|Van Pelt, Robert - Scott|
Submitted to: International Soil Conservation Organization (ISCO)
Publication Type: Proceedings
Publication Acceptance Date: 1/26/2006
Publication Date: 5/15/2006
Citation: Van Pelt, R.S., Zobeck, T.M., Popham, T.W. 2006. Factors influencing the stochastics of near-surface wind speeds in wind storms[abstract]. International Soil Conservation Organization Conference.
Interpretive Summary: Wind erosion results from the interaction of high velocity winds and unprotected soil surfaces. Computer models have been developed to assess the effects of soil management on potential wind erosion. The more recent models are mechanistic and have wind generators in them that calculate a range of instantaneous wind speeds based upon the reported average wind data for a location. Further, wind speed data are reported for the 2 m height rather than the soil surface. We looked at the statistical distributions of soil loss from 172 wind events at Big Spring, Texas. We also investigated the wind speed profiles over several agricultural field surfaces to determine the relationship between 2m mean values for wind speed and the instantaneous wind speed at 1 cm above the surface. We found that one half the total soil loss could be accounted for by the strongest 10% of the total number of wind events. We also found that a mathematical function adequately describes the instantaneous surface wind speed for a range of wind speeds up to a certain surface-specific 2m average. Above this surface-specific 2m average wind speed, the intensity of turbulence increases markedly and the wind speed at the surface and energy available for erosion increase exponentially. We believe this explains why certain wind storms result in much, much more soil loss and dust generation than the average event.
Technical Abstract: Wind erosion event intensities vary greatly from occasional saltation during afternoon wind gusts to very intense storms that result in large amounts of soil loss and fugitive dust. We examined the soil loss data from 172 wind erosion events during eight years of wind erosion measurements made at Big Spring, Texas, USA to determine the range of variability. In order to understand the effects of surface soil conditions and crop residue cover on near-surface wind speeds, we instrumented several fields with different soil surface conditions and residue covers with wind profile masts with anemometers placed at 0.01, 0.5, 1, and 2 m heights. Wind speed data were collected at 1 s intervals when the hourly average wind speed at 2m was > 3.5 m s-1. We found that 2m 1 minute mean wind speeds < 13 m s-1 resulted in normally distributed near surface wind speeds with variances that increased linearly with 2 m wind speed and that the 95th percentile near surface wind speed was always less that the 2 m mean. However, for 2 m 1 minute mean wind speeds > 13 m s-1, the surface wind speeds were no longer normally distributed, the variance increased much more rapidly, and the 95th percentile of near surface wind speed was often greater than the 2 m mean wind speed. The presence of soil surface roughness tended to reduce this effect slightly and the presence of surface crop residues appeared to eliminate it entirely.