The temporal variability of centimeter-scale surface roughness in a playa dust source: Synthetic aperture radar investigation of playa surface dynamics

Abstract

Emission of mineral dust aerosols is highly dynamic, in part due to variability in surface erodibility. Investigation of the variations of surfaces within dust source regions has the potential to elucidate the processes that control erodibility and to improve model representations of dust emission. In this study, we investigate surface temporal variability in the Black Rock playa (Nevada, USA) using synthetic aperture radar (SAR) satellite data from between 2004 and 2010. The SAR backscatter (σ0) observations of the playa surface are compared to weather station observations and temporally-resolved measurements of water inundation derived from multi-spectral MODIS satellite data. The data illustrate that centimeter-scale surface roughness on the playa is surprisingly heterogeneous over multi-meter length scales and evolves dynamically on annual time scales. Interannual changes in surface roughness on the playa are quite large compared to the variability of surface roughness during the summer months (July–September), suggesting that summer anthropogenic and aeolian processes are substantially less important than water-related processes for controlling the evolution of the playa surface. Playa median σ0 is relatively low in years with high annual precipitation (>140mm), suggesting that surface water controls centimeter-scale surface roughness and, potentially, erodibility. Spatially, there is a relationship between σ0 and the time of year at which a surface dries. Areas drying in July have significantly higher σ0 than surfaces that dry earlier in the year, suggesting that spatial heterogeneity in σ0 is controlled by cycles of wetting and drying at the edge of the playa lake. The implications in the Black Rock playa, and potentially more broadly, are that water is a critical factor controlling playa surface evolution, and the annual time scale is crucial for investigations of playa erodibility.