We develop a novel Normalized Harvest Phenology Index (NHPI) that integrates the Normalized Difference Vegetation Index (NDVI) and the Near-Infrared (NIR) reflectance to accurately monitor whether fields of corn and soybean have been harvested. Leveraging the distinct separability of NIR reflectance for corn and soybean before harvesting (senescent plants) and after harvesting (crop residue), combined with the contrasting trends between NIR and NDVI during this transition, the NIR-to-NDVI ratio amplifies the harvesting signal in its time series, making it a robust indicator of harvesting events. As the first spectral index designed for scalable identification of crop harvesting stage, the developed NHPI is applied to map harvesting dates for corn and soybean fields across the U.S. Midwest from 2020 to 2023 using Landsat and Sentinel-2 imagery via Google Earth Engine (GEE). The paper can be found in the following link: https://doi.org/10.1016/j.rse.2025.115016
