**6. Conclusions**

In this paper, the recharge resources of two lakes were explored based on remote sensing data and site data, and we studied to the reason for degradation of the lakes. We found that Daihai Lake is supplied with external groundwater, which alleviated the decline of the lake in arid areas owing to the existence of a water diversion structure. Huangqihai Lake lacks recharge of external groundwater directly, and humans pumped too much groundwater from the basin, resulting in the lake drying up. Most of the lakes in the Inner Mongolia Plateau are arid or semi-arid climatic conditions, but there are still many lakes. These lakes are basically dependent on direct or indirect recharge of exogenous

groundwater. However, these lakes are facing degradation in different degree. Although exogenous water can alleviate the decline of these lakes, the increasing human demand for water cannot prevent these lakes from disappearing eventually. These lakes play an extremely important role in semi-arid fragile ecosystems. People seem to be aware of the urgency of this problem, actions are being taken to curb the lake shrinkage and even to restore the lake to some extent, which we need to keep watch. Due to the limitation of the data quantity of remote sensing images, this paper did not calculate the material around the surface entirely according to the year of the lake area Figure 5 change, but selected the phenomenon of image observation about every five years, which may bring deviation to the experimental results. Besides, this paper cannot estimate the annual direct loss volume of the Huangqihai Lake due to the lack of the lake water level change data, which will make the conclusion insufficient. In the future, we will continue to monitor the changes of these lakes and whether human rescue measures can serve as a model for their protection.

**Author Contributions:** Conceptualization, J.C. and J.L.; Methodology, J.C. and J.L.; Software, J.L.; Validation, J.C., J.L., and Q.W.; Formal analysis, J.W.; Investigation, J.C. and J.L.; Resources, J.C.; Data curation, J.L.; Writing—original draft preparation, J.L., J.C., and N.L.; Writing—review and editing, N.L. and J.C.; Visualization, Q.W. and J.W.; Supervision, N.L.; Project administration, J.C.; Funding acquisition, J.C. and N.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded in part by the National Key R&D Program of China (no. 2017YFB0502700), in part by the National Natural Science Foundation of China (no. 61771183, 61601437), in part by the Fundamental Research Funds for the Central Universities (no. 2016B07114), in part by the Plan of Science and Technology of Henan Province (no. 192102210082), in part by the Youth Talent Lifting Project of Henan Province (no. 2019HYTP006), and in part by the China Postdoctoral Science Foundation (no. 2013M541035).

**Acknowledgments:** Thanks for Jiansheng Chen to furnish with professional idea on water hydrochemistry work; thanks for Zhanyang Zhang that provided assistance for Figure 11.

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