Fetch Effect Of Aeolian Sand Transport On Farmlands In Bashang,hebei Province

Wind erosion on farmlands is the main factor that hinders the development of agriculture and animal husbandry and the improvement of ecological environment in northern China.Bashang area of Hebei Province is the important area of wind erosion control in northern China.It is particularly essential to understand the wind erosion on farmlands.Because of the prompt mode of wind erosion and the lack of definite boundary in the actual environment,the estimation of soil wind erosion on farmlands have always been the core of the research,the crucial phase of wind erosion quantitative research is to obtain the rule of fetch effect accurately.The fetch effect means that the aeolian sediment flux increases gradually with the change of the sand bed surface or the observed length,then it tends to be stable.The main parameters of fetch effect include maximum sand flux(f_max)and corresponding critical fetch length(L_sat).Based on previous studies,this paper took typical farmlands in Kangbao County,Hebei Province as the object of studies.Field experiments and models were used to simulate the variation characteristics of fetch effect on different sampling height and temporal scale on typical farmlands in Bashang,Hebei Province by model method.Discussing the temporal and spatial variation rule of fetch effect,and providing scientific reference for further evaluation of aeolian sediment transport.The results are as follows:（1）The data of aeolian sediment transport in 105 literatures were obtained from 5countries studies,which was fitted by fetch models included Stout,Andreotti,Fryrear,Visser,Logistic,Zhang Chunlai.The verification of the measured data shows that Logistic model are more suitable for describing the fetch effect of wind erosion on farmlands.（2）Calculating the sediment transport rate of continuous and discrete sampling heights.The results show that the sediment transport rate increases with the increase of the fetch length.Logistic model was used to analyze the measured data of sediment transport at different heights on the farmlands,and the important parameters of fetch effect including the maximum sand flux(f_max)and corresponding critical fetch length(L_sat)were obtained.There is a power function relationship between the maximum sand flux(f_max)and the sampling heights.With the increase of sampling height,the maximum sand flux(f_max)decrease significantly.However,the corresponding critical fetch length(L_sat)tends to change with the increase of sampling height,there are differences about the corresponding critical fetch length(L_sat)between continuous sampling heights and discrete sampling heights,which indicate that the type and standard of sediment collectors should be unified in the observation of fetch effect on farmlands,and the unified mechanism of field observation should be established.（3）The maximum sand flux(f_max)and the corresponding critical fetch length(L_sat)on farmlands of Bashang,Hebei Province both vary significantly with the temporal resolution,but the coefficient of variation（C.V）decreases with the increase of the temporal scale.Lower temporal resolution,the more significant between the maximum sand flux(f_max)and the friction velocity（u_*）,the maximum sand flux(f_max)and the friction velocity（u_*）may be expressed as a power function.However,the corresponding critical fetch length(L_sat)was not depended on natural wind for the two wind erosion events,which indicated the mechanism of the corresponding critical fetch length(L_sat)was more complex under the condition of unsteady wind on farmlands with turbulence.The exploration about factors affecting the change of the corresponding critical fetch length(L_sat)still need a large number of field measured data.