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Increasing shrinkage and salinization is occurring in China's inland lakes. Lake Chenghai is located in the southwestern plateau of China and it is one of only three lakes in the world where Spirulina is found naturally, which has experienced severe decline in water level in recent decades (especially between 2006 and 2016). In addition, in China, there is a lack of hydrological observation data collected for many of the similar plateau inland lakes, which has led to a research gap regarding water-salt balance. This study aimed to investigate the lake watersalt balance, identify the causes for the decline in water volume and the adverse effects of the transition of a freshwater lake to a saline lake. A coupled water-salt balance mathematical model of the lake was constructed to gain insight into inland water-salt variation in this data scarce region. For Lake Chenghai, there was a 6.43-mdecline in water level with an average rate of decrease of 7.0 cm a(-1) between 1970 and 2016; the multi-year average water shortage was 2945 x 10(4) m(3) a(-1) from 2006 to 2016. The variation in the water level was positively correlated with precipitation (R=0.81, P < 0.0001) and surface runoff inflow (R=0.76, P < 0.0001) during the wet season, and it was negatively correlated with evaporation (R=0.71, P < 0.0001) during the dry season. The warm, dry climate in the dry, hot valley area may be the main reason for the decline in the lake level. Changes in water volume and lake salinity affected the presence and abundance of phytoplankton species and the pH of the aquatic ecosystems. The developed methodology can be used to predict the long-term water and salinity evolution of inland lakes to support sustainable water resource planning in data-scarce regions. (c) 2018 Elsevier B.V. All rights reserved.