Retrieval of Green-up Onset Date From MODIS Derived NDVI in Grasslands of Inner Mongolia

In this paper, moderate resolution imaging spectroradiometer (MODIS) derived normalized difference vegetation index (NDVI) was used to retrieve the onset date of spring phenology in grasslands of Inner Mongolia from 2002 to 2017. To validate the application of MODIS derived data, long-term retrieval and investigation on its reliability was performed. The correlation between MODIS derived green-up onset date and ground observations from 2002 to 2012 were given medium to high strength (R²: 0.58-0.81). Besides, the relationship between seasonal climate variables, including winter and spring precipitation and temperature, and MODIS derived green-up onset date from 2002 to 2017 were established. Overall, linear regression analysis shows that there are 8-12 days RMSE for climate variables explaining green-up onset date shifting. This satellite method might have a poor performance in sparse and snow-cover grasslands but is fitted well in medium to dense vegetation and snow-free grasslands. These suggest that MODIS estimated spring phenology has the potential to be applied in grass resources utilization in Inner Mongolia.