Diurnal variations of radar backscatter from a vegetation canopy

Measurements of the radar backscattering coefficient of densely planted fields of sorghum were conducted using a radar spectrometer mounted atop a 20 m truck-mounted boom. Data were collected at 8 frequencies between 2 and 8 GHz, 2 polarizations (HH, VV) and 6 incidence angles (0\degto50\degfrom nadir in10\degsteps). The objective of the experiment was to determine the radar response to soil moisture variations underneath a continuous vegetation canopy with soil surface roughness and canopy height as parameters. Four fields were prepared having different surface roughness-plant height combinations. After sprinkling the four fields with water for about 12 h and allowing adequate time for the water droplets on the leaves to evaporate, radar backscattering measurements were recorded for the four fileds on a rotating basis for several days. Analysis of the scattering data indicates no discernible sensitivity to soil moisture variation, which is attributed to severe attenuation by the continuous cover vegetation canopies (the shortest plant height was 90 cm). It was observed, however, that the backscattering coefficient exhibited a definite diurnal variation pattern. Furthermore, this pattern appears to be sensitive to frequency and incidence angle but lacks sensitivity to polarization.