盖度与冠层水深对沉水植物水盾草光谱特性的影响

遥感技术可应用于大尺度实时监测沉水植物的分布与生长状况。然而沉水植物的光谱特征受其冠层在水下深度的影响,从而影响湖泊和河流中沉水植物的遥感影像解译与信息提取。应用地物光谱仪,通过野外原位测定和室外控制试验,实测了沉水植物水盾草(Cabomba caroliniana)群落冠层在水下不同深度的反射光谱,分析了冠层水深对水盾草反射光谱的影响,并建立了基于光谱反射率和冠层水深的水盾草群落盖度反演模型。研究结果表明(1)不同盖度的水盾草群落光谱反射率的基本特征主要体现在绿光和近红外波段;(2)水盾草群落的光谱反射率与冠层水深基本呈负相关,相同盖度水盾草群落的光谱反射率随冠层水深的增加而减小,在近红外波段尤其明显;(3)水盾草群落冠层水深越小,其盖度与光谱反射率的相关性越强,且水盾草群落盖度越大,其光谱反射率与冠层水深的相关性越显著;(4)水盾草光谱反射率与盖度相关的最佳波段在692—898 nm,与冠层水深相关最佳的波段在710 nm和806 nm附近;(5)在710 nm和806 nm处建立的结合冠层水深的修正模型,无论是回归方程决定系数(R2),还是水盾草群落盖度的反演精度都明显高于仅用光谱反射率反演盖度的简单模型,因此可有效减除冠层水深对反演精度的影响。本研究的结果可为遥感监测沉水植物的分布和动态变化,以及沉水植物生物物理参量反演提供科学依据。

Impacts of coverage and canopy water depth on the spectral characteristics for a submerged plant Cabomba caroliniana

Remote sensing technology can be used to timely monitor the distribution and growth status of submerged aquatic vegetation (SAV) on a large scale. However, the water depth from water surface to plant canopy may affect the spectral characteristics of submersed plants, resulting in an inaccuracy to interpret the related remote sensing images. In this study, the spectral characteristics of a submerged plant Cabomba caroliniana with varying coverage and canopy water depth were measured using a FieldSpec^TM Pro JR Spectroradiometer in the field and a control experiment. The influence of canopy water depth on the spectral reflectance of Cabomba caroliniana was analyzed and a model was established to deduce the coverage of Cabomba caroliniana community based on the spectral reflectance and the canopy water depth. The results showed that (1) the basic characteristics of spectral reflectance for varying coverage of Cabomba caroliniana appeared mainly at the bands of green and NIR;(2)the spectral reflectance of Cabomba caroliniana community showed a negative correlation with the canopy water depth, and the spectral reflectance of Cabomba caroliniana community with same coverage decreased with increasing canopy water depth, especially at the band of NIR;(3)the shallower the canopy water depth, the more significant the correlation between the coverage and the spectral reflectance of Cabomba caroliniana community, which enhanced with the increasing coverage of Cabomba caroliniana; (4) the most significant correlations between the coverage and the spectral reflectance of Cabomba caroliniana community were observed at the band of 692-898 nm, while the most significant correlations between the canopy water depth and the spectral reflectance of Cabomba caroliniana community were near the bands of 710 nm and 806 nm; (5) a revised model established at the bands of 710 nm and 806 nm and adopted both the parameters of canopy water depth and spectral reflectance was much better than a simple model established at the bands of 710 nm and 806 nm and adopted only the parameter of spectral reflectance to deduce the coverage of Cabomba caroliniana community. The determination coefficients (R²) between the predicted and measured coverage of Cabomba caroliniana community were 0.742 and 0.777 for the simple model at the band of 710 nm and 806 nm,while were 0.852 and 0.935 for the revised model at the band of 710 nm and 806 nm. The revised model adopted both the parameters of canopy water depth and spectral reflectance could effectively reduce the influence of canopy water depth on the deductive accuracy for the coverage of submerge vegetation. The results from this study could be helpful to timely and accurately monitor the distribution and dynamics of submerged vegetation on a large scale using remote sensing technology.