Season-long daily measurements of multifrequency (Ka, Ku, X, C, and L) and full-polarization backscatter signatures over paddy rice field and their relationship with biological variables

The objective of this study was to investigate the interaction between microwave backscatter signatures and rice canopy growth variables, as well as to provide definitive insight into the interaction between backscatter and vegetation based on a comprehensive data set collected under the unique crop conditions of paddy rice (background is water surface). Our unique data consisted of daily microwave backscattering coefficients at all combinations of five frequencies (Ka, Ku, X, C, and L), all polarizations (HH, VH, HV, and VV), and four incident angles (25°, 35°, 45°, and 55°) for the entire rice crop period, from before transplantation until postharvest cultivation. A wide range of plant variables, such as leaf area index (LAI), and biomass of the whole plant and plant parts were measured periodically throughout the season. Analyses based on statistical correlation and a simple backscatter process model (the water cloud model) showed that LAI was best correlated with HH- and cross-polarization of the C-band, while fresh biomass was best correlated with HH- and cross-polarization of the L-band. Contrarily, the higher frequency bands (Ka, Ku, and X) were poorly correlated with LAI and biomass. Interestingly, the weight of heads (ultimately the grain yield) was highly correlated with the backscattering coefficient of the Ka- and Ku-bands, while the others were poorly correlated. The simple scattering process model may be applicable for C- and L-bands in rice canopies, while it may not be suitable for Ka- and Ku-bands. In the model, LAI was a better canopy descriptor for the C-band, while total fresh biomass was a better canopy descriptor for the L-band.