In order to improve WUE in table grape production for sustainable production, a balance between vegetative and reproductive growth must be maintained. Block D was a vigorous block as concluded from shoot growth, high pruning mass and the standardised opening of the canopy at véraison to increase light interception for improved colour development. For Block D, yield of the first season was the highest, but the second season’s yield was reduced probably due to stricter crop control in the second season in an effort to have fewer but higher quality bunches. Block A
154 had poor production as well as productivity and this could be ascribed to the wetness of the soil, as well as saline soil conditions found in this block. This is an indication that soil conditions and canopy management are important factors affecting WUE. It is a common practice in viticulture to remove leaves at véraison to improve light interception for improved colour development. It is important to avoid severe leaf removal that can have a negative impact on physiological mechanisms, as well as productivity. Also, in the second season, which was drier and warmer, better fruit quality was obtained, indicating that a certain level of water stress during the growing season can improve fruit quality.
The two blocks that were irrigated with a micro-sprinkler irrigation system had a higher irrigation volume and ET. This was in agreement with Myburgh (2012), who reported a higher ET in Thompson Seedless table grapes irrigated with a micro-sprinkler irrigation system compared to drip irrigation in the Lower Orange River region. In a water stressed country such as South Africa, it is probably necessary to consider drip irrigation systems that use less irrigation volumes and reduces evaporation. Furthermore, lower ΨS measured in this study indicates that drip irrigation
systems can successfully be implemented without forfeiting yield and quality, provided it is well managed. Additionally, deficit irrigation systems can control vigorous vegetative growth, while reproductive growth is maintained or improved, resulting in improved WUE. Thus, tools and methods are needed to measure plant and soil water status in order to schedule irrigation properly and contribute to a higher WUE. The two micro-sprinkler irrigated blocks had a tendency towards a higher WUEy in the 2014/15 season due to the higher ET and yield measured in these blocks,
while Block D, which was irrigated with a drip irrigation system, had a higher WUEirr. Calculations
included in this study also indicated over-irrigation in some blocks, especially for the micro- sprinkler irrigated blocks, meaning that irrigation scheduling is not optimal and more needs to be done to reduce irrigation volumes in order to improve WUE in table grape production.
Water footprints provide useful information on the water use of a specific area and strategies to improve WUE can be developed based on this information. When combined with crop yield data, it creates the basis for identifying existing levels of water use efficiency (so called ‘crop per drop’, for example kg/ton of table grapes produced per m3 of water used). Water footprint analysis is a
good starting point for determining the quantity of water needed for a certain crop in a specific area. This information can aid in decision making as to which crop can be produced sustainably with better economic benefits to the production area. Thus, WF can be used as a tool to raise awareness, as well as determine crop efficiency, which can be used in debates and decision making regarding water allocations.
7.3.1 Limitations
The main limitation of this study was that only mature established blocks were used and due to human capacity and equipment availability, more blocks could not be included to have a specified statistical layout, hence resorting to scenario evaluation. Equipment to measure physiological measurements was limited, due to the fact that there was only one IRGA to measure four blocks and all blocks could not be measured at the same time in order to make comparisons between them. This was the reason why only two blocks were selected for the second season to conduct diurnal cycles. The other limitation was equipment calibration issues. The leaf water use efficiency could not be determined, because there was probably a calibration issue with the IRGA, which affected the vapour measurements which may also have affected stomatal conductance, transpiration and VPD values obtained with the IRGA. Furthermore, two different porometers were used to determine stomatal conductance in the second season and one of these porometers had
155 calibration issues. Therefore, that data had to be excluded as well. This is an indication that calibration issues can hamper the adaptability and use of equipment in the field, therefore simple and less sophisticated equipment should be used by producers for field evaluation to guide them with irrigation scheduling. Obtaining reliable weather data was also very problematic. There were three automatic weather stations (AWS) available for the De Doorns area. The AWS situated on the Hex River experimental farm, which was used for the first season, was not available for the second season, due to vandalism and therefore the Modderdrift AWS was used in the second season. At times, weather data of a specific station also seemed to be incorrect or stuck at a certain value for consecutive days, making it unreliable.
7.3.2 Novelty value
Few studies have been conducted on table grape WUE and blue WF and this study can contribute to that limited information availability. For sustainable table grape production, it is very important to determine the WF and WUE in order to fulfil the concept of more crop per drop without forfeiting yield and fruit quality. Most of the studies conducted on table grapes WUE and WF were desktop studies and did not include actual plant growth and physiological measurements. Additionally, most of the global data (Mekonnen & Hoekstra, 2010; Pahlow et al., 2015) available did not make a distinction between the different grape types (table grapes, raisin & wine grapes), which have different trellising systems, as well as different canopy sizes and structures, with different management styles, which can have an effect on vegetative and reproductive growth, as well as crop water use, hence influencing WUE and WF. Furthermore, a comparison of table grape blocks grown under different climatic conditions were made to determine the effect of growing conditions and weather conditions on WF and WUE. This study has proven that higher evaporative demand increases blue water use, which leads to a higher blue WF with a lower WUEy. The plant based
measurements in this study also contribute to the scientific knowledge and understanding of how the grapevine’s performance is affected by different soil types and irrigation systems, through direct plant based measurements during critical phenological stages.
Water footprint analysis requires information on crop water use and production at a range of spatial and temporal scales, which is not always available. As already discussed under section 2.4.3 in the literature review, there are different methods to estimate ET. However, remote sensing offers the advantage of being able to estimate crop water use for each pixel of a satellite image. Furthermore, this technique also estimates crop water use based on the grapevine growth tempo and canopy characteristics and does not need to rely on the generalised crop coefficient often used in the industry. Hence, indicating the novelty of FruitLook in irrigation water management.