Temporal development of surface roughness and its relationship with water in a playa dust source: the impact of precipitation and inundation on surface properties

To model the role of dust in the Earth system and in geochemical cycles, it is critical to understand fundamental controls on dust emission in dust source areas. Surface properties such as roughness and surface strength affect dust emission by changing the erodibility of the surface. Accordingly, knowledge of temporal controls on surface properties is important. In this study, we investigate the temporal evolution of a playa dust source (the Black Rock Desert, NV, USA) over a seven year time period to elucidate the effects of water on surface roughness. We then compare the observations in the natural system to simple lab experiments simulating the development of surface roughness in the playa to improve our understanding of the processes controlling soil erodibility. In dynamic playa systems, we hypothesize that inundation by surface water is critical to surface development. To evaluate this hypothesis, we utilize radar backscatter data (ESA Advanced Synthetic Aperture Radar (ASAR) instrument) to characterize centimeter scale surface roughness and its evolution over time. Radar data from 65 images between 2004 and 2010 are compared to inundation maps derived from 472 MODIS images (1640 nm, band 6) from the seasons preceding the ASAR images. Regions of the playa that dry after May exhibit systematically increased backscatter (-13.3 dB to -11.6 dB, p < 10^-5). One potential explanation is that late drying areas have the potential to undergo more rewetting, and wetting-drying cycles, increasing surface disruption. Backscatter of areas in the playa not inundated after May are negatively correlated with annual precipitation (p = 0.09); the average backscatter in years with <150 mm of precipitation is -12.2 dB, versus -13.0 dB in years with >150 mm. A hypothesized mechanism that explains the data is smoothing of the playa surface via rain splash disruption. The association between drying conditions and surface roughness in the Black Rock Desert suggests that cycles of wetting and drying increase playa roughness. To test the hypothesis that wetting-drying cycles disrupt the playa surface, increasing surface roughness, we performed laboratory experiments. Semi-quantitative X-ray diffraction analysis of surface sediments from the Black Rock Desert demonstrates that the mineralogical composition by weight of the playa is ~30% quartz, 45% clay, 10% calcite, and 5%; thus we used a mixture of quartz and clay for our experiments, and added a solution with dissolved halite (5 wt%) or calcite (10 wt%) to simulate evaporites. Relative changes in the thickness and roughness of the experimental surfaces over time were quantified in 2D using an optical profilometer (Zygo NV7300). Over 12 wetting-drying cycles, thickness increased by 10-20%, and root mean square roughness doubled. This supports the contention that wetting and drying influence the development of density and surface roughness in the playa. Additionally, we investigated the effect of solution composition by varying the dissolved ion composition of the initial solution. Thickness increased more with solutions rich in dissolved halite; this suggests that the presence of halite on the playa can have a significant impact on surface roughness evolution.