Hyper-spectral Estimation Models Of Biophysical And Biochemical Parameters Of Winter Wheat In Different Stages

In this study,the winter wheat Xiaoyan 22 was selected as the research object in northwest region.During different growth period,the hyperspectral reflectance of winter wheat canopy measurements were collected by SVC HR-1024 I field-portable spectroradiometer,and at the same time,chlorophyll content of wheat canopy was obtained by using plant polyphenol portable measuring instrument,Leaf Area Index was obtained by using Sunscan.Analyze chlorophyll content and Leaf Area Index change characteristics and rules of winter wheat in different growth stages,first derivative transformation and continuum removal of winter wheat canopy spectra were processed,extract sensitive bands from original,first derivative and continuum removal spectrum respectively,established quantitative estimation model for chlorophyll content and LAI of winter wheat canopy based on multiple spectral bands and spectral absorption parameters.Caculate DVI,RVI,NDVI and SAVI vegetation index respectively within the scope of 350nm-1300 nm,400nm-2000 nm,looking for sensitive optimal band combination of chlorophyll content and LAI of winter wheat canopy.Building a variety of models and testing the accuracy of models,The main conclusions are as follows.(1)Analyze the change rules of original reflectance,first derivative spectrum,continuum removal spectrum of winter wheat canopy in different nitrogen levels at different growth stages,the study found that chlorophyll content of winter wheat canopy showed a rising trend from the standing stage to heading stage,chlorophyll content started to decline in filling stage,reach the minimum in milk period.chlorophyll content of winter wheat canopy increased with the increase of nitrogen content in every stage;(2)LAI of winter wheat canopy first rose and then declined with the growth period advancing,reaching the maxmum value in the heading stage,decreasing in the milk period,and the minimum value of LAI is at the standing period.The plant morphology of winter wheat changed greatly,the plant coverage increased and the LAI value increased,at the same time the difference of LAI of winter wheat canopy was obvious from the standting stage to the heading stage.With nitrogen content increasing at different nitrogen levels,the trend decline after rising,reaching the maximum value in the level of N2,then decreased gradually,whitch shows similar changes in each growth stage.(3)During the whole growth stage,the spectral reflectance of winter wheat canopy is low,reflectance is greater than 0.34 in the visible range of 350nm-680 nm,formed a high reflective platform in the near infrared range of 700nm-1300 nm,reflectance decreased progressively in the shortwave infrared range of 1300nm-2500 nm.The variation of the original spectral reflectance of winter wheat canopy was not obvious in the visible wavelength.From the starting period to the heading stage,the change law of the winter wheat was obvious in the near infrared wavelength.With the growth stage advancing,the original reflectance increased gradually.The difference of the winter wheat was significant in the visible light wavelength range from the filling stage to the milk stage.The reflectance increases gradually in the visible light range and decreases gradually in the near-infrared range.From N0-N5,with the increase of nitrogen fertilizer level,the difference of spectral curves of wheat canopy was significant,and the reflectance in the near infrared band increased gradually,but there was no significant difference in the visible light band.At different growth stages and under different nitrogen levels,the red edge of winter wheat has "double peaks" or "multi peaks" phenomenon.(4)The sensitive spectral bands of chlorophyll content of winter wheat canopy are in the range of 350nm-740 nm in each growth period,the red edge position gradually move to the long wavelength,that the "red shift";green peak reflectance near 550 nm decreased gradually,the absorption depth increased gradually.Chlorophyll strongly absorb blue light(500nm)and red light(670nm),whitch formed two absorption valley.With the range of 410nm-770 nm in the continuum removed spectrum,forming a "double valley" structure,with the increase of chlorophyll content,the spectral absorption valley position in the range of 410nm-550 nm continuum removed spectrum moved to the long wavelength.The maximum absorption depth increased,the absorption peak area was significantly reduced.In the 410nm-770 nm and 550nm-770 nm band range of the continuum removal spectrum,the absorption valley area is significantly increased,the change of the absorption valley position and the maximum absorption depth is similar to that of the 410nm~550nm band continuum removal spectrum.(5)The original spectrum and chlorophyll content showed a significant negative correlation with the 350nm-740 nm wavelength range in the growth period,the correlation between chlorophyll content and other band is weak.And the correlation between chlorophyll content and first derivative spectrum is significantly negative in 400nm-550 nm,positive in 580nm-800nm;The correlation betwwen chlorophyll content and continuum removed spectrum is significantly negative in 400nm-760 nm,significantly positive in 780nm-820 nm,the correlation between chlorophyll content and other band is weak.In the models of estimating chlorophyll content of winter wheat canopy,the BP artificial neural network models of principal component analysis based on spectral absorption characteristic parameters has the hightest estimation accuracy.(6)In the different growth stage,the correlation coefficients between spectrum of winter wheat canopy and LAI showed a significant negative correlation in 400nm-700 nm,a significantly positive correlation within the range of 730nm-1330 nm and 1844nm-1914 nm,the correlation between LAI and other band is weak.The correlation between LAI and first derivative spectrum is significantly negative within the range of 450nm-1433 nm.There is a significantly positive correlation between LAI and first derivative spectrum in the range of 650nm-1836 nm,a weak correlation in other band.The correlation coefficient between LAI and continuum removal spectrum showed a significant negative correlation in 350nm-730 nm,1160nm-1860 nm and 1984nm-2397 nm,a significant positive correlation in 823nm-1914 nm,a significant positive correlation in 823nm-1914 nm,a weak correlation in the other bands.In the models of estimating LAI of winter wheat canopy,the BP artificial neural network models of principal component analysis based on spectral index has the hightest estimation accuracy.