不同水分条件下棉花冠层含氮量高光谱监测研究

【目的】建立1种适用于不同水分条件的棉田氮肥高光谱监测模型。【方法】通过设置包含灌溉梯度和施氮梯度的大田水肥试验,在生育期内同步测定棉花冠层光谱反射率、冠层含氮量(Canopy nitrogen content, CNC)、冠层等效水厚度(Canopy equivalent water thickness, CEWT)等信息,综合分析棉花冠层含氮量及冠层等效水厚度与光谱指数的相关性,确定最优光谱指数并构建棉花CNC的高光谱监测模型。【结果】冠层光谱与CNC在可见光波段附近出现连续的敏感区域,其中最大相关系数|r|max为0.53,位于718nm;在不考虑CEWT对模型精度影响时,NDSI(800,770)的建模效果最佳(R^2=0.76),但是进入花铃后期其预测精度偏低,出现了低估现象;综合考虑CEWT的影响后,本研究选取NDSI(570,500)作为最优光谱指数,所建模型有效改善了棉花含水率变化而造成模型精度偏低的现象(RRMSE=0.18)。【结论】本研究建立的新型水分钝感光谱指数NDSI(570,500)可以有效提升棉花CNC的估算精度,为高光谱技术在棉田氮肥监测的应用提供技术依据。

Estimating Nitrogen Content in Cotton Canopy Using Hyperspectral Imaging

【Objective】Timely monitoring the change in nitrogen content in plant canopy can improve fertilization,and in this paper we investigated the feasibility of using hyperspectral imagining to estimate nitrogen in carbon canopy under different irrigation and fertilization gradients.【Method】The irrigation and nitrogen application gradients were established in a field.During the growth period of the cotton,we measured spectral reflectance,canopy nitrogen content and canopy equivalent water thickness(CEWT)concurrently.The impacts of nitrogen content and the equivalent water thickness on the spectral index were quantitatively analyzed to determine the optimal spectral index that can be used to estimate the nitrogen content in the cotton canopy.【Result】The spectra that were most sensitive to the canopy nitrogen content were the wave between the visible and near-infrared bands,and the maximum correlation coefficient between them was 0.53 at the band of 718 nm.The NDSI(800,770)gave the best results with R2=0.76 without considering the influence of CEWT,but it was less accurate for estimating canopy nitrogen content at the flower-boll stage.Considering the influence of CEWT,we chose NDSI(570,500)as the optimal spectral index which can reduce errors caused by water content(RRMSE=0.18).【Conclusion】The NDSI(570,500),a moisture-insensitive spectral index established in this paper,can effectively improve estimation of nitrogen content in cotton canopy,providing a quick way to estimate nitrogen content in plant canopy of cotton at large scales.