Sensitivity of playa windblown-dust emissions to climatic and anthropogenic change

Windblown dust is a significant component of atmospheric PM (particulate matter) in arid regions worldwide, with adverse effects on human health and visibility. In the future, windblown-dust emissions are likely to increase if water tables drop as a result of climatic or anthropogenic changes. To manage this hazard, air-quality managers need quantitative models that predict the impact of climatic and anthropogenic change on dust emissions. To meet this need, we constructed a process-based numerical model that includes Richards’ equation for vertical moisture flow in the unsaturated zone, Chepil's model for the effect of surface soil moisture on threshold wind speed, and the saltation equation, which also predicts the rate of dust emission from the surface to within a multiplicative factor. This model is solved analytically for a Weibull distribution of wind speeds under steady-state moisture conditions, providing a single predictive equation for the long-term average saltation flux based on local meteorological and hydrological parameters. The model equations are used to predict the increase in saltation flux and dust emissions resulting from the dessication of a wet playa by climatic change, stream diversion, or groundwater withdrawal. The model is calibrated using CLIM-MET station data collected near Soda (dry) Lake, California. The model results identify a critical range of water-table depths between 3 and 10 m (depending on hydrological parameters) in which small increases in water-table depth cause large, nonlinear increases in windblown-dust emissions. For water tables deeper than 10 m, dust emissions are close to their maximum value and are largely independent of water-table depth. This analysis highlights the importance of preserving the hydrological balance of wet playas in order to minimize windblown-dust emissions. Future climatic changes may also influence dust emissions through changes in the mean or variability of wind speeds. For representative model parameters, 10% increases in the mean and variability of wind speeds, for example, are predicted to increase dust emissions by 80% and 20% within this model framework.