降雨影响下的水稻长势极化特征提取

Extraction of polarization characteristics of Oryza sativa growth under the rainfall fluctuation

【目的】直接利用合成孔径雷达(SAR)后向散射系数监测水稻Oryza sativa等南方作物长势的方法易受降雨影响,在时序曲线上易产生大幅波动。研究降低这一影响的方法,以期获得更准确反映水稻长势规律的结果。【方法】以广西桂平市水稻栽植区为研究对象,采用2018年3−11月共23期Sentinel-1双极化SAR数据,以及同期降雨数据,分析水稻长势在降雨影响下的同向极化(VV)和交叉极化(VH)后向散射系数时序特征,在此基础上提出基于VV和VH后向散射系数组合的改进方法——归一化差分后向散射指数(NDBI),并结合光学卫星影像数据对结果加以验证。【结果】NDBI时序曲线的平均曲率为0.86,低于VV和VH时序曲线的平均曲率(分别为2.90和2.31),表明NDBI时序曲线的变化趋势更为平稳,受降雨不规则波动的影响更小。早、晚2季水稻的NDBI时序曲线具有相似的变化趋势,能够更准确地反映水稻幼苗期、分蘖拔节期、长穗期和结实期各阶段的极化特征,即幼苗期主要以二次反射为主,而从分蘖拔节时期开始到长穗期再到结实期直至收割,体反射和漫反射不断增强,二次散射逐渐减小。【结论】利用NDBI方法降低了降雨对极化特征时序曲线扰动的影响,获得更准确的水稻长势规律。

[Objective]The method of directly using synthetic aperture radar(SAR)backscatter coefficient to monitor the growth trend of Oryza sativa and other crops in south China is susceptible to rainfall and prone to produce large fluctuations on the time series curve.The objective of this study is to explore methods to reduce this impact and obtain more accurate results reflecting the law of O.sativa growth.[Method]Taking the O.sativa planting area in Guiping City of Guangxi as sample area,the temporal characteristics of VV and VHpolarized backscattering coefficient of O.sativa growth under the influence of rainfall were analyzed using Sentinel-1 dual-polarization SAR data of 23 periods from March to November 2018 and the corresponding rainfall data.On this basis,an improved method named“Normalized Differential Backward Scattering Index(NDBI)”based on the combination of VV and VH-polarized backscatter coefficients was proposed and the results were verified by optical satellite image data.[Result]The mean curvature of the NDBI curve was 0.86,which was lower than that of VV and VH curves(2.90 and 2.31,respectively),indicating that the NDBI curve was more stable and less affected by irregular rainfall fluctuations.The NDBI curves of O.sativa in the early and late seasons had similar trends,which could more accurately reflect the polarization characteristics of O.sativa at the seedling stage,tiller jointing stage,long spike stage and fruiting stage.The secondary reflection was the main part in seedling stage,and from tiller jointing stage to long spike stage to fruiting stage to harvest stage,the body reflection and diffuse reflection increased continuously and the secondary scattering decreased gradually.[Conclusion]It is feasible and applicable to use the NDBI method to reduce the influence of rainfall on the disturbance of polarization characteristic time series curve and to obtain more accurate O.sativa growth law.