Monsoon Asia Rice Calendar: 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 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currently available global- or continental-scale rice calendars suffer from coarse resolution, poor recording, and outdated information, which do not provide detailed and consistent information on rice phenology. To address this limitation, this study mapped a new (2019 to 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 spatially explicit characterization of the rice transplanting date, harvest date, and number of rice croppings. The methodological framework incorporates two steps: (1) detection of rice phenological dates and number of rice croppings through combination of a feature-based algorithm and the fitted Weibull function, and (2) spatio-temporal integration of the detected transplanting and harvest dates 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 rice calendar, the proposed rice calendar outperformed the MODIS-based RICA rice calendar. It exhibited bias of 4 and −6 days for transplanting and harvest dates, respectively, with marked improvement in MAE by 10 and 15 days, and in RMSE by 6 and 15 days for transplanting and harvest dates, respectively. In total, the proposed rice calendar can detect single, double, and triple rice cropping with area of 5.3 × 10⁶, 4.5 × 10⁶, and 0.9 × 10⁶ km², respectively. This novel gridded rice calendar fills the gaps in finer-resolution rice calendars across major global rice production areas, facilitating research on rice phenology that is relevant to the climate change.