Quantifying the unauthorized lake water withdrawals and their impacts on the water budget of eutrophic lake Dianchi, China

Water withdrawn from lakes is one of pathways to support economic development in China. Yet lake water withdrawals, especially through unauthorized behaviors, have not been accurately quantified. Here we proposed a coupled simulation-optimization model to determine the unauthorized water withdrawals from Lake Dianchi (the sixth largest freshwater lake in China) and assessed their impacts on lake hydrology. Together the results of systematic observations, on-site surveys, and local statistics reveal that lake water withdrawals played a non-negligible role in lake water budget, accounting for 19.6% of total outputs from the lake over the period 1999-2011. Most water withdrawal events occurred from April to October when toxic blooms of cyanobacteria happened. Lake water withdrawals reshuffled water budget components of the lake itself, leading to a substantial decrease in lake outflows and water levels and an increase of highly-polluted return flows, which might further intensify local lake eutrophication and downstream water scarcity. In the light of the importance for sustainable water management, we suggest that a new and expanding monitoring network across China's lakes to understand the process of lake water withdrawals would be beneficial to meet water demands without compromising lake ecosystems.