**6. Conclusions**

To improve the degraded vegetation ecosystem in the arid areas of northwest China, caused by human activities and natural factors, artificial water delivery has become the main method of human intervention. Due to the contradiction between water use for agriculture and vegetation water demand in spring and summer, water transfer is generally carried out in autumn. However, the impact and mechanism of artificial water transfer on vegetation and wetland ecosystem restoration have not been thoroughly investigated, especially in northwest China. Taking the Qingtu Lake wetland system in the lower reach of Shiyang River in northwestern China as the study area, this study illustrated how water transfer a ffects the surface water, groundwater, and soil water interaction, and improves the vegetation ecosystem.

The surface area of Qingtu Lake expanded to the maximum in autumn and winter after water was transferred, and largely decreased the next summer of each year after 2010. The coverage of vegetation around Qingtu Lake area the next spring and summer also increased significantly each year from 2010. A positive correlation was found between the surface area of the lake body area in autumn and the vegetation coverage in each FVC interval the following summer, suggesting that water transfer improved the vegetation ecosystem in the study area. Further, we also explored the mechanism of how the water transfer in autumn, when the vegetation stops growing, improves the vegetation coverage in the following year. We found that the groundwater level increased after water transfer, and the soil water content increased and remained relatively high for the following months, which suggests that transferred water from upstream can be stored as groundwater or soil water in the subsurface through surface water. These water sources can provide the water for vegetation growth the next spring, or support the plants in the summer. Thus, ecological water transfer plays an important role in improving the vegetation ecosystem, even when the water is transferred in seasons when vegetation does not grow.

**Author Contributions:** Conceptualization, R.M.; methodology, S.Q.; validation, S.Q., R.M. and Z.S.; formal analysis, S.Q.; investigation, M.G., J.B., J.W., Z.W. and H.N.; resources, R.M. and Z.S.; data curation, M.G. and J.B.; writing—Original draft preparation, S.Q.; writing—Review and editing, R.M. and Z.S.; visualization, S.Q.; supervision, R.M.; project administration, R.M. and Z.S.; funding acquisition, R.M. and Z.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the National Key Research and Development Program of China (2017YFC0406105), National Natural Science Foundations of China (NSFC-41722208 and 41907177), and the Fundamental Research Funds for the Central Universities (CUGL180817).

**Acknowledgments:** We thank the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geosciences for their support for field trip and project coordination.

**Conflicts of Interest:** The authors declare no conflict of interest.
