Controls of dust emission fluxes and wind erosion threshold on a wet playa

The control of dust emissions from crusted surfaces is both highly variable and difficult to measure directly. Seasonal changes in moisture availability, temperature, evaporation, surface roughness, and sediment supply result in a highly complex surface condition that remains to be fully described in the context of wind erosion potential. A highly intensive project on Sua Pan, Botswana using the PI-SWERL (portable wind tunnel) combined with surface measurements of crust and soil properties has led to a new understanding of the controls on wind erosion from these surfaces. The PI-SWERL is a highly portable wind tunnel that applies a wind shear to the surface using a motor-controlled rotating annular blade and measures resulting dust emissions with a DustTrak dust monitor. We undertook a sequence of tests with the PI-SWERL to obtain both the wind erosion threshold (using a slowly increasing shear velocity) and a dust emission flux (using a constant shear velocity) across a 12 km by 12 km grid across the pan surface. A total of just under 1000 wind tunnel tests and 2000 correlated measurements of a variety of surface properties including crust thickness, surface and subsurface soil moisture, shearing strength (shear vane), normal stress resistance (penetrometer), and surface roughness were conducted in August 2011. These results show that wind erosion potential is best described by measurements of normal stress resistance rather than shearing strength at low dust emission fluxes, but despite their frequent use in wind erosion studies of crusted surfaces neither metric provided a good explanation of higher dust emission fluxes. Surface soil moisture explained the most variation in both dust emissions and wind erosion threshold although much variation remains unexplained. Our results suggested that combining measurements of surface roughness, soil moisture, and crust thickness provided a reasonable explanation of wind erosion potential on the salt pan surface. As pan surfaces can exhibit a range of aerodynamic roughness lengths over three orders of magnitude the small-scale partition of wind stress could be considered. Surface soil moisture also had a very large range in which a relatively sharp threshold was found to increase dust emissions when combined with other surface factors. Although the role of surface moisture in dust emissions is understood it remains a very difficult (yet critical) parameter to measure and a call for more precise estimations of this metric is highly encouraged.