Erhai Lake in Dali Bai Autonomous Prefecture, southwest China's Yunnan province, Nov 21, 2015. [Photo by Xue Dan/for chinadaily.com.cn]

Erhai, the mother lake of Dali in Yunnan province, is the source of 800,000 people's drinking water. Pollution has become serious in recent years and blue algae has broken out twice since the 1990s.

President Xi Jinping expressed the importance of giving a higher priority to ecological and environmental protection during his visit to the Dali Bai Autonomous Prefecture in January. He said local officials should improve the quality of the water in the lake, and he had a photo taken with local officials to "keep a record".

Yunnan is to spend 26.4 billion yuan ($4.1 billion) to tackle pollution in the lake.