**6. Conclusions**

Ephemeral rivers, as an essential part of surface runo ff, maintain the stability of the watershed ecosystem and fulfill the water requirements of humans living in arid ungauged regions. With the advantage of high-resolution data and flexible application, the UAV technique has provided more perspectives for runo ff monitoring in addition to the traditional hydrological model and satellite remote sensing methods. In this study, a method was proposed to estimate the flood peak discharge of ephemeral rivers using the sediment mobility and UAV data derived in dry seasons. The main conclusions are as follows:


3. The effects of using small moving stones or large boulders in the proposed method depend on the discharge in the ephemeral river. In the river with a little flow, identifying smaller moving stones would increase the estimation accuracy. In large ephemeral rivers, estimation results are greatly influenced by using smaller stones or large boulders.

The proposed methodology provides a fast and flexible way of estimating the peak discharge of ephemeral rivers in arid ungauged regions with appropriate accuracy. The method is worthy of further development and local calibration for extensive applications in obtaining runoff information of ephemeral rivers in ungauged regions in the future, especially with the quick advance of UAV technology.

**Author Contributions:** Conceptualization, S.Y., C.L. and H.L.; Methodology, C.L. and P.W.; Software, J.W. and X.R.; Visualization, P.W. and J.W.; Writing – original draft, C.L.; Writing – review & editing, S.Y. and H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China (grant number U1603241, 41801334, and U1812401), China Postdoctoral Science Foundation (grant number 2017M620663), and National Young Scientist Natural Science Foundation of China (grant number 41601569).

**Acknowledgments:** We thank Glenn Pennycook for editing the English text of the original draft of this paper.

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