| Chlorophyll content in crop canopy is vertically heterogeneous,and lower leaves tend to fade green and turn yellow first when crops were short of nitrogen.Therefore,timely and accurate monitoring of vertical distribution of chlorophyll content in crop canopy is of great significance for accurate management of crop growth.Remote sensing has several advantages over the traditional ground acquisition methods in determining the physical and chemical parameters of surface crops,including the characteristics of timely acquisition,low cost and wide coverage,etc.Therefore,remote sensing technology has been widely used in crop growth monitoring.Remote sensing technology has several advantages over the traditional ground acquisition methods in obtaining and analyzing the physical and chemical parameters of crops on the surface,including timely acquisition,low cost and wide coverage,etc.Therefore,remote sensing technology has been widely used in crop growth monitoring.However,the traditional remote sensing is mostly observed in a single vertical direction,and its ability to detect different layers of the vertical canopy of crops is still limited.The multi-angular remote sensing technology can greatly increase the three-dimensional spectral information of the target observation through the observation of multiple angles of the target crop,so as to realize the remote sensing monitoring of the vertical distribution of chlorophyll content in the canopy of crop leaves.In addition,UAV remote sensing has the advantages of low cost and low requirements on weather conditions,and it is gradually applied widely in the monitoring of crop growth.To some extent,the pure pixel spectrum of high-resolution images can be used as a substitute for spectral data at canopy scale.In this study,maize(Zea mays L.)and wheat(Triticum aestivum L.)were taken as the research objects,and the remote sensing monitoring method of vertical distribution of chlorophyll content in crop canopy was established by using multi-angular hyperspectral observation methods on the ground and low-altitude unmanned aerial vehicle hyperspectral observation methods,so as to achieve timely monitoring of chlorophyll content of crops.In this study,the following studies were carried out to solve the above problems.(1)Aiming at the remote sensing monitoring problem of the vertical distribution of chlorophyll content in the wheat canopy,this paper established a remote sensing monitoring method for the chlorophyll content in the wheat canopy based on the combined strategy of VZAs through the multi-angular observation experiment of wheat,and analyzed different observations based on the TCARI(Transformed chlorophyll absorption ratio index)and MCARI/OSAVI(Modified chlorophyll absorption ratio index/Optimized soil adjusted vegetation index)indices.The degree of information redundancy between the spectral indices and the contribution of different layers of chlorophyll content to the overall spectral response were determined.The results showed that the VZA 10° was used as the VZA for determining the chlorophyll content of the upper-layer,and 10° and 30° were used as the VZA for the middle-layer.The VZA 10°,30° and 50° were used for the chlorophyll content of the bottom-layer leaves.The monitoring model established in the research could weaken the influence of the chlorophyll content of the upper-layer on the middle-and bottom-layers to a certain extent,so as to obtain a more accurate monitoring effect of the chlorophyll content of the middle-and bottom layer leaves.(2)In this study,the sensitive characteristics and sensitive view zenith angle(VZA)of vertical distribution of chlorophyll content in maize canopy at different growth stages were studied through remote sensing observation experiments from multiple angles(VZA 0°,10°,20°,30°,40°,50° and 60°).The results showed that there was a good correlation between EPI(Eucalyptus pigment index)and REIP(Red edge inflection point)index and the chlorophyll content(SPAD value)of maize canopy.For the early growth stage(before and excluding the tasseling stage),the optimal VZAs of chlorophyll content in the 1st-,2nd-and 3rd-layers of maize canopy were 0°,30° and 40°,respectively.For the late growth stage(after and including the tasseling stage),the optimal VZA of chlorophyll content in the 1st-,2nd-and3rd-layers of maize canopy was 50°,and the optimal VZA of chlorophyll content in the 4th-layer was 40°.Through the screening of sensitive VZAs,EPI and REIP could well estimate the chlorophyll content in each layer of the canopy.Therefore,the canopy multi-angular spectral reflectance could accurately estimate the vertical distribution of chlorophyll content in maize canopy.The optimal VZA of different vertical layers varied in different growth stages,and the remote sensing monitoring ability of chlorophyll content in leaves of the 4th-layer at the late growth stage was limited.(3)For multi-angular spectrum on the depth of different maize canopy chlorophyll content monitoring method and ability also open research question,this study based on the different growth stages of the maize of multi-angular remote sensing observation experiment,discussed the different canopy depth on maize leaf chlorophyll content(SPAD value)of the maximum depth monitoring,and based on the VZA screening to determine the best monitoring model.The results showed that EPI index could be used to monitor all layers of canopy in the early growth stage.In the late growth stage,the VZA 50° could achieve the maximum detection depth of the canopy,and the detection depth could reach the upper-3-layer.However,with the increase of detection depth,the monitoring ability was continuously declining,and the inclusion of the 4th-layer with poor detection ability into the overall canopy(i.e.,the monitoring of the upper-4-layer)would lead to the decline of monitoring accuracy.(4)In view of the detection ability of near-ground unmanned aerial vehicle hyperspectral to the chlorophyll content in different layers of maize canopy,this study studied the monitoring ability of airborne hyperspectral imaging data to the vertical distribution of chlorophyll in maize canopy leaves through the hyperspectral observation experiment of unmanned aerial vehicle maize canopy.The results showed that the chlorophyll content in the 1st-layer to the 3rd-layer of maize could be effectively monitored based on the airborne hyperspectral imaging data from vertical observation of the milking stage of maize,while the chlorophyll content in the4th-layer was poorly monitored.The main innovative contributions of the paper are as follows.(1)A multi-angle remote sensing monitoring method with different observation zenith angle combination strategies for different vertical layers of wheat has been established.This study fully considered the effect of leaf chlorophyll content in different vertical layers of wheat canopy on the spectral index changing under different VZAs.In view of the difference in the difficulty of remote sensing monitoring at different vertical layers,different numbers of VZAs were used for different vertical layers.Combining strategies to eliminate the impact of non-target layers has achieved a more accurate monitoring of the vertical distribution of chlorophyll content in wheat canopy leaves.(2)A multi-angular remote sensing monitoring method for the vertical distribution of canopy chlorophyll content of maize at different growth stages was established.This study proposes a remote sensing method for monitoring the vertical distribution of chlorophyll content in maize canopy leaves at different observation angles during the early and late growth stages of maize(before the tasseling stage)and the late growth stage(after the tasseling stage).The impact of layer structure characteristics changes on the remote sensing monitoring of chlorophyll content,realizing high-precision monitoring of the vertical distribution of chlorophyll content. |
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