Correlation between field-measured grain moisture content and C-band SAR backscatter: Potential use of Sentinel-1 SAR data to estimate optimal harvest timing in rice crop cultivation

Increases in temperature associated with climate change can potentially damage rice crops. Accurate prediction of the optimal time for harvesting therefore represents a key precautionary countermeasure that can mitigate against the damage caused by heat exposure and excessive heat-induced grain dryness. The remote sensing of grain moisture content, which is an important indicator for accurate harvest timings, has been rarely studied using microwave Synthetic Aperture Radar (SAR). Here we examine the relationship between crop biophysical parameters and C-band SAR backscatter in order to determine the correlation between field-measured grain moisture content and satellite SAR backscatter. This study takes advantage of the specific properties of microwave radar signals, and, in particular, their sensitivity to moisture content. We first pre-processed Sentinel-1 data taken in 2020 and 2022 to extract radar backscatter values averaged within rice crop fields. Partial least squares regression was used to determine the relation between field-measured crop biophysical parameters gathered over a 2-year period and backscatter data acquired during the ripening stage. This statistical analysis revealed that co-polarized radar backscatter strengths obtained with shallow incidence angles (of around 45°) exhibited a significant correlation to grain moisture content and leaf and tiller moisture content ( R 2 = 0.669), with grain moisture showing the highest variable importance. On the other hand, the correlation with these biophysical parameters was not statistically significant for backscatter at the steeper incidence angle (of around 35°). The ripening phenomenon restricts stem elongation in order to furnish nutrients to the grain so that the physical structure of the plant changes little and grain moisture substantially decreases. This most likely explains why backscatter signals obtained at shallower incidence angles, which are more susceptible to interaction with the crop canopy layer, can be strongly affected by grains. Our study indicates the great potential of synoptic C-band SAR observations for the estimation of grain moisture content in rice within individual rice-crop fields and thus points the way toward improved prediction of optimal harvest timings. These results should eventually lead to a better adaptation of rice cultivation to climate change and hence contribute to the establishment of more stable rice supplies.