Managing vineyards rootzone salinity and maximising water saving by sub-surface irrigation techniques
The results of a five-year field experiment in the South Australian Riverland indicated that under non-restricted irrigation allocations there were no water use savings or improvements in rootzone salinity with the use of subsurface irrigation; either using conventional subsurface drip line or when the subsurface drip line was enclosed in a porous fabric strip designed to improve the lateral movement of water. Only under severely reduced irrigation volume was there some yield advantage with the two types of subsurface irrigation. There was deposition of fine colloidal clay within the fabric covering which may have influenced the long-term performance of this irrigation system with the water source used in this experiment and this warrants further investigation.
During winter 2009 a large field experiment was established by Treasury Wine Estates at the Markaranka vineyard in the South Australian Riverland to determine whether subsurface irrigation resulted in any water use savings and improved rootzone salinity. Support from Wine Australia enabled the installation of monitoring equipment in 2010 and collection of soil water and salinity data over ensuing seasons.
Three types of irrigation were installed: standard drip irrigation, subsurface drip and subsurface drip within a porous fabric cover designed to increase the lateral movement of water away from the emitter. Four irrigation treatments were established, resulting in applications of about 50, 67, and 84% of the standard irrigation (100%) used in the remainder to the block of Chardonnay grapevines, which were planted on to Ramsey rootstock in 2004. The soil type varied from sandy to loamy sand across the trial site.
A split-plot design was used with irrigation volume as main plot along a single row of vines, and irrigation type the sub-plot within the row. Each subplot consisted of nine vines irrigated using the same lateral, and treatments were replicated five times across the width of the block. In-line water meters were installed strategically to enable the calculation of water volume applied to each treatment. Irrigation treatments were applied during five consecutive seasons with conclusion of field work at pruning in winter 2015.
Aggregated data for season and irrigation volume indicated that the yield of subsurface irrigated vines was significantly lower than standard drip or fabric covered subsurface, which were similar. However, there was a significant interaction between irrigation type and volume. For 100% irrigation volume, subsurface drip yielded significantly less fruit than the other two irrigation types. Under severely restricted irrigation (50%) vines irrigated with the standard drip were significantly lower in yield than the other irrigation types, which were similar. The yield of 50% subsurface irrigated vines was less than all irrigation types at 67%, with vines indicating that a change to subsurface irrigation does not maintain yield with less water. This is a significant outcome that is somewhat contrary to previous findings.
The lower yield of subsurface irrigated vines may have been in part due to higher rootzone salinity. The four-season mean of water samples collected from SoluSamplers indicated the three irrigation types were significantly different to each other with subsurface drip being about 50% higher in overall rootzone salinity than standard irrigation and fabric covered subsurface drip being intermediate and significantly different to the other two irrigation types. Within each season, subsurface drip recorded the highest overall rootzone salinity and standard irrigation the lowest. The horizontal location of the drip line determined soil water content with the upper layers of the profile being wetter with standard irrigation compared with subsurface drip, while at depth the pattern of ‘wet’ or ‘dry’ soil was similar. In years one and two the profile under fabric covered subsurface drip was more often dry than wet however, at the time, it was hypothesised that this may have been the result of soil water sensor location alongside the fabric cover. However in the third season the pattern reversed to more closely resemble subsurface drip. This unexpected change in water distribution may indicate a change in properties of the fabric cover and hence may highlight potential longevity issues of this system, depending on the physical properties of the irrigation water at the site.
In conclusion, this long term field experiment demonstrated that under non-restricted irrigation allocations there were no water use savings or improved root zone salinity with the use of subsurface irrigation, or when the subsurface drip line was enclosed in a porous fabric strip designed to improve the lateral movement of water compared with standard drip irrigation. Subsurface irrigation resulted in the highest overall rootzone salinity for the four years of data. There was a yield penalty using subsurface irrigation under non-restricted or reduced irrigation volume. Only under severely reduced irrigation volume there was some yield advantage with the two types of subsurface irrigation.