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

摘要

High-resolution (ca. 1m) 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 (σ~0) 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 4years 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 σ~0 was clearly demonstrated. The baseline σ~0 values (minimum σ~0 for zero-biomass paddy fields) were determined to be -28.5dB in VH and -21.1dB in HH and VV, respectively. The dynamic change in σ~0 during the full range of rice growth was similar (ca. 12dB) in all polarizations. A comprehensive analysis revealed the response of C-band σ~0 to biophysical canopy variables. High or moderate sensitivity of σ~0 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 σ~0 throughout all growth stages. These relationships were expressed by exponential curves with high coefficients of determination, although σ~0 saturates at around a LAI of 3 and a leaf biomass of 180gDWm-2. The response of σ~0 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 σ~0 had the highest correlation with fAPAR, and the σ~0-fAPAR relationship was linear throughout the growth stages. The results suggest the suitability of C-band σ~0 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.