Moisture mapping in the Amazon Basin using near-nadir and far-range backscatter slopes
The electromagnetic scattering from vegetated targets has both coherent and non-coherent scattering components that correspond to contribution from underlying soil and vegetation canopy, respectively. The non-coherent contribution is higher at far-range incidence angles (θ) than near-nadir θ. The relative contribution of coherent component increases for lower canopy thickness. The relative contribution of coherent and non-coherent scattering components is also effected by the moisture content of canopy and soil. We use normalized radar backscatter (σ°) measurements from ERS Scatterometer (ESCAT), NASA Scatterometer (NSCAT) and Tropical Rainfall Measuring Mission Precipitation Radar (TRMM-PR) to analyze the moisture characteristics and its spatial and temporal variability in the Amazon basin. The amount of coherent scattering is indicated by high near-nadir slopes of σ° θ-response which depends on surface scattering contribution. The rate of decrease in σ° from nadir direction to near nadir directions (θ < 5°) is found to be very sensitive to the vegetation density. The slope drops significantly as the θ increases to 10°, beyond which the non-coherent scattering dominates. For very dense vegetation the near nadir σ° has only non-coherent scattering and hence slope is low. Densely vegetated targets do not exhibit near nadir specular reflection and thus slope increases gradually with increase in θ. We define the Normalized Near-nadir and Far-range Slope Difference (NNFSD) as a discriminant to quantify the relative contribution of coherent and non-coherent scattering. The presence of water bodies (rivers and ponds) significantly increase the specular reflection of electromagnetic waves at near-nadir θ. NNFSD is used to identify large scale water-bodies in the Amazon basin and monitor their spatial and temporal behavior.
Environmental Monitoring | Geographic Information Sciences
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Long, D. G.,
Moisture mapping in the Amazon Basin using near-nadir and far-range backscatter slopes.
2005 AGU Spring Meeting
American Geophysical Union.