A framework of field observations and spatial data for understanding dust emissions in the Mojave Desert

Modeling dust events at landscape to regional scales requires field observations of dust-source characteristics, mapping of source types by remote sensing, and wind fields representing the conditions that mobilize dust from the surface. A conceptual framework has been built for understanding dust-source types and their dynamics in the Mojave Desert. Observations of dust events in the Mojave indicate five general source types: 1) Sparsely vegetated surfaces that are vulnerable during periods of drought; 2) wet playas where a near-surface groundwater table generates "fluffy" (very soft sediment) conditions; 3) transitional playas where groundwater extraction has lowered the water table, and playa surface composition produces sediments that are vulnerable to erosion; 4) ephemeral flood deposits; and 5) anthropogenic sources where off-road vehicles, military training exercises, and dirt roads create a disturbed surface. Some sources are perennial and others are strongly influenced (sometimes in opposite ways) by precipitation cycles. A multi-year study of precipitation, vegetation, winds, and saltation at several plots in the Mojave National Preserve shows that blooms of annual vegetation in wet years can leave biomass that protects the surface for more than a year after the rains. Monitoring of the wet Franklin Lake Playa shows that a shallow ground-water table is associated with more vulnerable conditions for dust emission. Repeat photography of the relations between winds and dustiness at transitional Mesquite Lake Playa shows that dust is mobilized during the spring when winds are greater than about 5 m/s. Satellite images reveal dust emission from ephemeral fluvial systems, such as the Mojave River Sink, at the end of wet spring seasons. Satellite images also document dust emissions from areas of heavy military and off-road vehicle activity. Landsat imagery was used to map perennial vegetation cover for the Mojave Desert, calibrated to 250 field transects. The perennial vegetation map was used in conjunction with a Landsat-derived surface-sheltering map to identify areas vulnerable to dust emissions due to sparse vegetation. A map of playa lakes, derived from USGS 1:100,000 maps, satellite imagery, and field observations establishes the boundaries of wet, transitional, and dry playas. Wind fields generated from a regional climate model show that intense winds in vulnerable regions are associated with contemporaneous dust plumes observed from geostationary satellites during the major dust storms of January 5, 2007 and March 27, 2007. Through systematic collection of data from the scale of field plots to satellite images, a framework for the modeling of dust generation for a geographically complex and temporally dynamic region like the Mojave Desert is being created.