Measurement of water content as a control of particle entrainment by wind



Of all controls on particle transport by wind, which include texture, crusting, vegetation cover and roughness, the role of water content is one of the most difficult to parameterize because of its high degree of spatial and temporal variability and its operation at a particle-scale level directly at the surface. This study demonstrates that measurement of the distribution of brightness for all pixels in an image, now routinely employed in digital photography, is strongly correlated with gravimetric water content. Wind tunnel experiments further suggest that measurement of the distribution of β, as normalized against the brightness of the dry sand surface, is very useful in determining the order of magnitude of the mass transport rate (q). Finer resolution will likely never be achieved because of the heterogeneity of the particle transport phenomenon. Analysis of the variability in surface brightness does suggest that q is governed by the partitioning of momentum to particle motion that terminates in adhesion to surrounding areas of the surface that remain relatively wet. The proportion of surface particles that becomes dry appears to be of less importance. Preliminary work suggests that field application of digital photography in tracking spatial and temporal changes in the water content of beach deposits looks promising. Copyright © 2006 John Wiley & Sons, Ltd.