SHENZHEN, China, Dec 9 (Reuters) - China"s major southern cities Guangzhou and Shenzhen have warned of severe water shortages lasting into next spring as the East River, a tributary of Guangdong"s Pearl River, continues to be hit by its most severe drought in decades. Authorities in both cities are asking citizens to reduce water consumption, with rainfall between January to October this year down by a quarter compared to average levels over the last decade, according to the Guangzhou government. The inflow of water into the East River Basin, a major supply of water for both cities, will remain at around 50-60% its usual level into next spring, according to an official notice on Monday. The company in charge of Guangzhou"s water supply is taking emergency measures to deal with increased salt tides, where the water supply becomes increasingly saline due to a lack of fresh water, it said on Wednesday. Hong Kong also imports much of its water from the East River.