LARGE-SCALE VARIABILITY OF WIND EROSION MASS FLUX RATES AT OWENS LAKE.2. ROLE OF ROUGHNESS CHANGE, PARTICLE LIMITATION, CHANGE OF THRESHOLD FRICTION VELOCITY, AND THE OWEN EFFECT

Variability in airborne particles larger than sand flux for distance s cales larger than 200 m was observed during wind erosion episodes on t he northeast side of a dry lake bed (Owens Lake). Measurements were ma de during erosion episodes on a similar to 3-km line of wind measuring and sand flux collecting instruments. Data were selected for winds th at (1) aligned with the instrument line and (2) had mean speeds cubed at 4-m heights at the beginning, middle, and end of the line that diff ered by less than 5% (i.e., mean wind speeds differed by less than 1.7 %). Four mechanisms were determined to cause the large-scale differenc es in the mass flux profiles. In order of their importance, the mechan isms are as follows: (I) change of the drag coefficient (or the ratio u/U, where u* is wind friction velocity and U is mean wind speed from place to place), this is a measure of variability in roughness height ; (2) particle limitation (depletion of the loose ''available'' erodib le material on the surface); (3) variation of the threshold friction v elocity; and (4)the Owen effect (the increase of u/U with U).