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. (authors)