Municipal governments began limiting water supply Thursday from the Sameura Dam reservoir in Kochi Prefecture after its level dropped to under 15 percent, officials said.
It is the fourth time water-supply curbs have been taken since the dam began operation and the first time in 11 years the area has experienced a drought.
The Sameura Dam reservoir is a major source of water for Shikoku. According to the officials, water to Kagawa will be cut by 75 percent, while supplies for Tokushima will be reduced by 22 percent.
The measures took effect at 9 a.m. Thursday, they said.
According to the land ministry's Shikoku bureau, the reservoir is likely to dry up by Aug. 18 if the drought continues.
In Kagawa Prefecture, which depends heavily on the dam, five cities and 12 towns had reduced their water pressure by Thursday. The town of Miki plans to cut off water at night, starting Thursday.
The city of Takamatsu, which depends on the dam for half its water, will wait until the reservoir dries up.
Given the emergency, residents of the area began looking for other means of securing water.
On Thursday, a dental clinic in Takamatsu finished digging a well to prepare for a possible water suspension. The clinic spent 200,000 yen on the well.
"Demand for wells for private water supply is increasing," said an official of the company that dug the well.
There are also moves by local residents who already have wells to distribute water to neighbors.
According to the city of Takamatsu, about 120 people have already expressed a willingness to let neighbors use their wells.