# Validation Analysis of Drought Monitoring Based on FY-4 Satellite
## 5 Conclusions  
The FY-4A satellite is the second generation of geostationary meteorological satellites in China. We proposed a novel method for calculating daily TVDI using FY-4A geostationary meteorological satellites and conducted a verification analysis. The main findings include as follows:  
1. The strong negative correlation between TVDI and site MCI was 15% in the Huanghuai region, 6.5% in Yunnan Province, and 51.9% in Guangdong Province. The percentage of negative correlation between TVDI and pixel SHRI were 24.8%, 28.7%, and 40.7% in the Huanghuai region, Yunnan Province, and Guangdong Province. TVDI had a strong negative correlation with MCI and SHRI in the severe drought area. 2. The trends of TVDI, MCI, and SHRI were similar. The pixel difference between increased TVDI and decreased SHRI is \(-\)13.5% in the Huanghuai region, 12.8% in Yunnan Province, and 28.1% in Guangdong Province. The trends of proportion in drought areas on TVDI and SHRI during the drought process were consistent. 3. The drought area on TVDI was generally greater than the area of SHRI. The pixels difference between TVDI and SHRI are mainly in the Huanghuai region, Yunnan Province, and Guangdong Province, which are [5.16-28.3%], [16.93-25.22%], and [6.66-18.93%]. 4. Comparing FY-4A TVDI and MODIS TVDI, the spatial distribution is consistent, but there are differences in the severe drought areas, and the monitored severe drought area was larger than MODIS, similar to MCI. FY-4A yielded a higher number of valid values, with 14,149 and 13,712 pixels, respectively. Similarly, we observed consistent spatial distribution of TVDI between Landsat-8 and FY-4A. FY-4A TVDI shows similar numerical distributions with Landsat-8 TVDI. 5. In a comparative analysis of TVDI calculations by FY-4A and FY-4B, the LST's differences were mainly concentrated in [\(-\)10, 15], with the median around 3. The NDVI's differences were concentrated in [\(-\)0.05, 0.27], with the median around 0.15. This study is based on a geostationary meteorological satellite and accomplishes the data fusion of multi-imagery within one day, enabling the implementation of the daily-scale TVDI calculation. Through a comparative analysis of different drought indices (MCI and SRHI) and TVDI based on earth observation satellites data (MODIS and Landsat-8), the dynamic drought monitoring capability of FY-4A has been validated. Furthermore, FY-4A TVDI has demonstrated the ability to provide higher-frequency drought monitoring compared to earth observation satellites, ensuring the timeliness and accuracy of drought monitoring. It can provide high-frequency drought monitoring for the national meteorological department, enabling timely drought warnings for rapidly occurring droughs in agricultural production. It aids in reducing agricultural losses, safeguarding food security, and mitigating socio-economic losses. High-frequency drought monitoring can help to get accurate assessments of drought duration, frequency, and phases, and make appropriate decisions by policymakers. In this study, TVDI exhibits a lag compared to meteorological drought indices, and its accuracy in drought monitoring is compromised by cloud cover. These factors introduce disturbances to the precision of drought monitoring. Therefore, in future research, it is worth exploring the other types of drought indices by FY-4 series satellite and investigating combined applications between FY-4A and FY-4B. Additionally, the impact of cloud cover on drought indices might be addressed by improving cloud detection accuracy and utilizing intelligent algorithms to reconstruct data in cloud-covered areas. Conceptualization, H.L. and L.H. ; methodology, Z.M. ; software, Z.M. ; validation, Y.L. ([PERSON]), H.W. and B.L. ; formal analysis, H.L. ; investigation, Y.L. ([PERSON]); resources, H.W. ; data curation, B.L. ; writing--original draft preparation, Z.M. ; writing--review and editing, H.L. and L.H. ; visualization, Y.L. ([PERSON]); supervision, Y.L. ([PERSON]); project administration, L.H. and H.L. ; funding acquisition, H.W.