In the middle of the millennium drought, conventional wisdom was that sub-surface irrigation was the best way forward if water restrictions were to become the norm. Five years of good science supported by Wine Australia now suggests otherwise.
The final report of a project led by Dr Mike McCarthy, Principal Scientist with the South Australian Research and Development Institute (SARDI)*, shows that there is no value in going underground except possibly in times of extreme and sustained water shortages.
‘We couldn’t demonstrate any improvement in yield or water use savings by switching to sub-surface irrigation, which was contrary to what we had been led to believe’, Dr McCarthy said. ‘In fact, there was actually a yield penalty in going to sub-surface at the normal rates of irrigation that growers might be using.
‘To be honest, we don’t know why. We don’t think it’s due to blocking of the drip lines, although in some areas there’s an issue with colloidal clay material in the water, which might have some long-term impact.
‘What we are thinking is whether sub-surface drip is not keeping the root zone cool. If you put water on the surface it’s cooling the upper part of the root zone, but that’s not happening if the drippers are about 30 centimetres below ground.’
The initiative to test the claims of sub-surface irrigation advocates in Australian conditions was instigated by Treasury Wine Estates, which worked with SARDI to set up a large field experiment at its Markaranka vineyard in the Riverland in 2009.
Funding from Wine Australia expanded the project, including the installation of monitoring equipment in 2010 and the collection of soil and water salinity data over five seasons.
Three types of irrigation were installed: standard drip irrigation, sub-surface drip and sub-surface 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 per cent of the standard irrigation (100 per cent) 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 the main plot along a single row of vines and irrigation type the sub-plot within the row. Each sub-plot consisted of nine vines irrigated using the same lateral line and treatments were replicated five times across the width of the block.
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 per cent irrigation volume, subsurface drip yielded significantly less fruit than the other two irrigation types,’ the final report says. ‘Under severely restricted irrigation (50 per cent), vines irrigated with the standard drip were significantly lower in yield than the other irrigation types, which were similar. The yield of 50 per cent subsurface irrigated vines was less than all irrigation types at 67 per cent, indicating that a change to subsurface irrigation does not maintain yield with less water.’
* SARDI is a division of Primary Industries and Regions SA (PIRSA).