Development and analysis of spectral signatures of onion crop using ground-based hyperspectral measurements

Accurate crop monitoring requires precise characterization of crop reflectance behavior across the electromagnetic spectrum. This study presents a detailed development and interpretation of weekly spectral signatures of onion (Allium cepa L.) captured using an SVC HR-1024i spectroradiometer during the 2023-24 growing season. Measurements were taken from 11 to 99 Days After Sowing (DAS), covering major phenological stages. Reflectance values showed the highest visible-band peak at ~550 nm, and strong absorption near 450 nm and 680 nm. The near-infrared (NIR) region (>700 nm) exhibited a marked increase in reflectance corresponding to canopy development, while characteristic water absorption dips were observed near 1400 nm and 1900 nm. NDVI rose from 0.49 at 11 DAS to 0.90 at 67 DAS, before declining to 0.37 at 99 DAS. Polynomial NDVI estimation models achieved a coefficient of determination exceeding R² = 0.96, demonstrating the reliability of spectral signatures as indicators of crop physiological trends. Results confirm that hyperspectral ground sensing is effective for monitoring onion crop vigor, biomass progression, water status, and senescence dynamics.