Study Of Agricultural Drought Monitoring In South Asia Based On FengYun-3D MERSI-2 Data

South Asia,with its dense population and heavy reliance on agriculture,is vulnerable to the impacts of climate change.Extreme drought events are likely to increase in frequency and intensity,pose significant challenges to South Asian countries.Therefore,effective drought monitoring plays a critical concern in mitigating the adverse impacts of droughts and supporting sustainable agricultural practices.Satellite remote sensing has been proven to be an effective way for providing timely and accurate information about drought.Feng Yun 3D Medium Resolution Spectral Imager 2(FY-3D MERSI-2)is the second-generation polar-orbiting satellite launched by China in 2017,which has similar band settings to MODIS data,suitable for regional and global monitoring.Hence,this study utilizes FY-3D MERSI-2data for agricultural drought monitoring in South Asia first time.The main research objectives include:(1)developing a Two-Factor Split-Window(TFSW)land surface temperature retrieval algorithm for MERSI-2 data,(2)analyzing the spatiotemporal patterns of land surface temperature in South Asian countries and(3)conducting agricultural drought monitoring in South Asia during time span of 2018 to2021.In the first phase of the study,a Two-Factor Split-Window(TFSW)land surface temperature retrieval algorithm was developed for MERSI-2 data to obtain land surface temperature.The basic coefficients of the TFSW algorithm were fitted based on the thermal infrared bands.The near-infrared(NIR)bands 16,17,and 18 were selected as atmospheric water vapor absorption bands,and band 4 was chosen as a non-absorption band to establish an atmospheric water vapor retrieval algorithm.Additionally,considering the impact of the viewing zenith angle(VZA)on atmospheric transmittance(AT),an equation for atmospheric transmittance inversion and angle correction was proposed.Finally,the retrieved MERIS-2 land surface temperature was cross-validated with MODIS(Moderate-resolution Imaging Spectroradiometer)land surface temperature product.The results indicate that the accuracy of MERSI-2 land surface temperature is relatively high.When compare with simulated Radiative Transfer Equation LST retrieved from MODTRAN a Mean absolute error(MAE)of <1℃ was observed,the cross-validation with MODIS LST product indicating error ranges from 1.74 to 2.76℃,and the correlation coefficients(R2)are between 0.84 and 0.93.In the second phase of the study,a comprehensive spatiotemporal analysis of LST across south Asian countries from 2018 to 2021 was conducted at annual,seasonal,and monthly intervals.V Various statistical techniques were applied to the MERSI-2 images,including Man-Kendall trend analysis,Sen’s slope estimator,and Anomaly Index for time-series analysis.The results indicate that the trends and spatiotemporal patterns of MERSI-2 land surface temperature are generally consistent with MODIS land surface temperature.Among different land cover types,bare land exhibits the highest temperatures,followed by grassland and built-up areas.The land surface temperature of different land cover types is negatively correlated with NDVI(Normalized Difference Vegetation Index).Finally,agricultural drought monitoring was conducted based on the Vegetation Health Index(VHI),Standardized Anomaly Index(SAI),and Evaporative Stress Index(ESI)indices.The monthly datasets were divided into three crop phenological stages using Food and Agriculture Organization(FAO)crop calendar information: Start of Season(SOS),Length of Season(LOS),and End of Season(EOS).The statistically significant trends were observed through Man-Kendall trend analyses and drought-affected areas were estimated considering severity classes of the VHI index over phenology stages.Additionally,FAO crop production data emerged as a crucial parameter,corroborating drought intensity observed through MERSI-2.Ultimately,the intensity of agricultural drought was validated using crop annual yield retrieved from FAO data.The results indicate that countries such as Afghanistan,Bangladesh,India,Pakistan,and Sri Lanka are significantly affected by drought.Overall,this study confirms the feasibility and applicability of FY-3D MERSI-2 in studying the spatiotemporal patterns of land surface temperature and regional drought monitoring.