## 4 Conclusions  
The first comprehensive PWV retrieval and application platform (CPRAP) was established in this study. The CPRAP can be used to obtain PWV from GNSS, RS, ERA5,  
Figure 11: Relationship between hourly PWV and rainfall at ZJXC station over the period from 19 February to 10 March 2015, where (**a**) refers the time series of PWV and rainfall over the period of 19 February to 10 March 2015, (**b,c**) are the hourly PWV and rainfall on 26 February and 7 March, respectively.  
Figure 12: Relationship between hourly PWV and rainfall at ZHOS station over the period from 19 February to 10 March 2015, where (**a**) refers the time series of PWV and rainfall over the period of 19 February to 10 March 2015, (**b,c**) are the hourly PWV and rainfall on 26 February and 7 March, respectively.  
Sentinel-3A and FY-3A, respectively and can be applied for drought and rainfall monitoring. In retrieving PWV, the RMS, Bias and MAE of GNSS-derived PWV are 2.15, 0.05 and 1.65 mm, respectively, when compared with that of RS. The satellite-based PWV was also evaluated and the averaged RMS, Bias and MAE for FY-3A and Sentinel-3A are 4.46/0.56/3.61 mm and 2.95/0.01/1.37 mm, respectively, when compared with the values derived from GNSS and RS. Furthermore, the ERA5-derived PWV data were also compared with that from GNSS and RS, and the statistical result shows a good accuracy of ERA5 with the RMS of less than 2 mm in China over the period of 2012 to 2020. Experimental results show that in the Chinese region, the overall accuracy of PWV inversion of GNSS and ERA5 is higher. The overall accuracy of FY-3A and Sentinel-3A is lower, especially in the southern region of China, so the accuracy of GNSS and ERA5 in China is higher. In the aspect of PWV application, the CPRAP-derived PWV using different techniques was used for drought monitoring in Yunnan Province. The CPRAP-derived SPCI was found to have a good correlation with SPEI and the correlation coefficient is larger than 0.83 (\(p<0.05\)), which proves the ability of CPRAP-derived SPCI for drought monitoring. Furthermore, CPRAP-derived PWV was also used for rainfall monitoring in Zhejiang province, which shows that PWV generally increased with the different magnitudes and sharply decreased before and after the rainfall event, respectively. Such evident PWV change before rainfall is a very important signal for rainfall forecasting, and verifies the potential of established CPRAP for rainfall monitoring and forecasting.  
Conceptualization, Q.Z., X.Z. and K.W.; methodology, Q.Z. and X.Z.; software, Q.Z. and K.W.; validation, K.W., X.L., Z.L. and Y.S.; writing--original draft preparation, Q.Z. and X.Z.; writing--review and editing, X.Z., Y.S. and Y.L.; visualization, X.Z., K.W., Y.S. and Y.L.; All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Interdisciplinary joint research program of the department of aerospace medicine, Air Force Military Medical University (2021 SZJC1004) and key research and development plan of Shaanxi Province (2022 SF-190).  
Not applicable.  
The author would like to thank Crustal Movement Observation Network of China (CMONOC) for providing GNSS observations, the ECMWF, European Space Agency (ESA); National Satellite Meteorological Centre (NCMC) and IGRA are also thanked for providing the corresponding data.  
The authors declare no conflict of interest.