The man-made lake is held back by a dam built on the Yoshino River on the island of Shikoku in Kochi prefecture and services three neighboring prefectures. It is usually 80% full in summer, according to Japanese public broadcaster NHK, but scorching heat has increased consumption, leaving it 31% full.
Kochi authorities have ordered water pressure to be lowered in public spaces and allowed the use of water from alternative dam reservoirs.
Resources-poor Japan has been battered by sweltering heat since June. Temperatures in Tokyo rose to 36.4 degrees Celsius (97.5 degrees Fahrenheit), setting a new monthly record, while parts of Japan saw temperatures surge past 40 degrees.