Study On Spectral Characteristics And Radiation Modeling Of Leaf-Ear Composite Canopy In Wheat

Wheat is one of the most important rations in China.The development of wheat industry is directly related to national food security and social stability.The establishment of remote sensing monitoring technology for wheat growth is of great significance for wheat management and food security in China.Crop canopy spectral characteristics and radiation modeling are the key technologies of quantitative remote sensing.In recent years,scholars have done a lot of research on the construction and application of radiative transfer model,but there are still many deficiencies.One of the significance points is that in the process of building the existing models,only the vegetation leaves and soil background are considered,while other organs or components(such as ear organs,returned straw,etc.)are not considered.Wheat ear is an important component of wheat canopy in the late growth stage,and straw is a part of the background,which also affects the spectral reflectance of wheat canopy.In this study,wheat was used as the research object.Through two years of field experiments with different varieties,densities and nitrogen levels,the reflectance of wheat canopy was measured by Field Spec Pro FR2500 back mounted hyperspectral radiometer,and the reflectance of single leaf and ear were measured simultaneously.Meanwhile,the relevant agronomic parameters were obtained destructively,Considering the effects of straw background and wheat ear,the radiative transport model of leaf-ear canopy(ACRM-s)was established.By comparing the spectral differences of straw,soil,leaf and wheat ear,the effects of straw soil background and wheat ear on canopy reflectance spectrum were clarified.Digital camera was used to take photos,and threshold segmentation was used to determine straw soil background coverage parameters.Based on the analysis of spectral characteristics and geometric shape of wheat ear,the reflectance and transmittance of wheat ear were simulated by using LIBERTY model.Considering the actual geometric shape of wheat ear,the geometric optical model of wheat ear is divided into three parts,the upper part is a cone,the middle part is a cylinder.Finally,a new radiative transport model of leaf-ear canopy(ACRM-s)was constructed by combining two-layer canopy bidirectional reflectance model(ACRM)with shading coefficient and straw-soil background.The ACRM-s model was verified and evaluated by using the measured bidirectional and vertical reflectance of wheat canopy at different growth stages and under different cultivation conditions.The results showed that the simulated values of ACRM-s model were significantly higher than those of PROSAIL model from tillering stage to jointing stage after the modified background effect,which might be because the two periods were greatly affected by straw background.RRMSE values of ACRM-s model and measured values were concentrated in the range of 20%-40%,while RRMSE values of PROSAIL model and measured values were concentrated in the range of 40%-60%.The former can significantly improve the blue band(450 nm),green band(550 nm),red band(600 nm),red-edge band(700 nm)and near-infrared band(800 nm)at the heading stage(including heading,flowering and grain filling)of wheat,which is closer to the measured spectrum.The mean R~2 of ACRM-s model was 0.88,and the mean RRMSE was 32%.The mean R~2of PROSAIL model detection results was 0.83,and the mean RRMSE was 36%.Moreover,the simulation accuracy of the former was better than that of the latter in the middle and late growth period of wheat(i.e.booting stage to grain filling stage),especially in the heading stage.Meanwhile,the R~2 of the simulated and measured vertical spectra of typical bands(Blue-450 nm,Green-550 nm,Red-600 nm,Red edge-700 nm,and NIR-800 nm for example)are all higher than 0.8.Taking the red-edge band heading date data as an example,the R~2of the simulated and measured values of ACRM-s model and PROSAIL model are0.90 and 0.85 respectively,and the relative root mean square error is 0.23 and 0.34respectively.In general,ACRM-s model can more accurately simulate wheat canopy reflectance than PROSAIL model,which provides a certain technical support for the simulation of wheat canopy spectrum and the inversion of growth parameters.