**6. Conclusions**

Remote sensing techniques, model calculations, and statistical analyses were used to analyze lake surface area changes in response to climate change and groundwater in Qaidam Basin, and stale isotopes were used to identify potential sources of groundwater. Our analysis suggests that long-term increases in temperature and precipitation had a certain promotion effect on lake expansion, with higher temperatures accelerating glacier melting rather than promoting lake evaporation. However, the significant increase in the rate of lake expansion also indicated the important contribution of groundwater to lake expansion, which includes not only glacial meltwater that infiltrates the piedmont plain but also other sources of water. Isotope data revealed that confined groundwater can circulate rapidly and is recharged by modern water since the global nuclear explosion tests. The more depleted isotopic signature of the confined groundwater compared to that of local

meteoric precipitation and glacial meltwater suggested recharge by water sources with more isotopic depletion from other basins. Therefore, it is speculated that the 2003 Ms 6.4 earthquake in the northwest of Delingha may be a possible mechanism for the expansion of the lakes in the study area by enhancing crustal permeability and keeping fractures open, which promotes the groundwater contribution to lakes and in turn causes rapid lake expansion and increased groundwater levels.

The expansion of Gahai Lake has caused an increase in surrounding groundwater levels, which threatens the lives and livelihoods of residents. Under the long-term trend of climate warming, Gahai Lake will inevitably continue to expand in the future. Therefore, to ensure the safety of residents and continued operation of the irrigation area, it is suggested that drainage channels can be excavated at the end of the alluvial fan to divert groundwater to the Bayin River and eventually Tuosu Lake. This study emphasizes the important role of groundwater in lake expansion and improves our understanding of groundwater sources, circulation, and evolution patterns in Qaidam Basin and the arid area of northwest China. Currently, the future contribution of groundwater to lake expansion cannot be predicted because of a lack of groundwater monitoring data in the study area; therefore, future work should include long-term monitoring of the groundwater contribution to lakes in Qaidam Basin.

**Author Contributions:** Conceptualization, J.C. (Jiaqi Chen) and X.Z.; methodology, X.Z.; investigation, X.Z. and F.M.; writing—original draft preparation, X.Z.; writing—review and editing, J.C. (Jiaqi Chen), J.C. (Jiansheng Chen), and T.W.; supervision, J.C. (Jiansheng Chen). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the National Key Research and Development Program of China (Grant/Award Number: 2018YFC0406601), National Natural Science Foundation of China (Grant/Award Number: 61771183, and 42101021), and Fundamental Research Funds for the Central Universities (Grant/Award Number: B200202184).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are grateful for the help of the State Key Laboratory of Hydrology– Water Resources and Hydraulic Engineering at Hohai University, where all analyses were performed.

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