WIND EROSION

1. WIND EROSION

4. Mechanisms

5. Saltation serious if v > 21 kph @ 0.3 m 0.1 to 0.5 mm Suspension dust storms can transport dust across continents (as happened at Bangor in 1988) size usually < 0.1 mm

6. Contributing factors Climate Becomes more important in lower rainfall areas. Wind erosivity Record wind speed & total time for each velocity increment in each segment, 1 to 16, of the compass. Minimalist approach - plot number of days for which wind blows in each direction to guide. Make length of the radius equal to number of days. Result is known as a "wind rose".

7. The histogram will look something like the following for each section of the compass (1 to 16)

8. Analysis of wind speed for one direction winds below 19 ms-1 would not contribute

9. Wind erosivity index for direction j is calculated from: where i designates velocity (m s-1) band and j the direction vij is the mid-point of the velocity band less 19 m s-1, the threshold velocity) fij is the total amount of time wind in that direction that is within a particular velocity band, i.e. for each compass point, j In the example, Ij would be t19.5x 0.53 + t20.5 x 1.53+ t21.52.53 etc This is repeated for each section of the compass.

10. This is an empirical equation; wind kinetic energy is proportional to v2 but v3 seems to work better.

11. Texture More erosion if soil high in particles between 40 µ - 300 µ ...

12. silt size

13. OM Worse if high unless un-decomposed Lime presence of calcrete (petrocalcic horizon) on surface leads to reduced erosion but if rock or calcic soil becomes powdery, increased erosion results Aggregate stability

15. Wind erodibility of soil One equation is given as follows but does not include factor for aggregate stability, organic matter content or lime: Kw = 100 - (34.7 + 0.9 X1 - 0.3 X2 - 0.4X3) where X1 = < 1µ size range X2 = 50 - 250 µ X3 = > 250 µ See web site for WEPS - wind erosion prediction system linked from the Wind Erosion Research Unit

16. Prevention • Stubble mulch • increased drag effect on wind traps particles travelling through saltation

18. Effect of vegetation on wind velocity near the surface Plane of zero wind velocity occurs at height of about 70 per cent of the height of vegetation above the ground surface

19. (b) crop cover raises height of the effective surface by a distance D and also increases the value of z0 (a) zero wind velocity occurs at a height z0 which lies above height of mean surface;

20. (c) velocity profiles plotted on linear scale (d) velocity profiles plotted on logarithmic scale

21. Minimum tillage Reduces wind erosion as well as water erosion though little hard research data of measured erosion rates • Wind strip cropping • Plant @ right angles to average wind direction • Alternate tall and short crops

25. Wind breaks • Protect for 10 to 20 x height of trees leeward and 5 x windward • Design considerations: • make as long as possible • do not plant too densely or eddies will form and increase erosion • plant several rows with smaller bushes / trees on either side of trees • align roads at angle to wind

26. Wind breaks: • reduce evaporation • increase yields (10 to 20% in average year, 50 to 60% in dry year in parts of Kenya) • stone walls and hedges also reduce erosion and evaporation

29. Windbreaks which are too wide will reduce the efficiency -----

30. It is important that wind breaks consist of species at different levels - not just one (usually tall) species

31. Crop yield normally declines in the vicinity of a wind break but many researchers have found that away (about 1 to 1.5 tree heights) away from the wind break, yields increase, mostly because of reduction of evapotranspiration losses.

35. Dune stabilisation • fences, reeds, etc. for micro wind breaks • chemical mulching • re-vegetation