Author
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Hagen, Lawrence |
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Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/7/2007 Publication Date: 12/5/2007 Citation: Hagen, L.J. 2007. Ratios of erosive wind energies on dry days and all days in the western United States. Transactions of the ASABE. 50(6):1981-1986. Interpretive Summary: Do wind speeds differ significantly on wet and dry days? If so, could these differences affect the accuracy of models that use simulated wind speeds to predict wind erosion? Statistical tests were made on hourly wind speed distributions for each month on wet and dry days from 46 weather stations in the western United States. About 87% of the wet and dry day monthly pairs of distributions were different. To determine the potential impact of differing distributions on erosion predictions, monthly erosive wind energies were calculated at each station. Over large areas in the western United States, the erosive energy in the dry day distributions was less than that in the all day distributions. In contrast, at a few stations with low precipitation, the all day distributions had the most erosive wind energy. Because wind erosion occurs primarily on dry days, erosion models that use simulated winds could be modestly improved by accounting for differences in wet and dry day wind speed distributions. Technical Abstract: Stochastic wind speed generators generally use statistical data derived from all available hourly wind speeds. This study was undertaken to determine if wet and dry day distributions of hourly wind speeds were different. Differences might cause errors in wind erosion predictions when modelers assume wind and rain events are not correlated. Hourly wind speed and precipitation data were obtained from the National Climatic Data Center SAMSON data set at 46 selected observation stations in the western United States. Wind speeds were sorted into 25 classes and a calm class (0 to 0.5 m s-1). After removal of calm periods, distributions were created for all days, dry days, and wet days. The wet days comprised wind speeds from the initial hour of precipitation and the 23 succeeding hours. Among 552 pairs of wet-day and dry-day cumulative monthly wind speed distributions, 87 percent were from different populations (significant at 0.10 level or less based on a Kolmogorov-Smirnov test). To determine the importance of these differences, monthly ratios of erosive wind energy were calculated from dry-day and all-day distributions. These differed among months, stations, and erosion threshold wind speeds. Over much of the area, however, the erosive wind energy was lower on dry days than on wet days. The eastern Great Plains and western Washington near Spokane had the lowest ratios. Hence, use of an all-day wind speed distribution at these locations likely overestimates potential soil loss from wind erosion. In contrast, a few of the low- precipitation areas in the west had ratios that were consistently greater than one. In summary, accuracy of predicted wind erosion from physically-based models can be modestly improved by accounting for differences in wind speed distributions on wet and dry days. |
