Passive Microwave Remote Sensing of Soil Moisture: The Effect of Tilled Row Structure

The tilled row structure is known to be one of the important factors affecting the observations of the microwave emission from a natural surface. Measurements of this effect were carried out with both L-and X-band radiometers mounted on a mobile truck on a bare 40 m × 45 m row tilled field. The soil moisture content during the measurements ranged from ~10 to ~30 percent by dry weight. The results of these measurements showed that the variations of the antenna temperatures with incident angle ¿ changed with the azimuthal angle ¿ measured from the row direction. In particular, at ¿ = 0° and ¿ ¿ 45°, the observed horizontally and vertically polarized antenna temperatures, TBH(¿, ¿) and TBV(¿, ¿), were not equal. In general, TBH(¿°, ¿) > TBV(¿°, ¿) when 0° ¿ ¿ < 45° and TBH(¿°, ¿) < TBV(0°, ¿) when 45° < ¿ ¿ 90°. The difference between TBH(0°, ¿) and TBV(0°, ¿) was observed to decrease with ¿ approaching 45° and/or with soil moisture content. A numerical calculation based on a composite surface roughness-a small-scale RMS height variations superimposed on a large periodic row structure-was made and found to predict the observed features within the model´s limit of accuracy. It was concluded that the difference between TBV(0°, ¿) and TBH(0°, ¿) was due to the change in the local angle of field emission within the antenna field of view caused by the large-scale row structure.