Evolution of the hydro-ecological environment and its natural and anthropogenic causes during 1985-2019 in the Nenjiang River basin.

Lakes and vegetation are important components of the hydro-ecological environment and sensitive indicators of anthropogenic climate change and human activities. For the first time, the lake area, vegetation cover change (obtained from Landsat satellite images), terrestrial water storage (TWS; estimated from the gravity recovery and climate experiment), and hydrochemistry, stable isotopes of different waterbodies were combined to analyze the long-term evolution and controlling factors of lake area and vegetation cover in the Nenjiang River basin, China. Between 1985 and 1999, the lake area experienced natural shrinkage due to climatic and hydrological changes, whereas artificial shrinkage related to increased human activities dominated during 2000-2019. The normalized difference vegetation index (NDVI) increased during 1988-1999 and was affected by hydrometeorological factors in the same year and lake area in the previous year. The rate of increase in the NDVI accelerated during 2000-2012 owing to the impact of human activities which is reflected by GDP, and the related artificial shrinkage of the lake area. The main hydrochemical type of lake water changed from Ca-HCO3 before 2000, which was similar to that of river water, to Na-HCO3 after 2000, which was the main hydrochemical type of groundwater. In addition, most of the lake water and groundwater samples were rich in H2SiO3, and a strong correlation between the TWS and lake area for the period 2003-2019 indicated that lake water was strongly affected by groundwater, which was related to deep circulation. Therefore, the decreased river water recharge owing to human activities made groundwater contribute more to lake water. This study can substantially improve the decision-making support for environmental protection and water resource management in ecologically diverse areas, specifically in arid and semi-arid areas.