Vineyard water management can be the most influential cultural practice that determines grapevine vigour, yield, fruit quality and general vine health. It is also a practice that, to be effective, requires tailoring to suit site conditions (soil type and depth, terrain and climate), current vine size (canopy leaf area, rooting depth) and vine spacing and variety and to achieve other management goals, including cover crop maintenance, frost protection and disease and pest control.
For most crops, irrigation principles are aimed at attaining maximum crop growth and yield while minimizing water waste. These principles guide the selection of irrigation equipment and the placement and amount of water applied in order to closely match water delivery to uptake from the root zone.
In vineyards, irrigation principles differ because water management is used, much like pruning and training, to manipulate vegetative growth, berry development and fruit exposure so as to achieve high fruit quality for winemaking. These manipulations often rely on the creation of mild to moderate water stress after fruit set.
The divergent goals of attaining maximum fruit quality and maintaining vine growth, development and productivity (yield) can be difficult to achieve. The greater importance of fruit quality over yield may emphasize the desire to create stress. However, growers should be aware that deficit irrigation methods intended to improve fruit quality can result in excess vine stress, poor fruit quality, damaged or abscised leaves and weakened vines that are susceptible to winter injury and perform poorly in the following growing season.
Irrigation management should be based on local experience and with a goal to balance vine health with vigour control.
Water management for table grapes should promote production of large clusters with large berries. Water stress should be avoided though the growing season, and deficit irrigation techniques should not be used. Growers should frequently monitor vineyards for water stress symptoms and increase irrigation frequencies if stress is detected.
If the water supply to the vineyard is less than vineyard requirements during critical periods, an on-site reservoir may be needed. If the total water supply is limited or costly, a deficit irrigation technique should be considered.
Irrigation water should be analyzed if there is any doubt about the alkalinity or salinity of the water.
Vineyard water requirements over the growing season are estimated to be 25-75 cm, depending mainly on vine canopy size and climatic conditions, which determine total transpiration. The amount of water transpired by the cover crop and evaporated from the soil surface can increase water requirements substantially.
Water uptake during different periods of the growing season is generally related to crop transpiration during that period:
Rainy periods that provide extended cloud cover and raise air humidity will reduce evapo-transpiration and the rate of soil dry-down. Rainfall that penetrates to the root zone will offset irrigation requirements.
Soil type and condition, particularly soil texture and bulk density, affect the water holding capacity and pressure with which water is held by the soil.
The field capacity of a soil is the maximum water content the soil can retain when allowed to drain freely. Soil water holding capacity is usually expressed as the percent moisture at field capacity. Storage capacity is the difference between field capacity and the low limit of plant-available water, known as the permanent wilting point.
The difference between the soil water content and the permanent wilting point is considered to be the plant-available water. However, a vineyard can suffer from water stress at soil moisture levels significantly higher than the permanent wilting point.
Finer textured soils, containing silt and/or clay, have larger storage capacities and therefore require larger water applications than sandy or gravely, coarse textured soils to wet a given soil volume to field capacity. Soils that are shallow, above a shallow water table or with coarse subsoil require less water to wet the root zone to field capacity.
| Coarse | Medium | Fine | |
|---|---|---|---|
| Permanent wilting point (%) | 3 | 6 | 14 |
| Field capacity (%) | 7 | 16 | 28 |
| Storage capacity (%) | 4 | 10 | 14 |
| Storage capacity per metre (mm) | 100-300 | 145-210 | 200 |
| Storage capacity per foot (in) | 1.0-1.3 | 1.5-2.2 | 2.2 |
The amount of water applied during irrigation should be sufficient to wet the targeted soil volume — usually the root zone, unless partial root zone drying is implemented — to field capacity.
If all irrigations are applied when the soil in the root zone reaches the same threshold water content, the amount of water required per application will always be the same. However, if regulated deficit irrigation (see below) is implemented, the threshold soil water content is varied with phenological stage and this changes the amount of water required to thoroughly wet the root zone.
A problem encountered in many vineyards is variable soil texture. Under such conditions, irrigating as appropriate for the coarsest textured soil in the vineyard block will be adequate for the finer textured soils that will likely have a larger moisture reserve. However, such a situation may lead to variable vigour within the vineyard, often due not only to differences in moisture availability but to associated differences in nutrient availability. Ideally these conditions should be dealt with in the irrigation design and with precision management of fertilizers.
There are various irrigation systems to consider. All have advantages and disadvantages, depending on site conditions, water availability and cultural goals.
An overhead sprinkler system uses up to 50 times more water than a drip system but is simple to maintain, applies large amounts of water in a short period and can be used for frost protection. Another advantage of sprinkler irrigation is the water applied between planted rows helps maintain cover crop growth throughout the growing season. A disadvantage is that large areas of the vineyard are watered at the same rate, sometimes including areas that do not require water. Sprinkler systems are thus most suitable for large sites with level terrain, little variation soil types and uniform vine vigour.
The system should be designed to apply water evenly over the field surface. It is important to know the rate of water application per area for calculating the amount of time required to wet through the vine rooting depth. A run time that is too short will deprive deeper roots of water and may lead to buildup of salts within the root zone. A run time that is too long may leach fertilizers, particularly nitrate, from the root zone.
The appropriate water application rate may be limited by the rate of water absorption by the soil. Water infiltration through soils is influenced by the soil bulk density and texture: coarse textured soils have higher infiltration rates.
Infiltration is substantially reduced by the presence of a shallow water table or hardpan. It is also influenced by the condition of the soil surface. Surface crusting and puddling can result from frequent cultivation without addition of organic matter, cultivation of fine textured soils when wet and extreme water application rates. Cover crops can greatly improve soil surface permeability.
Without surface impedances, water application rates should not normally exceed 5 mm per hour on fine textured soils or 10 mm per hour on coarse textured soils.
There are two main types of low-pressure irrigation systems: drip or trickle systems and microsprinkler systems.
Drip systems can be installed above or below ground, but above-ground systems are most common and are easier to access for maintenance. Both systems can be installed to vary water application rates according to variations in soil texture or vine vigour within the vineyard.
Drip irrigation wets a discrete soil volume within which root growth is restricted unless significant rainwater is retained by the soil or the wetted volume is in contact with high water table. The restricted wetted soil volume and size of the root system can provide an advantage in controlling canopy vigour.
This may be particularly useful in vineyards with deep, fine textured soils. However, a root system that is too small may not take up sufficient water to satisfy canopy requirements during hot, dry weather. If prolonged, this water stress can result in permanent leaf damage both from lack of water and overheating. A shallow root system is also particularly prone to winter freeze damage.
Thus, it is important to install a drip system with appropriate emitter numbers, spacing and delivery rates and to operate the system at run durations that lead to a wetted soil volume appropriate for the vine canopy size.
Micro-sprinkler systems operate under low pressure but use emitters with flow rates that are higher than those of drip emitters. Higher water application rates are thus achieved with fewer emitters.
The simplest scheduling method is to apply water when the total available moisture in the root zone reaches a low threshold. For example, irrigating when 40% of the available water remains in the root zone in coarse textured soils or when 50% of the available water remains in fine textured soils will provide adequate water to satisfy vine requirements while avoiding excess drainage to below the root zone.
Although this method is effective for preventing vine water stress, it may result in excess vigour and delayed fruit ripening, particularly on deep, fine textured soils. Two methods are used for estimating available moisture in the root zone: measurements of soil moisture content and estimates of crop water consumption.
Methods and instruments for measuring soil moisture content include:
Recent research conducted in a sprinkler-irrigated vineyard on deep, fine textured, silty soil near Penticton revealed the importance of properly locating soil moisture monitors.
The study found that, when moisture measured through the soil profile down to 60 cm depth was depleted to well below 15% over more than three weeks, leaf stomatal conductance and photosynthesis rate remained as high as when irrigation had been applied at a soil moisture threshold of 15%. These results indicate that a significant portion of the root system was below 60 cm depth. In such a vineyard, soil moisture monitors should be placed deep within the profile where the active roots are located.
Determining the optimum placement depth for monitors may require excavation to determine the depth of the active root system and a study of the relationship between monitored moisture at different depths and signs of vine water stress.
Vineyard water consumption or evapotranspiration (ETc) can be estimated using the following relationship:
Kc = ETc/ET0 or ETc = Kc × ET0, where ET0 is the evapotranspiration of a short, green crop completely shading the ground and Kc is the crop coefficient (the fraction of water a non-stressed crop uses in relation to that ET0).
K0 varies through the growing season as vines develop active leaf area. There have been tabulated values of K0 published for grapevines, and these can be modified depending on the vigour level of a vineyard:
To calculate ETc using tabulated values of Kc, growers need current ET0 values.
One method to determine ET0 uses a black Bellani plate atmometer. This is an inexpensive flat, black, porous plate which is kept wet. The amount of water evaporating from its surface is used to calculate ET0.
Current ET0 values can also be obtained by contacting a local weather office that provides daily Class A pan evaporation data. This is used to calculate ET0 by using a pan coefficient which varies with wind speed, relative humidity and position of the pan.
Growers applying irrigations based on estimated crop water consumption should be aware that, even when good data can be accessed for ET0, estimates of Kc can be inaccurate and lead to poor estimates of ETc. If ETc is overestimated and excess irrigation is applied, excess vigour and an increased risk of nitrate leaching may result. If ETc is underestimated, moisture deficits may become progressively worse through the season and lead to delayed ripening and poor fruit quality.
Another irrigation scheduling method uses a measured indicator of vine water status as a threshold, rather than soil water content or estimated water consumption. This technique may overcome the problem of not knowing whether soil moisture measurements are representative of the whole root zone or whether estimated consumption estimates are accurate.
Measurements of petiole xylem water tension, known as the leaf water potential, are taken at a specific time of day (i.e. solar noon). Irrigating at a midday threshold of -1.0 megapascals for white varieties or -1.2 megapascals for red varieties is recommended to avoid water stress and to ensure adequate water is provided to satisfy vine requirements. Growers should be aware of their soil dry-down dynamics (speed with which soil moisture is being depleted) and the corresponding effects on vine water status to ensure the crop is monitored at the appropriate frequency to catch threshold vine water potentials.
Also, the time of day when leaf water potential measurements are taken may be critical. Recent research conducted in a vineyard on sandy soil in the Oliver-Osoyoos area found that mid-day leaf water potential measurements can be a poor indicator of vine water status.
The study found that, when soil moisture deficits were prolonged for a week beyond when irrigation was normally applied, mid-morning and early-afternoon leaf water potential measurements were not significantly different from those taken on irrigated vines even though stomatal conductance was reduced by one-third and photosynthesis rate was halved.
The explanation is that the reduced stomatal conductance in response to the soil moisture deficit reduced the leaf transpiration rate and decreased xylem water tension to the same level as the irrigated vines.
This technique makes use of the variable effects of vine water status at different phenological stages over the growing season. The goal is attain high fruit quality by optimizing vine vegetative growth, early cluster development and fruit set and then encouraging maximum resource supply to maturing clusters. In other words, water management is used to achieve vine balance.
An additional benefit of RDI is improved water conservation compared with that of standard irrigation scheduling for root zone replenishment.
One strategy for achieving high-quality wine grapes is aimed at maintaining low to moderate vine vigour and small berries without delaying fruit ripening or reducing fruit bud initiation for the following year.
The recommendation begins with irrigating through the bloom set period at a rate similar to vineyard evapotranspiration (ETc). This ensures that fruit set and fruit bud initiation will not be hindered. After fruit set and up to veraison, irrigation is held back until the crop is under mild water stress indicated by leaf water potentials of about 1.2-1.4 megapascals, which corresponds to about a 35% reduction in water applied compared to fully replenishing ETc. At sites with fine textured soil, partial rather than full irrigations are applied during this time so that periods of mild water stress are frequent.
Only minor reductions in leaf stomatal conductance and photosynthesis occur with mild water stress, while growth of shoots and green berries can be reduced substantially. Because the supply of sugars is not reduced relative to the utilization by developing sinks, veraison is not delayed. On the contrary, with a reduced crop load, time of veraison may be advanced.
After veraison, it is recommended that irrigation is increased to maintain high photosynthesis rates. However, recent research has found that there is little benefit from resuming irrigations at full ET. Scheduled irrigations should continue after harvest so vine carbon reserves are replenished and root systems are protected against desiccation and cold injury.
Caution is required while implementing RDI in vineyards, particularly for those on coarse textured soils that have low moisture storage capacities or in drip irrigated vineyards with small root systems. In such vineyards, it is particularly important to monitor soil moisture and vine water status frequently and at several locations in the vineyard.
Both scion and rootstock varieties vary in their response to soil moisture levels, so experience is needed in setting irrigation thresholds. Excess water stress can reduce vine vigour and yield for several seasons. Too little stress may not produce the desired results. Controlled water stress can be enhanced by reducing the amount of water applied per irrigation and reducing the length of an irrigation cycle (increasing irrigation frequency).
This irrigation technique was first developed in Australia to conserve water but has been found to be effective in reducing vine vigour while maintaining vine water status. In some vineyards, PRD has improved wine grape quality.
The theoretical basis for the benefits of PRD is that the positive effects of mild water stress and high vine water status are brought simultaneously via two parts of the root system. This is achieved by having part of the vine root system in moist soil and the other part in relatively dry soil. The roots in the drier soil produce less cytokinin and more abscisic acid than roots in moist soil. The lowered cytokinin levels reduce shoot growth while the increased ABA levels cause partial stomatal closure. The net effect is reduced vegetative growth and water consumption.
Another effect is enhanced root growth in the drier soil. Some growers believe that a larger root system will access more of the minerals needed for development of complex flavours, but there is currently no scientific basis for this theory.
There are several ways to implement PRD.
In the Australian region where the technique was developed, vineyards have shallow soils, and effective separation of the root system was achieved by drip irrigating the area between pairs of vines down the row while the areas between the vines in each pair were allowed to dry. When the rate of soil drying in the non-irrigated areas slowed or stopped, these areas were then irrigated on a normal schedule while the previously irrigated areas were allowed to dry.
Switching the irrigated versus non-irrigated areas was done to maintain the beneficial levels of cytokinins and ABA. The first PDR irrigation systems installed consisted of two drip lines per row, each corresponding to an irrigation zone. Currently on the market are double, fused drip tapes that ease installation and keep each zone’s emitters properly separated.
On deep or fine textured soils where vines have extended root systems, the configuration of wet and dry zones between and within pairs of vines in the planted row may not be effective for splitting vine root systems. Root systems can extend well into the space between planted rows and beyond neighbouring vines within the row. An effective PRD drip system developed for use in this type of vineyard has three zones: one in the planted rows, another halfway between every second planted row and the third halfway between the remaining planted rows. The three zones are irrigated one at a time.
It is important that vineyards be well established before implementing PRD. Growers should watch for signs of significant water stress and adjust the frequency of irrigations and switchover events to avoid levels of stress that lead to reduced vine performance.
In vineyards with deep-rooted vines on rich, fine textured soils, it may be difficult to achieve an effective zone of dry soil. It may be worthwhile to first try PRD on a small area to observe whether there are benefits before converting an entire vineyard.
Recent research comparing deficit irrigation techniques has found no benefit of PRD over RDI. It is thought that both techniques create mild water stress and result in portions of the root zone in wet and dry soil over the course of irrigation cycles. Further research is needed to gain a better understanding of how these techniques affect the distribution of soil moisture within the root zone and the development and physiology of vines growing on a range of soil textures and depths.
The following briefly describes the most common equipment used to irrigate today’s vineyards. More details are available from your irrigation dealer.
Until about 20 years ago, most overhead systems utilized impact sprinklers. These have now been replaced mostly by rotating sprinklers (rotators). Having only one moving part, they are generally more reliable than impact sprinklers and need less maintenance.
Overhead systems meet the water demand of the crop and maintain an actively growing cover crop but generally use more water than any other system.
Micro sprinklers (or spinners) are low-pressure, low-volume sprinklers positioned overhead or below the cordon. They use relatively little water compared to impact sprinklers, but water distribution is not very uniform under windy conditions.
Due to the small droplet size, a substantial proportion of the water may evaporate in hot and low relative humidity conditions. They also need relatively frequent checking, as they tend to plug easily when the water is not clean or when algae grow in the lateral lines. Like overhead systems, they supply water to both the vine and the cover crop.
There are a number of options to consider when installing drip for vineyards. The most common system these days is based on flow controlled, or pressure compensating, emitters. The emitters are either in-line (manufacturer integrated into the drip line) or externally inserted into punched holes. External emitters are easily replaced if they are plugged, but are now less common than the integrated emitters.
Subsoil drip lines or soaker hoses have been installed in some vineyards. They are buried about 10 cm below the surface, thereby eliminating puddling, or pooling of water below the drip line. They also reduce some weed growth. The major disadvantage is the difficulty in monitoring evenness of discharge rate. They may also be damaged by rodents such as pocket gophers.
Drip systems in coarse soils tend to limit the root system of the vine to the actual wetted area. They are usually not suitable for supporting the growth of a cover crop but use less water per acre than overhead systems.
All drip systems using surface water require a good filtration system to avoid premature plugging of emitters. Filtration is also recommended for well water systems.
Self-flushing sand filters are very efficient at removing debris from the water supply and are appropriate for surface water. Apart from periodic change of the filtration media (sand), they require little maintenance. Depending on the size of the system, they can be relatively costly.
There are a number of disk filters on the market, which are less costly but require more maintenance. They are suitable for relatively clean well water.
A number of different systems are available for injecting fertilizer or other water soluble materials into the drip irrigation. Some fertilizers can also be applied through overhead irrigation systems, but depending on materials and concentration, there is a risk of phytotoxic effect.
Venturi based injectors require no external power source as they are based on the Bernoulli principle. They are commonly used where only smaller quantities of liquid are injected into the system.
Injector pumps are used where power is available and where larger volumes of fertilizer need to be applied in shorter time. Piston pumps are very accurate and are not affected by variations in line pressure. An inexpensive alternative is the use of a crop sprayer to inject soluble materials into the irrigation system.
A broad range of timers are on the market, from simple lawn irrigation timers to very sophisticated devices able to use sensors for automated decision making. Most timers connect to household current but operate the valve control signals at 24 volt AC.
Inexpensive lawn irrigation timers are well suited for use with drip systems. Most of them allow for season adjustment of irrigation length of time and are usually fairly simple to program. The limited maximum length of watering time for each zone, however, makes some of these timers less suitable for overhead systems.
All these relatively inexpensive systems need two wires for each electric valve: a common wire and a control wire. Wire can become a fairly substantial portion of the expense for large acreages or for blocks remote from the controller. All these timers come with a master valve control, which allows automatic pump start when an irrigation cycle starts, which is useful for systems with their own delivery pumps.
For mid-range timers, there are some interesting new developments. One is a timer programmed from a computer connected to the controller through wireless communication. It requires a line of sight from computer to controller, and the maximum range is about 300 m. This controller also comes with a wireless manual remote control, which is very handy for testing and troubleshooting the system. Programming abilities with these systems are almost unlimited in terms of length of watering cycle and cycle frequency.
Another new timing device available uses the same two wires to control a large number of zones, considerably reducing wire cost.
Most of the mid-range timers allow for the use of soil moisture and/or rain sensors to automatically suspend irrigation when a certain threshold is reached. The use of sensors, however, adds complexities which require more than average tech savvy from the operator. Familiarity with soil and crop variability and appropriate siting of the sensors can improve the effectiveness of using these timers. Many mid- and high-end timers, with the appropriate add-ons, can be controlled from a cell phone.
There are new systems on the market which control the electric valves wirelessly through radio signals. These systems, still under development, can be very expensive.
Automatic timers play an important role in water conservation since each zone will only run for a predetermined length of time.
“Add a zone” is a device that allows the splitting of a zone controlled by one electric valve into two separate zones without having to add an extra wire. This is useful where blocks originally serviced by drip systems are converted to overhead or where flow rates in a drip system are increased by the reduction of emitter spacing. They require an additional, unused zone on the controller to work.
A wide array of soil moisture sensors are available, including the expensive neutron probe, TDR sensors, sensors based on electrical conductivity or resistance and tensiometers. All these sensors have their advantages and drawbacks, but interpretation of the collected data requires experience and skill.
The simplest and cheapest tool for assessing soil moisture is still the shovel, used for digging pits in various locations in the vineyard, along with tactile and visual observation of the soil.
Other methods for determining whether the plants need water are visual cues (tendrils, basal leaf coloration), thermal indicators (leaf temperature at predetermined times of day) or pressure-based measurements (stem or leaf water potential) taken with a pressure bomb.
