Characterization of saline soils using airborne radar imagery

Complex dielectric constants determined by inversion of the polarized returns of AIRSAR images acquired in wet conditions delineate the distribution of saline soils in irrigated regions around the town of Pyramid Hill, in western Victoria, Australia. There is good agreement between the areas delineated as having anomalous dielectric constants by the radar backscatter inversion techniques with saline areas as defined by electrical conductivity and as inferred from dielectric constants determined in the field. Surface roughness maps and vegetation classification maps derived from AIRSAR data provide useful ancillary information regarding the extent of salinity in the region, but they are not as diagnostic as the determined dielectric constants. The magnitudes of P band radar-determined dielectric constants most closely approach those expected from field determinations, although the distribution of L-band-determined dielectric constants gives the best discrimination between saline and nonsaline areas as seen at the surface. C-band-determined dielectric constants are much lower than expected from field determinations and show saline areas as anomalous only in the bare, most severely affected, areas. The more general regional distribution of high dielectric areas seen at P band may be a more accurate indication of the salinity at depth. The AIRSAR image inversion techniques generally underestimate the magnitude of the complex dielectric constant. AIRSAR data for salinity mapping should be acquired under wet conditions. In such conditions there is an electrical continuum between the wet ground surface and saline water within the pores in the upper soil horizons. A wet ground surface results in a higher total power return than that which would occur under dry conditions and in a higher contrast between the radar-dark and radar bright areas within the scene.