Comparison of crop growth monitoring based on FY-3B/D NDVI and MODIS NDVI

Objective evaluation of NDVI data, the crop growth monitoring data from Chinese Fengyun 3 (FY-3) that is open to the worldwide users, is helpful for the application and promotion of operational products, algorithm improvement and calibration of long-term time series datasets. In the study, the operational 10-day and monthly composite NDVI data sets of FY-3B and FY-3D are compared with the similar operational MODIS NDVI data sets of MOD13A1 and MOD13A3 and assessed for crop growth monitoring based on the interannual difference comparative model, vegetation condition index model and the NDVI-based crop growing progress. The results of the interannual difference comparative model suggest that the dynamic range of monitoring based on FY-3 NDVI is about 0.1—0.15 narrower than that based on MODIS NDVI, the model output differences are between −0.02 and 0.02, and their standard deviations are between 0.08 and 0.09, which can be ignored in cataloguing evaluation of the crop growth condition. But the narrower dynamic range of FY-3 suggests it is necessary to grade the crop level with a narrower threshold. As for the vegetation condition index model, the differences between FY-3B NDVI and MODIS NDVI are close to zero, which indicates FY-3B NDVI can generate the approximately equal model output to MODIS NDVI. Therefore, FY-3B NDVI, like MODIS NVDI, can make a distinction of different crop growth conditions among different years. Additionally, FY-3 and MODIS monthly composite NDVIs produce approximately the same change trend along with the crop growing season, but during the rapid change period of crop NDVI, MODIS monthly composite NDVI usually yields a different trend, which is resulted from its aggregation method using three 16-day composites with a weighted average approach. FY-3 10-day composite NDVI and MODIS 16-day composite NDVI have the same crop progress monitoring result, but the former provides better interannual differentiation during the exuberant period of crop growth, while the latter has a smoother monitoring curve. However, there is an overall difference between FY-3 NDVI and MODIS NDVI, which needs to be eliminated through further data processing, quality control and objective calibration to produce standardized NDVI products consistent with MODIS NDVI for long-term quantitative assessment of crop growth condition.