Lower-than-average rainfall is affecting 85 Waimanalo farmers who will be under mandatory conservation orders that began yesterday.

The reservoir that supplies the state Department of Agriculture's irrigation system for the Waimanalo farmers is low already, officials said, even though winter is usually the rainy season.

Typically, an order to conserve water is issued in the summer after months of little or no rain, said Brian Kau, administrator of the Department of Agriculture's Resource Management Division. But rainfall so far this year in Waimanalo is already 36 percent below normal, he said.

The Waimanalo farmers are being asked to use 20 percent less water.

"It's a precautionary measure at the request of our water users," Kau said. "The farmers wanted to take a more cautious approach to ensure that they had the maximum opportunity to make the water last."

Statewide, about 650 farmers tap into the state's irrigation systems.