wind erosion mechanisms saltation serious if v > 21 kph @ 0.3 m 0.1 to 0.5 mm suspension dust...

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  • WIND EROSION

  • Mechanisms

  • Saltationserious if v > 21 kph @ 0.3 m 0.1 to 0.5 mm

    Suspensiondust storms can transport dust across continents (as happened at Bangor in 1988)size usually < 0.1 mm

  • 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".

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

  • winds below 19 ms-1 would notcontributeAnalysis of wind speed for one direction

  • Wind erosivity index for direction j is calculated from:where i designates velocity (m s-1) band and j the directionvij 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, jIn the example, Ij would be t19.5x 0.53 + t20.5 x 1.53+ t21.52.53 etcThis is repeated for each section of the compass.

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

  • Texture

    More erosion if soil high in particles between 40 - 300

    ...

  • silt size

  • OMWorse if high unless un-decomposed

    Limepresence of calcrete (petrocalcic horizon) on surface leads to reduced erosionbut if rock or calcic soil becomes powdery, increased erosion results

    Aggregate stability

  • Wind erodibility of soilOne 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)

    whereX1 = < 1 size range X2 = 50 - 250 X3 = > 250

    See web site for WEPS - wind erosion prediction system linked from the Wind Erosion Research Unit

  • Prevention

    Stubble mulch increased drag effect on wind traps particles travelling through saltation

  • 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

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

  • (c) velocity profiles plotted onlinear scale(d) velocity profiles plotted onlogarithmicscale

  • Minimum tillage

    Reduces wind erosion as well as water erosion though little hard research data of measured erosion ratesWind strip cropping

    Plant @ right angles to average wind direction Alternate tall and short crops

  • Soil texture

    Recommended width (meters)

    sand

    6

    loamy sand

    7.5

    sandy loam

    30

    silty clay

    45

    loam

    75

    silt loam

    85

    clay loam

    106

    silty clay loam

    137

    Table of recommended widths of strips for the control of wind erosion

  • Wind breaks

    Protect for 10 to 20 x height of trees leeward and 5 x windward

    Design considerations:make as long as possibledo not plant too densely or eddies will form and increase erosionplant several rows with smaller bushes / trees on either side of treesalign roads at angle to wind

  • 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

  • Windbreaks which are too wide will reduce the efficiency -----

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

  • 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.

  • Dune stabilisation

    fences, reeds, etc. for micro wind breaks

    chemical mulching

    re-vegetation

  • after Troeh, Hobbs and Donahue, 1980 and & Morgan p 35