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Analysis of Wind Data Used for Predicting Soil Erosion - USDA-ARS ...

Analysis of Wind Data Used for Predicting Soil Erosion - USDA-ARS ...

Table 1. Summary data

Table 1. Summary data for 14 analyzes wind events. Data in red are shown in Figure’s 3-6.Event Date Total Exceedance U 3m U t-eventminutes minutes ms -1 ms -15/23/96 2977 417 8.82 7.876/11/96 1565 480 9.81 8.546/26/96 2103 187 9.34 8.797/16/96 1613 184 9.01 8.547/19/96 1545 211 9.40 8.688/5/96 1194 575 8.01 6.989/17/96 1527 395 8.41 7.345/30/95 1940 179 9.93 8.896/12/95 1767 493 9.50 8.197/7/95 3896 1249 9.47 7.497/24/95 1736 722 8.29 7.267/31/95 1725 844 8.62 7.268/14/95 3252 990 8.78 7.588/31/95 3446 1658 8.15 7.33Results from 2 wind events (6-26-96 and 8-14-95) using Equation (5) to averageexceedance wind speed data over the 15 and 60 minute averaging periods are shown in Figure’s3, 4, 5 and 6. The 2 wind events used for these figure’s span a time period from minimumerosion to the beginning of the most severe erosion on the Columbia Plateau. Field surfaceconditions also change significantly over the summer fallow season as tillage reduced surfaceroughness and residue cover. Therefore, wind erosion events which occur in late summer andearly fall are often exacerbated compared to spring and early summer erosion events.These figure’s show that the 3.0 m erosive wind energies based on 15 minute averagingperiods (Fig’s 3, 5) are significantly correlated when compared to those based on a 60 minuteaverage (Fig’s 4, 6). Table 2 contains ratios between W* e based on 1 minute averages and W* ebased on 15 and 60 minute averages for all 14 wind events analyzed. These ratios indicate themagnitude which 15 and 60 minute W* e values would have to be multiplied by to result in thesame wind energy as when using 1 minute averages. In general, regardless of exceedanceminutes, W* e based on 15 minute averages contain the least variance from the 1 minute average.W* e based on 60 minute averages fluctuate over a wide range and alone, are not good predictorsof the energy contained in the wind. Figure 7 shows these ratios plotted as a function ofexceedance minutes and indicates that as exceedance minutes increase, the ratios of the 1:15 and1:60 minute averaging periods decrease. There is, however, a 3-fold difference between the ratesof decrease. On the Columbia Plateau, an increase in wind speed is a common characteristic ofan increase in event duration. Accompanied with the higher wind speed is the likely hood that thenumber of 15 minute averaging periods above threshold also increases. Therefore, the 1:15 ratioasymptotically approaches unity as a function of exceedance minutes and provides the highestcorrelation to 1 minute wind energies.6

2001203.0 m Energy, 1 min16012080400y = 1.9x + 3R 2 = 0.950 50 1003.0 m Energy, 15 miny = 5.4x + 12R 2 = 0.21Figure 3. 1 minute vs. 15 minute wind energies Figure 4. 1 minute vs. 60 minute windfor event of 6-23-96. energies for event of 6-23-96.3.0 m Energy, 1 min1008060402000 5 10 153.0 m Energy, 60 min100300250200150100500y = 1.04x + 6.1R 2 = 0.930 100 200 3003.0 m Energy, 15 min3.0 m Energy, 1 min806040200y = 1.3x + 5.2R 2 = 0.740 20 40 603.0 m Energy, 60 minFigure 5. 1 minute vs. 15 minute wind energiesFigure 6. 1 minute vs. 60 minute windfor event of 8-14-95. energies for event of 8-14-95.Calculated wind energy is also significantly affected by the total minutes in the averaging periodwhere threshold conditions were exceeded. The frequency distribution of minutes abovethreshold for 15 and 60 minute averaging periods for a typical wind erosion event (8-14-95) isshown in Figure 8. Over the duration of this event there was a total of 188 periods from which a15 minute average was calculated. By contrast, there was 49 hourly periods from which an hourlyaverage was calculated. This figure shows the distribution of total exceedance minutes containedin each of the 15 and 60 minute averaging periods.7

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