A Method for Summer Maize Phenology Monitoring by MODIS Data

Crop phenology period is an important feature of the agricultural eco-system. It is important to investigate the crop phenology period in large area for precision crop management and yield forecast by remote sensing technical. However, there are still some limitations in this approach, such as the investigation precision is restricted by the investigation area, and different types of crop growth curves don't match with the crop phenology period very well.Therefore, data from moderate-resolution imaging spectroradiometer (MODIS), and summer maize growth stages by field observations from 23 agricultural meteorological stations in Henan Province are adopted to improve the identification efficiency and accuracy. Normalized difference vegetation index (NDVI) growth curve with the time resolution of 1 d is reconstructed by denoising, smooth processing and logistic curve fitting. Crop growth feature points on the reconstructed growth curve are extracted by using dynamic threshold method and curvature extremum method. The optimum matching relationship between feature points and maize growth stages is constructed by based on the feature points, their occurrence date, and observed dates of growth stages. By matching with the investigated growth stage dates in 2013-2014, values of dynamic threshold 1 is chosen to extract the 7-leaf stage, with the root mean square error (RMSE) of 5.4 d. Values of minimum curvature is chosen to extract the jointing stage, with the RMSE of 6.4 d. Values of dynamic threshold 2 is chosen to extract the tasseling stage, with the RMSE of 6.0 d. By validation with the investigated growth stage dates in 2015, RMSE with the selected 3 key growth stages are all less than 6 d. The accuracy is higher than the earlier proposed methods to extract maize growth stages by using MODIS or other similar lower/medium spatial resolution remote sensing data. Map of summer maize critical phenology in Henan Province demonstrates that most of summer maize in the research areas enter 7-leaf stage, jointing stage and tasseling stage in June 25 to 27 June, 10 July to 17 July and 27 July to 2 August, respectively. Numbers of pixels, which enter 7-leaf stage, jointing stage and tasseling stage on the above-mentioned dates, account for 45.6%, 66.8% and 71% of the total, respectively. Maize growth stages obtained by the method proposed by this research can be used in crop management and grain yield forecast.