基于角度指数的油菜叶片等效水厚度估算研究

叶片等效水厚度(EWT)是评估油菜生长状态的一个重要参数。为快速准确估算油菜叶片EWT,选择9个常用的植被水分指数(WI、PRI、NDVI、NDII、NDWI、MSI、PWI、GVMI、NDMI),在6个已有角度指数(β_SWIR1、SANI、SASI、ANIR、NANI、NASI)基础上,提出2种角度比值指数(SARI、NARI),并根据油菜叶片水吸收谷峰高光谱特征,提出基于水吸收谷1 450 nm和1 930 nm的8种改进型角度指数,利用以上25种角度指数估算不同施氮水平下苗期、蕾薹期以及不区分苗期、蕾薹期情况下的油菜叶片EWT。结果表明,苗期ANI₁₄₅₀、ASI₁₄₅₀、MSI、GVMI、NDII估算效果较好,R^2均达到0.81以上;蕾薹期ANI₁₉₃₀、ASI₁₉₃₀、NASI、SANI、GVMI、SARI效果最好,R^2均达到0.71以上;在不区分苗期、蕾薹期的情况下,改进型角度指数ANI₁₄₅₀、ASI₁₄₅₀效果最好,R^2均达到0.832,可以在不区分苗期、蕾薹期情况下对油菜叶片EWT进行估算,适用性更广。本研究提出的改进型角度指数不仅丰富了已有角度指数,且提高了其反演油菜叶片EWT的精度,为快速精确估计油菜叶片EWT提供了新的研究思路。

Estimation of Equivalent Water Thickness of Rapeseed Leaves Based on Angle Index

Leaf equivalent water thickness (EWT) is an important parameter to evaluate the growth state of rapeseed. To estimate EWT of rapeseed leaves quickly and accurately, 9 commonly used vegetation water indices (WI, PRI, NDVI, NDII, NDWI, MSI, PWI, GVMI, NDMI) are selected, and two kinds of angle ratio indices (SARI, NARI) are proposed based on the six existing angle indices (β_SWIR1, SANI, SASI, ANIR, NANI, NASI). According to the hyperspectral characteristics of the water absorption valleys and peaks of rapeseed, 8 improved angle indices are brought forward based on water absorption valley at the wavelengths of 1450 nm and 1930 nm. The above 25 angel indices are used to estimate leaf EWT of rapeseed under different nitrogen levels at the seeding stage, bud stage and without distinguishing between seedling stage and bud stage. The results show that ANI₁₄₅₀, ASI₁₄₅₀, MSI, GVMI and NDII performed better at the seeding stage, and their R² both achieved 0.81 or above. ANI₁₉₃₀, ASI₁₉₃₀, NASI, SANI, GVMI and SARI are the best at the bud stage as their R² are all larger than 0.71. Improved angle indices ANI₁₄₅₀ and ASI₁₄₅₀ have the best effect, with their R² at 0.832, which can be used to estimate the EWT of rapeseed leaves without distinguishing between seedling stage and bud stage, and the applicability is wider. The improved angle indices in this research can not only enrich the existing angle indices, but also improve the accuracy of rapeseed leaf EWT inversion. Moreover, the indices provide a new idea for estimation of rapeseed leaves EWT instantly and precisely.