Issue link: http://winesandvines.uberflip.com/i/264184
54 p r a c t i c a l w i n e r y & v i n e ya r d M a r c H 2 0 1 4 g r a p e g r o w i n g An alternative method is the use of evapotranspiration figures derived from a reference crop (ETo) in conjunction with crop coeffi- cients (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 irriga- tion 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 irri- GATEWAY 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 irri- gation-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 neces- sarily 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 con- cluded that canopy architecture is an effective parameter for cre- ating "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 dif- ferential (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 irriga- tion 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 vig- orous 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 accompa- nied 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 Selective harvesting, based on zones of contrasting vine perfor- mance, provides an opportunity to potentially increase the WUEv through an increase in the price of fruit and/or wine as a conse- quence of providing separate parcels of fruit of uniform quality to the winery. Where a lack of water is limiting production, this strategy may be particularly beneficial in the context of risk man- agement. 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