Abstract
Lodging, a commonly occurring rice crop disaster, seriously reduces rice quality and production. Monitoring rice lodging after a typhoon event is essential for evaluating yield loss and formulating suitable remedial policies. The availability of Sentinel-1 and Sentinel-2 open-access remote sensing data provides large-scale information with a short revisit time to be freely accessed. Data from these sources have been previously shown to identify lodged crops. In this study, therefore, Sentinel-1 and Sentinel-2 data after a typhoon event were combined to enable monitoring of lodging rice to be quickly undertaken. In this context, the sensitivity of synthetic aperture radar (SAR) features (SF) and spectral indices (SI) extracted from Sentinel-1 and Sentinel-2 to lodged rice were analyzed, and a model was constructed for selecting optimal sensitive parameters for lodging rice (OSPL). OSPL has high sensitivity to lodged rice and strong ability to distinguish lodged rice from healthy rice. After screening, Band 11 (SWIR-1) and Band 12 (SWIR-2) were identified as optimal spectral indices (OSI), and VV, VV + VH and Shannon Entropy were optimal SAR features (OSF). Three classification results of lodging rice were acquired using the Random Forest classification (RFC) method based on OSI, OSF and integrated OSI–OSF stack images, respectively. Results indicate that an overall level of accuracy of 91.29% was achieved with the combination of SAR and optical optimal parameters. The result was 2.91% and 6.05% better than solely using optical or SAR processes, respectively.
Highlights
Rice is a major food source for nearly half of the world’s population
The significant increase in backscatter coefficients on 7 September was mainly caused by heavy rain (63 mm/d, Figure 4c), similar to findings from previous studies [29,30]
The significant increase in backscatter results recorded on 7 September associated to heavy rain, resulted in rice lodging to be overwhelmed
Summary
Rice is a major food source for nearly half of the world’s population. It is widely planted inChina [1], making China the largest global rice producer [2]. Rice is a major food source for nearly half of the world’s population. Rice lodging is a common disaster which is mainly caused by typhoons or hurricanes. The magnitude of rice production and grain quality will decrease as photosynthesis processes are disrupted due to self-shading, and translocation of nutrients and carbon from roots to grain is interpreted after lodging [4,5]. It is, Sensors 2020, 20, 7346; doi:10.3390/s20247346 www.mdpi.com/journal/sensors
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