The sun and wind are sucking up water from Upper Clutha land faster than the pivot irrigators can put it on.
In one of the driest summers farmers can remember, irrigation systems are failing to keep pace with evapotranspiration.
Farmer Bruce Jolly, whose land is included in the new Ardgour irrigation system, said his pivots were designed to deliver 5mm-6mm a day, but evapotranspiration at Wanaka Airport had been measured at 12mm a day.
"You could get a design to put that much water on, but it would cost lots of money to have that sort of capacity.''
The grass was still growing, he said, but not at the rate it would normally.
Most farmers in his area used irrigated land for grazing over summer, and he was having to turn down requests from farmers running out of feed, Mr Jolly said.
"I'm getting calls all the time. I'd imagine all the irrigated farmers would be getting the same calls.''
This summer's dry spell had been "one of the worst'' he had encountered, Mr Jolly said.
The Lindis River had already dried up, restricting by half Tarras farmers' ability to irrigate from that source, he said.
And the Upper Clutha is expected to miss out today on rain forecast for the South.
MetService meteorologist Lisa Murray told the Otago Daily Times this week some places would get up to 15mm, "but there are going to be places that get hardly anything''.
And Wanaka was one of those places, she said.
Wanaka Airport has recorded just 2mm of rain so far this year.
That came on top of a December total of 14.2mm, compared with the long-term average of 62mm.
Wanaka agricultural contractor Richard Woodhead said even the normally wetter Matukituki and Makarora Valleys were as dry as he had seen them in 15 years, and he was unable to sow any winter crops at present.
Otago Regional Council records show 7mm of rain in the Makarora Valley in the past week, none in the Matukituki, and no substantial rain in either valley for the past six weeks.
Makarora resident Raymond Edwards said he had planted 300 eucalypts on his 17ha property in the spring but most had died from the heat.
His micro hydro-electric system had stopped working because of a lack of flow in the stream he used, and he was having to irrigate his raspberries and blueberries.
Wilkin River Jets operations manager Danyel Hutton said the level of the Wilkin River was holding up thanks to rain in the headwaters earlier this week, and it was "business as usual''.