wind erosion mechanisms saltation serious if v > 21 kph @ 0.3 m 0.1 to 0.5 mm suspension dust...
Post on 16-Jan-2016
Embed Size (px)
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
Becomes more important in lower rainfall areas.
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.
More erosion if soil high in particles between 40 - 300
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
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
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
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
Recommended width (meters)
silty clay loam
Table of recommended widths of strips for the control of wind erosion
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.
fences, reeds, etc. for micro wind breaks
after Troeh, Hobbs and Donahue, 1980 and & Morgan p 35