Identification of optimal single date for rice crop discrimination and relationships between backscatter and biophysical parameters using RISAT-1 hybrid polarimetric SAR data

Synthetic aperture radar (SAR) sensors operate in all weather conditions and have a great potential for agricultural applications. Its usefulness in rainy (kharif) season during cloud cover when optical data cannot provide information is proven. The SAR images were used to discriminate cropped area, specifically rice cropped area using temporal images. These temporal images are costly and analysis intensive. The selection of date or growth stage is crucial part while using single date polarimetric SAR images for rice crop mapping. In this study, a single date was identified from a set of imagery during the rainy season for accurate rice mapping using C-band Radar Imaging Satellite-1 (RISAT-1) hybrid polarimetric data. The single optimal date imagery was selected by understanding the scattering mechanism, backscattering response and crop-based canopy architecture. The backscatter values and scattering mechanism of rice varying with growth stages were studied. The backscatter was low at transplanting stage and then as the crop grows, its backscatter increases. The saturation of backscatter was observed when the crop reaches harvest stage. At transplanting and harvest stages, odd and volume scattering dominates than double bounce scattering. Double bounce scattering was dominant at vegetative stage than other scattering mechanisms due to near vertical orientation of rice canopy and the crop height also reached maximum stage. Temporal variations of backscatter and sensitivity of backscatter to Leaf Area Index (LAI) were analyzed, and found that the relationship between backscatter and LAI showed good correlation. The analysis of multidate SAR data reflected good performance for LAI quantification and identification of optimal date for rice crop discrimination.