Combining multifrequency microwave and optical data for crop management

The potential for the combined use of microwave and optical data for crop management is explored with the use of images acquired in the visible, near-infrared, and thermal spectrum and the synthetic aperture radar (SAR) wavelengths in the Ku (14.85 GHz) and C (5.3 GHz) bands. The images were obtained during June 1994 and covered an agricultural site composed of large fields of partial-cover cotton, near-full-cover alfalfa, and bare soil fields of varying roughness. Results showed that the SAR Ku backscatter coefficient (Ku-band σ°) was sensitive to soil roughness and insensitive to soil moisture conditions when vegetation was present. When soil roughness conditions were relatively similar (e.g., for cotton fields of similar row direction and for all alfalfa fields), Ku-band σ°was sensitive to the fraction of the surface covered by vegetation. Under these conditions, the Ku-band σ°and the optical normalized difference vegetation index (NDVI) were generally correlated. The SAR C backscatter coefficient (C-band σ°) was found to be sensitive to soil moisture conditions for cotton fields with green leaf area index (GLAI) less than 1.0 and alfalfa fields with GLAI nearly 2.0. For both low-GLAI cotton and alfalfa, C-band σ°was correlated with measurements of surface temperature (T(s)). A theoretical basis for the relations between Ku-band σ°and NDVI and between C-band σ°and T(s) was presented and supported with on-site measurements. On the basis of these findings, some combined optical and radar approaches are suggested for crop management applications.