MAGNITUDE AND FREQUENCY OF BLOWING DUST ON THE SOUTHERN HIGH-PLAINS OF THE UNITED-STATES, 1947-1989

The concept of magnitude and frequency in geomorphology maintains that the amount of work done on a landscape (the amount of sediment moved) by geomorphic events is determined by the product of the magnitude of the event and the frequency with which that sized event occurs. For m any geomorphic systems, it has been found that events of moderate size produce the most sediment transport. This seems not to be the case fo r aeolian processes on the Southern High Plains. Using meteorological records on visibility reductions caused by blowing dust-a record of th e relative amount of dust transported by the wind-at Lubbock, Texas on the Southern High Plains for the period 1947-1989, we find that small magnitude and high frequency events move the most material; that rare , large magnitude events account for considerable transport as well; a nd, contrary to findings for many other geomorphic systems, moderate e vents move relatively little sediment. Magnitude and frequency analysi s conventionally assumes an identifiable relationship between force ap plied and the amount of sediment moved. On the Southern High Plains, t he soil's resistance to erosion and the vegetation's modification of w ind energy are both highly variable in time and space making it unlike ly that the sediment transported by two winds of the same speed will b e identical. In cases such as these where the resistance to erosion an d the effectiveness of the force vary in space and time, magnitude and frequency relations cannot be easily predicted.