The Mediterranean coastal dunes of Israel underwent a land-use change during the second half of the 20th century. Due to intense agricultural and pastoral activity, the coastal dunes were stripped of natural vegetation until the end of the first half of the 20th century. The barchan and transverse dunes were shaped by strong southwesterly winter winds. A decrease in human activity during the second half of the 20th century brought about a renewal of natural vegetation on the dune crest -the only area with neither erosion nor deposition. The establishment of vegetation on the crest changes the dynamics of these barchan and transverse dunes, so that not all of the sand eroded from the windward side is carried to the lee slip-face; some is trapped by plants. Consequently, there is a change in the shape of the windward slope from convex to concave, and the dune gradually becomes parabolic. In this study we trace the morphodynamics of the dunes by analysing 12 sets of aerial photographs, which were taken from 1944 to 1995. The average rate of advance of 15 dunes has decreased from 3Ð4 m a 1 to 1Ð9 m a 1 , while the vegetation cover has increased from 4Ð3 to 17 per cent during this period.

A field experiment was conducted to measure the flux and speed of wind-blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary-layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0-5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an 'egg-box' pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand-trap systems used in previous experiments, 1 2 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high-speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under-collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0-5-7.0 m s-', compared to a wind friction velocity of 0.32-0.43 m s-l and a wind velocity of 2.7-3.9 m s-' at the height of the particle measurements. Descending particles tended to exceed the speeds of ascending particles by -0.5 m s?. study the flux of wind-blown sand, including field measurements (e.

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Abstract. Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coetficient cr ø are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between cr ø and z0 for L band (X: 24 cm) HV (H, vertically and V,vertically polarized modes) L band HH, and C band (X -5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems. IntroductionAeolian processes are prominent in arid and coastal regions but can occur wherever loose particles are available for movemcnt by the wind. In addition to hazards created by sand and dust storms, the migration of sand over roads, agricultural fields, and other cultural features has negative economic impacts. Moreover, because aeolian processes involve the interaction of the atmosphere and Earth's surface. they can influence the climate. Aeolian processes can occur frequently and on short timescales, resulting in rapid surface changes. Remote sensing affords the opportunity to monitor these changes and, in some cases, allows the prediction of potential aeolian activity. Remote sensing in the visible spectrum has been used to map sand dune fields [e.g., Breed et al.