Effect of grain size on remotely sensed spectral reflectance of sandy desert surfaces

The effect of soil surface texture on spectral reflectance is reported for a site in the Mojave Desert. Abandoned central-pivot agricultural fields in the Manix Basin of southeastern California have introduced deflationary surfaces into the otherwise stable, armored desert surface. This has resulted in sand plumes, eroded from the fields by wind, transported by saltation, and deposited downwind of the fields. Grain size analysis of this wind-transported material reveals a fractionation by size within the plume, with smaller effective particle size toward the toe of the plume. This fractionation results from the greater mobility of smaller particles, and the longer saltation paths they take once airborne. Radiative transfer modeling of quartz grains with absorbing rinds indicates that the differences in grain size observed in the field should be revealed in apparent surface reflectance and are resolvable within the noise-equivalent delta-reflectance of the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) instrument. Analysis of AVIRIS-derived apparent surface reflectance demonstrates the expected negative correlation between effective grain size of sand in the plume and reflectance, with the most significant correlations in the short-wave infrared. The change in reflectance per mm change in particle diameter was −0.06 at λ∼1.7 μm and −0.08 at λ∼2.2 μm with R2=0.89 and 0.93, respectively.