**5. Conclusions**

This study focuses on the spatial and temporal variability of precipitation in the SRTR over the past two decades based on satellite observations and reanalysis data. The annual precipitation shows a slight reduction in the north and west of the SRTR and a slight increase in the east and south parts. In most regions of central SRTR, there is little variation in precipitation. For the annual variation, JJA contributes the most increases, while the remaining three seasons dominate the decrease in precipitation in most areas of the SRTR. In the west of the Yangtze River headwater region, although the precipitation amount does not increase, the precipitation occurrences ratio significantly raises. The higher spatial correlation between the precipitation amount and precipitation occurrences ratio in MAM and SON suggests the possibility of precipitation reduction caused by the decreasing precipitation frequency in these two seasons, while this is not the same in JJA and DJF. In contrast with the 2000s, the afternoon precipitation slightly reduced in the 2010s, while there is a strong increase shown for the nighttime precipitation. The spatial correlation coefficients between nighttime and total precipitation differences between the two decades are around 0.9 for almost all time periods, suggesting a dominant contribution of nighttime precipitation to the total precipitation changes. The water vapor transport changes dominate the precipitation changes in different seasons and months in summer, while the decreasing vertical velocity depresses precipitation to some extent. The changes in nighttime precipitation, especially its intensity, associated with the water vapor transport, contribute to the changes in precipitation over the SRTR region.

**Author Contributions:** Data curation, M.D. and Y.L.; Formal analysis, Z.L.; Project administration, X.M.; Software, M.D. and Y.L.; Writing—original draft, X.M.; Writing—review and editing, Z.L. and L.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was conducted under the auspices of the National Natural Science Foundation of China (41930759, 91837209, 42075089, 41975012), the Science and Technology Research Plan of Gansu Province (20JR10RA070).

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to that the GSMaP science team for making the satellite precipitation data available and thanks to the ECMWF for providing ERA5 reanalysis data.

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