Research On Vegetation Canopy Water Content Inversion Based On Microwave Remote Sensing Dat

Vegetation water content(VWC)is an important factor affecting photosynthesis and biomass of green plants.Traditional methods of vegetation water content inversion are usually based on leaf reflection properties in the visible and near-infrared wavelengths.These methods are limited in areas with frequent cloud and rainfall.Passive microwaves are increasingly being used for vegetation characterisation due to their ability to pass through haze particles and cloud cover.To cope with these limitations in the visible and near-infrared bands,this paper takes advantage of AMSR2’s multi-frequency observations of vegetation and conducts three studies,using the critical vegetation growth period of April and July 2022 as the study period,for conditions such as cloudy and rainy weather:firstly,the AIEM model is used to simulate the surface emissivity under various conditions(angle of incidence,soil moisture,Rms and correlation length)on bare ground Firstly,the AIEM model was used to simulate surface emissivity under various conditions(incidence angle,soil moisture,Rms and length of interest)and to find the relationship between surface emissivity and the adjacent frequency.A total of 80,960 simulations were carried out.Next,AMSR2 passive microwave data were used to invert the vegetation water content in the Chinese region.The VWC is solved by combining the radiative transfer model(ω-τmodel)and the passive microwave vegetation index(MVI)to derive an expression for the optical thickness of the vegetation.It is based on the assumption that the vegetation signal is uniformly distributed and unaffected by differences in polarisation.The method of multi-frequency passive microwave monitoring of vegetation is validated using ground truth data;finally,a nationwide vegetation moisture inversion is performed.The VWC retrieval accuracy was compared with the Normalised Vegetation Index(NDVI)obtained from optical sensor MODIS data.Finally,the specific emissivity vegetation index(EDVI)was calculated for different combinations of frequency bands at a national scale for the decade 2002 to 2011 based on Microwave Land Surface Emissivity(MLSE)data and its relationship with NDVI was explored.Finally,the Sen-Hurst test and Sen-MK test were used to evaluate the application potential of EDVI for different channel combinations in different vegetation type situations.The conclusions and findings are as follows:(1)The difference between the horizontal and vertical polarisation of surface emissivity between adjacent frequencies simulated using AIEM shows a linear relationship,which is determined by the frequency pairs,the closer the frequencies are to each other,the more obvious the linear relationship is,and vice versa,the weaker the linear relationship is.The root mean square error(RMSE)of the linear relationship between the L-band and C-band is 0.44%,and the RMSE of the linear relationship between the C-band and X-band is 0.10%.(2)The accuracy of the ground truth data for validation of the method in this paper R~2=0.71,RMSE=12.31.The vegetation water content retrieved from the AMSR data is well consistent and correlated with NDV on both spatial and temporal scales.At higher latitudes,vegetation water content was lower in April and NDVI values were smaller or even negative due to snow cover and vegetation dieback.During the vegetation growing season in July,vegetation water content and NDVI values increase significantly.At mid-latitudes,the seasonal variation in vegetation water content and NDVI is significant from April to July.At lower latitudes,the range of variation is not as pronounced.Both vegetation water content and NDVI values show high levels.(3)In order to reflect the characteristics and trends of annual-scale changes in vegetation water content,we calculated the EDVI,an index that can represent vegetation water content in China,for the decade from 2002 to 2011:1.The water content of vegetation in the central and western regions of China has continued to decline in this decade,but the magnitude of this decline is not significant;2.The water content of vegetation in the eastern and southeastern coastal regions of China has continued to increase in this decade,and the magnitude of this increase is more significant;3.The overall trend of change in the water content of vegetation in China is relatively stable.