A Novel Rape ( <i>Brassica Napus L.</i> ) Parameter Canopy Area Index and Its Inversion Using Full- and Compact-Polarization SAR

The canopy parameters play a critical role in determining crop productivity, resource use efficiency, and adaptation to environmental conditions. Traditional crop canopy parameters predominantly focus on measuring Leaf Area Index (LAI) and neglect other canopy components, leading to substantial uncertainty of crop yield estimation by canopy parameter. To fill this gap, a novel rape crop parameter—Canopy Area Index (CAI)—was proposed in this study. CAI accounts for both leaf area and the structure of nonfoliar green components, and was inverted using compact-polarization Synthetic Aperture Radar (SAR) data. First, by analyzing the succession characteristics of rape crop photosynthetic organs and canopy components at different growth stages, CAI was defined as the sum of LAI and Non-Foliar Area Index (NFAI), the latter derived from the structure of siliques. Then, using the compact-polarization SAR data simulated from fully polarized data as the data source, regional rape crop CAI inversion was performed using the RF model, and its accuracy was compared to that obtained using full-polarization SAR data. Results indicate that LAI underestimated canopy area by 29.1% during the silique stage and 42.1% during the maturity stage. Inversion model using full-polarization SAR achieved a normalized root mean squared error (nRMSE) of 6.7%, while compact-polarization SAR produced comparable accuracy (nRMSE: 7.7% to 5.5%), suggesting that compact-polarization SAR could provide sufficient information for CAI inversion with simpler design and lower data volume. These results offer a robust technical basis for monitoring crop growth and estimating yield, particularly in regions prone to cloudy and rainy weather.