River Murray irrigators are facing their most challenging year since the millennium drought after learning their opening water allocations could be as low as 14 per cent.
Even in a best-case scenario, irrigators would likely receive only 21pc of their entitlements from July 1, the state government announced on Monday.
That would be the lowest opening allocation since 2009.
The projections take into account a 50-gigalitre reduction in the amount of water to be taken out for domestic use in Adelaide, a change introduced this year to encourage SA Water to operate its desalination plant.
Without that, allocations would have been eight per cent lower.
The department's River Murray water operations manager, Jarrod Eaton, said it was important for irrigators to remember that the figures were only an early projection.
"This projection ... assumes conditions across the remainder of 2018-19 will continue to be extremely dry," he said.
He said allocations were also likely to increase throughout the year as more water flowed into the river system.
There was a 60pc likelihood that allocations would increase to 100pc during the year, and a 90pc chance allocations would increase to 75pc at some point.
"Most inflows to the River Murray system historically occur between July and November, so the main inflow season is yet to arrive," he said.
"The Bureau of Meteorology's outlook across the Murray-Darling Basin shows there is no strong indication either way towards a wetter or drier than average three months ahead."
Environment and Water Minister David Speirs said food producers could plan for the worst but continue to hope for improvements.
Irrigators who have unused water from previous years will be able to carry over up to 20 per cent of their entitlement volume.
The government will provide another update by May 15.