Monsoon Asia Rice Calendar (MARC): a gridded rice calendar in monsoon Asia based on Sentinel-1 and Sentinel-2 images

Abstract. An accurate and spatially explicit large-scale rice calendar can enhance the understanding of agricultural practices and their ecological services, particularly in monsoon Asia. However, currently available global- or continental-scale rice calendars suffer from coarse resolution, poor recording, and outdated information; thus, they do not provide detailed and consistent information on rice phenology. To address this limitation, this study mapped a new (2019–2020) gridded (0.5°×0.5° resolution) rice calendar for monsoon Asia based on Sentinel-1 and Sentinel-2 satellite images. The novelty of this rice calendar lies in the development of a consistent optimal methodological framework that allows the spatially explicit characterization of the rice transplanting date, harvesting date, and number of rice cropping seasons. The methodological framework incorporates two steps: (1) detection of rice phenological dates and number of rice cropping seasons via the combination of a feature-based algorithm and the fitted Weibull function; (2) spatiotemporal integration of the detected transplanting and harvesting dates derived from Step 1 using von Mises maximum likelihood estimates. Results revealed that the proposed rice calendar can accurately identify the rice phenological dates for three croppings in monsoon Asia. When compared with single rice data from the census-based RiceAtlas calendar, the proposed calendar exhibited better results than the MODIS-based Rice Crop Calendar for Asia (RICA). It exhibited biases of 4 and −6 d for the transplanting and harvesting dates, respectively, with 10 and 15 d lower MAE values and 6 and 15 d lower RMSE values for the transplanting and harvesting dates, respectively. In total, the proposed rice calendar can detect single-, double-, and triple-rice-cropping seasons with an area of 0.53×106, 0.45×106, and 0.09×106 km2, respectively. This novel gridded rice calendar fills the gaps in 0.5° rice calendars across major global rice production areas, facilitating research on rice phenology that is relevant to climate change. The developed gridded Monsoon Asia Rice Calendar (MARC) is available at https://doi.org/10.17595/20230728.001 (Zhao and Nishina, 2023).