Capability of C-band backscattering coefficients from high-resolution satellite SAR sensors to assess biophysical variables in paddy rice

High-resolution (ca. 1 m) synthetic aperture radar (SAR) sensors have great potential for all-weather monitoring of crop biophysical variables in small and mosaic crop fields in Asia. Rice is the most important staple crop in monsoon Asia, and the timely monitoring of rice growth is critical for precision farming and the assessment of productivity. The objective of this study was to determine the potential capability of backscattering coefficients (σ0) from satellite C-band SAR sensors for the assessment of biophysical variables in rice. SAR images were acquired by a Radarsat-2 sensor in spotlight mode during the critical growth stages over 4 years in one of the major rice-producing areas of Japan. Detailed plant biophysical measurements were made concurrently with the SAR observations. The seasonal consistency of C-band σ0 was clearly demonstrated. The baseline σ0 values (minimum σ0 for zero-biomass paddy fields) were determined to be − 28.5 dB in VH and − 21.1 dB in HH and VV, respectively. The dynamic change in σ0 during the full range of rice growth was similar (ca. 12 dB) in all polarizations. A comprehensive analysis revealed the response of C-band σ0 to biophysical canopy variables. High or moderate sensitivity of σ0 to canopy height, water content, or chlorophyll content was superficial and was attributable to the change in leaf biomass and structure. Both the leaf area index (LAI) and leaf biomass were significantly and consistently correlated with σ0 throughout all growth stages. These relationships were expressed by exponential curves with high coefficients of determination, although σ0 saturates at around a LAI of 3 and a leaf biomass of 180 gDW m− 2. The response of σ0 to total biomass was expressed by an exponential function with a high coefficient of determination, but the sensitivity was clear only within the lower 20% range of the seasonal maximum biomass. The C-band σ0 had the highest correlation with fAPAR, and the σ0–fAPAR relationship was linear throughout the growth stages. The results suggest the suitability of C-band σ0 for the assessment of LAI or fAPAR and show promise for the timely monitoring of rice growth by C-band SAR and/or through its constellation with optical sensors.