Study On Coupling Of Soil Moisture And Vegetation Phenology Based On Multi-frequency Passive Microwave Remote Sensing

Soil moisture(SM)and vegetation optical depth(VOD)are important parameters in land surface energy balance,global water cycle,carbon cycle,and large-scale climate models.Currently,the use of passive microwave remote sensing technology to obtain large-scale SM and VOD has become the most promising approach.Although numerous SM and VOD products have been developed and used,the data quality of these products under different underlying surfaces and climatic conditions is still not fully understood due to retrieval algorithms and differences in observation frequencies,which makes it still challenging for their applications in nature disaster.The coupling mode between SM and vegetation phenology before being affected by natural disasters such as bushfires has not yet been accurately obtained.Moreover,bushfires can affect not only the green part of vegetation but also its non-green components,thus affecting the local vegetation phenology and rendering existing coupling models unsuitable during wildfire periods.Therefore,based on current multi-frequency passive microwave remote sensing data for SM and VOD,this thesis investigates long-term stable coupling between SM and vegetation phenology in Australia,taking the 2019-2020 forest fires into consideration.The main results are as follows:(1)Evaluation and inter-comparison of the data quality of multi-frequency passive microwave SM and VOD products is essential to provide applicability analyses of these data quality under different conditions.To address the problem of incomplete evaluation of data quality for SM and VOD products,in-situ measured data and optical vegetation indices were used as reference datasets.Quality metrics such as R,bias,RMSE and ub RMSE were considered under dynamic and static conditions to evaluate the data quality comprehensively.The study found that SM and VOD products based on L-band observation has better performance and is more suitable for monitoring dense vegetation areas,but this advantage is not applicable to all regions.The performance of multi-frequency passive microwave products is complementary under different underlying surface conditions.Verification revealed that the accuracy of SM retrieval results increases with increasing soil temperature,but the wetness degree of the soil is not highly correlated with the accuracy of SM retrieval results.(2)Obtain the stable seasonal coupling pattern between surface soil moisture(SSM)and vegetation leaf phenology under different vegetation communities that have not been affected by bushfires.In response to the problem of unsynchronized SM and vegetation phenology,the most suitable SM product for Australia and the leaf area index that can characterize vegetation phenology were selected based on the existing data quality verification,and the coupling model of their seasonal changes was studied.The study found that the seasonal variation of SSM is synchronized with the leaf phenology in the north and northeast of Australia.However,in the tropical forests of Southeast Asia,the growth of leaf area lags behind SSM by up to 90 days.The time lag between LAI development and SSM in the southwest is about 60 days.This pattern is verified by using three SSM products.(3)Obtain the coupling pattern between vegetation phnology and SM during the fire,and evaluate the severity of the 2019-2020 Australian bushfires.By dividing the fire into different stages and using a combination of optical vegetation indices and multi-frequency passive microwave remote sensing data,the impact of the fire on vegetation and soil was evaluated,and the coupling mode of the fire at different stages was obtained.Experimental results showed that bushfires resulted in a more synchronous seasonal variation between plant water storage and leaf phenology.In evergreen broadleaf forests and evergreen coniferous forests,the regeneration of leaf area lags behind plant water storage by 120 days.After bushfires,vegetation regeneration in shrublands and woody savannas was not significant.Bushfires would shorten the seasonal time lag between the development vegetation leaves and soil moisture.There are 27 figures,28 tables and 188 references.