Optimization of limited water resources in irrigated vineyards
Irrigation budgeting and strategies to optimize both the utilization and acquisition of water resources are critical to maximize vineyard profitability.
Maximizing the returns (crop yield and fruit quality) and water-use efficiency (WUE) are not only compatible objectives, they are also necessary objectives, particularly in environments with limited (and potentially diminishing) water resources. In such environments, irrigation is likely to move from being a yield/growth strategy to a risk-management strategy.
Irrigation is a management tool that allows growers to directly manipulate vine growth, crop yield and fruit quality. Approximately 87% of the Australian wine grape production comes from regions that rely wholly or partly on irrigation water. Water costs can be as high as AU$3,000 per megaliter (ML) and power costs to move water continually increase.
In recent years, wine grape producers in southeast Australia have started the growing season with anywhere between 0% and 70% water allocations. Similarly, in southern parts of southwest Western Australia, irrigation resources have been limited and irrigation allocations have been based on risk minimization strategies.
Optimizing utilization of water
PRODUCTION WATER-USE EFFICIENCY
Production water-use efficiency (WUEp) is how water utilization is generally measured and compared. It is defined as the amount of crop produced per unit of water applied as irrigation and effective rainfall and is therefore an outcome of the efficiency by which water reaches vines and how the vines access and transpire water in fixing carbon.
M.R. Gibberd et al. and S. Tyerman et al. proposed that there are several sources of variation associated with WUEp.6,7,27
1) Irrigation efficiency: This relates to the proportion of applied water transpired by the vine. For this efficiency to be optimal, water loss through runoff, drainage, leakage, evaporation and cover crop transpiration must be minimal. Water-management practices must also be optimal since this efficiency is dependent on both the water delivery infrastructure and how water is applied to optimize production.
2) Transpiration efficiency: the amount of carbon fixed during photosynthesis per unit of water transpired by the vine. Much of the variation in this efficiency can be attributed to variation in vine genotype through differences in stomatal conductance and photosynthetic capacity.
3) Harvest index: The amount of total dry matter removed as harvested product. It is dependent on the supply of sugar from both photosynthesis and reserves and is important as it determines yield from the product of transpiration efficiency and the volume of water transpired. It is strongly influenced by scion and rootstock genotypes.
VALUE WATER-USE EFFICIENCY
Water use can also be considered on a value basis. Value water-use efficiency (WUEv) is defined as the monetary value of the fruit or wine per unit of water applied as irrigation and effective rainfall. Whereas WUEp relates to the vineyard, WUEv relates to the vineyard-winery-marketplace continuum. This form of WUE also needs to be considered in the decision-making process since a reduction in productivity as a consequence of deliberate or otherwise under-irrigation practices could be offset by a potential improvement in fruit and wine quality (and, hence, monetary value).
When comparing irrigation strategies and water use across a single vineyard or vineyards in close proximity to each other for any given year, the effective rainfall component can be excluded. WUEp and WUEv can then be expressed as tonnes per ML and $ per ML respectively.
WHEN DOES THE VINE REQUIRE WATER?
Minimizing the risk of crop loss when insufficient water is available requires an understanding of the vine’s water requirements during the growing season. Improvements in irrigation efficiency can be achieved by avoiding waste (over-irrigation) and by ensuring sufficient water is applied during key growth periods (avoiding under-irrigation when the risk of yield reduction is high). The amount of water required during different stages of the growing season will depend on:
• climatic conditions,
• variety/rootstock combinations,
• soil type and depth,
• crop load.
Table I provides guidelines on vine water requirements to ensure that productivity and fruit quality are not compromised for the current and subsequent growing seasons.
The effect of water stress on vine growth and berry development will depend on the timing and severity of water deficit during the growing season. P. Iland et al. provided guidelines for qualitatively assessing shoots, berries and soil in relation to the degree of vine water stress:12
1) Severe water stress prior to flowering and fruit set is likely to have a negative impact on crop potential for both the current and subsequent growing seasons.
2) Timing of irrigations is critical, and an individual block assessment at key phenological growth stages is imperative. This assessment should take into consideration vine vigor, root distribution, target crop yield and wine style.
3) Environmental factors to consider include current and forecast temperature and evaporation rates, water quality, soil moisture status and water-holding capacity of the soil. Each factor needs to be assessed progressively throughout the growing season and will carry a different degree of importance at various times.
Under extreme cases of water shortage, minimizing the risk of a loss in crop and/or wine value may be more important than a loss in productivity. In this case, water should be allocated to those blocks from which the greatest monetary value will be gained.
HOW MUCH WATER SHOULD BE APPLIED AND WHEN?
In order to make informed decisions about how much water to apply and timing of those applications, a water budget is required, along with a measure of vine water status. Water budgeting is simply the process of balancing the output of water lost from vines through evapotranspiration and the amount of water readily available to vines via the so il. Determining the water status of vines is generally done directly using plant-based measures or indirectly using soil-based and/or climate-based measures.
H.R. Schultz and M. Stoll and S. Tyerman et al. discussed current and emerging technologies for measuring vine physiological parameters.22,27 A variety of instruments currently available provide information that can be used to determine irrigation requirements and include measures of:
• water potential (a pressure chamber or a psychrometer),
• stomatal conductance (a porometer),sap flow,
• leaf temperature,
• trunk diameter.
While these devices are generally used for research rather than commercial purposes, this is likely to change as some have the potential to be developed into user-friendly irrigation scheduling tools.
Since relationships can be established between vine physiological parameters and changes in soil water measurements, it is generally easier to assess vine-water status using devices that measure either soil water suction or volumetric soil water content. The correct installation and positioning of such equipment is important if the data is to be representative of the vineyard.
Water use by a vineyard is closely related to evaporation from an exposed water surface. The standard meteorological device to measure evaporation is the Class A pan evaporimeter. Crop factors are used to convert pan evaporation (Epan) to vineyard water use. However, this is not the preferred method when the canopy cover is incomplete.
An alternative method is the use of evapotranspiration figures derived from a reference crop (ETo) in conjunction with crop coefficients (Kc) to convert ETo to vineyard water use (ETc). Kc values are defined in reference to particular calculations of evaporative demand. Long-term ETo can be used to plan an approximate irrigation frequency based on average climatic conditions, and forecast ETo can be used to predict when the next irrigation is due.
The Bureau of Meteorology provides recent ETo values on its website (bom.gov.au/watl/eto/), and registered users can access forecast ETo via the meteogram service. CSIRO provides recent and forecast ETo values for selected locations via the irriGATEWAY website (weather.irrigateway.net/).
Recent research has shown that by adjusting Kc for grapevine canopy cover, it is now possible to improve ETc calculations.28 This approach is being trialed by CSIRO for the Irrisat-SMS irrigation-scheduling service (irrigateway.net/tools/sms/).11
A free water budgeting tool is available from the SA Rural Solutions website (solutions.pir.sa.gov.au/markets/water_management).
Spatial variability and the effect on water-use efficiency
Another question to consider in relation to optimizing WUE is where to apply the water. Land is variable, and because of this vine performance usually has a distinct spatial structure.2,3,5,21,26 The range of variation in crop yield obtained from blocks under uniform management is typically 10-fold.2
Indices of fruit quality (total soluble solids, titratable acidity, pH and anthocyanins), and sensory attributes of finished wine have also been shown to be spatially variable,3,5,25 but not necessarily temporally stable as they are more susceptible to climatic conditions.
Differential management of irrigation water
The majority of vineyards are managed on the assumption that they are homogenous. When a variable block is managed in this way, inputs such as irrigation water are uniformly applied. A number of studies have shown that there is significant within-vineyard variability in grapevine water status.1,24
J.A. Taylor et al. found that canopy size, which is associated with vine vigor, and soil type are the dominant drivers of this variation, particularly as water becomes restricted.23 They concluded that canopy architecture is an effective parameter for creating “zones” of similar vine performance as a basis for differentially applying irrigation water. Numerous studies have used airborne remote sensing to identify relative differences in canopy characteristics at véraison.9,13,20
I. Goodwin et al. and L. McClymont et al. showed how the differential (as opposed to uniform) application of limited water supplies improved crop productivity, WUEp and certain fruit quality parameters within areas of a 6.4-hectare Shiraz block located in the Sunraysia region of Victoria, Australia.8,17
Until recently it was not clear whether the utilization of irrigation water varied spatially in accordance with vine performance or whether it was uniform across a vineyard block.
In a 6.8-hectare drip-irrigated Shiraz block located in the Great Southern region of Western Australia, I. Goodwin et al. and A. Zerihun et al. showed that vine vigor can have a considerable influence on vine physiological processes and water use.8,29 Leaf photosynthetic rates and stomatal conductance for the least vigorous vines were about 40% of the equivalent values for the most vigorous vines.
As expected, it was found that stomatal conductance increased after irrigation was applied. However, this was not accompanied by a corresponding increase in photosynthesis. This resulted in a lowering of WUEp, which was more apparent in low-vigor vines than in high-vigor vines. In the same study, it was shown that crop yield was linearly correlated to the vine’s capacity to utilize the applied water.
The dependence of crop yield on water availability and use suggests that to increase yield, a vine must be able to utilize all available water. The fact that low-vigor vines did not use all the irrigation water indicates that other factors must limit a vine’s response. One hypothesis is that this may have been related to the death of fine roots in spring, when weaker vines were under severe water stress prior to weekly irrigations. This indicates that severe water stress early in the season for weaker sections of a vineyard block must be avoided.
Selective harvesting, based on zones of contrasting vine performance, provides an opportunity to potentially increase the WUEv through an increase in the price of fruit and/or wine as a consequence of providing separate parcels of fruit of unifor m quality to the winery. Where a lack of water is limiting production, this strategy may be particularly beneficial in the context of risk management. Several commercial examples exist that demonstrate an increased profitability using a differential harvesting approach.3,20
Optimizing the acquisition of water
Many Australian soils have characteristics that potentially restrict both the volume and distribution of vine roots. Where this occurs, vines may be unable to access and utilize water and nutrients efficiently from either the tractor row area or within the vine row below restricted root zones. This is likely to have a negative impact on WUE and hence the long-term sustainability of the vineyard.
L. McClymont et al. showed that under non-limiting water and nutrient conditions, vine growth and crop yield decreased as the available soil volume was reduced.16 This implies that root volume and/or distribution have a direct effect on vine performance. Table II illustrates how soil type and root distribution can influence the amount of readily available water (RAW) that is potentially accessible to a grapevine.
By restricting roots to the vine row in sand and sandy loam examples, the volume of RAW, expressed as either per vine or per area, is reduced by about 33%. This situation occurs in vineyards where soil compaction within the tractor row prevents roots from growing laterally.18 In the clay soil example where root growth is restricted both horizontally and vertically, the volume of RAW is reduced by about 80%. Limited vertical root growth is also commonly found in vineyards.18
Several soil modification techniques are available to overcome restrictive root zones and reduce vine dependence on irrigation by improving water acquisition including:
•ripping to overcome physical restrictions such as compaction layers and rock,
•drainage to overcome anaerobic environments due to water-logging,
•mounding to increase the volume of soil for root development,
•application of surface and sub-surface amendments such as lime to correct pH imbalances and gypsum to ameliorate soil structure by exchanging calcium for sodium,
• application of surface mulches and/or compost to improve soil organic matter levels and reduce evaporative losses.
Soil modification techniques have been applied across a range of soil types, and such practices should be aligned to variations in soil characteristics to optimize their effect.14,15,19
There is a need to know how responses to different modification techniques vary in relation to the spatial variability of the vineyard. The use of high-resolution spatial data and geostatistical techniques provides a solution to this problem by enabling highly replicated designs in experiments to be conducted over whole vineyard blocks.
Access a full version of the Final Report and Adoptable Outcome 8, which forms the basis of this article, at the GWRDC website: gwrdc.com.au/completed_projects/optimising-the-utilisation-and-acquisition-of-water-resources-in-irrigated-vineyards-under-water-limiting-conditions.
Access an audio-visual presentation of Adoptable Outcome 8 at gwrdc.com.au/resource_type/videos/.
This text was first published in the December 2013 Australian & New Zealand Grapegrower & Winemaker and is edited and reproduced here with kind permission of the publisher, Winetitles, winebiz.com.au.
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