165

2017

181–189

Soil & Tillage Research

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Sand bed length is an important parameter that affects soil wind erosion. Although there has been a series of studies about the effect of sand bed length on wind erosion rate, many discrepancies have been found on how to express the change of wind erosion rate along bed length. To explore these discrepancies, a Trimble CX 3D laser scanner was used to continuously scan for changes in a sand bed at three friction velocities with four durations and the wind erosion mass was measured by weighing bed sediments before and after each experiment at four friction velocities with different durations in a wind tunnel. The results showed: (1) The bed form consisted of three zones, a pre-bowl zone, a bowl zone and a post-bowl zone, respectively at friction velocities of 0.35 and 0.41 m/s or two zones with the pre-bowl zone missing at high friction velocity of 0.47 m/s. The length of the pre-bowl change zone decreased with wind velocity and was independent of time; the length of the bowl zone decreased with wind velocity and increased with time; the length of the post-bowl zone occupied over 70% of the bed characterized by regular ripples with wavelengths and heights increasing with wind velocity and run duration. (2) The wave length and height of sand ripples increased with friction velocity and time. The frequency distribution of wave lengths followedf ðxÞ ¼ aeðlnðx=bÞÞ2=2c2 , (a, b and c are the fitting coefficients); while we did not find a uniform function to characterize frequency distribution of wave height. 3) Wind erosion rate along the sand bed could be described by f(x) = fmax(1 exp(xx0)/b) + f0, where fmax is the wind erosion rate at the dynamic equilibrium stage, b is the corresponding sand bed where wind erosion rate is equal to 0.63 fmax and f0 is the wind erosion rate for the sand bed between the initial bed position and above the first inflection point of wind erosion rate along the bed. These results can help to describe the effect of sand bed on soil wind erosion.

EA 40

1397/10/03

Dashagha

41