sion — no more than one to two leaves per shoot — may be allowable if it occurs late in the season.

Leaf blade-petiole angles will change under water stress, as the vine

is programmed to avoid sun exposure at such times. This mechanism helps prevent overheating of the leaves. Under high water status (not stressed), the petiole makes a 90° angle with the leaf blade. The angle becomes smaller and the leaf folds slightly under high levels of water status. This is another indicator of an over-stressed condition — vines in this state are not very pro-ductive.

However, some varieties, such as Cabernet Sauvignon, do not express this symptom readily. Thus, it is not a very reliable indicator of water status. Actually, all of the visual symptoms of water stress are "lagging indicators" in that they express themselves after the vine has experienced some stress.

The fruit can be used as an indicator of water status although, again, fruit-stress symptoms are often an indica-tion of an over-stressed condition. Before veraison, fruit water-stress symptoms indude flaccid or shriveled berries in the afternoon. This is not nec-essarily an over-stressed symptom, though, if the berries regain turgidity in the evening. In fact, this condition can lead to small berries that will make a more highly structured red wine, if desired.

After veraison, puckering or shriv-eling berries are usually an over-stressed condition. During ripening, flaccid or shriveling berries will never recover their turgidity and can result in severe loss of both yield and wine quality.

However, it is important not to con-fuse shrivel due to water stress with shrivel due to other conditions. Shrivel due to water stress will be seen on fruit all over the vine, while fruit shrivel due to overexposure to sunlight (heat stress) will be most prevalent on the afternoon-sun side of the canopy.

Fruit shrivel due to bunch-stem necrosis will be accompanied by a dried out rachis, while water-stress shrivel will retain a green rachis. Since fruit shrivel during ripening is poten-tially catastrophic, it should be moni-tored but not used routinely as an irri-gation guidance tool. Late in the season, the pressure chamber is really the only reliable tool for vine water sta-tus feedback. ■

Section C — Irrigation Strategy will appear in May/June 2005 PWV.

The author wishes to thank the Gallo family for their tremendous support and the staff of Gallo of Sonoma Winery & Vineyards for their substantial contribu-tions. Special thanks to Jim O'Donnell for his invaluable efforts on the research pro-jects. Additionally, the content reviews from Stan Grant, Jeff Lyon, Steve Matthi-asson, and Kirk Grace are sincerely appre-ciated.

References 1. C. M. Burt and S. W. Styles. 1999. Drip

and Micro Irrigation for Trees, Vines, and Row Crops. ITRC, Cal Poly, San Luis Obispo, CA.

2. Irrigation Scheduling: A guide for Efficient On-Farm Water Management. D. A. Goldhamer and R. L. Snyder, Eds. 1989. University of California publication 21454.

3. Micro-irrigation of Trees and Vines. L. Schwankl, B. Hanson and T. Prichard, Eds. 1996. University of California publication 93-03.

4. Scheduling Irrigations: When and How Much Water to Apply. B. Hanson, L. Schwankl and A. Fulton, Eds. 1999. Uni-versity of California ANR publication 3396.

5. Snyder, R. L., B. J. Lanini, D. A. Shaw, and W. 0. Pruitt. 1989. Using reference evapo-transpiration (ETo) and crop coefficients to esti-mate crop evapotranspiration (ETc)for trees and vines. Cooperative Extension, Univ. California, Berkeley, CA, Leaflet No. 21428.

6. Williams, L. E. 2001. "Irrigation of Winegrapes in California." Practical Winery & Vineyard. Pp.42-55.

7. Monthly Station Normals of Tem-perature, Precipitation, and Heating and Cooling Degree Days 1971-2000. 04 Cali-fornia. National Oceanic and Atmospheric Administration. National Environmental Satellite Data and Information Service. National Climatic Data Center. Asheville, NC

8. Historical Climatology Series 6-4. Climates of the World. 1991. National Oceanic and Atmospheric Administration. National Environmental Satellite Data and Information Service. National Climatic Data Center. Asheville, NC.

Web Sites California Irrigation Management Infor-

mation System (CIMIS): www.cimis.water. ca.gov/

UC Davis IPM — California Weather Databases: www.ipm.ucdavis.edu/WEA THER/weatherl.html

ZOI

6

ININEGROWING JULY/AUGUST 1995 PWV

by Michael Porter

Vine stress problems encountered in North Coast, California vine-yards late in the season are frequently misdiagnosed. A mis-diagnosed problem cannot be

effectively treated. For example, you've made it through

budbreak, frost, bloom, and veraison. The crop looks good as you tour the vineyard a couple of weeks before har-vest, but then you notice some leaves turning yellow. Or maybe you have a problem with slow ripening or high pH fruit. Maybe the vines are suffering from "stress." But what's the real problem? What's causing the stress? What should you do?

Too often people assume the above symptoms (or others) of vine stress re-sult from water deficit and irrigate, but lack of water may not be the problem at all. With grapevines, as always in biol-ogy, it is useful to take in the big picture — to step back and recognize that any problems are part of a complex web of interaction of many diverse factors. The real cause of the symptoms may not be simple to find, and the solution might not be obvious, but once you've figured out the cause, the solution you apply is much less likely to compound your dif-ficulties!

Late season symptoms in North Coast vineyards often fall into three general groups: visual foliar "stress," slow rip-ening/delayed harvest, and high pH fruit/wine. These symptoms have sev-eral possible causes, but in each case, we have a tendency to blame one cause and treat for it. As a result, the true problem goes uncorrected, and the vines suffer the consequences of unnecessary —sometimes harmful — treatment.

Foliar stress symptoms Few commonly used terms in viticul-

ture are in greater need of clarification than "stress."' The word is most often used when some leaves turn prematurely yellow, usually after veraison (but some-times before). The possible causes of the symptom are many: insufficient soil moisture, excessive soil moisture promot-ing root pathogens, potassium deficiency, salt or boron toxicity, or phylloxera, to name a few. Too often they get lumped together as having one likely cause — soil moisture deficit stress.'

Dealing with late-season vine stress

The fact that many people assume that vine stress is synonymous with a soil moisture deficit may be due to the history of viticulture research and train-ing in California. In the San Joaquin and southern desert valleys, where most viti-culture research has been done, the most common kind of stress may well be moisture deficit. But in the North Coast, it is just one of many kinds of stress, and it's not the most common.

Therefore, irrigating in response to stress symptoms is appropriate to com-modity viticulture in the San Joaquin and southern desert valleys, but it should be considered carefully before being applied in cooler and higher-rain-fall regions, particularly when the goal is producing fine wine.

Misdiagnosis of the source of foliar-stress symptoms often leads to inappro-priate action. At best, the nostrum may be of psychological benefit to the grower, but only a bandaid for the vines. At worst, one can do significant harm. (Irrigating stress symptoms caused by root pathogens, such as phy-topthora, pythium, or verticillium, is like putting out a fire with gasoline!)

The cause of stress symptoms must be discovered in order to determine appro-

'For background and some detail on these issues, read the first three articles in this series in PWV:

"Soil Origin" in March/April '94, "Soil Fertility and Vine Nutrition" in May/June '94, and "Soil Moisture and Water Management" in March/April '95. Lucie Morton and Richard Smart are also rec-ommended reading.

priate action. This means confirming one (or more) basic cause(s) and ruling out others. Diagnosis should be based on observation and/or analysis (as op-posed to speculation). Untangling a knotty problem can be difficult.

Beware of the quick fix. Unfortu-nately, all too often, someone unfamiliar with a vineyard arrives on the scene and reaches a quick diagnosis based on su-perficial evidence. It is difficult to diag-nose stress symptoms based on one visit. How the stress pattern develops can be diagnostic of the cause. While vi-sual symptoms sometimes point to an obvious cause, just as often they are am-biguous, pointing to several possibili-ties. When in doubt, use analysis and observation.

Appropriate diagnoses To diagnose the cause of nutrient

problems, soil and tissue analysis are ex-cellent tools when combined with ob-serving foliar symptoms. However, neither lab results nor visual observa-tions are sufficiently reliable to be used alone. Labs can make mistakes, and their interpretive standards are typically based on Thompson seedless — which is particularly inappropriate with regard to potassium and magnesium for fine wine grapes in the North Coast.

Too little or too much soil moisture can be a problem, but looking at the soil surface provides minimal information. You must actually check soil moisture in the root zone when problems arise (if not sooner!). Monitoring moisture at different depths over time, a neutron probe provides excellent information. For spot checking, the minimal test re-quires a shovel, a backhoe or auger would be better (to determine the loca-tion of the root zone).

Diagnosing root pests and pathogens normally requires sampling roots on problem vines (with the possible excep-tion of young vines damaged by go-phers). In the North Coast, phylloxera is a common root pest that can often be di-agnosed this way.

Of the root pathogens, oak-root fun-gus can sometimes be diagnosed in the field. It produces a white fuzzy coating easily visible on the roots and a charac-teristic fresh mushroom odor. Where vine symptoms suggest possible infec-tion with phytopthora, pythium, or ver-ticillium, fresh samples are best delivered to aathologq lab for culture

DISTURBING SYMPTOMS IN YOUNG VINES AA t this time, some young vine

yards in the North Coast are ex- periencing severe decline and vine loss due to one or more unconfirmed causes. Foliar symptoms on these vines often resemble leaf-roll virus (LRV), except that shoot-stunting and vine death far exceed the patho-logical consequences normally asso-ciated with LRV. In addition, many dying vines test negative for LRV, and some are certified clean stock

The vineyard rumor mill is cur rently chugging along at full steam, claiming the new disease is a virus, fueled by arguments such as "It looks like virus, and we don't even know how many viruses there are beyond those few that we know how to test for." This debate will only be resolved through research (with in-ternational collaboration, I hope) to discover the real cause of this dis-ease, not through speculation.

Other possible infective agents and modes (see below) are being vigorously scrutinized, but none as yet, including the "latent virus" con-jecture, have satisfied the require-ments of Koch's postulates for con-firming what disease is affecting these vines: 1) isolation of pathogen from` rom diseased tissue, 2) character-ized in pure form, 3) introduced into a healthy individual, 4) that indi-vidual develops the disease, 5) the pathogen is recovered from the now-diseased, formerly-healthy indi-vidual. Until these criteria are met, no theory can be considered defini-tive. "Virus-like symptoms" and even the presence of the virus itself do not prove that it is causing the vine decline.

Other possible causes include se-vere potassium deficiency (on Clear Lake clay, for example), which can cause stunting and curled leaves. Also blockage of xylem, can lead to severe foliar potassium/leaf-roll symptoms, even where soil potas-sium is quite adequate.

One common symptom associated with decline of young vines is the

presence of dark spots in the xylem of the rootstock, which appear black vi-sually, but are orange or golden in thin section when back-lighted. Many of these vines show little or no callus-ing at one or more disbudded nodes and apparent "heart-rot" of the pith is present. These vines test positive for saprophytes (decomposers).

Don't simply assume that if your young vines are dying, this "new disease" is the culprit, however. Many young rootstocks fail the cri-teria for selecting wood for root-stock cuttings in A.J. Winkler's Gen-eral Viticulture (p. 199 [2nd edition]). Winkler warns against "very long internodes indicate very rapid growth; such canes are usually soft and poorly nourished, hence low in stored reserves (starches and sug-ars)," and says, "When the cane is cut, the inner bark should appear green and full of sap; the wood firm, well-stored with reserves, and free from dark specks. ... Canes that are unusually flat or angular in cross section should be avoided." The io-dine test for starch described in the footnote on the same page is highly recommended.

It is unclear, at this time, why these failing vines show poor callus-ing and/or xylem blockage. It may be that immature, poorly nourished cuttings are slow to callus and easily infected by saprophytes in moist soil in the North Coast. It may be that callus formation is being inhibited in some vines due to improper con-ditions at the nursery, or by infec-tion by virus, fungi, or bacteria.

It may be that we have wide-spread, but as yet unrecognized, diseases such as new and more viru-lent forms of eutypa, esca, myco-plasmas, botryosphaeria, dema-tophora, flavescence doree, botryo-diplodia, or others. The jury is still out, but until an infective agent is confirmed as the cause, I favor the simpler theory that thin, flat, imma-ture plant material is the principle culprit.

8

WINEGROWING JULY / AUGUST '995 PWV

and identification. Among above-ground pathogens, eutypa and Pierce's disease are often recognizable on-site. On the other hand, you will likely need help from the pathology lab to confirm botryodiplodia, and possibly for pho-mopsis on shoots and petioles.

Slow ripening/delayed harvest Factors affecting the rate of ripening

include crop and canopy management, LRV, weather, soil moisture, and mineral nutrition. Thinning the crop at veraison or later can help compensate for the prob-lem, but throwing away fruit has an eco-nomic down-side that is difficult to ignore. Diagnosing and correcting the source of the problem is generally prefer-able to dropping grapes on the ground.

Problems arising from poor canopy microclimate have been thoroughly dis-cussed by Richard Smart and need not be repeated here. The desert shade trel-lis (the three-wire T trellis designed to shade fruit from the burning desert sun, which fosters mildew and botrytis in the North Coast region) should not have been adopted as the "standard" in the North Coast. Its problems are exacer-bated by over-stimulation with irriga-tion and application of nitrogen, as well as the use of 'kicker' canes above the main fruit zone. Severe hedging and leaf pulling, however, can also inhibit ripen-ing by leaving too little leaf surface area in proportion to crop load.

Leaf-roll virus has long been known to inhibit normal ripening, and more re-cently mealy bugs have been identified as a vector. Fortunately one can test for LRV, and when it's confirmed, can re-move diseased vines.

Unfortunately, many people choose to assume LRV is present because of foliar symptoms, which can be misleading (see sidebar on young vines).

Other problems do look like LRV and can be tested for. (Testability is basic to scientific methods; untestability is more in the realm of religion. Those having faith in unconfirmable conjectures should recognize the thin ice on which they skate.)

Potassium deficiency The most common North Coast prob-

lem visually mistaken for LRV is potas-sium deficiency, with phosphorous deficiency a distant second. The prob-lem is often primary, resulting from low soil K and/or very high magnesium. We

220

he horse is here to stay,

but the automobile is only

a novelty -a fad. Or•sid•nt of Michigan

Savings Bank advising

Henry Ford's lawyer not

to inv•st In th• Ford

Motor Company.

[ 451, SUPREMECORQ

The new tradition

For information about the benefits of SupremeCorq, call 800 794 - 4160.

10

WINEGROWING

have been successful in rescuing some vineyards from the bulldozer by correct-ing the nutrient problem and watching the "virus" symptoms disappear.

K deficiency is also often secondary; that is, it is frequently caused by root damage (e.g., very wet soil, phylloxera, phytopthora) or xylem blockage (see sidebar on young vines). As before, fix-ing the problem requires that the cause be correctly diagnosed. There are no "standard" actions, no panaceas.

Potassium deficiency is often misdiag-nosed as soil moisture deficit. This often occurs where K deficiency has been wrongly ruled out — typically by a lab or advisor relying on criteria established for Thompson Seedless — or where no analysis has been done at all!

Those relying on desert commodity nutrient criteria routinely rely on desert commodity irrigation criteria, too, which can be summarized by "when in doubt, drown 'em out." (In fact, it would be just as inadvisable to adopt a prohibition on late irrigation.)

The decision to apply water late in the

season (or not to) should be made on a site-specific basis, based on analysis of available soil moisture, crop load, peti-ole K/Mg, the weather, wine style/mar-ket niche, and so on, and may change from year to year. In any case, curing your vines' K deficiency, if they are de-ficient will go a long way toward reduc-ing the "need" for late-season irrigation.

Weather Excessive soil moisture available to

vines late in the season also delays rip-ening. Inappropriate irrigation may be the problem yet again, but natural causes could be the problem, too. In some years, a sudden and dramatic cooling of the weather (often accompa-nied by cloud cover and some rain) leaves many North Coast grapes hang-ing on the vine, gaining sugar at a snail's pace. This typically happens some time in October and is a particular nuisance when it is early in the month (1994) or when most vineyards are late due to a cool, wet spring (1991).

In vineyards where this occurs nearly

every year, however, we typically find one or more other problems: high water table, poor canopy, over-irrigation, LRV, and so on. Where these other potential problems have been addressed, one can expect to be caught by the changing sea-sons less often and for the effects to be less severe.

High pH Much has been said and written about

the problems associated with high pH fruit and wine, somewhat less about how they come about and how to avoid the problem.

Unusually slow ripening (discussed above) can lead to high pH, as the nor-mal rise in pH and associated decline in titratable acidity (TA) are not balanced by a commensurate increase in sugars. Conversely, a too-rapid rise in pH, due to excessive accumulation of K ions in the fruit, can outpace the gain in sugars. (We should note that both can occur si-multaneously.) The latter subject de-serves closer examination and, perhaps, more research.

Many people are familiar with Roger Boulton's work on the subject, especially regarding the movement of K ions into the fruit and substitution for H ions, re-sulting in a drop in TA and rise in pH. Far fewer have heard that the study vines were subjected to severe moisture deficit — to the point of defoliation.

As the vines underwent premature se-nescence (leaves turned yellow and fell off), K was moving out of the leaves at an abnormal pre-harvest rate, with some of it ending up in the fruit. Many people have come to view K as the source of the problem, but I am con-vinced that K movement into the fruit is secondary to the premature senescence. Had the leaves stayed green and re-mained on the vines, far less K would have ended up in the grapes!

Consider for a moment Richard Smart's work with dense, shaded cano-pies. He observed that interior leaves re-ceiving too little light turned yellow and fell off — and in the process moved K back into the vine, some of which ended up in the fruit, resulting in undesirable high pH.

It would be incorrect to blame the problem on K, as it is merely a link in the shade/senescence/K mobilization/ high pH process, but it is not the origin of the problem. High fruit K and pH are effects — leaf senescence due to shading

JULY /AUGUST 1995 PWV

MOVING ??? Tell us...Write us: Pracfical Winery & Vineyard 15 Grande Paseo San Rafael, CA 94903-1534

Let us know so that you will continue to receive your subscription at your new address.

• • • • • • • • • • • •

• • • • • • • • • • • •

BOTTLING FILTRATION &

PROCESSING EQUIPMENT

KHS ORION FILTER

ARMBRUSTER

KHS FILLERS

• KHS/H&K - Bottling & Packaging Equipment • PRIORITY ONE - Palletizers/Conveyors/& Labellers • KHS - Filtration Equipment • OMESS - Pressure Leaf D.E. Filters • ARMBRUSTER - Stemmer/Crushers & Mash Pumps • BEER - Capsule Dispensers & Heat Tunnels

rty -u-- KHS MACHINES, INC.

1350 INDUSTRIAL AVE., SUITE G, PETALUMA, CA 94952

TELEPHONE: 707/763-4844 TELEFAX: 707/763-6997

SUBSIDARY OF KHS AG • BAD KREUZNACH, GERMANY • East Coast Representative: JUERGEN LOENHOLDT - R.D.1, ROUTE 14, Himrod, NY 14842 (607) 243-7568

PWV JULY/AUGUST 1995

11

or water stress is the cause! What the two above problems have in

common is premature senescence of leaves, leading to increased mobilization of K out of the leaves and into the fruit and, ultimately, to high pH fruit and wine. I propose that the problem is pre-mature leaf senescence and that K is guilty only by association! High wine K and pH are symptoms of a more com-plex process; K is not the culprit but rather one link in a web of interactions.

I have analyzed bloom and veraison petioles and observed foliar appearance, then compared them with fruit/must pH in many North Coast vineyards for a number of years. The results are quite intriguing.

Contrary to the simple model that high vine K causes high wine pH, we have ob-served little direct correlation between those two variables. In fact, the most problematic, high pH vineyards com-monly have moderate to low petiole K.

More significantly, these high pH vines usually have very low K/Mg ra-tios and show post-veraison K defi-ciency symptoms — including premature senescence! Though it sounds counter-intuitive, low vine K (relative to Mg) can result in high fruit K, hence high wine pH. In such a case, improving vine K/Mg status can help avoid premature leaf senescence, result-ing in a lower wine pH. (For more on K/Mg ratios, see PWV, May/June 1994.)

The eminent biologist Garrett Hardin long ago noted that, "Counter-intuitive solutions are common when we are dealing with biological problems." Of course, the best place to do field trials is in your own vineyard.

A word of caution If your vines show pre-harvest yellow-

ing due to moisture deficit (a la Roger Boulton) or dense shade (a la Richard Smart), increasing the vine K content could easily result in more K being moved into the fruit and higher wine pH. If your vines experience premature senes-cence for any reason other than low K/ Mg status, adding K could backfire.

If your vineyard has a low K/Mg status and high vigor on a San Joaquin "shade trellis," you should fix the de-ficiency and the canopy! Adding K and keeping the shade could make things worse — and serve to propa-gate the mistaken notion that "K is bad for wine pH." ■

Michael Porter has worked as a winemak ing assistant, and as a research assistant at Crocker Nuclear Lab, UC Davis while study-ing Physics at Cal State University-Chico, followed by a masters degree in Earth Sciences from Chico. He has taught geology, meteorol-ogy, oceanography, physics, and astronomy in junior colleges. Since 1984, he has worked with Bob Uttermohlen to provide soil fertility, vine nutrition, and water management con-sulting services.

121

WASH IN GTON STATE'S RONALD IRVINE WINEMAKING

WITH WALTER J. CLORE

H 'STORY - - - -

THE IRRIGATION EXPERIMENT PROOF IS IN THE WINEGLASS

-11■ClIt44, APRIL 1994— Four glasses of Sauvignon Blanc sat in front of me waiting to be tasted. Their strawlike yellow color twinkled under the glare of the over-head lights.

I was attending a two-day seminar entitled "Winemaldng in the Vineyard," put on by the Central Washington Wine Technical Group with support from Washington State University. Toward the middle of the remaining day, we were tasting wines presented to us by Dr. Bob Wample. The four wines had been made at Columbia Crest Winery by winemaker Doug Gore.

Wample jumped up on the stage with the enthusiasm of a thirty-year-old, although he was likely closing in on fifty. He was fit and tanned and slightly balding with brown hair brushed to the side. He wore stylish clothes with reading glasses hanging on his chest by a strap. His demeanor was more coach than scholar.

Wample was about to accomplish an amazing feat. He was going to distill all of the detailed information that had been laboriously pre-sented to us over the last day and a half into four glasses of wine. In taste, smell, and appearance, tile four glasses of wine would be a pow-erful metaphor for his ongoing study about the effects of irrigation on grape and wine production. It is a classic (and forceful) teaching tool.

Long after the seminar was over we would all remember tasting the four wines. They spoke volumes. They were the end result of a study begun in 1982 when Wample joined with Dr. Sara Spayd and Dr. Bob Evans to establish a complicated monitoring system, in a vineyard

located at the Prosser research center, to measure moisture use, establish water requirements of wine grapes at varying crop levels, and determine their effect on vine growth, yield, wine quality, and winter hardiness.

Wample had taken the study to the next level of refinement. With the help of Stimson Lane Vineyards and Estates and winemaker Doug Gore of Columbia Crest Winery, together they used a circular 64-acre Sauvignon Blanc vineyard near the Columbia Crest Winery overlook-ing the Columbia River on the lower Horse Heaven Hills slope. They divided the vineyard into quadrants. Within each quadrant, vineyard rows were treated with four varying levels of irrigation. Under this system, four different water treatments were replicated in four different locations. Wines wee then made from each of these different treat-ments, beginning in 1992. Gore made sixteen different wines in batches of 3,000 gallons each.

The water treatments were Low Low (LL), Low High (LH), High Low (HL), and High High (HH). LL referred to a low water treat-ment applied throughout the growing season. LH was low irrigation early in the growing season, followed by increased irrigation from the point where control of canopy growth had been achieved through har-vest. HL was high irrigation during the early season, followed by low irrigation from the point where the low irrigation treatments showed control of canopy development through harvest. This treat-ment was similar to standard irrigation practices. And finally, the HH treatment referred to high irrigation maintained throughout the growing season.

High and low levels of irrigation were defined as 2.2 and 1.2 inches of water per foot of soil, respectively, in the top three feet of the soil profile. Even at the high levels, not a lot of water was being added to the naturally occurring 7 to 8 inches of annual rainfall. Thus, in the high-level treatment, the soil received an additional 6.6 inches of water, or a total of less than 15 inches of water, including nature's addition. At low levels the total water, including treatment and rainfall, was less than 12 inches.

Wample provided an aerial photo of the vineyard on the overhead projector. It looked like a giant dartboard with concentric circles and lines. Originally named Circle 100, in 1993 the vineyard was renamed the Clore Vineyard; in honor of Dr. Clore. What a fitting tribute! The vineyard encapsulates all that Dr. Clore set out to accomplish so many years ago.

Wample also showed a number of graphs to highlight photosyn-

thesis of the vi ne during different times of the growing season. He told how different water-level treatments affected the vine growth.

His talk had three elements of immediate importance to Washington growers and winemakers. First, by regulating a low irriga-tion treatment early in the season, the grower can better manage vine-yard canopy, which reduces physical management strategies, decreases vineyard diseases and pests, and sets the vineyard up for better hardi-ness by limiting the growth of the wood on the vine. Second, the vine-yard uses much less water than the standard irrigation practice has suggested. And third, the results of the various treatments could be easily tasted by growers and winemakers.

We tasted the four Sauvignon Blancs from the 1993 harvest. The four wines were from only one vineyard quadrant. Wample asked par-ticipants to indicate which wines they liked best. A clear preference was shown for the wines using the LL and LH. water treatments, with roughly an equal split in votes. Wample smiled broadly; these were the same results he gets whenever he has people taste these wines.

I voted for the LH wine. It was fruitier in the aroma, highlighting melonlike fruit aromas with just a bit of the grassy character of this grape. On the palate the wine was rounder and fruitier, with a mouth-filling grapiness. The LL wine had a tarter, more austere finish. The HL and HH wines tended to be leafier and more austere, higher in acidity and more tart.

Literally, the proof was in the wine. It was there to taste. It was an exciting culmination of all the overwhelming data and input. The taste of the fruity, tart Sauvignon Blancs brought it to an immediate and powerful conclusion.

Clearly, Wample was excited. And the results of the work were exciting. The wine growers, faced with another year of drought, could use his study to better manage limited water resources by using less water at the beginning of the season and slightly more at the end to help the vine survive potential cold weather conditions. Beyond the drought conditions loom even greater water-use issues such as draw-downs of the Columbia and Snake Rivers, and more restricted water usages in the future are going to make Wample's work much more elisimportant.

As water becomes limited, only those crops that use less water and are of higher value will be allowed to use the available water supply. Clearly, we were tasting the future of wine grapes: it was bright and fresh.

The results of this promising study showed that the Columbia

Valley is one area in the world where growth, production, and quality can be manipulated by the grower to produce distinctive, quality wines.

■

sa MARCH/APRIL 1991 PWV GRAPEGROWING

techniques for vineyard water management •

By Charles Krauter, Keith Striegler California State University, Fresno

Irrigation is one of the most important techniques in viticulture. Applying the right amount of water at the right time is vital to the yield and quality of the grapes. Because water is increasingly scarce and expensive, improving the efficiency of its application is becoming an important economic factor. There are three basic methods of man-

aging vineyard irrigation. Each has ad-vantages and disadvantages, but each is best applied to a particular situation. The first and oldest method is soil mois-

ture monitoring. If used properly, instru- I ments such as tensiometers, gypsum blocks, and neutron probes can provide an accurate measurement of the amount of water available in the vine's root zone. A second method is direct measurement

of plant water stress. This is the newest and potentially the most valuable if it can be developed for commercial use. The third method -is water budgeting,

often called Et scheduling. Water budget-ing jccertainly the easiest, but it is also the least direct. It can estimate both the timing and amount of irrigation but one of the other methods usually will be re-quired for verification.

Soil moisture monitoring The use of instruments to monitor soil

moisture is similar to checking a fuel gauge in an automobile. They provide a quick, easily measured indication of the amount of water remaining in the root zone of the vines. Devices like tensiometers and conduc-

tivity blocks are inexpensive and reliable if used properly. They respond to the same force of attraction between water and soil particles that the root must over-come to take up the water. Consequently, they are best used for deciding when that force has become too high for suffi-cient uptake and an irrigation is needed. These instruments are not as useful for measuring the amount of water that needs to be applied.

A tensiometer indicates when the sup-ply of water is almost gone. However, some experimentation may be necessary to determine the amount of water re-quired to fill up the root zone. Conductivity devices such as gypsum

blocks respond to the same soil moisture conditions as tensiometers but they are usually not as accurate in very moist soil. A tensiometer can only read soil mois-ture when the root zone is fairly moist. It goes off-scale and fails before the soil moisture is completely used up. The range of soil moisture at which the con-ductivity block is most accurate is at about the point where the tensiometer quits. Most grape vines will begin to stress

when the soil water content is still in the tensiometer's range of measurement, so they are a good choice for viticultural ap-plications. Gypsum blocks are more often used for field crops or where the manager wishes to dry the soil to a point beyond the range of a tensiometer. Some new conductivity blocks have a better range of sensitivity and since blocks are less vulnerable to damage during cul-tural practices and freezes, they may be an ideal choice for vineyard moisture monitoring.

The neutron probe is quite different in _ that it very accurately measures the amount of soil moisture rather than the strength of the attraction holding the water in the soil. The probe can indicate precisely how much water is in the soil, but not the point at which the soil mois-ture is low enough to cause water stress in the vines. Different soils will hold dif-ferent amounts of water. The user of a neutron probe must cali-

brate the instrument for a particular soil and root zone depth for the readings to be of maximum value. Once that calibra-tion has been done for the soil and crop, the neutron probe will provide the best data for irrigation management if maxi-mum accuracy is needed. Tensiometers and gypsum blocks, however, have been successfully used for decades and will often provide adequate information at a substantially rower cost.

Monitoring plant water stress A device that measures vine water stress

is potentially the best method of water management, since that is the element that irrigation is supposed to control. Water stress will occur at different soil ' water contents, or evapotranspiration rates, so management methods that rely on these factors may at times be in error. When a vine is water-stressed, its growth

rate will be reduced, and that will often decrease yield. There may be benefits to Mildly stressing the vine at some growth stages but those benefits can only be consistejttly achieved if the stress level can be measured and the irrigations pre-cisely controlled. Unfortunately, most of the devices that

detect water stress in the vine are expen-sive, delicate research instruments, such as the porometer and the pressure cham-ber, that may only work under certain conditions. They subject the leaf to me-chanical sensors in order to measure changes in leaf water potential. The pres-sure chamber is used for management of some field crops but no method has been developed for its use in commercial viticulture. In the past several years, however, a

technique using an infrared thermom-eter to measure leaf temperature has been tried with some success. The tech-nique is promising and research is cur- rently in progress to develop it for use in commercial viticulture. Unstressed vines transpire a great deal

from .their leaves in hot weather and therefore reduce their temperature sig-nificantly by evaporative cooling. Stressed vines cannot evaporate as much water, so their leaves will not be cooled as much. It is possible to predict the temperature

of an unstressed leaf from measurements of the air temperature and humidity. The infrared thermometer can take the actual temperature of a large number of vine leaves in a short time and compare it to the predicted leaf temperature. The ratio of the predicted, unstressed

leaf temperature to the actual measured leaf temperature can be expressed as a number called the Crop Water Stress In-dex (CWSI). If the leaves are act ally as za

Water Budgeting-- The third basic method of water man-

agement is water budgeting, often called 'Et scheduling'. It is essentially an appli-cation of accounting with water instead of money. The manager tracks irrigations, water stored in the root zone, and evapo-transpiration just as an accountant tracks income, bank balance, and expenses. Et scheduling is best done on a small

computer with programs very much like the accounting systems available for PC's. The manager estimates the amount of water in the root zone of the vine and then deducts the amount used each day by evapotranspiration. When the soil water has been depleted to the point where an irrigation is needed, the amount missing from the soil is applied in an ir-rigation and the root zone account bal-ance is replenished.

The advantage of this method is speed and adaptability to computerization once the system is set up with the initial infor-mation and estimates. The major prob-lem with the method is acquiring accur-ate initial information and making pre-cise estimates of the water available to the vines and daily evapotranspiration. The total amount of water in the root zone is not difficult to estimate, but the portion of that amount that the vine can use without being stressed is not so easy to calculate. Usually, only 25% to 50% of the water

in the root zone is readily available, and an irrigation should take place when that amount has been taken up by the vine. One of the more difficult decisions to be made is the selection of this 'allowable depletion' of the vine root zone. The other difficult estimation the manager must make is deciding just how much water the vine used each day, i.e., the evapotranspiration or Et. The Et of the vines depends on both the

weather conditions for that day and the growth characteristics of the particular vineyard. Temperature, humidity, wind, and sunlight can be used in a formula to calculate a generic value called the ref-erence Et. This process is complex and expensive

for an individual grower, but there is a _ network of weather stations operated by the state of California called alvIIS. These stations are located in all major growing areas and can be used to obtain a refer-ence Et for that region. (The Et informa-tion may also be available from private or public sources in other states, or in re-gions in California not served by CIMIS. Data from these other sources may have to bet onverted to the proper Et form by the user.)

cool as the predicted value, the CWSI will be 0 for the unstressed vines. If it is stressed, the CWSI will be more than 0; the higher the level of stress, the higher the number. The manager must know the vineyard

well enough to be able to determine the reason for a stress detected by the CWSI method. Insect damage and disease can result in plant stress that irrigation can-not relieve. When the problem is actually water stress, it may not be controllable by better irrigation management. For example, on a hot or windy day, a

vine may be stressed to some degree even though the soil is filled with water. Extreme atmospheric conditions can draw more water out of the leaves than the root system is capable of supplying. Since the CWSI should be measured in

the middle of the day, this unavoidable stress may often be apparent and the manager will have to be familiar enough with the phenomenon to know that it cannot be prevented. Exceptionally cool temperatures or fog

can reduce the transpiration rate of the vines to the point where the plant can get sufficient water from a relatively dry soil for those conditions. No stress will be measurable on such a day, but the manager must be aware that when normal temperatures return, the vines may sud-denly be short of water. The measurement of leaf temperature

with an infrared thermometer can easily be in error unless -the instrument is care-fully aimed to read only sunlit leaves. If surfaces such as the sky, soil, or wood are included in the field of view of the sensor, large errors can result. Correct technique can provide accurate readings, allowing the CWSI to be properly calculated. The advantage of the method is its ability

to quickly measure conditions over a large area. It is the only device than can provide the manager with sufficient, real time, water stress information to allow daily adjustments in the irrigation schedule. If an intentional or programmed stress

level is desired, a drip system can be used to vary the application by reducing the irrigation when the CWSI is too low and increasing it when the CWSI exceeds the desired level. This is the only way that changes in water stress due to changes in growth stage or evapotranspiration can be taken into account. Direct measurement of plant water

stress is, at the moment, the most dif-ficult method, but it could become the most useful once proper techniques and management interpretations have been established.

The Et from CIMIS must then be cor-rected for the particular growth charac-teristics of a vineyard to get the daily water use of the vines being managed. This daily crop Et is then deducted from the estimated water supply in the root zone. When an irrigation is called for, the amount to be applied is the total of the daily Et's since the last irrigation. This process can be used in its simplest

form to make rough estimates of the ir-rigation schedule, or it can use carefully measured inputs to make the estimates more precise. Extra effort in correcting the reference Et from the weather station with crop and stress coefficients is the best way to increase the accuracy of the water budget. Conclusion The question of which of these three

basic methods is best for vineyard water management has no simple answer. They can each be used effectively once the manager has properly set them up and become experienced in their operation. None of them can deliver precise irriga-tion schedules immediately; they all re-quire some fine-tuning, for a season or more, to reach maximum utility. One generalization that can be made is

that the best management often combines two of the methods. Many professional consultants use the water budget to make estimates of the irrigation schedule and then use soil moisture instruments to regularly check the accuracy of the Et program. The computer-based Et schedule is easy

to generate and the soil instruments need only to be read once or twice a week to verify it. As this plant stress measure-ment method becomes more established, it could replace soil moisture monitoring in the hierachy of water management methods. ■

OSA fin59

9/7 2

DRIP IRRIGATION The authors are Albert W. Marsh, Extension Irrigation and Soils Specialist; Roy L. Branson,

Extension Soils and Water Specialist, Riverside; C. Don Gustafson, Farm Advisor, San Diego

County; and Sterling Davis, Agricultural Engineer, ARS, USDA, Riverside

What is drip irrigation

Drip irrigation is the frequent slow application

of water to soil through mechanical devices

called emitters located at selected points along

water delivery lines. The application rate must

be slow enough so that the flow of water across

the soil surface is limited, and runoff in the

usual sense does not occur; most of the move-

ment of water to wet the soil between emitters

occurs by capillarity beneath the soil surface.

The volume of soil wetted in an orchard or field

is usually much less than that wetted by other

methods of irrigation, varying from 10% for newly

planted permanent crops to as much as 50% for

some mature crops. The amount of soil wetted

depends on soil characteristics and the number

of emitters. The number of emitters used ranges

from less than one per plant for row crops, to

eight or more for large trees.

How is it done

The emitter, the key element in a system, pro-

vides the slow flow needed. Most emitters are

manufactured to provide a fixed rate of output

with a specified water pressure. A few have

adjustable rates. Rates available usually range

from 1/, to 2 gallons per hour (gph). One gph is

the most common.

Most emitters provide a point source of water,

usually placed directly on the soil surface; some

are buried at shallow depth. Others provide a

line source of water from a tube having perfo-

rated or porous walls. This type is usually

buried.

The emitters are connected to or are part of a

small diameter ( 3/s to 3/4 inch) plastic lateral line.

The laterals generally lie on the soil surface

but may be buried at shallow depth for protection.

They connect to a buried plastic main line that

receives water from a head. The head is the

control station for the system, where water is

measured, filtered, treated with fertilizer in

solution, and regulated as to pressure and timing of application.

Why the great interest

Drip irrigation has created wide interest in a

short time in spite of a small amount of experi-

ence, information, and development. The main

factor behind this interest has been the potential

of drip irrigation to reduce operating costs. Early

reports indicated that a grower could irrigate his

crop with less than half the usual water con-

sumption and obtain greater yields. Labor costs

for irrigating could also be cut, since water

applied by drip irrigation presumably does not

have to be tended, but merely regulated by turn-

ing valves. Many growers, faced with scarcity

and high prices for both water and labor, imme-

diately embraced the prospect that drip irrigation

would be financially beneficial. Drip irrigation

systems also offer the opportunity to inject

fertilizers into the irrigation water, avoiding

labor requirement for ground application.

Other advantages

• Since drip rates are slow, main line and

lateral line sizes can be smaller than those

for sprinkler or surface irrigation.

• Because much of the surface soil never be-

comes wet, orchard operations are not inter-

rupted. Weed growth is reduced so control

efforts are reduced.

• With row crops on beds, the furrows in which

pickers walk remain relatively dr y and provide

firm footing.

• Frequent irrigations maintain a soi I moi sture

condition that does not fluctuate between wet

and dry extremes and keeps most of the soil

well aerated.

The University of California's Agricultural Extension Programs are available to all, without regard to race, color, or national origin.

ONE-SHEET ANSWERS Ul AGRICULTURAL EXTENSION UNIVERSITY OF CALIFORNIA

a-operative Exten s ion work i n Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriultur• 1,0

aerating. Distributed In furtherance of the Acts of Congress of May 8, and June 30, 1914. George B. Alcorn, Director, California Agricultural Extension Servico.

• Avoidance of intermittent drying permits use of more saline water than can be used with other methods of irrigation.

Potential problems

Because the emitter outlets through which water

must flow are very small, they can become

plugged by particles of mineral or organic matter

carried in the water. Dissolved salts can also

crystallize around the external perimeter of the

orifices reducing the size of opening. These re-strictions may reduce the emission rate, upset the uniformity of distribution of irrigation water,

and cause plant damage before plugging is de-

tected.

Most emitters operate at low pressures, 3 to 20

psi. If the field slopes steeply the nozzle pres-

sure and discharge during irrigation may differ

up to 50% from that intended, and the lines drain

through lower emitters after shut off. Some plants

would receive too much, others too little water.

Some soils may not have sufficient infiltration

capacity to absorb water at the usual discharge rate without runoff or undersirable ponding. At one gph discharge, the soil must have in infiltra-

tion capacity of 0.5 inch per hour to keep the

pool of free water around the emitter from ex-

ceeding 2 feet in diameter. Sandy soils are prob-

ably best adapted to drip irrigation, especially

those with slight horizontal stratification. Such

stratification is beneficial for drip irrigation

because it promotes lateral water movement and

wets a greater volume of soil. Experience has

shown that medium textured soils usually per-

form well, but some fine textured soils have

created problems.

Salts tend to concentrate at the soil surface and

also at the perimeter of the soil volume wetted

by each emitter. Since light rains will leach

surface accumulated salts into the root zone,

irrigations should continue on schedule until

substantial rain has fallen. Drying of the soil

between irrigations may cause a reverse move-

ment of soil water resulting in the transfer of

salt from the perimeter back toward the emitter.

Kodents are known to chew polyethylene laterals.

Rodent control or use of PVC laterals are pos-

sible solutions.

Operational requirements

• Cleaning the water is essential. Most failures

observed are caused by inadequate filtering. Multiple screens as fine as 200 mesh or sand filters should be used.

• Irrigations must be frequent and light.

• Duration of each irrigation must not be too

lengthy, helping avoid local excessive wet-

ness in soils lacking rapid permeability, and minimizing algae growth in laterals.

• Amount of water applied should be based on

measured or carefully observed soil-water conditions.

• Though water emission rates are small, lateral

lines must be designed with adequate capac-

ities to carry the maximum expected flow rate with little or no loss of pressure. Since lateral

lines are generally of uniformly small diam-

eter, their length should not exceed 330 to

400 feet. Additional main lines are preferable

to excessive lateral lengths.

• If flushing type emitters are used, lateral

lines must be designed to carry the larger initial flow rate needed for flushing.

Water movement must always be away from the

emitter to avoid salt damage.

The foraging ability of the roots for nutrients

and water is limited to the small volume of soil

wetted. Should uncontrolled events cause sus-

pension of water and fertilizer supplies, damage

to the cropcould occur rather qUickly.

0 xversity of California • s Agricultural Extension Programs are available to all, without regard to race, color, religion, sex, or national origi . __

• Pressure regulators are necessary on sloping

land. The laterals should be laid as level as possible.

• Phosphate fertilizers should not be applied

through a drip irrigation system; phosphate

reacts with calcium in irrigation waters, form-

ing a precipitate that can clog emitters.

FACTORS TO BE CONSIDERED IN THE SELECTION & APPLICATION OF IRRIGATION FILTERS

1. Introduction to irrigation filtration. The function of an irrigation filter is to remove suspended solids from irrigation water; to make th'at water suitable for low volume drip-trickle or sprinkler system application.

The nature or type of solids or particulate in the irrigation source water is a determinate factor in selecting the type of irrigation filter to use. Particulate or suspended solids in irrigation water consists of organic and inorganic matter. These are considered physical contaminants and are easily handled with proper filtration. The particulate would be items like algae, weed seed, snails, moss, etc. an certain forms of bacteria, i.e. anything that is or was alive. The inorganic matter would be the range of sand, soil, silt particles, minerals and some chemical contaminates that appear in a solid mineral form.

In addition to the organic and inorganic suspended solids, some irrigation water also contains (in solution) chemical and/or biological properties that through the injection of fertilizer or other chemicals can be converted to suspended solids. These are usually referred to as precipitates or precipitated matter. If these are precipitated prior to filtration, they can be effectively handled.

The selection of the proper irrigation filter type and size is dependent on several important factors:

a) The type, size and concentration of contamination in the irrigation source water.

b) The quality requirement for the filtered water. c) The flow rate or peak water requirement. d) An initial investment cost analysis.‘

2. Definition of filter equipment and recommended application. True filtration equipment is classed into two basic types.

a) Sand media filters. b) Screen filters.

In addition, centrifugal sand separators are available for certain limited applications in removal of heavy or larger inorganic particles from water, however, they are not effective in removal of organic or fine inorganic particulate. Sand separators are not applicable to low pressure systems since the pressure loss through a separator is very high.

A. Sand Media Filters: These filters are ideally suited for filtering water with either organic or inorganic particulate. Sand media filters are the proper selection for filtering source water that is heavy with organic matter. Sand media filters have the ability to entrap and hold large quantities of contaminate.

Figure A

- OUTLET -

THE FILTERING PROCESS

BACK FLUSH

THE BACKWASH PROCESS

This type of filter uses a bed of sand as the filter media. The size and type of sand can be specified to achieve the desired water quality. These filters are cleaned by reversing the water flow through the bed, expanding the media allowing it to release and eliminate the trapped particulate out through the backwash line.

The diagram in fig. A shows the flow and backwash action in a vertical tank filter system. Horizontal type sand media filters employ the same basic principles, however on high rate filtration applications they are not as effective as vertical filtration units. Horizontal filtration is generally recommended for gravity flow applications with flow rates of 10GPM ft2 or less.

One of the most important functions of the sand media filter is its ability to properly backwash and clean the particulate from the sand bed. Various methods for water distribution are used to accomplish this backwash function, i.e. slotted pvc lateral pipes, wedge wire tube distribution, etc. These methods all use a loose gravel pack around the water distribution system to support the sand bed. During backwash and while the sand is being expanded for cleaning, the loose gravel is also dislodged and a sand-gravel mixing occurs. After backwash some sand is co-mingled with the gravel and will pass out through the system or will wedge against or in the slots of the water distribution tubes causing a blockage.

The unique water distribution system used in the Free Flow filter's ceramic sand bed support structure, provides the most effective backwash of any filter in the industry.

This support structure eliminates the use of loose gravel pack and provides a barrier so that sand does not pass through the system.

7:1

OUTLET

=M.! •■••••• -am

fiE[THE 11-",-=1

INLET OUTLET

The Free Flow steel, epoxy coated screen filter employs a vortex-centrifugal water flow to separate the heavier particulate and collect this material in a bottom reservoir where it can be periodically flushed away. This filter further provides fine filtration through a screen mesh. This screen filter is extremely effective for filtering water that is heavy to inorganic particulate.

Although a variety of screen filters are available in a multitude of shapes, sizes and materials, the most popular irrigation screen filter is the Free Flow Thru Flush screen filter.

These stainless'steel filters employ a polymeric screen mesh (available in 30 to 200 mesh) for fine filtration or may be used with a stainless steel perforated screen for sprinkler system filtration.

This line of filters is quickly cleaned by opening the Thru Flush valve to allow the system water to wash the accumulated particulate from the screen and out through the Thru Flush port. The screen does not have to be removed for cleaning as with most other screen filters.

The easy field replaceable screen mesh allows a change in the mesh size if different filtered water quality is desired.

VALVE

77°

Prog]nl

INLET

Filtration Mode Flow enters the barrel, passes through the screen, depositing debris upon the inside of the supported mesh.

VALVE

THRU FLUSH

Thru Flush Mode The opening of the Thru Flush valve creates a high velocity flow along the inner walls of the mesh screen and out the Thru Flush port, affording maximum cleaning action.

Usually screen material is made of stainless steel or a polymeric mesh. While stainless steel screens are stronger the flexibility of a polymeric screen aids in the cleaning process.

23l

Sand media filters are generally employed in multiple tank sets that provide for filtered water backwash. This greatly reduces the chance for field emitter, tube and line contamination.

Backwashing the filters is the process by which clean water flows upward through the bed, lifting and expanding the media, allowing it to release the collected contaminate. The contaminate is then carried away with the backwash water. Excessive backwash flow rates will expand the media to the point that the media itself is expelled out of the tank. Insufficient backwash flow will not expand the media enough to purge all the entrapped contaminate. This could result in a residual pressure loss through the bed, even after backwash. To achieve maximum filter performance, the back-wash flow must be properly adjusted.

Although the amount of water required to backwash the filter bed is small compared to the amount of water filtered, it is discharged at a high rate for a short period. Provisions should be made to drain away, store, or otherwise dispose of the dirty backwash water. The backwash line should not be connected to a transfer pressure line and should be discharged to atmosphere.

The G.P.M. Flow capability through any sand media filter is determined by the square foot media surface area times the given flow rate.

Example: a 36" diameter set of 2 filter tanks has a square foot media surface area of 14.2 square feet. With a given flow rate of 25 GPM per square foot, the 36-2 filter can be operated at 355 GPM (14.2 ft.2 x 25 GPM flow rate = 355 GPM). A maximum flow rate of 25 GPM is recommended for the average irrigation source water, however, in cleaner than average water, rates up to 30 GPM have been successfully applied. (A subsequent section on filter sizing should be referred to prior to filter size determination). The same factors are applicable to other size filters from a 88 GPM (18"-2) filter system through the 1800 GPM (48"-6) filter system. It should be emphasized here that the minimum water available to the filters must be a quantity adequate to backflush the filters.

B. Screen Filters: This type of filter is recommended primarily for filtering water with inorganic particulate. The screen filter is not effective with water containing significant amounts of organic contamination. Unlike sand filters, screens do not have the ability to trap and hold large amounts of organic particulate without restricting the flow through the filters.

There are many varieties and design of screen filtration available. Bar screens employ a flat screen surface the water passes over and the particulate is caught in the screen and occasionally flushed off or the screen removed for cleaning. These type of screens are usually employed in gravity filtration applications. Continual cleaning screens have a pressure water flow constantly passing back through the screen to flush away the particulate from the screen, although effective, these are very water wasteful and have mechanical parts that can and do create maintenance problems.

13

It should be again emphasized that screen filters are most effective on inorganic contaminant. Large amounts of organic particulate tend to quickly block the screen, requiring frequent flushing or cleaning.

3. Proper Sizing of Irrigation Filters: All filters are designed and rated to provide specific maximum rated GPM flow of filtered water with average source water conditions. However, if the condition of the source water has a heavier than normal amount of particulate, additional filter surface area will be required to provide the same quantity and quality of filtered water. Thus, a larger filtering system should be specified to provide extra filtration capability in heavily contaminated water. Seasonal water quality fluctuation should also be considered when sizing irrigation filtration.

Example: As previously discussed, a 36"-2 tank sand media filter is rated with average water conditions at 355 GPM. If the source water condition is or could become heavier in particulate, it would be advisable to select a larger size filter to provide more filter surface area. In this example a 48"-2 would be selected and would provide quality filtered water without excessive backwash. (This is the same as reducing the flow rate per square foot of filter surface area.) The same principle holds true in the sizing of a screen filter. If a recommended flow rate for a filter is 400 GPM and the requirements of filtered water is 400 GPM and the water is heavy to particulate, then a larger screen filter system should be selected.

Most quality filtration products are designed so that if the filtered water requirements increase at a later date, additional units can be added to the filter system to increase the filtered water capability. A 48"-2 sand media filter system can be easily increased to a 48"-3 tank system; a single 400 GPM screen filter can become a two screen system, etc.

4. Sand or Mesh Media Size Selection: The filtered water quality is achieved by the filter media used. Too coarse screen or sand size will lead to clogging or plugging of the irrigation system and too fine filtration media will cause unnecessary, rapid and excessive cleaning of the filters. The type or size of the sprinkler heads, emitter nozzles or drip tubing used in the irrigation system should be determined and filter media sized for that application. Most quality filters are designed so that the media size can be changed, if at a later date, a change in the irrigation system would require a different filtered water quality.

The chart in fig. C shows the relationship between sand sizes and the equivalent screen mesh sizes.

Screens are classified by the size opening and the opening is defined by a mesh number. The mesh number refers to the number of openings per inch. The relationship between sand size, pore size, mesh number size are also shown in fig. C.

233

Filter sand is classified by two factors - mean effective size and uniformity coefficient. The effective size is the measure of the minimum sand size in that grade while uniformity coeffecient is the range of sand sizes within that grade. A more complete definition of these terms is given in fig. C.

Fig. C. Sand size, pore diameter, v/s screen mesh.

Sand Effective Approximate U.S. Designation Sand Size Pore Diameter Screen Mesh

Number inches mm inches mm Designation

8 .059 1.50 .0084 .214 70

11 .031 .78 .0044 .111 140

20 .018 .46 .0026 .066 230

Definitions: (Courtesy American Water Works Association - Standards for Filtering Material).

Effective Sand Size: That size opening that will just pass 10% of a rep-resentative sample of sand. An effective size of .78 mm means that 10% of the sample is finer than .78 mm.

Uniformity Coefficient: A ratio of the size opening that will just pass 60% of a representative sample of sand divided by that opening that will just pass 10% of the same sample. (A uniformity coeffecient close to 1.5 is considered good for irrigation filter sand grades.)

Sharp, crushed media is recommended for sand filter use. Two basic materials are used -sharp, crushed silica sand or sharp, crushed granite.

5. Manual or Automatic Operation: Most irrigation sand media and screen filters are available for either manual or automatic operation.

The manual flushing operation is activated by hand opening a flush valve or removing the screen and cleaning when a specified pressure differential is indicated on the pressure gauges. Regularly scheduled backwash or screen cleaning is necessary to assure proper operation.

Free Flow automatic systems provide for unattended flushing of the filters on a pre-scheduled time interval basis, and in addition include an automatic pressure differential override safety circuit. One of the advantages of an automatic system is that the filters are assured of being cleaned on a scheduled basis. However, should the source water quality vary and pressure differential develop prior to the scheduled flushing time, the pressure differential circuit will activate a flushing cycle.

The initial investment in automation can be compared to the repetitive labor cost of manual flushing, with additional consideration given fox the economical operation and safety that is provided by automation.

234

Most new filter controllers for automatic filter systems are designed with digital solid state components and are available for either 110 volt A.C. electrical input or for operation with a 12 volt D.C. power source. In addition, solar power collector units are available as an alternative power source.

Summary:

The selection and application of the proper filtration equipment is vital to the success of a modern irrigation system.

John Ribble of the University of California sums it up in his paper on filtration where he states in the final paragraph:

"The goal in selecting a filter is to achieve the necessary filtration and maximize efficiency of operation while minimizing cost, maintenance time, labor and operator inconvenience. To reach this goal certain guidelines must be established:

a. The size of the irrigation system (flow rate in gallons per minute, pressure, and volume of water required) — the capacity of the filter should exceed the demand of the system.

b. The physical, chemical and biological quality of the irrigation water to be used. The size and quantity of suspended solids to be removed; probabilities of chemical and/or biological clogging; stability of water quality with time.

c. The complexity of the- filter unit — what problems would be involved with cleaning or replacing the filter.

d. Availability of labor for cleaning and maintenance — for larger systems automatic flushing is generally used.

e. Location of the filter unit and disposition of the backwash and rinse water.

f. Flexibility of the filtration system — capability of enlargement or modification if it becomes desirable."

rardney WATER MANAGEMENT PRODUCTS DIVISION P.O. BOX 1566, CORONA, CA 91720 PHONE (714) 687-3901

235