Research On Retrieval Method Of Cotton Nitrogen And Chlorophyll Based On UAV Visible Light Image

As the main cash crop in China,cotton is widely used in fine chemical industry and textile industry,and plays an important role in the national economy.In the growth process of cotton,nitrogen and chlorophyll are important agronomic parameters to measure the growth status of cotton,but the actual measurement procedures are complex and time-sensitive.The rapid and non-destructive dynamic monitoring of cotton nitrogen and chlorophyll can be realized by UAV remote sensing platform,which has an important guiding role for formula fertilization and nutrition diagnosis of cotton.This study was carried out in the experimental station of Shandong Cotton Research Center from June to August in 2020.Field experiments of cotton with different varieties,film mulching treatments and nitrogen fertilizer gradients were designed.Visible light images of the main growth stages of cotton were obtained by Phantom 4 RTK visible aerial vehicle,and nitrogen and chlorophyll in the cotton canopy were measured simultaneously.The UAV visible light image is preprocessed to extract 22 kinds of visible light vegetation index and 24kinds of texture features.Five univariate regression(UR)models based on vegetation index were established,and multivariate stepwise regression(SR)and partial least squares regression(PLSR)models based on vegetation index,texture features and vegetation index+texture features were used to estimate nitrogen and chlorophyll quantitatively.The main results are as follows:(1)the nitrogen content of cotton increased first and then decreased,that is,it increased from bud stage to early flowering stage,and decreased from early flowering stage,flowering and boll stage to full boll stage.The absolute value of correlation coefficient of texture features with high correlation with nitrogen was between 0.6 and 0.69,and the absolute value of correlation coefficient of vegetation index with high correlation was between 0.6 and 0.84,and the overall correlation of vegetation index was higher.In the nitrogen UR model,the logarithmic regression model with vegetation index GBDI as the independent variable has the best inversion effect(R~2=0.7864,RMSE=0.2525,RPD=2.19);In the nitrogen SR and PLSR models,the vegetation index+texture feature model is better than the single vegetation index model or texture feature model,and the inversion effect of nitrogen PLSR vegetation index+texture feature model is better than that of nitrogen SR vegetation index+texture feature model.In the comprehensive analysis,PLSR vegetation index+texture feature model(R~2=0.9503,RMSE=0.1308,RPD=4.23)can be used to retrieve cotton nitrogen quickly and accurately,and generate nitrogen grade distribution map,which provides guidance for cotton field formula fertilization.(2)The chlorophyll content of cotton in the four growth stages decreased first and then increased,that is,decreased from bud stage to early flowering stage,and increased from early flowering stage,flowering and boll stage to full boll stage.The absolute value of correlation coefficient of texture features with high correlation with chlorophyll is between 0.6 and 0.78,and the absolute value of correlation coefficient of vegetation index with high correlation is between 0.6 and 0.92,and the overall correlation of vegetation index is higher.In the chlorophyll UR model,the polynomial regression model with vegetation index NGBDI as independent variable has the best inversion effect(R~2=0.8620,RMSE=2.4550,RPD=2.72);In the chlorophyll SR and PLSR models,the vegetation index+texture feature model is better than the single vegetation index model or texture feature model,and the retrieval effect of chlorophyll PLSR vegetation index+texture feature model is better than that of chlorophyll SR vegetation index+texture feature model.Based on the comprehensive analysis,PLSR vegetation index+texture feature model(R~2=0.8976,RMSE=2.1198,RPD=3.14)can be used to retrieve cotton chlorophyll quickly and accurately,and generate chlorophyll grade distribution map,which can provide guidance for the field nutrition diagnosis of cotton.