| Vegetation as the key factor of energy,water cycle and carbon balance of the earth’s ecosystem,the growth process and status are influenced by climate and environment.Vegetation parameters are highly sensitive,while microwave remote sensing technology enables continuous monitoring of the vegetation growth process.The accurate modeling of microwave radiative transfer model by modeling the geometric and physical parameters of vegetation growth process and analyzing its temporal variation pattern can improve the accuracy of model simulation,which is crucial to the dynamic monitoring of vegetation ecosystem.The microwave remote sensing parameters of vegetation canopy transmissivity()and canopy optical depth()describe the energy attenuation of microwave radiation energy through the vegetation canopy.The spatial and temporal variation characteristics of crop geometric and physical parameters are essential for the high accuracy simulation of microwave radiation parameters,and can also effectively explain the variation of water content(VWC)and canopy wilting process inside the vegetation canopy.The vegetation canopy is a layered 3-D structure with obvious horizontal and vertical dimensions,and the accurate construction of canopy geometry is important for remote sensing parametric simulation and growth state analysis.In order to effectively simulate the vegetation microwave radiation parameters,it is necessary to reproduce the canopy geometry characteristics and physical parameters changes of the entire growing season.The correlations and allocation between multiple parameters using multiple fitting relationships were analyzed in this study based on the datasets of crop parameters measured in the area of Nong’an County,Jilin Province for the whole growing season from 2021 to 2022.The analysis results found that leaf area index(LAI)had high correlations with other canopy parameters(the mean value of correlation coefficient R was greater than 0.79),which indicated that the LAI was an effective indicator for simulating other canopy parameters.Therefore,a semiempirical crop geometric and physical parameter model(GPMCM)with LAI as the input parameter was established in this study,which can not only analyze the spatial and temporal variation and allocation of crop canopy parameters,but also provide a reference for vegetation microwave radiative transfer model to improve the canopy parameters.Summarily,the spatial and temporal characteristics of vegetation canopy parameters were analyzed to improve the vegetation microwave radiation transmission model,and then analyze the variation patterns of vegetation transmittance()and optical depth()in the crop growth process.Analyzing the effects of vegetation microwave radiation characteristics on canopy structure at different periods of crop growth,the main research contents and conclusions of this study are as follows:(1)The canopy geometric and physical parameters were analyzed for their variation characteristics and allocation patterns for the entire growing season.The relationships between LAI and each parameter were established separately,and the results showed that the crop geometric parameters increased rapidly in the early growing season and stabilized in the latter part of the growing season.The fresh biomass and VWC of crop physical parameters showed the trend of rapid increase in the pre-growing season and decrease in the post-growing season,while the dry biomass showed a continuous accumulation process.The relative water content showed a linear decrease for the entire growing season,but the water loss in the leaves was significantly faster than that in the stem component in the latter growing season.(2)A model of crop geometric and physical parameters was established,and a segmented fitting method was selected to improve simulation accuracy based on the allocation of each crop parameter and the fitting relationships with LAI for the entire growing season.The simulation results of the geometric parameters of this segmented simulation modeling approach for crop height,long and short radius of stem diameter showed root mean square error(RMSE)of 28.96 cm,1.65 mm and 1.49 mm,respectively.The RMSE and mean absolute percentage error(MAPE)of physical parameters such as fresh biomass,dry biomass,water content and relative water content ranged from 0.04 to 0.51 kg/m2 and 3.68 to 27.28% in the stem and leaf fractions,respectively.(3)The vegetation microwave radiative transfer model(Tor Vergata)was improved by combining the crop geometric and physical parameter model(GPMCM).The canopy description parameters in the Tor Vergata model are improved by the crop model to simulate the vegetation canopy transmissivity()and optical depth()in the entire growing season.The characteristics of its variation at several simulated observation angles and polarization modes(H/V)were analyzed,as well as the effects of microwave radiation parameters on the variation of canopy components(stem and leaf)content.The results indicated that the stem component was the main source of contribution to and ,with the leaf component being the main factor contributing to the changes in t and τ for the post-growth stages.Finally,this study parameterized the entire growing season vegetation optical depth using crop LAI and VWC.The root mean square error and relative percentage error of the simulation results ranged from0.008 to 0.0192 and 2% to 10%,respectively.The high accuracy of the simulation results indicated the feasibility of this study. |
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